For Professional Engineers

Transcription

Professional Development Hours
for Professional Engineers
All the hours you need
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Additional topics are available at
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About your Professional Development Hours
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Review the course material in this book.
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Do you offer additional courses?
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How do I know my state will accept your courses?
Although most boards of Professional Engineers do not pre-approve courses or professional development providers,
we would like to reassure you that McKissock has developed these courses with your best interest in mind. We
have over 20 years of experience offering professional development courses to licensed professionals and pride
ourselves on the quality of education we offer.
We currently hold provider and/or course approvals in the following jurisdictions:
•
•
•
•
•
Florida Board of Professional Engineers, 0004610
Maryland Department of Labor, Licensing and Regulation
New York State Board for Engineering and Land Surveying, #49
North Carolina Board of Examiners for Engineers and Surveyors S-0502
Louisiana Professional Engineering and Land Surveying Board Pending approval in additional jurisdictions as of 1/15/14. Check our website for the most up-to-date approval
information available.
Course Instructors/Authors
In addition to state board approval status, we would like to share the credentials of a few of our highly qualified
course instructors:
EMAD HABIB, Ph.D., P.E.
Dr. Habib is a registered Professional Engineer in Louisiana and Texas, a professor in the Department of Civil
Engineering at the University of Louisiana at Lafayette, and a consultant engineer. Prior to beginning his career at
the University of Louisiana in 2003, he worked as a graduate assistant at the Iowa Institute of Hydraulic Research
as a PhD candidate, and then an assistant professor of Civil and Environmental Engineering at Tennessee
Technological University.
FREDERICK BLOETSCHER, Ph.D., P.E.
Dr. Bloetscher is a registered Professional Engineer in North Carolina, Florida, South Carolina, Utah, Colorado,
Tennessee, Michigan and Ohio. He is an assistant professor at the Department of Civil Engineering at Florida
Atlantic University. He received his BS in Civil Engineering from the University of Cincinnati, a Master of
Public Administration degree, 1984, University of North Carolina at Chapel Hill; emphasis on local government
management and finance, minor in planning, Ph.D. in Civil Engineering, 2001, University of Miami, with emphasis
on risk analysis, groundwater resources and utility management and planning.
WILLIAM HAMMACK, Ph.D., P.E.
Dr. Hammack is a tenured full professor of Engineering at the University of Illinois, an American Society of
Engineering Education award winner, a former Senior Science Advisor for the US Department of State, and a
regular commentator for NPR’s marketplace. Prior to his career at the University of Illinois, Dr. Hammack was a
professor in the Department of Engineering at Carnegie Mellon University, Pittsburgh, PA, and a professor in the
Department of Chemical & Biomolecular Engineering at the University of Illinois at Urbana-Champaign.
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State
Mandatory Topics
Hours
Required
AL
None
15
AK
None
24
AR
None
30
DE
3 hours of Ethics, not to surpass 6 hours in Ethics. Maximum of 9 hours related to business
24
FL*
4 hours Florida Laws & Rules
8
GA
None
30
ID
None
30
IL
None
30
IN
1 Hour of Ethics AND 1 hour of Indiana Statutes and Rules
30
IA
None
30
KS
None
30
KY
None
30
LA
1 hour of Ethics ***
30
ME
None
30
MD*
1 hour of Ethics
24
MN
None
24
MS
1 hour of Ethics
15
MO
None
30
MT
None
30
NE
None
30
NV
None
30
NH
None
30
NJ*
1 hours of Ethics
15
NM
4 hours of Ethics
30
36**
NY*
1 hour of Ethics
NC*
None
15
ND
Minimum of 20 hours of Technical, Maximum of 10 hours of non-technical (Ethics)
30
OH
None
30
OK
None
30
OR
None
30
PA
None
24
SC
None
30
SD
Minimum of 20 hours in health, safety and welfare (HSW)
30
TN
13 hours in health, safety and welfare including technical, ethical or managerial content
24
TX
1 hour of Ethics
15
UT
None
24
VA
None
16
WV
None
15
WI
2 hours of Ethics
WY
◊
None
30 ◊
30
Maximum 17 hours home study allowed by the state of Wisconsin.
***
For licensees who design buildings and/or building systems, a minimum of 8 PDHs shall be earned in Life Safety Code, building codes and/or
Americans with Disabilities Act Accessibility Guidelines.
**
Additional 6 hours available online.
*
Requires state approval. Provider numbers listed on page 2 (About Your PDHs - How do I know my state will accept your courses?).
NOTE
PDH Rules can change. Always check your state board for the most up-to-date information.
Professional Development Hours
for Professional Engineers
Table of Contents
Page Course Name
Hours
3
Energy: Its Effect on our World and Lives
3 hrs
$49.97
24
Amusing Ourselves Safely
3 hrs
$49.97
48
Practical Repair Materials for Roadway Pavements
2 hrs
$34.97
71
Building Safe Structures in Flood Zones
2 hrs
$34.97
88
Heavy Loads
4 hrs
$67.97
121
Designing Buildings to Mitigate Terrorist Attacks
8 hrs
$134.97
166
Municipal Wastewater Treatment Systems
2 hrs
$34.97
183
The History and Future of Domes
3 hrs
$49.97
206
Professional Engineering Ethics
3 hrs
$49.97
226
Continuing Professional Competency Log
227
Notes Pages
230
Book & Individual Course Evaluations Form
231
Registration Form
232
Student Assessment Answer Sheet
All your hours for only $197
Price à la carte
Energy: Its Effect on
our World and Lives
Course Description
The objective of this course is to provide the student with information regarding the effect of energy in our society, and roles engineers can play in making sure we have a secure energy future. It assesses the future impact
of each major energy source – oil, wind, solar, and nuclear – and it articulates a role for engineers in energy
conservation.
Chapters
• Chapter One: An overview of Energy
• Chapter Two: Solar Energy
• Chapter Three: Small Wind Turbines
Learning Objectives
Upon completion of this course, the participant will be able to:
• Understand the roles energy plays on the human element
• Identify the ways energy flows
• Recognize the current energy options
• Understand how to conserve energy and the three basic options for conservation
• Understand the best way for consumers to use solar power in homes and small businesses
• Discuss the basic geometry and definitions used in making solar energy calculations
• Understand the wind energy options available to home and small business owners
Page 3
Chapter One:
An Overview of Energy
Overview
•
•
•
•
Introduction
The Effect of Energy
Survey of Our Current Energy Options
Summary
Learning Objectives
• Understand the roles energy plays on the human
element
• Identify the ways energy flows and understand the
given examples
• Recognize the current energy options including
oil, wind, solar power, and nuclear energy
• Explain how to conserve energy and the three
basic options for conservation
Introduction
In this course we will discuss energy and sustainability, starting with an overview of energy: its role in our
society, its impact on our world, and the prospects for
alternative energies that move beyond fossil fuels. Engineers play a vital role in bringing energy security to
our country. Many people like to say they are “green.”
Buzzwords like “sustainability” and catch phrases
such as “simply ecoLOGICAL” abound. But engineering is the only profession that can move beyond
mere slogan into practical action.
Engineers can design solar devices, install windmills,
and pioneer the next wave of energy technologies. So,
in the second and third chapters of this course there
will be a focus on specific calculations that a practicing
engineer might need to make, to implement wind and
solar technologies. In Chapter Two, you will master
the necessary solar geometry used to size and choose
a solar device, such as a heater or a photovoltaic cell.
In Chapter 3 we’ll look at how to estimate the amount
of power a windmill can capture from the air.
In this first chapter, though, I want to survey the effect
of energy on our world and lives, and then look briefly
at oil and the alternatives of wind, solar, and nuclear
energy. This chapter ends by outlining the role engineers can play in educating the public about energy.
The Effect of Energy
As engineers, we like to quantify things, often without including an explicit human element. We use all
manner of measures to assess the impact of energy
– measures that appeal to engineers, that is. We talk
about conversion efficiencies, energy costs, per capita
utilization levels, growth rates, consumption elasticities, or output ratios. Yet the real impact is on the individual, on his or her ability to create a life worth
living. How can anyone assess that?
While no single variable measures human happiness
and welfare, there are three measures that dramatically
show the role energy plays in the quality of life. The
following Figures #01-101, #01-102, and #01-103
show infant mortality rates, female life expectancy,
and the human development index (HDI) versus annuPage 4
al per capita use of commercial energy. Infant mortality and life expectancy are, perhaps, the best indicators
of the physical quality of life.
From the graphs, we can see acceptable infant mortalities (less than 30 per 1,000 live births) correspond to
annual per capita energy use of at least 30-40 gigajoules of energy. The same holds true for life expectancy. The final graph charts the human development
index (HDI). The HDI is used to rank countries by
level of “human development.” It combines normalized measures of life expectancy, literacy, educational
attainment, and GDP per capita for a country. A higher
HDI means a wider range of life options for residents
– greater opportunities for education, health care, income, employment, etc. These are gross statistical
indicators of the impact of energy. We can see even
more clearly the impact of easily available energy on
the life of an individual by comparing life in this country c. 1900 to life in the United States today.
Figure #01-101
Figure 2.10
Comparison of infant
mortality with average
annual per capita use
of commercial energy.
Plotted from data in
UNDP (2001).
Source: Energy at the Crossroad: Global Perspectives and
Uncertainties by Vaclav Smil (MIT Press 2005)
Figure #01-102
Figure 2.11
Comparison of
average female life
expectancy with
average annual
per capita use of
commercial energy.
Plotted from data in
UNDP (2001).
Figure #01-103
Figure 2.13
Comparison of the
Human Development
Index (HDI) with
average annual
per capita use of
commercial energy.
Plotted from data un
UNDP (2001).
Source: Energy at the Crossroad: Global Perspectives
and Uncertainties by Vaclav Smil (MIT Press 2005)
Think for a moment of life without easily and readily available energy. As late as the early 20th century, many people in America – especially in the south
– were still living as their ancestors did. The main
meal was often a stew, just a big pot of meat and vegetables cooked in a liquid for a long time. A great deal
of human labor went into that stew. A man used handmade knives to butcher an animal; a woman carried
water to the house in wooden buckets, held together
by leather, likely made by her husband. They cooked
the stew over a wood fire; the was likely was chopped
by her husband or son, and the vegetables came from
the housewife’s garden. She thickened the stew with
grains, husked and threshed by hand. Any scraps or
garbage that was not used was moved outside, probably by another family member. All this was a tremendous amount of work. It highlights how energy
saves time; time which is then converted into innovation, economic power, and an improved quality of life.
Over the course of the 20th century, the energy flows
controlled by an individual changed dramatically.
Consider three examples of these changed energy
flows. A farmer at the turn of the 20th century used six
large horses to pull his plow.
Source: Energy at the Crossroad: Global Perspectives
and Uncertainties by Vaclav Smil (MIT Press 2005)
Page 5
This energy expenditure of about 5 kW would only
be sustained for a few hours before both human and
beast needed to rest. Today a farmer sits in an air-conditioned tractor cab with an upholstered seat, listening, perhaps, to music. At this farmer’s control is 300
kW of energy, which never tires. Think for a moment
of transportation: In 1900 an engineer running the locomotive of a transcontinental train controlled about 1
megawatt of steam power; today a 747 pilot controls
45 MW – and five miles about the Earth’s surface at
500 miles per hour. Lastly, in 1900 an engineer at a
coal-fired, electricity generating plant supplied a city
with 100,000 W; today the same engineer can dispatch
1,000,000,000 W, or over four orders of magnitude
more power. This last item highlights the greatest contribution of engineers to society: electricity.
The public easily forgets the impact of electricity until
a dramatic event such as a blackout occurs. For example, a few minutes after 4 p.m. Eastern time on August 14, 2003, the largest blackout in United States
history hit the east coast. An 800 megawatt power
surge roared from Ontario to New York City, shutting
down power grids across the region. We called this a
blackout, yet the real reason our world came to a halt
was not because of dimmed lights, but rather because
of stilled motors.
The motor is what made electricity a superstar
among energies. We don’t often think about it, but
electricity is best thought of as a way to move matter
from one place to another. The immediate source of
electricity is motion; the mechanical motor of a generator spins in order to create electricity. This motor
is driven by various fuels; mostly coal, petroleum
and nuclear. Converting these fuels into electricity
is a costly process. We do it, though, because of the
convenience.
Electricity is easily transmitted and can run motors
in our homes. Without electricity, each house would
have a loud motor churning away to run everything.
In fact, our houses would look much like the factories of the late 19th century, which were powered
by large steam engines. A steam engine spun a shaft
that ran through the factory; machines hooked onto
the rotating shaft through an elaborate system of
belts. That all changed in the early part of the 20th
century, when an electrical grid was laid across the
United States, and when Nikola Tesla invented a durable electric motor – a motor that runs our homes
today.
Our refrigerators have motors, as do hair dryers,
TVs, and air conditioners. And, of course, many of
the things in our homes exist because of electric motors. During the blackout, dairy farms lost the ability to milk their cows mechanically, and no motors
whirred in ATMs to spit out money. In Detroit the
blackout caused a gas shortage when it stilled the
electric motors at the pumps. And perhaps the most
devastating was in Cleveland, which lost fresh water because no electric pumps could move the water
from Lake Erie. Some areas ran the risk that their
major reservoirs would run dry and fire departments
would have no way to put out fires.
What were we left with, then, with no moving parts
that could get us through a blackout? A technology
almost exactly the age of the motor, in fact one which
grew up slightly before it: the 100-year-old technology of ham radio, which is run off of batteries. In the
New York area alone, about 100 ham radio operators
worked with the Red Cross to coordinate the emergency response of ambulances.
Page 6
Survey of our Current Energy Options
So, without cheap electricity we’ll return to the 19th
century. But what are our options for the future? Let’s
assess the main sources of energy: oil, and the alternatives of wind, solar, and nuclear. The focus here is
just to highlight the issues, promises, and pitfalls of
each.
Oil
A recent headline brought shocking news: The Shell
Oil Company “lost”
one-fifth of their oil
reserves overnight,
about four billion
barrels. That is, they
reduced their estimate of the amount
of oil in the ground.
It would seem clear
from this we are
quickly running out
of oil. Figuring out
how much longer we’ll have oil appears simple: Find
out how fast we use oil, find out the number of barrels
of oil under the ground, then make the appropriate calculation. This method yields an answer of twenty to
forty years of oil reserves. But of course it isn’t that
simple.
In the 1920s, observers calculated that by 1930 we’d
run out of oil. They overestimated the rate at which
we’d use oil, and they didn’t know about oil fields
in the Middle East, South American, Africa, Siberia,
Alaska, or the North Sea.
So, can we just take the current rate of consumption
and use the reserves reported by the oil companies
to calculate how long the oil will last? The oil companies report “proven reserves,” defined as “those
quantities of oil which are known to be in place and
are economically recoverable with present technologies.” Note those phrases “economically recoverable”
and “with present technologies.” This sentence makes
much more sense to us as engineers, because we know
engineering solutions are a fluid, continually changing
balance between economic resources and technologiEnergy: Its Effect on our World and Lives
cal limitations. But to a public terrified of soon running out of oil, this seems an abstraction not worth
considering.
Right now, we recover oil from porous underground
rock. A significant fraction of the oil sticks to the
rocks, so we recover, at best, about eighty percent, and
usually a lot less than that. So, when we estimate oil
reserves, should we include this oil? Perhaps a way
will be found to cheaply recover this left-over oil. The
same question applies to other sources: The Canadian
province of Alberta contains the Athabasca Tar Sands,
which have oil content close to current proven reserves. In America, Colorado’s oil shale also contains
vast oil reserves. Now, extracting the oil isn’t easy. For
example, the harsh climate in Alberta freezes the tar
solid, and it takes 30 tons of shale to make one ton of
oil.
So, the answer to when the oil really runs out comes
down to a question of faith about the limits of human inventiveness: Will our technological wizards
develop ways to cheaply tap other sources, or have
we reached a technological limit? In the past, those
who bet against the ingenuity of engineers have usually lost, and the prophets of doom have always been
nearly wrong. Perhaps, though, in this case, we should
follow the old, Russian proverb: Pray to God, but keep
rowing toward shore. In other words, engineers should
work toward solutions to our energy problems – to recovering more oil, for example – but at the same time
help the public with new technologies that help them
conserve energy.
Wind
Energy from the wind is sustainable and pollutes very
little. Yet, wind supplies only about 1.25% of the
United States’ electricity, according to the American
Wind Energy Association (AWEA). Why such a small
amount? There are several reasons wind energy hasn’t
been universally adopted in this country.
First, wind energy only recently became inexpensive. The most important piece of machinery in
turning wind into electricity is, of course, a turbine.
Understandably, early manufacturers of turbines for
capturing wind power based their designs on jet engines. This yielded wind turbines that were inefficient, making the cost of a kilowatt of wind energy
Page 7
about 40 cents in the early 1980s, which was many
times more than that of fossil fuels. A jet engine
turbine is optimized to fly an airliner, not to extract
energy from the wind. Today’s state-of-the-art windmill is fifteen stories tall, with blades 200 feet or
more across. These blades move very slowly, typically about fifteen revolutions per minute, a tenth of
that of older systems. New turbines are so efficient
wind energy costs about the same as coal, natural
gas, or nuclear power. With these advances, what’s
the problem now?
The problem is this: You have to build the windmills
where there is wind. Typical places for wind farms, as
they call banks of windmills, are plains, shorelines,
the tops of hills, and the narrow gaps between mountains. These places are rarely near transmission lines.
The United States transmission system was designed
to supply electricity to a local area, so power plants are
typically built near cities. Since we build most of our
cities where the wind doesn’t blow, there are no power
lines near wind farms. This calls for building costly
transmission lines over unforgiving terrain.
In addition, wind power differs from fossil and nuclear
fuels in a critical way: It can supply steady electricity,
but not a burst of electricity. In addition to peak plants,
utilities use coal and nuclear-powered plants that kick
in when demand is greatest. Engineers are designing
special batteries to store energy and supply it when the
wind dies down, but the problem hasn’t been solved
yet.
To find solutions, we might look to other countries.
For example, Denmark gets one-third of its electricity
Page 8
from wind. Yet, oddly, this highlights the scale of the
problem in bringing wind power to the United States.
Denmark is slightly smaller than Vermont and New
Hampshire combined and has a population about that
of Chicago. To generate their electrical energy from
wind takes over 6,000 wind turbines, located offshore.
So, wind power isn’t the panacea that will save us. The
most optimistic estimate found is from the American
Wind Energy Association. They think about thirty percent of America’s power will be from wind by 2030.
Mostly likely, wind power will be part of a patchwork
of many energy systems that, if all goes well, will supply the energy needs of the United States.
Solar
There are many ways solar energy can be used to
meet the needs of a
society: direct heating
of buildings or water,
solar biomass (using
tree, bacteria, algae,
etc. to make fuel,
chemicals, or
building materials, to
make food, or to
generate electricity
using photovoltaic
cells). In the next
chapter we will focus
on how solar, can
serve a local need, but here we’ll look at the big
picture, emphasizing use of solar energy to produce
electricity.
There are three key issues preventing solar power
from playing a large role in supplying U.S. energy.
The first is the efficiency of the photovoltaic cell itself. Nobel Laureate William Schockley showed in the
early 1960s that for a “single-junction” solar panel,
the highest efficiency is 31%, or about 41% efficient
with concentrating mirrors and lenses. The only way
to exceed this limit is to make complicated multiplejunction photovoltaics (junction refers to the layers of
semi-conductor material used). Recently researchers
at the University of Delaware reported a multi-junction device that, with concentrators, had an efficiency
of nearly 43%. There are large efficiency gaps between
these laboratory devices and their performance in the
field. Issues like mass producing high-purity single
crystals result in commercial versions with efficiencies
as low as 12% to 14%. The point here is these tour-deforce devices are difficult to mass produce. Thus, the
most likely conversion for a mass-produced device for
a very long time in the future will be, at best, 30%.
It will likely be lower since the cheapest panels tend
to be low efficiency. This conversion brings us to the
second issue with using solar energy.
When photovoltaics are used en masse to replace
conventional fossil fuel based power plants, the solar
farms take up a great deal of space. A recent estimate
by British physicist David McKay suggests that 5% of
Britain would need to be covered with panels. Furthermore, installing this many panels – enough to generate
only 50 kWh/day per person, a fraction of typical use
– would be about $130,000. If the solar farm lasted
20 years this would add up to about $0.35 per kWh about three times the cost today for fossil fuel.
Lastly, if solar power became dominant, the nation’s
power plant infrastructure would no longer be able to
accommodate power swings; real storage would become necessary. Today’s electrical grid has essentially
no storage.
Nuclear Energy
The key question here is not whether nuclear power is
viable – currently it provides about 16% of the world’s
electricity – but whether or not it will be a significant
part of the energy mix in the coming century. This is
not, then, a scientific question, but is engineering in
its broadest sense: responding to public needs, desires,
and fears with technological solution. To really thrive,
a technology needs more than a scientific side; it must
fit into our world socially and legally. The short answer about nuclear is that it could a major part of the
mix, but is unlikely to be. Why?
First, no power company in the Western World, except
for France, has built any new reactors for several decades. Since 1973, U.S. companies have placed orders
for 39 new reactors, every one of which was canceled.
Even France, a world leader in nuclear energy, began
constructing its most recent reactor back in 1991.
Second, many countries in Europe have legislated
deadlines for the abandonment of all nuclear generation. Sweden, for example, had 12 reactors which
made it a world leader in per capita nuclear power
generation. They had planned to decommission all reactors by 2010, although in 2009 they reversed this.
Still, the country has not committed to expanding its
nuclear industry.
Third, reason not to be optimistic about nuclear impact is persistent public concern about the safety of
nuclear power plants. Here, perhaps some education
might help. The public, for example, is exposed to
more radiation from natural sources than from the operations of a safe nuclear plant.
Finally, the political issues of where to store nuclear
waste remain unresolved. For years the U.S. Department of Energy has tried to create a disposal site at
Yucca Mountain, but this remains mired in controversy.
Until this technical-social-political problem is solved,
the fate of nuclear looks dire. We must wonder whether
nuclear will slowly fade away in the 21st century.
Conservation
The public hope lies with some great technological
marvel that will instantly supply all our energy needs.
Not so long ago hydrogen was the answer: Certainly, it
sounds amazing: “A single chemical reaction between
hydrogen and oxygen generates energy, which can be
used to power a car – producing only water, not exEnergy: Its Effect on our World and Lives
Page 9
haust fumes.” But this pollution-free miracle is currently, and for the foreseeable future, only a redirection of petroleum. Ninety-six percent of the hydrogen
produced comes from natural gas, oil and coal – exactly the fossil fuels we’d like to abandon.
Yet if you were to ask the public about hydrogen, they
would regard it as the wonder fuel for the future. This
highlights a role for engineers beyond our professional
work: striving to raise the level of energy literacy. We
can become involved in local government as advisors,
helping public servants who do not have an energy
background. This work would affect local laws, codes,
and zoning ordinances that influence energy usage.
We can help create better media coverage by being
resources for members of the press. Like much of the
rest of society, members of the media lack the training
and knowledge to understand energy issues without
expert help. We might even, in a sense, become public
educators ourselves by speaking at Rotary clubs, Kiwanis clubs, the League of Women Voters, etc.
Summary
This chapter outlined the effect of energy in our society and roles engineers can play in making sure we
have a secure energy future. First, we showed the impact of energy by plotting the human development index (HDI) against the annual per capita use of commercial energy. We then surveyed our current energy
options: oil, wind, solar, nuclear, and conservation.
Finally, the chapter outlined the role of engineers in
conservation: to be educators to the public though, for
example, becoming involved in local government and
making personal choices for conservation.
Additionally, we can lead by example, by upgrading
our homes, influencing energy use at work, by being
energy champions, or leading with our choices about
our own transportation. Here are the three best options
for conservation:
1. Conserving through personal choice. This focuses
on using existing energy technologies to save energy. For example, turning down the thermostat,
or “ganging” errands together in order to drive
less, or taking the bus more often.
2. Conserving by replacing end-use technology. Here
we use the same primary energy source, but employ
a new technology to use energy more efficiently.
We might install the latest thermostat that automatically lowers the temperature, or install dimmers
for fluorescent lights in large office buildings. We
might buy a car that gets better gas mileage.
3. Conserving by replacing energy conversion technology. Here we continue to use the same energy
source – most often fossil fuel – but conserve energy by using a more efficient device. In a home,
for example, we might upgrade a furnace to attain
greater efficiency.
Page 10
Chapter Two:
Solar Energy
Overview
• Introduction
• Solar Calculations
• Summary
Learning Objectives
• Understand the best way for consumers to use
solar power in their homes and small businesses
• Comprehend the basic geometry and definitions
used in making solar energy calculations
• Identify the average amount of energy available at
the surface of a solar device for any location and day
Introduction
Many consumers first think of solar energy as a way
to have an off-grid home, to be separate from the
power grid. Except in very specific circumstances,
this type of system is never economically reasonable. It is usually used either for locations far from
power lines, or for places that cannot experience a
power interruption. In essence, they use the solar
system as a back-up. The most common, and often
most financially reasonable, system is what is called
a “grid tie-back” or a “grid tied electrical system.”
With these types of systems, no solar energy from
the panels enters into the user’s home or business.
Rather, all of it is sold back to the power company.
The heart of these systems is an inverter that converts the DC from photovoltaic panels to the AC
used by the utility. Each local utility company has
very specific regulations on these inverts, and also
specific tax incentives for installing these types of
systems. The U.S. Department of Energy keeps
track of these incentives at the Database of State Incentives for Renewables & Efficiency (http://www.
dsireusa.org). For consumers looking for installers
for solar systems, the American Solar Energy Society maintains an updated list.
To many, the most exciting use of solar energy is
in a high-tech rooftop photovoltaic system. But,
anyone thinking of installing such a system should
look at simple solar heating, using the sun for direct
heating of a building or water. The cost of installing
photovoltaics is about four times the cost of installing solar thermal panels. The photovoltaics deliver
only half as much energy as the thermal panels, although it is high “quality” energy (electricity). If
the primary reason for using solar energy is heating,
it makes little sense to make electricity and then degrade it to heat.
But regardless of the type of solar energy conversion, an engineer needs to know how to calculate
the amount of energy falling on a solar device.
Page 11
“Provide the engineer, the architect and contractor alike, with a useful and reliable reference data
book relating to the art of heating and ventilating. A wide range of data within the scope of the
field is presented and every effort has been made
to present the material in a practical and useful
manner.”
Solar Calculations
The focus in this chapter
is calculating the solar
energy available for a
particular locale and time
of year.
Today, as we enter a new energy age, the Handbook is
still relevant. For work on solar installations, there is a
table for different latitudes and sky conditions. For example, shown below is the table for 48 degrees North
latitude with a clear sky.
While this would seem straightforward, it becomes
complicated because an engineer needs to take into
account the relationship of the sun to the earth;
that is, the constant motion of earth around the sun
and the Earth’s rotation on its axis make for some
complicated looking formulas. The upshot of this
is the amount of energy available to a solar device
depends on the time day, the time of year, and the
location of the device; in short, what is called solar
geometry.
There are several things to note:
1. The times shown are “solar times.” Solar time is
synchronous with local standard time, but leads
or lags it because of time zone differences and
the Earth’s rotation. The latter makes solar time
“speed up” and “slow down” relative to local time,
depending on the season. The difference is typically small; plus or minus a quarter hour at most.
2. The tables are “symmetric” in that the flux in the
morning and evening hours is the same.
Basic Geometry and Definitions
The American Society of Heating, Refrigerating
and Air Conditioning Engineers (ASHRAE) publish
tables in their Handbook of Fundamentals showing
the energy flux striking the Earth. The Handbook is
typical of the ingenuity of engineers and the care
with which they protect the public’s health. First
published in 1922 by ASHRAE predecessor the
American Society of Heating and Ventilating Engineers (ASH&VE), it was intended to:
The basic idea is that we use the flux values in the
chart above as the basis for calculating the actual flux
that reaches a solar collector. To do this, we need to
account for the position of the sun at various times of
Month
5 a.m.
7 p.m.
6 a.m.
6 p.m.
7 a.m.
5 p.m.
8 a.m.
4 p.m.
9 a.m.
3 p.m.
10 a.m.
2 p.m.
11 a.m.
1 p.m.
Noon
Jan
0
0
0
116
584
754
823
842
Feb
0
0
11
568
780
869
908
919
Mar
0
0
481
743
852
905
931
939
Apr
0
340
646
778
846
883
902
908
May
129
510
689
781
833
863
879
883
Jun
243
544
693
774
822
849
864
868
Jul
135
492
666
757
809
839
854
859
Aug
0
311
599
732
801
839
858
864
Sep
0
0
414
678
792
848
875
883
Oct
0
0
12
521
734
825
866
878
Nov
0
0
0
115
565
734
803
822
Dec
0
0
0
0
442
675
765
789
Page 12
the year, and times of the day. So, we’ll define various
trigonometric relationships between Earth and the Sun
and then use them to calculate a correction factor.
The basic equation gives the proportion of the incoming solar energy that is available to the device:
Fraction of incident solar energy available =
Where
is the angle of incident between the beam
of sunlight and the direction of the surface. While this
sounds simple, calculating it correctly involves understanding the relationship between Earth and the Sun.
This is a function of the solar altitude, tilt angle of the
device, the hour angle and the surface azimuth angle –
all of which will be defined.
Specifically, the relationship is
Point B in the drawing is on the Earth’s surface. If we
connect it with an imaginary point A in the atmosphere,
directly above point B, we can form the angle delineated
by A-B-C. This angle, called the zenith angle
is
closely related to the solar altitude we are looking for:
Thus, the solar altitude
is the angle between a line
from the sun and a line to the horizon. How is this calculated? This takes us deeper into solar geometry: We
need to consider the declination of the sun, which is a
measure of its angle to the equator. You might think of
this property as comparable to latitude on earth, but
projected onto the celestial sphere. It is measured in
degrees north and south of the celestial equator. Therefore, points north of the celestial equator have positive
declinations, while those to the south have negative
declinations. (See figure 02-102)
Figure #02-102
Let’s take each in turn, except for which is a property of the device and is set by the engineer building it.
Source: Energy Systems Engineering: Evaluation and
Implementation by Francis M. Vanek, Louis D. Albright
(McGraw-Hill 2008)
Solar Altitude
The declination can be calculated for any day of the
year using this formula:
Figure #02-101
Where N is the day of the year (N=1 being January 1st
and N=365 December 31st). We can see clearly that
calculating the amount of energy hitting a solar device
will depend on the day of the year.
Looking at the geometry in figure #02-101, we can see
the sun strikes the Earth at an oblique angle to a line
drawn directly above a section of the Earth’s surface.
Once the declination is known, we can calculate the
solar altitude as a function of declination and of the
“where” and “when” of the solar device. Solar altitude
depends on the latitude and the time:
(McGraw-Hill 2008)
Where L is the latitude in degrees and
is the hour
angle. The hour angle is calculated by subtracting or
adding 15 degrees for every hour before or after solar
Page 13
noon, respectively. Thus
= hour x 15 -180 where
the hour is given in 24-hour decimal format. For 1
p.m. solar time, then:
in the Northern Hemisphere is installed due south, so
the azimuth angle is zero in most calculations at solar noon, when the sun is the highest in the sky:
= 0.
At other times, we can calculate it from this equation:
Values are given in Table 02-101 below.
Table 02-101: Hour angle and Solar Azimuth
Time
(solar)
Hour angle
(degrees)
Solar
Azimuth
(degrees)*
10:00 a.m.
-30
-27.30
11:00 a.m.
-15
-13.74
noon
0
0
1:00 p.m.
15
13.74
2:00 p.m.
30
27.30
*For 35 degrees North latitude at the Winter Solstice on
December 21st.
Azimuths:
A solar device (for example, a solar photovoltaic panel) affixed to the Earth will have a fixed orientation
to the points of a compass. We designate this the azimuth angle , the angle between the device’s orientation and South on the Earth. In addition, we define ,
which is the angle between the direction of the Sun
and due south. See figure below.
Figure #02-103
Note: Since
depends on the declination
it varies with the day of the year, and since it depends on
the solar altitude
it varies with the time of day and
position on the Earth’s surface. In Table 02-102 I’ve
tabulated for 35 degrees North latitude at the winter
solstice, December 21st. The calculation is done as
follows for 1:00 p.m.:
Step 1: The winter solstice is the 355th day of the year,
which means the declination equals -23.45.
Step 2: The latitude is 35 degrees north and the hour
angle associated with 1:00 p.m. is 15 degrees. Thus:
which corresponds to an
which yields
of 29.84. This means that:
= 13.74.
Putting it all Together: Calculating the Energy
Incident on a Solar Device
Using the relationships above, which are based on solar geometry, we can now calculate for any given day,
at any hour, at any position on Earth, the actual energy
flux reaching a device from the sun.
We will calculate the fraction of energy actually hitting our collector using the following equation:
(McGraw-Hill 2008)
Angles to the west of due south are positive, and angles to the east are negative. Typically, a solar device
Page 14
We will use this to modify the values from the
ASHRAE Handbook of Fundamentals.
Let’s start by setting some parameters. We want to
make the calculation for a device that is near Devil’s
Lake, North Dakota. The latitude there is 48 degrees
North. The solar collector is orientated due south, so
; and it is tipped 30 degrees from the horizontal,
which means
. We will make our calculation for
March 21st.
We’ll put the result for every step in the calculation in
Table 02-101, outlining the steps below for 1 p.m., or
13:00 hours.
Step 1: Tabulate the data from the ASHRAE Tables.
For latitude 48 degrees, we see for March the Sun is
up from 7 a.m. (07:00 hours) to 5 p.m. (17:00 hours).
For example, at 3 p.m. (15:00 hours) the energy flux
is 852 W/m2. We’ll use values for every hour the sun
is up.
Step 2: Calculate the hour angle for every hour the
sun is up. For 15:00 hours:
This means:
Step 5: Calculate the solar azimuth
using the solar
altitude , the declination
= -0.4, and the hour
angle = 45:
Which yields
= 53.14.
Step 6: Calculate the incident angle using the value
above:
This value is the fraction of solar energy available to
the device.
Step 3: Calculate the declination using N = 80 because March 21st is the 80th day of the year.
Step 7: Multiply the value above by the flux listed in
the ASHRAE Tables to get the actual amount accessible to the device:
Step 4: Calculate the solar altitude . For 15:00 hours
using the declination = -0.4, the hour angle = 45
at latitude 48 degrees:
(852 W/m2)(0.68) = 578 W/m2
Table 02-102: Energy available (W/m2) at the surface
of a solar device located near Devil’s Lake, North
Dakota (latitude 48 degrees),
,
March 21st.
Hour
Available
Flux (W/m2)
Hour
angle
(degrees)
Solar
Altitude
(degrees)
Solar
Azimuth
(degrees)
Incident
Angle
(degrees)
Actual Flux
(W/m2)
Fraction
7
481
-75
9.7
-101.5
87.57
20.42
0.04
8
743
-60
19.2
-113.5
84.86
66.58
0.09
9
852
-45
27.9
-53.1
47.29
577.89
0.68
10
905
-30
35.0
-37.6
34.20
748.52
0.83
11
931
-15
39.9
-19.7
23.31
855
0.92
12
939
0
41.6
0.0
18.40
890.98
0.95
13
931
15
39.9
19.7
23.31
855
0.92
14
905
30
35.0
37.6
34.20
748.52
0.83
15
852
45
27.9
53.1
47.29
577.89
0.68
16
743
60
19.2
113.5
84.86
66.58
0.09
17
481
75
9.7
101.5
87.57
20.42
0.04
Page 15
Summary
In this chapter, we outlined the calculations necessary
for an engineer to size a solar panel for a home. We
began by pointing out that many consumers think of
solar energy as a way to have an off-grid property. In
reality it is used to be part of the power grid. We suggest an engineer look at both solar heating and photovoltaic systems for home use; the former is simpler
and often just as effective in reducing energy costs.
The remainder of the chapter detailed the calculations
necessary to estimate the solar energy available at a
particular locale and time. The chapter notes, while
this may seem straightforward, it is complicated by
the constant motion of the Sun and the Earth’s rotation
on its axis. Thus, we carefully defined the hour angle,
the solar altitude, the solar azimuth, and the incident
angle in order to calculate the available energy flux on
a particular day at a specified latitude and longitude.
Page 16
Chapter Three:
Small Wind Turbines
Overview
• Introduction
• The Energy of the Wind
• Summary
Learning Objectives
• Understand the wind energy options available to
home and small business owners
• Estimate the available energy from wind
for a particular location and season
• Explain windmill efficiency
• Recognize how to use the Rayleigh Distribution
to predict wind energy based on a measure
of average windspeed
Introduction
In the first chapter, we looked at wind energy from
the viewpoint of using it in wind farms to produce
mega-watts of energy to sustain our nation’s need
for electricity. One of the keys, though, to solving
our energy problems is to look at what makes sense
locally. Wind might be ideal for a particular location, and small wind turbines are electric generators
that use wind energy to produce clean, emissionsfree power for homes, farms, and small businesses.
In some cases, this allows an individual to generate
his or her own power and cut energy bills. The U.S.
leads the world in the production of small wind turbines, which the American Wind Energy Association (AWEA) defines as having rated capacities of
100 kilowatts or less. They predict the market will
experience strong growth through the next decade.
According to the AWEA, typical small wind energy
systems cost from $3,000 to $5,000 for every kilowatt of generating capacity, or about $40,000 for a
10-kw installed system. This is much cheaper than
solar-based systems, but the payback period (ROI)
can still be lengthy. The AWEA recommends taking advantage of rebates or tax credits available for
small wind system installations. Well-sited small
wind turbines can usually pay for themselves within
Page 17
15 years, about half their serviceable lifetimes, if
the right incentives are applied. The AWEA Website
offers a state-by-state listing with state, local, and
utility incentives. Small wind systems pay off, most
often for rural homes and businesses, with at least
an acre of property, Class 2 winds, and utility bills
averaging at least $150 monthly. Often, installing a
wind turbine is much cheaper than extending power
lines, which can cost as much as $20,000 to $30,000
per quarter mile.
As a rule of thumb, the AWEA estimates a 10-kilowatt wind turbine mounted on an 80-foot tower
should generate an average of 1,000 kilowatt-hours
(kWh) monthly. They suggest using a net metering
arrangement so each kWh generated can be valued
at the local electric rates. Smaller wind generators
with a 1-to-3-kilowatt capacity do not usually produce excess generation and are often used to power
specific applications such as water pumps or recreational vehicle lights and appliances. These small
turbines also can reduce energy bills. For example,
a 3-kW turbine mounted on a 60- to 80-foot tower
costs about $15,000 which can reduce a monthly
electricity bill by $60 to $100.
In placing a windmill, perhaps more accurately
called a wind turbine, an engineer needs to estimate
the amount of power available from the wind. In
this chapter we will learn to: a) estimate the amount
of energy in the wind, based on a typical average
velocity; b) refine this by using Rayleigh distribution; c) add in effect of seasons and of height; and
d) calculate the effect of placing more than one
wind turbine in an area. We will not focus on rotor
design – the speed, tip velocity, etc. Typically, an
engineer chooses an appropriate wind turbine from
a manufacturer who has considered all this in their
design. Rarely do engineers design a wind turbine
from scratch, but many erect and place purchased
wind devices.
The Energy of the Wind
The key design parameter in choosing and using a
wind turbine is the amount of power available in the
wind. This chapter focuses on estimating that amount
accurately.
Page 18
Simple Estimation of Wind Power Available
A wind turbine, of course, captures the wind’s kinetic
energy using a rotor of two or more blades. This rotor
is attached to an electrical generator. The key question
in choosing a windmill is the amount of energy in the
available air at a particular site. We can make a rough
estimate of the wind power available using basic physics.
Since wind is purely kinetic, its energy is given by the
familiar equation:
Kinetic energy = 1/2 mv2
Where m is mass and v is velocity. For wind energy
calculations, we can arrange this into a more useful
form:
Wind power per square meter of air =
Here is the air density. For typical values, say an air
density of 1.3 kg/m3 and an average air velocity of 6
m/s, the power in the wind would be:
= 1/2 * 1.3 kg/m3 * (6 m/s)3 = 140.4 W/ m2
Not all the power in the wind can be captured by the
windmill. In reality, a windmill only slows the air; if it
brought the wind to a dead stop, the slowed-down air
would get in the way.
Windmill Efficiency
In 1919 German physicist Albert Betz (1885-1968)
calculated the maximum amount of incoming energy that can be extracted from a windmill. Betz had
earned a PhD for his work on “ship propellers with
minimum loss of energy”, and was able to apply this
to find the “Theoretical Limit for Best Utilization of
Wind by Wind Motors” as his paper was entitled.
Betz’ Law, as it became known in wind power literature, states that independent of the design of a wind
turbine, only 16/27 (59%) of the kinetic energy of the
wind can be converted to mechanical energy. This, of
course, is theoretical limit. In practice it is a bit less.
A well-designed modern wind turbine can achieved
aerodynamic efficiencies of about 50%, although the
net value is often closer to 40% because of gearbox
and electrical losses. Thus, for the conditions described above, the best a windmill could do would be
about 70.2 W/m2.
Refining Wind Energy Calculations
While it would seem that using the average wind velocity would yield a good estimate of the power available, for design work, it is a poor choice. At any site,
wind speed varies throughout the day and throughout the seasons. More important is not the average
speed, but the details of its spectrum (or distribution
of speeds), which gives a power average. Since the
wind’s power is proportional to the cube of the velocity, this means the power calculated with just the average speed, differs by up to 50% from that calculated
from a power average. (We’ll see exactly how to do
this in a moment.)
To correct for this, the engineer could approach this
empirically, by placing a wind logger at the site and
at nearby locations to create a statistical wind map.
Typically sixteen “bins” are used, which break down
the windspeed into increments of 1 m/s. For example,
the first bin measures wind with a speed of 0 m/s, the
second measures winds with speeds between 0 and 1
m/s, the third between 1 m/s and 2 m/s, etc. To make
such a map requires either taking measurements at the
windmill location over a year or multiple years, or taking measurements around the area and then using a
statistical technique to obtain the average. Obviously
this costs a great deal, both in terms of money and
time. Luckily for the designer, there is a good way to
estimate the distribution of the wind; if we have the
average wind at a location, we can use statistics to
make a good estimate of the available power.
In 1872 Lord Rayleigh developed a statistic technique
that typically works well to measure wind. John William Strutt, 3rd Baron Rayleigh (1842-1919) was a
British physicist who co-discovered the element argon,
which earned him the Nobel Prize for Physics. Strutt
was a wealthy man, having inherited his father’s title
and land as a young man. At Terling Place, the family
estate in Essex, he converted the stables into a laboratory. There, he performed experiments in photography,
optics, electricity, and acoustics, working alone for the
next 50 years. He remained active in his laboratory until a few days before his death on June 30, 1919.
When he was 29 years old, poor health necessitated a
break from this life of experiment. He needed to recuperate in a warmer climate. During his convalescence,
which was spent traveling up the Nile in a houseboat,
Rayleigh wrote The Theory of Sound, which remains
a classic in the field of acoustics. He discovered a statistical distribution that related the peak power of a
sound to its average value, which is now called the
Rayleigh Distribution. While this seems far away
from wind energy, it is closely related.
Empirical observations have shown that the Rayleigh
distribution reliably predicts the probability of a particular windspeed at or below a particular value if the
average windspeed is known.
The distribution of wind velocities can be modeled
with the Rayleigh Distribution:
Note: This only depends on the average wind, an easily available number. In this equation p is the probability of winds with speed less than U, U is a particular
windspeed of interest, and Uaverage is the average windspeed in the region.
To illustrate how this would work for wind, let’s apply
it to the bin idea for measuring winds. (Keep in mind
we are estimating what experimental results would
show for a wind study. Our goal is to not have to do
the measurements, but simply to be able to calculate
them quickly.) Let’s look at a region which has an average windspeed of 5 m/s. If we look at bin 3, which
captures wind with speeds between 1 m/s and 2 m/s,
we can use the Rayleigh Distribution to estimate the
probability the wind will be in bin 3:
Thus the probability that wind will be in bin 3 is (0.12
- 0.03) = 0.09 or 9%.
Using the Rayleigh Distribution to Estimate
Wind Power
We now want to draw up a table that covers all of the
15 bins typically used in gathering wind power data.
This is the data that could be used to size and choose
a wind turbine. Let’s say that average speed has been
measured for the area, and it is 6.2 m/s. We then construct Table 03-101.
Page 19
Table 03-101: Estimated Wind power for a location with Uaverage = 6.2 m/s, air density = 1.15 kg/m3
Bin
Wind
speed
min (m/s)
Wind
Probability Probability
%
speed
for min
for max
(probability
max (m/s)
speed
speed
for bin)
Bin
average
speed
Hours
per year
1
0
0
0
0
2
0
1
0
3
1
2
4
2
5
Average
Power
Power
2
(kWh/m
)
W/m2 in bin
0
0
0
0
0
0.04
3.8
0.5
333.25
0.07
0.02
0.04
0.14
10.57
1.5
925.6
1.94
1.8
3
0.14
0.29
15.09
2.5
1322.31
8.98
11.88
3
4
0.29
0.46
16.77
3.5
1469.09
24.65
36.22
6
4
5
0.46
0.62
15.84
4.5
1387.74
52.4
72.71
7
5
6
0.62
0.75
13.17
5.5
1153.73
95.67
110.37
8
6
7
0.75
0.85
9.8
6.5
858.67
157.91
135.59
9
7
8
0.8505
0.9164
6.59
7.5
577.66
242.58
140.13
10
8
9
0.92
0.96
4.03
8.5
353.39
353.12
124.79
11
9
10
0.96
0.98
2.25
9.5
197.38
492.99
97.31
12
10
11
0.98
0.99
1.15
10.5
100.94
665.63
67.19
13
11
12
0.99
1
0.54
11.5
47.35
874.5
41.41
14
12
13
1
1
0.23
12.5
20.41
1123.05
22.92
15
13
14
1
1
0.09
13.5
8.09
1414.72
11.45
Total Power in Wind
Let’s review how the data was calculated, using bin 9
to illustrate.
Bin Numbers and Windspeeds
The bin numbers and their minimum and maximum
windspeeds are just convention. This is how windspeeds are measured and recorded.
Minimum and Maximum Probabilities
We’ve already done this calculation above. For this
particular set of conditions, the values are:
1940.55
Bin Average Speed. As the name implies, this is just
the arithmetic average of the minimum and maximum
speeds of the bin:
(7 m/s + 8 m/s)/2 = 7.5 m/s
Hours per Year
In a year there are 8760 hours. The probability of the
wind being in bin 9 is 9.7 %; thus
Hours per year = (8760 hours)(0.097) = 849.72 hours
Average Power
This is the average power in the bin. We calculate it
using the kinetic energy equation at the beginning of
this chapter, using the average speed in the bin:
Thus the probability wind will be in bin 3 is (0.7295 0.6325) = 0.097 or 9.7%.
Page 20
Power
This is the total power available from the bin over the
course of a year:
= average power * hours per year
= (242.58 W/ m2)*(849.72 hours)(1 kW/1000W) =
206.11 kW-hr/ m2
Comparing Power Calculations
As noted, estimating the wind power available using
just the average windspeed can vary a great deal from
using the power average of the Rayleigh Distribution.
We found above (see Table 03-101) the wind power
calculated using the Rayleigh Distribution for an average windspeed of 6.2 m/s is 1940.55 kWh/m2. Let’s
compare this to using just the average speed:
The difference is substantial: a percent difference of 61%.
There is another way we can quickly estimate the wind
power based on the average windspeed. We can use
the Rayleigh Distribution to get an adjusted value of
the average windspeed and then use that in our power
formula. It tells us that if we multiply the windspeed
by
and then use that in our power equation,
we’ll get a reasonable estimate of the power average:
This is a huge over-estimate compared to the more exact method we used in constructing the table. In general, as the average velocity gets smaller, this quick
estimation works better. For example, with an average windspeed of 4.5 m/s, the “table” method yields
a power average of 873.79 kW-hr/ m2, while using
gives 876.26 kW-hr/ m2.
Wind Variation with Height
In addition to varying with season, the wind varies
with height. For an ideal, smooth plane surface, the
average wind increases as:
where U(z) is at height z and z1 is a different height.
The goal here is to be able to use measurement at
one height to estimate the wind velocity at a different height so we don’t have to make another expensive experimental measurement. Perhaps we have a
measurement at a particular height, but need to build
a wind turbine higher off the ground. (Typically one
wants a windmill high in the sky since windspeed increases and the power output varies at the cube of the
velocity.) We could use this formula, and then use the
techniques outlined above to estimate the total power.
For flat terrain the value of
is 0.2. Thus, if we have
measured an average velocity of 5 m/s at 30 feet and
we want the velocity at 100 feet, we would use:
The value of
depends on the terrain, but many
things can decrease the windspeed; hills covered with
trees, topography, and vegetation all alter the wind
speed, as well as other wind turbines
Effect of Spacing of Wind Turbines
Grouping towers into large wind farms is a great advantage for operation and maintenance, but too close
a spacing leads to interference, reducing the power of
the downwind units. An engineer can estimate the decrease in available wind power based on the number
of rows of wind towers and the space of the rotors
using:
where Nd is the row spacing in terms of rotor diameters (this is dimensionless) and Nr is the downwind
row. Thus, for a windmill in row 10 (Nr = 10) located
5 (Nd = 5) rotor lengths away:
Thus, a wind turbine in the 10th row will yield on 45%
of the power of one in the front row.
Summary
In this chapter, we focused on the calculations necessary for small wind turbines. We began with a look
at the typical use of wind energy in a home or small
business, including rules of thumb for sizing a turbine. Then, we delineated the calculations an engineer needs to perform when placing a wind turbine
and showed how to estimate the amount of energy in
the wind, based on a typical average velocity; how to
refine this using the Rayleigh distribution; how to add
in the effect of seasons and height; and the effect of
placing more than one wind turbine in an area.
Page 22
Student Assessment
Select the best answer for each question and mark your answers on the Student Assessment Sheet
(last page of book) or complete your assessment online at www.McKissock.com/Engineering.
Final Exam
1. In what ways can solar energy be used as part
of the U.S energy mix?
a. To heat buildings
b. To provide electricity
c. To make chemicals
d. All of the above
2. The key factor limiting the wide-scale use of
solar to supply U.S. energy needs is:
a. The efficiency of mass-produced solar cells
b. Citizens’ resistance to having solar farms near
their homes
c. The huge cost of transmission lines
d. Very few areas of the U.S. receive enough sun
to make solar energy practical
3. What are the main categories for conserving
energy?
a. Conserving through personal choice
b. Conserving by replacing end-user technology
c. Conserving by replacing an energy
conversion technology
d. All of the above
4. Compared to solar photovoltaic systems, solar
thermal heating:
a. Creates a higher quality energy
b. Delivers twice as much energy
c. Requires more sunlight
d. Costs four times as much
5. Calculating the solar energy incident on a
device is complicated because:
a. The sun is at an angle to the earth.
b. The sun and the Earth are in constant motion
relative to each other).
c. The sun rotates.
d. No one has measured the amount of energy
from the sun striking the Earth.
6. Solar altitude is:
a. The angle between a solar device’s orientation
and South by the Earth’s compass
b. The angle between the direction of the Sun
and due south
c. A measure of its angle to the equator
d. The angle between a line from the sun and a
line to the horizon
7. The sun’s declination is:
a. The angle between a solar device’s orientation
and South by the Earth’s compass
b. The angle between the direction of the Sun
and due south
c. A measure of its angle to the equator
d. The angle between a line from the sun and a
line to the horizon
8. The American Wind Energy Association
defines a “small wind turbine” as one with a
rated capacity of less than:
a. 1 kilowatt
b. 10 kilowatts
c. 100 kilowatts
d. 1000 kilowatts
9. The best estimate of power available in the
wind is reached by using:
a. The average windspeed in the location
b. A corrected “power average” based on the
Rayleigh Distribution
c. The strongest wind in the area
d. None of the above
10. The maximum efficiency of a windmill, as
determined by German physicist Albert Betz, is:
a. 1%
b. 10%
c. 59%
d. Up to 100% if designed correctly
Page 23
Course Description
This course outlines the role of engineers in safeguarding the public, focusing on the techniques and designs
used to ensure the safety of what appears frivolous – amusement park rides and toys – in order to highlight the
importance of incorporating safety into design. It looks at essential design principles used to create the world’s
greatest roller coasters using the ASTM Standard F2291 Standard Practices for Design of Amusement Rides and
Devices. It then turns to toys, detailing the revolutionary design of several classic toys; discussing how the ASTM
Standard F1148 Standard Consumer Safety Performance Specifications for Home Playground Equipment applies
to designing a safe platform for children’s equipment; and studying ASTM F963 Standard Consumer Safety
Specifications for Toy Safety to show how to design toys for stability.
Chapters
• Chapter One: Engineering of Roller Coaster
• Chapter Two: Engineering Design Standards
• Chapter Three: The Engineering of Toys
Learning Objectives
After completion of this course, the participant will be able to:
• Understand the role of engineers in keeping the public safe
• Identify the elements employed to make roller coasters fail-safe
• Describe the phenomenon of vortex shedding
• Summarize the ASTM F24 Standards
• Apply ASTM F24 Standards to other engineering work
• Describe engineering toy standards
• Explain ASTM Standard F1148 as it applies to playground equipment
Page 24
Chapter One:
Engineering of Roller
Coasters
Overview
•
•
•
•
•
•
•
Introduction
Historical Perspective
The Roller Coaster
The World’s Greatest Roller Coaster Designers
Roller Coaster Failure: Vortex Shedding
The Top Thrill Dragster
Summary
Learning Objectives
• Explain the role of engineers in keeping
the public safe
• Determine the role of failure in design
• Identify the elements by which roller coasters
are designed to be fail-safe
• Describe the phenomenon of vortex shedding
Introduction
In this course we will focus on the professional skills
and ethical behavior required of an engineer to ensure
the safety of the public, outlining engineers’ historical
role in order to highlight and give texture and context to the role engineers play in society. There is also
thorough coverage of technical details and techniques
– whether they are the careful calculation needed to
design a playground platform for children, or the failsafe redundancy of a high-tech roller coaster. These
can be abstracted and applied to any engineering project. Even where specific techniques aren’t directly applicable, the examples used illustrate the proper approach to safe engineering design.
Today every aspect of our lives is touched by an engineered object; we live in a world filled with cell
phones, dishwashers, clothes dryers, automobiles, and
planes. Imagine what would happen if any of these
failed or did harm on a regular basis. So, it is clear that
engineers have an ethical duty to design safety into
their products. But in addition to this notion of ethical duty to be sure products do no harm; an engineer
needs the scientific and engineering knowledge to design safe objects. This course gives examples of this
engineering methodology.
By way of illustration, this course looks at devices
used during leisure time: amusement parks and toys.
This may seem lighthearted or even frivolous but, as
you will see, this study highlights how vital safety is.
As engineers, we like to celebrate achievements that
take one’s breath away, such as landing a man on the
moon, creating the world’s longest bridge, or constructing the world’s tallest skyscraper. At a deeper
level, we realize our essential role in providing the
necessities of life: clean water, afordable and safe energy, and durable shelter.
Looking at amusements might seem silly. Yet such diversions are a part of our human experience and will
always be an integral aspect of our existence. Visitors
to U.S. amusements parks get on and off a ride about
three billion times a year. Perhaps, then, our greatest
achievement might be that, today, a roller coaster is
so safe that insurance companies worry more about a
sprained ankle from the merry-go-round.
Page 25
Historical Perspective
For an engineer to focus on amusements for the public
is nothing new; engineers have always mixed the two.
For example, we think of the great aqueducts of Rome
as providing water for life, yet the Romans didn’t actually need them; for washing, drinking, and much
of their irrigation they could use nearby springs and
wells. Yet the Romans built some of the most amazing
engineered structures, the aqueducts, which transported water from miles away.
Human nature has not changed: Many, if not most, of
the engineered objects in our lives are luxuries used for
diversion and amusement. So, in this course we’ll start
with roller coasters, looking at their history and how
they work. Then we’ll see how the American Society
for Testing Materials (ASTM) Standards for Amusement Parks is essential in keeping the public safe. Finally, we’ll look at toys. We’ll peer inside the engineering
of some of the most famous ones and also review how
engineering standards are used to keep children safe.
The Roller Coaster
Roller coasters originated from wooden carriages which
slid on ice and were built near St. Petersburg, Russia, in
the 17th century. In 1804, the French adapted such sleds
for use on a system of ramps. Eventually they evolved into
the engineering tour de forces that thrill millions today.
Why? For the amusement of the public. Baths were
the key use for the water. Not bathing out of necessity,
but bathing as a social activity.
Romans made the baths with Imperial splendor: Sixteen hundred seats of marble and walls covered with
mosaics or Egyptian granite, encrusted with green
marble. Water flowed from bright silver basins. The
baths were not only a way to escape the blazing Mediterranean summer, but places to meet and converse.
Today we would compare them to coffee houses. Seneca, a leading intellectual in Rome, complained about
the noise; he objected to the splashing and roars of
bathers and the cries of sellers hawking their wares.
These baths, though, were “water pigs”, and so the
Romans had to build the aqueducts to supply them.
Page 26
The engineer’s goal in designing a roller coaster is to use
gravity and acceleration to confuse and delight a rider.
The main thrill is the g-force, which is defined as the
changes in weight from acceleration. When the cars take
off, a rider feels about two and a half g’s, nearly what
the astronauts feel when the space shuttle launches. By
understanding these g-forces, engineers have been able
to enhance riders’ thrills and prevent their deaths.
An early coaster called the “Flip-Flap” had the first loopthe-loop, which is a complete circle that turns riders upside down. Its designers understood g-forces so poorly
that as the cars went around the loop they exerted twelve
g’s on the riders. Fighter jet pilots usually black out at
ten g’s. This coaster occasionally snapped riders’ necks.
Today, we are able, of course, to calculate the forces on
an amusement park patron on every section of the track.
Flip Flap
But it is not only technical skill that creates a good
roller coaster. Knowing some psychology also helps.
Werner Stengel, one of the greatest coaster engineers,
notes:
“You must keep the element of surprise. The track
must be unpredictable and intense. The ride must
keep the rider’s attention from beginning to end.
The minute a rider gets bored in a roller coaster,
you can blame that on a design error. You should
always have a first drop as deep as possible, and
some very nice ‘high-g’ moves.”
ers in the history of amusement parks. They originally
worked for a Swiss amusement park ride manufacturer, Intamin AG, before starting their own consultancy
in 1990. They have designed and built now over 70
coasters. In the U.S. their rides include Vortex, Kumba, the Raptor, Insane Speed, and the Manta.
Vortex
He explains further:
“A lot of different factors together make a ride
thrilling. A ride can have a wonderful first drop,
but the first drop isn’t the most important part of
the ride. You must keep the suspense in the ride,
even after the first drop. It is very important to
work with the element of surprise, and not only
with g-forces. A large first drop is wonderful. I’d
love to make a ride with a first drop of 100 meters
deep. But that is not what makes a ride thrilling.
A smooth, exciting and unpredictable lay-out is
what makes a ride thrilling.”
So, engineers have learned over the years it is best to
have a slow ascent on the first hill, and then dangle
people at the top before the great plunge. On that
plunge, engineers try to make cars go as straight down
as possible and appear to curl underneath, giving riders the impression they’re about to jump off the track.
A good roller coaster engineer also makes use of the
supporting structure. Sending a car close to a column
gives the impression of speed, and a classic coaster
trick is the fine del cap – Latin for “end of the head.”
By this they mean shooting the cars toward a horizontal beam, then ducking at just the last moment.
The World’s Greatest Roller
Coaster Designers
Let’s take a look at the human face behind today’s
roller coaster. To those in the know, the names Bolliger & Mabillard are magical. This engineering duo
is acknowledged as two of the greatest coaster designAmusing Ourselves Safely
Raptor
Not surprisingly, the firm’s president, Walter Bolliger,
feels there cannot be too many coasters in the world,
but cautions that engineering expertise is needed every
step of the way. “I think we need to be careful in the
design,” Bolliger says. “We need to have something
very well balanced; what’s important is good balance
and the ride experience. I think there is still a lot which
can be done with coaster design.”
B&M coasters are known for their smoothness. Bolliger explains how they achieve that: “It’s a lot of dePage 27
sign,” he says. “It’s going in detail on the geometry of
the track, the geometry of the trains. To make sure the
fabrication is done in according to the drawings. The
precision of the manufacturing is one-sixteenth of an
inch. The elements are very precisely manufactured.”
B&M dazzled riders with its first ride, the Iron Wolf,
an innovative stand-up roller coaster at Six Flags Great
America. Since then, they have continued to define the
cutting edge of coaster design; for example, the floorless roller coaster.
The Iron Wolf
A defining element of many B&M coasters is a “predrop” – known in the industry as a “kicker.” Just after
the top of the first lift hill, the coaster drops a short
amount. This thrills riders waiting for that great downward plunge, but it also serves an engineering purpose.
It reduces tension on the lift chain. A flat section after
the first drop carries the weight of the passenger cars,
reducing stress on the chain. Compare this to most
coasters: as the first cars begin to drop, the chain is
still lifting the back of the train.
Roller Coaster Failure: Vortex
Shedding
Roller coasters do fail, but engineers design them so,
at worst, they fail safe. VertiGo, a Cedar Point ride,
collapsed in early 2002, only a few months after its
debut, when three of its steel support poles cracked.
Page 28
VertiGo packed a powerful punch by zipping riders up
to nearly 300 feet in the air at a speed of 50 miles per
hour. Suspended on steel cables from three 265-foot
towers, the ride vehicle looked like an oversized triangular shaped model rocket. It used compressed air to
launch a six-passenger car along the hair-raising ride.
Six passengers rode it in pairs, facing outward on the
vehicle’s three sides.
These unique vehicles allowed a rider to choose one
of three positions to enjoy the ride. The Hot Rocket
kept the riders upright and seated; the Cosmic Flip
kept them in an upright position for the ascent, but
as the ride reached its peak, the seats tipped forward
150 degrees to provide a “nose-dive” sensation for
the descent; and the Big Bang flipped riders forward
150 degrees shortly after launch and stayed that way
throughout the ride. Enhancing the thrill was that VertiGo used an innovative harness that held passengers
safely, yet left them feeling free and loose.
Three huge steel towers surrounded the “launch pad.”
Steel cables attached to the vehicle hoisted the car
slowly a few feet in the air, and then compressed air
“fired” the rocket by pulling it straight up it reached
almost 300 feet in the air. Riders could barely enjoy
the magnificent view before they free-fell back to the
platform; stopping, of course, just before the concrete
floor. The problems with this ride could be found in
the three steel towers.
In January of 2002, the carriage and steel cables that
supported the ride had been removed from the structure for winter storage. It was then discovered one of
the three support poles had broken off about 65 feet
above the ground. An investigation revealed that the
cause was “vortex shedding.”
Vortex shedding occurs in slow, steady winds over
long periods of time. Since Cedar Point is near Lake
Erie and often has cold temperatures, this type of wind
is not unusual. The effects of wind on a structure –
and the occurrence of vortex shedding – depend on the
cross-sectional shape of the structure, its vibrational
modes, and also the degree of damping. If the wind
speeds are such that the frequencies of its vortices
match any of the structural modes, these can be amplified and excited within the structure.
In fact, while the breaking of the first pole was unobserved, a second was seen to break. Engineers observed it
oscillating with amplitude of about fifteen feet. Note that
VertiGo, like most coasters, was designed to withstand
winds of up to 80 miles per hour. Yet on the day the pole
oscillated, the winds were only about 10 miles per hour.
Several prominent engineers think this same phenomenon caused the famous collapse of the Tacoma
Narrows Bridge near Puget Sound. (Others ascribe
the failure to aero-elastic flutter.) Opened to traffic on
July 1st, 1940, the Tacoma Narrows Bridge was the
third-longest suspension bridge in the United States at
the time, with a length of just over a mile. Because of
design flaws, the bridge frequently experienced rolling wind-driven undulations; in fact, locals gave it the
moniker “Galloping Gertie.” On November 7, 1940,
strong winds excited the bridge’s transverse vibration
mode, with amplitude of 1.5 feet. This motion lasted
three hours, until the bridge collapsed.
At low Reynolds numbers, the fluid flows smoothly
around the object. As the Reynolds number increases
to about 4, the flow separates downstream from the
object. The resultant wake, though, is formed by two
symmetrical eddies. Then, as the Reynolds number
gets larger (greater than 47), vortices are created at the
back of the body and detach periodically from either
side of the body. This creates alternating low-pressure
vortices on the downstream side of the object. These
force the object to move toward the low-pressure
zone. If the frequency of vortex shedding matches the
resonance frequency of the structure, then the structure will resonate, and the movement will become
self-sustaining.
The Strouhal number (S) helps to determine when vortex shedding will take place. The frequency at which
vortex shedding takes place for a cylinder can be derived using the following equation:
S = f D/V
Where f is the vortex shedding frequency, D is the diameter of the cylinder, and V is the flow velocity. The
Strouhal number depends on the body shape and on
the Reynolds number. Empirical relationships have
been derived for other shapes.
The Top Thrill Dragster
Vortex shedding can also occur when a fluid flows past
a blunt object. See Figure 1-101 below.
Let’s take a detailed look at the engineering of a modern-day roller coaster. The Top Thrill Dragster at Cedar Point is an amazing engineering achievement. Engineers prepared over 1,000 blueprints in three years
to design the Dragster. It used 90 truckloads of steel,
which traveled by boat, train, and truck from Europe
to Ohio. Two hundred workers anchored the massive
structure in 149 footers which used 9,000 cubic yards
of concrete and over 5,000 bolts hold the framework
Page 29
together. Then, to make the complex electrical system
engineers strung more than 100 miles of electrical cable, all to create a very peculiar roller coaster.
Unlike conventional coasters, the Top Thrill Dragster
doesn’t start slow, it has no clackety-clack climb, and
no scenery on the way: Sixteen passengers in a car
are shot out of the gate, reaching 120 miles per hour
in four seconds. They zip up a 420-foot hill, rotate 90
degrees, crest the top – losing all momentum right at
the peak – then plunge back down. The complete ride
takes less than 30 seconds!
Let’s look at the extensive engineering that goes into
those 30 thrilling seconds.
Hydraulic System
Traditional roller coasters gradually ascend a hill so
they can build up enough potential energy to coast up
and down the remaining track. The designers of this
coaster chose to have the car burst out of the gate instead.
They considered high acceleration linear induction
motors (LM), which would be extremely efficient and
Page 30
require little maintenance. LM-powered coasters essentially ride a magnetic wave down a track; metal
fins on the bottom of the train are propelled by stator
coils along the track. But the engineers determined that
such motors could not provide quite the acceleration;
the coaster would have needed a longer approach to
the hill. This would require Cedar Point, which is built
on a narrow peninsula jutting into Lake Erie, to use up
precious land. Additionally, they would have incurred
expenses in relocating or even removing other rides.
Instead, they turned to a hydraulic system.
The hydraulics at the heart of the Top Thrill Dragster accelerates a 15,000 pound coaster train from zero to 120
mph in four seconds. This is essentially the same technology used for stamping presses, and used for the same
reasons: an applied force can be controlled precisely.
The hydraulic launch system transmits about 10,000
horsepower to accelerate the coaster. A hydraulic system is ideal because the power is needed for only a
few minutes. Hydraulics can store huge amounts of
energy and release them in a rapid but controlled
manner. Two identical sets of sixteen hydraulic motors drive an internal ring gear. This pair of ring gears,
in turn, drives a dual-input planetary gearbox, which
drives a sheave that pulls a cable to launch the coaster.
The cable is wound onto drums turning at 500 revolutions per minute. Fluid to the motors flows through a
two-inch, six-wire house and is returned with a sixinch hose to minimize pressure drop.
The system operates at about 4,500 psi with a peak
of 10,000 hp. This means that total flow to the pumps
must be about 3,650 gallons per minute. That is over
100 gpm for each pump, which is too hefty, so the
pump-motor combination uses a piston-type accumulator and gas pressure vessels. These supply most
of the flow for the four-second launch. Right before
launch, each of the 32 pumps forces oil into the wet
side of an accumulator. This volume pushes back the
piston and forces nitrogen into the dry side and into
the gas bottle. Because the volume of the gas bottles
is fixed, the pressure of the nitrogen increases. The
nitrogen compresses and acts like a spring, while the
hydraulic oil acts like potential energy. It takes less
than a minute for the gas bottles to get a full charge.
Design of the Ride
The hydraulics impart just enough momentum so the
Dragster can make it over the top. Unlike a regular
roller coaster, the Dragster has no anti-rollback devices. Instead, it is designed to roll backwards if it doesn’t
get enough “oomph” from the hydraulics. The Dragster is just one large “up and down” ride, so rather than
have cars stuck at the top, some 420 feet in the air, it
is designed so the cars are not stable at the top; they
either roll back or continue the ride and finish. (Many
riders feel lucky if they roll back downhill because
they get to ride all over again.)
At the top are three sensors spaced a known distance
apart, so the speed of the ride can be calculated for every trip. The impulse needed at the beginning can vary
because of weather conditions. So, using the average
of the last three trips, a computer calculates whether
the ride has been too slow or too fast and then adjusts
the speed of the next launch. It can happen that three
cars full of lightweight riders are followed by a car of
heavyweights. In that case, the coaster likely wouldn’t
make it up the hill. This, though, rarely happens, much
more likely is a change in environmental conditions;
heat or rain, for example.
Stopping the Top Thrill Dragster
A permanent magnet braking system decelerates and
stops the train at the end of the ride. (There is an identical system at the launch in case a train slips back.)
This type of braking system is essential because a conventional system would give passengers too much of
a jolt. Each car has copper fins that fit into the slots of
the permanent magnets underneath the tracks. When
ready to launch, pneumatics pop the magnets out of
the way as the train blasts off, snapping back into place
immediately in case the train returns. The permanent
magnets make for a fail-safe system. If power fails,
they are actuated pneumatically into position so that it
requires an electrical signal to move the magnets out
of the way. Thus the brakes are automatically in position to stop the coaster.
G-Forces
Unlike earlier rides, the Top Thrill Dragster’s g-forces
are carefully calibrated. Using the methods set out by
the ASTM Standards, they are first measured using
water-filled crash test dummies that simulate the average weight of riders. These are equipped with accelerometers to measure the effects on the human body,
even though they were very well understood at the design phase. It would seem that rising 420 feet in a few
seconds, twisting, and then descending would have
exerted lethal g-forces, yet the Dragster is so cleverly
designed that riders feel about the equivalent of jumping on a pogo stick. The key to safe g-forces is duration and direction. For example, on this coaster riders
might feel 4 to 4.5 g’s, but only for a fraction of a second. They are well inside the limits set by the ASTM.
In the next chapter we will look carefully at how the
ASTM Standard for amusement park rides is used.
Summary
Our first chapter outlined the role of engineers in safeguarding the public, focusing on the techniques and
designs used to ensure that what appears frivolous –
amusement park rides – are safe for public use. We
examined the history of the roller coaster, noting that
many early roller coasters had severe design flaws. We
discussed the essential design principles used in creating the world’s greatest coasters. Next we turned to
the modern-day failure of a coaster, that is, the vortex
shedding of a Cedar Point ride. We closed by taking a
careful and detailed look at the safety systems on the
Top Thrill Dragster at Cedar Point.
Page 31
Chapter Two:
Engineering Design
Standards
Overview
•
•
•
•
•
•
•
Introduction
ASTM Committee F24
Waiting In Line
Acceleration
Safe Construction
Standards for Metal Framework
Summary
Learning Objectives
• Summarize the ASTM F24 Standards
• Apply ASTM F24 standards to other
engineering work
• Distinguish the limits of acceleration in
a roller coaster
• Show how to use the ACI Standard 318
to design a footer
• Apply the ASTM Standards for the appropriate
level of safety for a metal framework
Introduction
In the previous chapter we outlined the engineering of
roller coasters, particularly the Top Thrill Dragster. In
this chapter we will look at how the ASTM Standard
F2291 Standard Practice for Design of Amusement
Rides and Devices tackles these issues. We’ll look at
some of the design standards for the coasters themselves, but also at the many ways – unsuspected by the
thrill-seeking public – that engineers make that trip to
the amusement park safe. Some ways may seem mundane, but slipping on a step or falling over a guardrail
is not inconsequential, if it results in injury.
ASTM Committee F24
The ASTM Committee F24 puts this standard together. One might think their motto is “Billions of Happy
Screams.” This committee started to really push the
envelope for setting standards when it formed in 1978,
paving the way for the truly innovative and exciting
new amusement park rides we enjoy today. The committee has about 500 members who currently have jurisdiction over 17 ASTM Standards. The committee
consists of manufacturers, coaster operators, and those
with a general interest in coaster safety. The scope of
the Committee is in developing standard methods
of testing, performance specifications, definitions,
maintenance, operations, and practices and guides for
amusement rides and devices to ensure enjoyable and
safe diversions for the public.
Waiting in Line
During peak season at any popular park, a patron may
wait an hour or more for one of the bigger, bolder roller coasters. Waiting in line seems mundane but in an
amusement park, patrons are crowded together, eager
to ride a coaster, perhaps angry at the wait and tired
from the “sun and fun.” This creates a potent mix that
can result in injury.
Fencing, guardrails, and handrails must be designed to
prevent patrons from falling as they approach a ride.
Typically, the entrance to a ride is elevated and a patron slowly walks uphill in line to the ride’s entrance.
Page 32
The ASTM Standard recommends that guardrails be
no less than 42 inches high to keep patrons from contacting the coaster and from falling over the edge.
Guardrails also must be built with openings small
enough that a four-inch-diameter sphere will not fit
through them. (See Figure 1.)
Source: ASTM Standard F2291 Standard Practice for
Design of Amusement Rides and Devices
tive and negative x, y and z, where z aligns with pull
of gravity. (See Figure 2.)
Measuring the Dynamic Characteristics of Amusement
Rides and Devices
The triangular openings formed by riser, tread, and
fence need to be small enough that a six-inch sphere
cannot fit through them.
They designate acceleration along the z axis as “eyes
up” for negatives g’s and “eyes down” for positive
g’s. Let’s look for a moment at the limits for a roller
coaster.
Acceleration
Figure 3 shows the allowed reversals from -Gz (eyes
up) to +Gz (eyes down).
In the first chapter of this course we noted the limits on acceleration set by the ASTM Standard. The
inclusion of acceleration standards is unique. Even
though the committee formed in 1978, the g-force
limits took root only in 1987. While discussing
them, members of the sub-committee in charge of
drafting the acceleration standard realized it would
not mean much unless they also specified how acceleration was to be measured. So they developed
ASTM F2137 Standard Practice for Measuring the
Dynamic Characteristics of Amusement Rides and
Devices. This standard provides experts around the
world with a common language and also allows detailed comparisons from year to year and from ride
to ride.
ASTM F2137 states, for a ride where patrons (with an
average weight of 150 pounds each) are seated, the accelerometer must be between 13 and 16 inches above
the seat and 3 to 5 inches behind the upper torso contact surface. The key isn’t just the magnitude of the
acceleration, but also its duration and direction. The
standard looks at acceleration in six directions; posiAmusing Ourselves Safely
Measuring the Dynamic Characteristics of Amusement
Rides and Devices.
It is acceptable to change from zero (at the peak for
some rides) to 2g in 200 ms. A further increase would
require a 133 ms pause. Generally coasters, as indicated by the rightmost graph in Figure 3, use about
15g/sec as an acceptable limit.
Page 33
Safe Construction
An amusement park ride is, of course, seated in the
ground, usually with concrete. The ASTM Standard
F2291 suggests that the concrete meet the criteria laid
out in the American Concrete Institute’s (ACI) Standard 301 or 318. (These are good standards for an engineer to become familiar with.) Obviously a key to a
safe coaster is that it stays in the ground! ACI 318 is a
long and complicated standard, and here we will look
at just one aspect: footings.
Footings
As we’ve noted, roller coasters typically are set in
concrete footings. The ACI Standard spells out how to
design the base area of a footing to support a particular
load. (See Figure 4.)
Thus, the required base area for the footing is:
Af = (400 + 200)/3.75 = 160 square feet
which is about 13 feet by 13 feet (169 ft2).
In order to determine the proper depth of such a
footing, ACI 318 requires an estimation of the shear
strength by treating the action on the footings in two
ways. First, using wide-beam action; that is, assuming
the footing acts as a wide beam with a critical section
across its entire width. Second, assuming two-way
action for the footing, which checks for “punching”
shear strength. The final design depends on which type
of action is most severe. Rarely does the wide-beam
action control the shear strength, but still an engineer
must be sure the beam action strength associated with
this action is not exceeded.
Wide Beam Action
Source: ACI 318: Building Code Requirements for
Structural Concrete and Commentary (American Concrete
Institute 2005)
For the situation shown in this figure, the service dead
load is 400 kips; the service live load is 200 kips; and
the service surcharge (from the soil) is 100 psf. We’ll
use a typical value for the weight of soil and concrete
above the footing base of 130 pcf (pounds per cubic
foot). For this area the permissible soil pressure is 4.5
ksf and the column is 35 inches by 15 inches. Using
these values, we can calculate the base area.
The calculation begins with estimating the factored
loads and the soil reaction. The Standard uses the
idea of “required strength” (U) which is calculated
by combining the service loads according to factors.
Specifically, equations 9-1 to 9-7 of ACI 318-05 (the
2005 version of the code) show various ways this can
be calculated. It spells out carefully which equation
to use; for our example we’ll use equation 9-1 of the
Standard:
U = 1.4 (D + F)
where D is service dead load and F is service live load.
For our example, this means:
The soil reaction then, is:
We first make a calculation using unfactored service
loads. The total weight from the surcharge is:
(0.130 x 5) + 0.1 = 0.750 ksf
The net permissible soil pressure is then:
4.5 - 0.75 = 3.75 ksf
Page 34
We then calculate the shear strength required for
wide-beam action. To make these calculations, the
engineer usually chooses a typical depth and average effective thickness, based on experience or on
the standard practice of a particular engineering
firm, and then checks to be sure the shear force is
less than effective shear strength. For this example
we’ll use an overall footing thickness of 35 inches,
assuming an average effective thickness of 30 inches (2.5 feet).
We then need to compare this to the shear strength for
the concrete, which can be calculated using:
We first calculate the factored shear force using
For the wide beam, we use the area indicated in Figure 5.
where
is the shear reduction factor, which section
C.3.2.3 of the codes assigns a value of 0.85 for causes
of “shear and torsion.”
is the compressive strength of concrete; we’ll use
a typical value of 3,000 psi.
is the width of the critical area for the wide beam; in
this case, 13 feet (156 inches - the width of the footing).
d is the effective thickness (30 inches, or 2.5 feet).
Using these numbers yields shear strength of:
= 435.7 kpis
Note that the shear strength is greater than that calculated by wide beam action.
435.7 > 218 kips
This thickness is acceptable. Next we must check the
two-way action.
Two- Way Action
One side of this area is simply the length L (13 feet in
this case); the other side is calculated using the size of
the column and the depth. (Note that the depth is used
as a parameter for sizing the area; the actual depth
would, of course, be into the page in Figure 5.)
Calculating two-way action is a little more complicated. As before, we start with the factored shear force,
but use a different area. In this case we use the area
designated in Figure 5: the area of the footing minus
a critical section with sides of lengths
and
; that is, an area larger than the column by
the thickness d in each dimension.
For this example, this is:
Note that this is calculated using the smaller dimension of the column; this gives a larger area and hence
a greater shear.
Page 35
Thus,
Standards for Metal Framework
We must, of course, compare this to the proper shear
strength of the concrete, designated as
. In this
case we need to make three separate calculations and
choose the minimum of them. ACI 318-05 (the 2005
version of the code, which is the most current as this is
written) lists equations 11-33, 11-34, and 11-35
Where
is a perimeter for the critical section of slabs and
footings, its calculation using:
For our column, this is:
= 2(35 + 15) + 4*30 = 197
is the ratio of the long side to the short side of the
column
is 40 for interior columns, 30 for edge columns,
and 20 for corner columns. (We’ll assume an interior
column.)
Thus, for our column the three possibilities are:
The governing value is 3.71; thus:
A roller coaster such as the Top Thrill Dragster is
held in place by a massive metal framework. To ensure an appropriate level of safety and longevity of
this support system, ASTM Standard F2991 suggests
using the mean fatigue property data downgraded by
two standard deviations when doing design calculations. Often, a two-standard deviation reduction from
the mean fatigue strength based on rigorous statistical
analysis cannot be done; in lieu of that, the Standard
recommends using an alternative method based on a
strength reduction factor. Because of the larger uncertainty associated with the “reliability factor” approach
as compared to rigorous statistical analysis, the Standard recommends using a reliability factor of 0.75,
which is associated with three-standard deviations;
i.e., 99.9% reliability.
To be reliable, the ASTM Standard requires the design to use a 35,000 operational hour criteria. This
means all of the primary structures of a ride – track,
columns, hubs, etc. – must be designed such that
they will operate for a minimum of 35,000 hours.
The goal here is to ensure that an engineer has designed all the main structures of an amusement park
ride for at least a minimum fatigue life. As you can
imagine, the notion of an “operational hour” needs
to be carefully defined. Obviously, there often is a
great deal of time when a park is closed and the ride
is not operating. Beyond this common-sense idea,
the batch nature of roller coaster rides implies that a
ride isn’t being continuously fatigued, even during
a busy day. So, the “operational hours” are allowed
to be reduced by accounting for the loading and unloading of patrons.
The ASTM Standards limit this to a maximum of 50%
of the 35,000 operational hours. To calculate the general time reduction for loading and unloading, an engineer uses:
(Total load/unload time for one ride cycle) / [(Total load/unload for one ride cycle) + (Time for one
ride cycle)]
Again, the factored shear strength is less than the calculated shear. So, the depth of the footing meets code.
Page 36
Then to determine the operational hours to be used in
design calculations, the engineer uses:
[(35,000 operational hours) x (1.00 - general
reduction for load/unload times]
For example, the Top Thrill Dragster takes about five
minutes to load, and then has an incredible 30-second
ride. The general reduction in operational hours is
(5 min)/(5 min + 1/2 min) = 0.90
Thus, the operational hours used for design are:
(35,000)(1.00 -0.90) = 3,500 hours.
Summary
In this chapter we looked at ASTM Standard F2291
Standard Practices for Design of Amusement Rides
and Devices. We examined specific design issues of
roller coasters: Designing safe waiting areas for park
visitors, the limits of acceleration on a roller coaster,
the ACI 318 Standard’s criteria for a concrete footer,
and the longevity and safety of the metal framework
used to hold up a coaster were all discussed.
Page 37
Chapter Three:
The Engineering of Toys
Overview
•
•
•
•
•
Introduction
Standards for Toys
Engineering Secrets of Toys
ASTM Standards that Apply to Safety of Children
Summary
Learning Objectives
• Identify the essential role of engineers in ensuring
the safety of children
• Describe how the engineering toy standards have
come about
• Explain how toys are designed
• Describe the details of ASTM Standard F1148,
which applies to playground equipment
Introduction
In 1960, many children loved the Zulu Toy. Although
politically incorrect today, it was a peashooter of sorts,
a mouth-powered way to shoot plastic darts. Its black
plastic tube was about as long as a stick ballpoint pen
, and perhaps, just a little thicker. It cost a dime and
came with three, one-inch rubber darts, each with a
suction cup on the end. It foreshadowed the caution of
Ralphie’s mother in the movie “The Christmas Story”:
She wouldn’t buy him a toy rifle because, she said,
“You’ll shoot your eye out.” In a classic life-followsfiction tale, soon after the release of the Zulu Gun
emergency rooms saw a surge in accidental ingestion
of plastic darts. In 1969, it was one of eight toys banned
by National Commission on Product Safety. Today,
when we watch Ralphie’s mother admonish him to be
careful it seems amusing, only because we’ve become
so accustomed to having safe toys for children.
Safe toys, though, were developed only after World
War II. In the first half of the 20th century, children
had very few toys and most were homemade. The economic boom after the War created an enormous market for toys. Five thousand new ones appeared every
year; in fact, by 1972 150,000 toys were on the market. Manufacturers with a poor understanding of the
behavior and even typical physical dimensions of a
child filled the market with dangerous toys.
For example, in 1955 New York banned fake Davy
Crockett coonskin caps, and rightly so; because they
were made of shredded paper, the caps could burst
into flame “in seconds after the most casual exposure
to a live cigarette or to any spark,” as the New York
Director of Safety to the State’s fire chiefs said.
Page 38
Standards for Toys
Since the 1981 Consumer Product Safety Act, the federal government has supported the development of
voluntary standards whenever those standards will be
effective in addressing a risk of injury. This gave impetus to write the ASTM Standards that describe the
essential principles for building safe toys. To me, they
represent engineers’ great contribution to the safety
and welfare of our children.
This contribution is significant as the global retail toy
sales total over seventy billion dollars, and the U.S.
accounts for over 30% of this figure. The U.S. is also
the world’s largest toy producer. U.S.-based toy manufacturers develop more than half of toys sold around
the globe. This position of worldwide leadership in
manufacturing has propelled the U.S. toy industry to a
leadership position in ensuring toy safety.
In 1971, the U.S. toy industry association drafted a
comprehensive voluntary toy safety standard. Five
years later, in 1976, the trade association led a group
of pediatricians, national safety council, retail organizations, and industry experts to publish a comprehensive standard under the auspices of the National
Bureau of Standards. The voluntary standard supplements the U.S. federal mandatory standards for toys.
Together, they cover more than 100 separate tests and
design specifications intended to reduce or eliminate
hazards to children from their toys.
Engineering Secrets of Toys
Twenty years ago, as I was looking for some clever
examples of engineering design and manufacturing, a
mechanical engineering colleague suggested I look at
toys. He said they “cut the edge” in manufacturing innovation and design. He was right. Let’s look at how
clever engineers have been in designing toys. We’ll
start with the Magic 8 Ball.
Magic 8 Ball
Many of us had a “Magic 8 Ball” when we were children. First introduced in the 1950s, the toy is an approximately four-inch diameter replica of a billiard
ball, painted to look like an “eight ball” – the “black
ball of “death” in the most popular type of pool. Although the ultimate object of the game “Eight Ball” is
to legally pocket the eight ball in a called pocket, it can
only be done after all a player’s assigned balls have
been cleared from the table. The player who pockets it
any time before that time loses the game. The makers
of this toy built on these dire connections to design
this simple, yet very popular children’s toy.
The ball has a small window at the bottom. Hold this
window to the ground, ask a “yes” or “no” question,
wait 10 seconds, then turn the ball upside down and
up pops an “answer” in the window. Sometimes the
ball responds, “Ask again later” or “Don’t count on
it”, or “Outlook good.” This bare outline doesn’t begin
to capture the attraction this toy had for children (and
I speak from personal experience), but the cold hard
sales numbers do. To this day, about a million Magic 8
Balls are still sold yearly.
Although the toy’s origins are shrouded in mystery, it
appears a Cincinnati clairvoyant inspired this pseudofortune telling device. She used a “Psychic Slate”, a
blackboard with a lid. The clairvoyant would snap
shut the blackboard and then ask it a question of interest to her client. The board made squeaking noises,
so-called “spirit writing”; the psychic then opened the
blackboard to reveal an answer. This device inspired
her son to create the Magic 8 ball in the 1940s. It is essentially a self-contained, but limited, “psychic slate.”
The psychic’s son partnered with his brother-in-law, Abe
Bookman, and a few others to form a novelty company
to build and market the Magic Eight Ball. Bookman,
who had graduated from the Ohio Mechanics Institute,
was a meticulous genius. He patented a brilliant design
for the ball, then called the “Syco-Seer”, and billed it as
a “Miracle home fortune teller.” In his patent, he called
his invention a “Liquid Filled Dice Agitator”, which is
a pretty good description of today’s ball.
Page 39
Inside the ball is a tube filled with blue fluid; the rest
of the ball contains no fluid. Floating in this liquid is
an icosahedron (a twenty-sided shape) which is only
slightly less dense than the fluid. The icosahedron is
hollow, and is narrow enough to allow fluid to float
by it. Inscribed on the faces of this solid shape are 20
phrases that provide vague but “enlightening” answers
– at least, children consider them enlightening. A professor of psychology from the University of Cincinnati provided the phrases for the magic ball. Half are
negative and vague, and half are affirmative. The liquid is a mixture of alcohol and blue dye.
The cylinder contains three parts: 1) the cap; 2) the
main body; and 3) a bubble eliminator. The device
works because there is just enough air to form bubbles
which, when the ball is turned over, rise and cause
the liquid to shift and the icosahedron to twirl. Over
the years, engineers have worked to refine the bubble
eliminator.
In earlier versions bubbles would obscure the viewing port. In today’s Magic 8 Ball, the bubble eliminator collects the bubbles in the fluid, blocking their
return to the main body: When one turns the ball over
to get a new answer, bubbles inside will float to the
top and pass through the hole in a funnel in the collector, where they are then trapped. Only when the ball is
shaken can they re-enter the main chamber of the tube.
Copyright © 1997-1999, Dan Egnor and Heath Hunnicutt
“Magic 8-Ball” and “Tyco Toys, Inc.” are registered
trademarks of Tyco Toys, Inc., and are used without
permission. Quotations from the Magic 8-Ball® are
Copyright © by Tyco Toys, Inc. No affiliation exists
between Tyco Toys, Inc., and this publication. Opinions herein are solely those of the author. Other trademarks are the property of their respective owners, and
are used without permission.
Super Soaker Water Gun
It may be cliché to say of an invention, it takes a rocket scientist to think of it, but it really did take one to
bring the squirt gun into the realm of high tech. I’m
speaking of the Super Soaker, the most popular toy
of the 1990s. For those who haven’t seen it, it’s been
described as “a squirt gun on steroids.” It holds some
Page 40
two gallons of water, and can drench an opponent up
to forty feet away – enough to send anyone crying to
mommy. It was invented by engineer Lonnie Johnson,
who worked at NASA’s Jet Propulsion Lab. He’d designed, for example, the power supplies for the Galileo space probe.
Lonnie Johnson
One day, Lonnie was experimenting with a new type
of refrigerator that used water instead of freon. He
hooked a nozzle to a faucet in his bathroom and, when
he turned on the water, it shot across the bathroom,
making air currents so strong his shower curtain started to swirl. His first thought, and I quote him here,
was, “Boy, this would really make the neatest water
gun.” His key squirt gun insight was to use pressurized air to drive the water through a narrow hole in the
nozzle.
“From that point”, Johnson said, building a high-tech
squirt gun “was an engineering problem.” Where the
engineering came in was to come up with a way that
“a small kid would be able to pump the gun up to
a very high pressure.” Johnson went to work in his
home workshop.
Using a small hobbyist’s lathe, he built a model out
of PVC pipe, an empty plastic Coke bottle, and Plexiglass. Next came test marketing; he let his daughter,
age six, try it out on neighbors. The result was a great
success, at least for her. Next, Johnson had to interest
a manufacturer.
He first approached Daisy Manufacturing, the maker
of BB guns, but they passed on his idea after two years
of discussion. In 1989 Johnson met with the Larami
Corporation. He walked into the meeting, opened his
suitcase, and pulled out his prototype of PVC tubing,
Plexiglas and plastic soda bottles. A split second later,
he fired a giant stream of water across the room. Larami’s president had just one word: “Wow!” But would
they sell? In the past, a squirt gun sold for twenty-nine
cents: Would any one be willing to pay ten dollars for
a squirt gun?
The first year startled the industry. Sales took off when
Johnny Carson, on the Tonight Show, used a Super
Soaker to drench Ed McMahon. A year later, it was
the most popular water gun in American retail history,
sold not only by toy stores, but also by upscale adult
stores like Sharper Image. By the late 1990s, about
250 million Super Soakers had been sold – enough for
each person in the United States.
Slinky
There is a song, a jingle, that’s recognized by a large
percentage of American adults. It’s this: “It’s Slinky,
it’s Slinky, oh what a wonderful toy. It’s Slinky, it’s
Slinky, fun for a girl and boy.” Of course, that’s the
song for the slinky toy, a coil spring that “walks”
down steps. In 1943, Richard James, a naval engineer,
was developing a spring that could keep sensitive instruments aboard warships steady in rough seas. Accidentally, he knocked one of his test springs off a shelf.
It crawled, coil by coil, to a lower shelf, then onto a
pile of books, finally coming to rest on a table. This so
enchanted James that when he got home he said to his
wife: “I think, if I could get the tension right, I could
make it walk.”
For two years Richard James worked to find the proper length and tension so it could walk perfectly down
stairs. His wife, Betty, helped name this new spring.
Flipping through the directory, she came across a
word that meant “stealthy, sleek, and sinuous.” That
word was, of course, “slinky.”
By 1946, James had his Slinky ready to sell. On a
snowy day, he set off for Gimbels department store in
Philadelphia. His wife worried that no one would want
a Slinky. She gave a friend a dollar to buy the first one,
so her husband wouldn’t feel bad. When Richard and
Betty James stepped off the elevator that day, they saw
across the sales floor a mass of people waving dollar
bills. Within ninety minutes, they’d sold 400 Slinkys.
For the next fifteen years, Slinky continued to sell
well. But Betty James saw little of the profit. Richard
had, in her words, joined a “religious cult” and was
giving it all the profits. By 1960, he left his wife and
family to join this group in Bolivia, never to return to
his family again. Betty took over the Slinky Company,
now nearly bankrupt, and turned it into a multimilliondollar enterprise.
Page 41
The company uses the same machines that Betty’s
husband designed in 1945. No one has ever been able
to build better ones. So important are they to making Slinkys that no one is ever allowed to photograph
them, for fear foreign competitors could copy the machines and bootleg Slinkys.
The machines make a Slinky in about ten seconds by
coiling a sixty-three foot metal wire into eighty-nine
coils. When finished, the machine drops the slinky,
which pops up, walks down a step, and steps into its
own box. This isn’t just for fun. A slinky wound too
tight or too loose won’t walk down the step and into
the box correctly, and is rejected.
Two hundred and fifty million Slinkys have walked
into their own boxes since 1945 and Slinky shows no
signs of slowing down. Sales have risen twenty-five
percent in the last several years. It’s likely a whole
new generation will remember that Slinky jingle.
Certainly these toys have improved the human condition; they have helped us raise happy children. In addition, each has been designed with children’s safety
in mind. Let’s look now at specific areas where the
standards have helped improve children’s health, and
even prevent death.
ASTM Standards that Apply to
Children
ASTM Standard F 1148 Standard Consumer Safety
Performance Specifications for Home Playground
Equipment sets the safety specifications for home playground equipment for use by children from eighteen
months to ten years of age. The cognitive abilities and
sizes of children vary greatly with age. The purpose of
this standard is not to eliminate parental responsibility, but instead to remove possible hazards that are not
readily recognized by the public. The standards, build
on years of work by engineers to identify such hazards.
They grew out of a Voluntary Product Standard (1976),
supervised by the National Bureau of Standards, which
is now the National Institute for Standards and Technology (NIST). The standards make clever use of the typical sizes of children in articulating the size of things.
Let’s look at how the standards make use of statistical
data to ensure the health and safety of children.
Page 42
Platforms
A platform forms the center of most home playground equipment. It must, of course, support the
weight of several children playing and jumping all
at the same time. Specifically, the standard requires
that a platform be able to be loaded vertically without
shock, and the total load shall remain for five minutes. When applying the load, the platform is divided
into four equal quadrants, with the load calculated at
five points: the center of each quadrant and the center
of the platform. (This is done whether the platform
is square, round or triangular.) These loads are calculated based on the maximum number of users and
the most likely largest weights of such users. It is a
two-step process:
1. Calculate the maximum number of users as follows:
where N is the maximum number of users and X
is the area of the maximum age user using Table 1
“Structural Integrity Loading Chart.”
2. Using the maximum number of users, apply the
load for two 95th percentile maximum age users
and the balance for users in the 50th percentile
maximum age.
Table 1: Structural Integrity Loading Chart
Age
(years)
50th
Percentile,
lbs
95th
Percentile,
lbs
Area
occurred
by user ft2
1.5
22.7
26.8
0.6
2
28
29
0.7
3
32.8
42
0.8
4
35.36
43
0.8
5
39.7
50
0.9
6
44.1
59
1.0
7
50.5
69
1.1
8
56.2
81
1.2
3
63.1
89
1.3
10
70.5
105
Source: ASTM Standard F1148-03
1.4
For example, consider a piece of playground equipment with a 5-foot by 5-foot platform intended for use
by children seven years or younger. The maximum
number of users would be:
The maximum load to test with would then be
= 2 x 69 lbs (95th percentile weight) +
+ 19 x 50.5 lbs (50th percentile weight)
= 1097.5 lbs
Stabilization of Toys
Table 2: Height of Fifth Percentile Children (Values given
for boys or girls, whichever is lower)
Age, years
Height, inches
1
27
2
29
3
33
4
37
5
Source: ASTM Standard F963
40
Table 3: Weight of 95th Percentile Children (Values given
for boys or girls, whichever is higher)
This type of statistical information forms the core for
making all manner of toys safe. ASTM F 963 Standard Consumer Safety Specifications for Toy Safety
uses similar data on height to specify the appropriate
design of toys for stability. For example, in any toy a
child can sit on or ride, the child’s feet must be able
to easily reach the ground for stabilization. If a rideon toy has a seat where the height of the seat from
the ground is one-third, or less than one-third, of the
height given in Table 2, there is no need for a stability
test as long as the child’s legs are unrestricted in their
sideways motion and can be used for stabilization.
Table 2 is made from the lower of these two numbers:
(1) the fifth percentile group of boys at each age, from
age one up to and including five years; (2) the fifth
percentile group for girls at each age, from age one up
to and including five years.
Age, years
Weight, lbs
1
28
2
29
3
42
4
49
5
50
6
59
7
69
8
81
9
89
10
105
11
121
12
120
If the seat is higher than the values in this table, then
an engineer must do a test for stability. Namely, place
the toys on a smooth surface which is inclined, usually, 10 degrees to the horizontal. (If the child’s feet are
not accessible then it needs to be 15 degrees.) Then,
turn the steering mechanism to the position where it
is most likely to tip and apply to the seat a static load
equal to the weight of a child in the 95th percentile
(see Table 3). The load must be applied in such a way
that the height of its center of gravity is 8.7 +/- 0.5 in.
The center of gravity for the load for all ride-ons will
be secure both 1.7 inches rearward of the front-most
position of the seating area and 1.7 inches forward of
the rear-most seating area.
13
140
14
Source: ASTM Standard F963
153
Merry-Go-Rounds
While we’re on the playground, let’s look at what
engineers have learned about making safe merrygo-rounds, as articulated in ASTM Standard F 1487
Standard Consumer Safety Performance Specification
for Playground Equipment for Public Use. Over the
years, engineers have isolated the key features that
make a merry-go-round reasonably safe, without killing all the fun.
Page 43
First, it must be nearly circular: The minimum and
maximum radii must differ by no more than two inches. This way, a young child can clearly see where to
stand in order not to get hit; a highly non-circular device would be very confusing. Second, the platform
must be continuous so there are no “pinch, crush, and
shear points.” The vertical clearance under the revolving wheel may be no less than nine inches. The movement of the merry-go-round must match a child’s simple vision of how it would move; thus, no oscillations
and no up-and-down movement. Lastly, the speed of
the merry-go-round will be according to this formula:
Figure 1 outlines the key relationships to be used in
designing a chute.
where
Note: The rotation per minute formula is to be used only for
diameters less than 10.5 feet and the velocity formula for
diameters less than 13.0 feet.
Slides
Another area where attention must be given to safety
is slide chutes. Engineers must design these so a child
does not easily fall from a height that could cause injury.
Page 44
Source: ASTM Standard F1148 Standard Consumer
Safety Performance Specifications for Home Playground
Equipment
First, the height/length ratio must not exceed 0.577;
otherwise, it will become too steep for a child to use
safely. In addition to the overall (“gross”) criteria,
there needs to be specification for how steep any section of the slide can be. The ASTM code recommends
no sliding surface have a slope greater than 50 degrees. To prevent a child from rolling off the edge of
a slide with an open chute, the side walls must be at
least four inches high; the inside of the chute needs to
be at less 12 inches for children aged two to five and
16 inches for 5- to 12-year-olds. The bottom of the
slide may be curved instead of flat. This means that
a method must be specified for where to measure the
necessary four-inch side wall. So, a semi-circular or
curved cross-section chute must have:
1. Side walls that are at least four inches or greater (y
in Figure 1) when measured at right angles above
a horizontal line (x in Figure 1) that is 12.0 inches
long when intended for 2- to 5-year-olds or 16
inches when intended for 5- to 12-year-olds.
Or
2. The vertical sidewall height (H in Figure 1) may
be a minimum of 4 inches minus two times the
width of the bedway (W in Figure 1) divided by
the radius (R) of the bedway curvature:
force of a return spring, the point shall be identified as sharp.”
An example of such an apparatus is shown in Figure 2.
Source: 16 Code of Federal Regulations § 1500.48
Technical requirements for determining a sharp point in
toys and other articles intended for use by children under
8 years of age
H (inches) = 4 - 2W/R
The test point enters on the right side of the device,
as shown in the drawing. If it is able to pass through
the gauging slot and press the sensing head 0.005”, an
electrical circuit is completed and powers an indicator
light, thus showing the point is too sharp for use in
toys.
Another area of concern is projections from toys, particularly sharp points. For children under eight years
of age, the Code of Federal Regulations (CFR) mandates under section 1500.48 of their code:
Similarly, CFR 1500.49 lays out the test for determining the proper sharpness of an edge for any toys intended for use by children under eight years of age.
The code defines very carefully how to determine if
an edge is too sharp. A hard mandrel, which can apply
a precise amount of force while spinning, forms the
heart of the test.
Projections
“A rectangular opening measuring 0.040 inch
(1.02 millimeters) wide by 0.045 inch (1.15 millimeters) long in the end of the slotted cap establishes two reference dimensions. Depth of
penetration of the point being tested determines
sharpness. If the point being tested can contact a
sensing head that is recessed a distance of 0.015
inch (0.38 millimeter) below the end cap and can
move the sensing head a further 0.005 inch (0.12
millimeter) against a 0.5-pound (2.2-newton)
An engineer needs to wrap the full circumference of
the mandrel with a single layer of polytetrafluoroethylene (TFE) tape. As the mandrel spins, it must be set
to apply a force of 1.35 pounds to the toy edge whose
sharpness is being measured. (See Figure 3.)
Page 45
Source: 16 Code of Federal Regulations § 1500.48 Technical requirements for determining a sharp point in
toys and other articles intended for use by children under
8 years of age
Rotating with a tangential velocity of 1.00 +/- 0.8 inches per second, the mandrel must complete one revolution as the edge touches the mandrel in a “worst case”
situation; i.e., as a child is most likely to cut himself
or herself with the edge. The edge is labeled “sharp” if
it completely cuts through the tape for a length of not
less than 1/2 inch at any force up to 1.35 pounds.
Summary
This chapter opened by outlining the essential role
of engineers in designing safe toys for children. We
then detailed the revolutionary design of several classic toy designs: The Magic 8 Ball, the Super Soaker,
and the Slinky. We studied how the ASTM Standard
F1148 Standard Consumer Safety Performance Specifications for Home Playground Equipment applies to
designing a safe platform for children’s playground
equipment. Next, we looked at ASTM F963 Standard
Consumer Safety Specifications for Toy Safety to show
how to design toys for stability and further used this
standard to outline safe designs for merry-go-rounds,
slides, and projections in toys.
Page 46
Student Assessment
Final Exam
1. Engineers have had a role in ensuring public
safety:
a. Only recently.
b. Since the profession started.
c. Since 1900.
d. Never.
c. Water slides
d. Arcade games
2. The Roman aqueducts were used for:
a. Transporting the water necessary for living.
b. Transporting water for livestock.
c. Industrial water supply.
d. The amusement of the public.
7. The organization that writes the standards used
for designing amusement park rides and toys is:
a. The American Institute of Chemical
Engineers (AIChE).
b. The American Concrete Institute (ACI).
c. The American Society for Testing Materials
(ASTM).
d. The American Society of Mechanical
Engineers (ASME).
3. A well-engineered roller coaster has
the following characteristics:
a. Is safe; uses gravity and acceleration to
confuse and delight
b. Has high g-forces; is made of metal
c. Is cheap to build; is made of wood
d. Riders enjoy it; it can be constructed quickly
8. In addition to technical skill, the most
important kind of knowledge for an engineer
to use in designing a good roller coaster is:
a. Sociology.
b. Kinesiology.
c. Physics.
d. Psychology.
4. Cedar Point’s Top Thrill Dragster:
a. Has an anti-rollback mechanism.
b. Is designed so the cars roll back if
they do not reach the top.
c. Uses a chain mechanism to power the cars.
d. Uses air brakes.
9. What is the pre-drop in a roller coaster
designed to do?
a. Thrill riders waiting for the great downward
plunge.
b. Reduce tension on the lift chain.
c. Make the coaster cheaper to build.
d. Both a and b.
5. Roman engineers built the aqueducts:
a. For water for irrigation.
b. To provide drinking water.
c. To transport water for washing clothes.
d. As a luxury for their baths.
6. Insurance companies are most concerned
about _________ at an amusement park.
a. Merry-go-rounds
b. Roller coasters
10. The structure of VertiGo, a roller coaster at
Cedar Point, broke because of:
a. Vortex shedding caused by the wind.
b. Freezing and thawing.
c. An earthquake.
d. Failure of a concrete footer.
Page 47
Practical Repair Materials for
Roadway Pavements
Course Description
This course is designed to fulfill professional development requirements for professional engineers and will
cover practical repair materials for roadway pavements. This includes the materials that are used and the ways
they are tested. Unless otherwise noted, all information in this course was taken from the U.S. Army Corps of
Engineers Website.
Chapters
• Chapter One: Background Information And Concrete Materials
• Chapter Two: Hot Mix Asphalt Pavements And Testing
Learning Objectives
• Understand the background of repair materials
• Identify that the goal of repair is to meet requirements for roadway pavements while reducing the logistics and
material requirements associated with conventional methods
• Understand the evaluation parameters used to determine the materials to be used
• Recognize the various types of concrete materials
• Identify the material types associated with Hot-Mix Asphalt Pavement
• Understand the importance of the strength and workability of the material being used
• Comprehend triaxial compression testing, the equipment needed and the testing protocol
• Identify the procedures and materials used to test pavement
• Compare and contrast rigid repair materials and asphalt repair materials
Page 48
Roadway Pavement
Chapter One:
Background Information
and Concrete Materials
Overview
•
•
•
•
•
•
•
•
•
•
•
Introduction
Repair Parameters
Evaluation Parameters
Temperature
Moisture
Cost
Safety
Shelf life
Equipment
Aggregate
Concrete Materials
Learning Objectives
• Understand the background of repair materials
• Identify that the goal of repair is to meet
requirements for roadway pavements while
reducing the logistics and material requirements
associated with conventional methods
• Understand the evaluation parameters used to
determine the materials to be used
• Recognize the various types of concrete materials
Introduction
Background
U.S. Army operation requirements currently focus on
increasing force maneuver support. The term “maneuver support” can be defined as the means to gain and
maintain freedom of maneuver and force protection
within a theater of operations. In particular, the focus is to establish, sustain, and secure objective force
lines of communication (LOC). Future forces will put
a premium on mobility, and the network of ground
LOCs in the theater will be a critical component of
theater campaigns and tactical operations. Ground
LOCs will be the foundation of force movement for
force sustainment and will link the majority of operational forces within shifting areas of operations.
Previous work has been done to evaluate the requirements involved in the deployment of forces from their
bases into a theater of operations. However, to provide for maximization of the mobility and force protection of arriving forces, information and procedures
are also required in a variety of areas to maintain the
transportation infrastructure within the area of operations. Conventional repair procedures often require
the use of large, specialized construction equipment
and tightly specified materials, and the length of time
of repairs is not critical. These procedures often require a substantial amount of mobilization resources
to transport the personnel and equipment needed to
achieve the repairs which are necessary for maintaining the LOC. In many instances, the mobilization resources are not available, nor are conventional equipment and materials available locally. In maximizing
an objective force’s LOC, time is a critical factor. Reducing the time required for each repair and increasing the length of time between repair procedures will
greatly increase the objective force’s LOC.
Objective
The objective of this study is to investigate materials and methods with a reduced logistics footprint to
achieve rapid repair for the sustainment of theater
roadways. Material and method requirements were
investigated with the goal of reduced material and
time requirements when compared with standard repair techniques used to maintain roadways. The reduction in time could include both installation time
and curing requirements.
Practical Repair Materials for Roadway Pavement
Page 49
Scope
The overall scope of this study was to identify material types available and their material properties for
repair of both asphalt and Portland cement concrete
pavements. The repair requirements for un-surfaced
roads were not addressed in this study. The distresses
considered were related to load or environmental factors and are not related to military actions (i.e., bomb
craters).
Repair Parameters
General
The goal is to meet expedient repair requirements for
roadway pavements while reducing the logistics and
material requirements often associated with conventional methods. The term “expedient”, as it is used
in this study, is defined by Webster as “suitable for
achieving a particular end; a means to an end or to
meet a need.” The selection of the expedient repair
materials for roadways may result in the sacrifice
of some long-term durability for a decrease in time
and material requirements. This possible reduction
in durability is necessary to provide the military immediate and as close to continuous on-ground LOCs
as possible. Using the term “expedient” to describe
repair materials for roadway repairs is based on the
immediate tactical situation as well as factors such as
urgency and availability of manpower, materials, and
equipment. Current and future emphasis for the use
of expedient repair materials is to provide a suitable
repair, while using the absolute minimum resources
required to perform the mission.
A reduction in the logistic footprint can be achieved
by reducing the weight or volume of materials required for repair and by increasing the durability of
the repair. This can be achieved by developing higher
strength materials, resulting in thinner sections that
can carry the required loads. Another method would
be through the use of locally available materials, often to extend the expedient repair materials. The nonavailability of gravel or other standard aggregates has
been a severe problem during many military deployments. The ability to use non-standard or marginal
aggregates as part of the repair materials, without a
decrease in durability, would result in the desired reduction of the logistic footprint.
Page 50
The ideal repair material could be: prepared and
placed in any weather by personnel without extensive training; be no more toxic or dangerous than, and
have a minimum performance equal to, conventional
pavement repair materials; and have a minimum shelf
life of several years. This material probably does not
exist, but it is possible a combination of materials can
be used to meet the same requirements. The goal is to
develop a few materials that can be used in combination to meet the requirements for repairs at any time,
anywhere in the world.
Concrete repair materials are formed through a chemical process in which various mixture components
combine to form a hardened matrix. Concrete repair
materials often use sand-sized aggregate particles to
produce a mortar mixture for small-volume repairs.
For larger volume repairs, this mixture can be extended with the addition of coarse aggregate to produce a
concrete mixture.
The asphalt cement binder used to make repair materials is a thermoplastic material that increases in
viscosity as the temperature decreases. This asphalt
cement coats the aggregate particles and holds them
together after cooling. Asphalt cement can be made
fluid at ambient temperatures, thereby eliminating
the need for heating by cutting or diluting it with petroleum-based solvent materials or by emulsifying it
with water. Cutbacks are asphalt cements that have
been diluted with a petroleum solvent (i.e., gasoline,
kerosene, or oil). Emulsions are asphalt cements that
have been emulsified or contained in an aqueous solution.
Design Failure Criteria
(Thresholds for Reconstruction)
Roadway pavements requiring reconstruction could
be constructed as either rigid (Portland cement concrete, PCC) or flexible (hot-mix asphalt, HMA) pavements. The roadways could also be classified as unfinished (i.e., gravel-surfaced roads). The thresholds
normally associated with failure of the pavement
design for the respective type of road surfacing are
given below:
a. PCC pavement. Pavement failure of a slab occurs
when at least 50 percent of the slabs in the traffic
area have a crack which divides the slabs into two
or more pieces.
b. HMA pavement. HMA surfaces can be considered
to have failed when the rut depth exceeds 25 mm
(1 in.).
Thresholds for Maintenance and Repair
The type and amount of distress that a pavement surface can develop before maintenance and repair is required will vary with the type and number of vehicles
traveling on the road. The distress levels listed above
consider only the failure method used for design of
the pavement. At the levels of distress listed above
for the various surfaces, passage would still be possible for most types of vehicles; however, these levels
could limit the speed at which a vehicle could traverse the road.
There are distresses, other than those considered for
design criteria, which must be considered for maintenance and repair. The major distresses for PCC
pavement include cracking, spalling, and faulting.
For HMA pavements, the major distresses include
cracking (due to both traffic loading and environmental conditions), shearing movement (rutting, shoving, and corrugation), surface distress (raveling and
weathering), and potholes. For un-surfaced roads, the
major distresses include improper grade and smoothness, potholes, and ruts. The pavement condition
index (PCI) is a method used to define the current
condition of a pavement surface in regard to various
distress types and levels. Minimum PCI levels for
airfield pavements are ≥70 for runways, ≥60 for primary taxi-ways, and ≥55 for aprons and secondary
taxiways. For roadways, the minimum PCI levels are
60 for primary roads, 50 for secondary roads, and 45
for tertiary roads.
Evaluation Parameters
Field placement parameters were established to provide a framework for comparison of the various asphalt repair materials (Table 1). The evaluation parameters include the environmental conditions of
temperature and moisture, equipment and aggregate.
Other parameters such as cost, safety, and shelf life
are important, although they may not be controlling
factors in material selection.
Table 1
User Parameters for Repair
Evaluation
Parameter
Pavement
temperature
Air temperature
Range of Placement Parameters
Minimum
Below
freezing
Below
freezing
Maximum
71 °C (160 °F)
≈49 °C
(120 °F)
Pavement
moisture
Dry
Under water
Air moisture
Dry (low
humidity)
Rain (100%
humidity)
Aggregate
Low quality,
uncrushed
High quality,
crushed & graded
Equipment
Hand tools
Specialized
mechanized
equip.
Temperature
The ambient temperature at the time of placement of
materials is of critical importance to their placement.
The temperature of the pavement upon which the repair is to be made can vary from below zero (point of
freezing) to over 60 °C (140 °F). Air temperatures can
also vary from below zero to occasionally over 49 °C
(120 °F). Variations in air temperature are mimicked
by the pavement, although somewhat delayed and
with less overall change, except for solar heating. The
effect of the sun shining on a pavement surface often
results in the pavement temperature greatly exceeding
the air temperature, especially for darker colored surfaces. Dark pavement surfaces (HMA), in some locations, can reach temperatures approaching 71 °C (160
°F) on hot sunny days. Considering solar heating, the
largest change in temperatures occurs in about an 8-hr
time span, from lows at about 0500 hr to highs about
1400 hr (Shoenberger 2001).
Conventional construction practices normally require
temperatures of about 7 °C (45 °F) and rising for asphalt
materials and range from about 7 to 30 °C (45 to 85 °F) for
concrete materials. These temperature limits show that
working with many conventional materials at extremes
of either high or low temperatures can cause problems.
Expedient repair materials need to have the ability to be
applied at temperatures outside of these ranges.
Page 51
Moisture
The amount of moisture or humidity present at the
time of placement of materials is of critical importance
to their performance. The amount of humidity in the
air will vary with the geographical region, the time of
year, and local weather occurrences. Humidity levels
from very low to around 100 percent will have a considerable effect on the rate of evaporation of moisture
and other volatiles. Repairs attempted during rainfall
and immediately after rainfall are greatly affected by
the excess moisture. Conventional repair materials
require a dry surface or, at most, only a slightly wetted surface in order to bond to the existing surface.
Conventional repair materials cannot be placed during
rainfall. Compaction of conventional materials is difficult when excess water is present because the water
results in excess pore water pressure to counteract the
compaction effort. Hot mixtures have the same problem and will quickly cool due to heat loss from the
moisture. Hard rains will flood repair areas and require
repair materials to displace the water. Mixtures can be
compacted in water only if they are cohesive enough
to displace and not absorb the water prior to compaction. Chemically setting or curing materials, such as
Portland cement and other concrete materials can be
placed while displacing the water, provided that they
are designed for this purpose.
Cost
The cost of the material, while still important, is not
as critical a factor as it would be for conventional repair applications. The overall cost of using the material within a theater of operations would include the
purchase price plus the logistics cost of shipping the
materials. In many situations, the material cost is not
extravagant; the logistics cost would constitute the
largest percentage of the overall cost. This is certainly
true when the higher cost material results in increased
operation capability.
Safety
The safety concerns of each material, relative to handling, mixing, and placement, are important considerations. Safety concerns are reported based on laboPage 52
ratory and field experience and information available
from the manufacturer’s material safety data sheet.
Shelf Life
The shelf life, as provided by the manufacturer, is reported for each material. The shelf life of a material is
important to the military because of the need to stockpile materials for availability with minimal warning
of impending need. When possible, the overall effect
of performance on using expired materials will be reported. The shelf life of many materials is dependent
upon the conditions existing during storage; generally,
materials will last longer if stored under controlled
conditions rather than out in the elements. Some materials are greatly affected by temperature or moisture
extremes, while others are affected only by extreme
conditions in one direction. For example, a cutback
asphalt mixture will not be greatly affected by long
periods of extreme cold, but it will lose overall workability if exposed to high temperatures for long periods of time.
Equipment
The availability of equipment, in various types and
amounts, can have a significant effect on what methods and materials can be used for repairs. In situations
where no machines or specialized mechanized construction equipment is available, the possibilities are
limited and not all maintenance options can be considered. Therefore, when equipment is not available,
only repairs can be considered, and the size of these
is governed by the number of personnel available. In
these situations, the types of materials that can be used
are also limited. Materials that require heating or large
mixers, or those which cannot be placed by hand can
be used only when satisfactory equipment is available.
When equipment is not available locally, repair practices may have to be considered in regard to initial and
long-duration phases. In the initial phase, construction equipment is severely limited or not available,
and only limited repairs are possible. After a period of
time, it is anticipated that more mechanized equipment
will become available, making possible more complex
maintenance and repair procedures with a wider range
of repair materials.
Aggregate
Concrete Materials
Aggregate is a critical item in most repairs, especially
when the volume of the repair is large. Recent military operations have shown that the availability of
high-quality aggregates in many locations is severely
limited. Conventional aggregate used for both asphalt
and concrete repair materials have requirements for
relatively clean, hard, angular, and durable particles
within a desired gradation band. Even in areas with
ample stone or rock resources, unless crushers or
washers are available, sufficient quantities of highquality aggregates will not be available. Gravel may
be available locally, but the quality or consistency is
often unknown.
Material Types
A portable aggregate crushing plant would be a useful piece of equipment in establishing a stable LOC. A
crushing plant would also require equipment and accessories for supplying parent rock, processing, and
delivering the crushed aggregate. The amount of processing could be minimal using only what is crushed,
or it could be screened and gradations controlled. Particular gradations provide for optimum use of the cementing or binding materials; however, nonstandard
gradations could be used for expedient applications.
The non-availability of aggregates meeting standard
requirements provides an emphasis for investigating
the use of lower quality or marginal aggregates.
The use and the effect of marginal materials for asphalt and Portland cement concrete have previously
been investigated for the construction of pavements
(Grau 1979; Rollings 1988; Ahlrich 1997a, 1997b).
These investigations generally found that some variation from specified requirements could be accomplished with only minimal decreases in performance;
however, varying too far from established requirements would cause substantial decreases in performance. The difficulty of using these materials arises in
knowing how far variations may be allowed without a
substantial decrease in performance. Often, local conditions could affect performance, which would require
engineering judgment to prevent failures
The need for rapid pavement repair necessitates the
use of fast-setting or rapid-hardening cementitious
materials. The terms “fast-setting” or “rapid-hardening” vary somewhat between manufacturers and are
often not well defined. Generally, they include materials that achieve an initial set in less than one hour and
a minimum compressive strength of 3.5 MPa (500 psi)
within three hours. The following is a list of the some
of the available types of rapid-hardening cements:
a. Magnesium-phosphate cement.
b. High-alumina cement.
c. Regulated-set portland cement.
d. Gypsum cement.
e. Special blended cements.
f. Type III portland cement with accelerating admixtures.
g. Polymer cements.
1. Epoxies.
2. Methacrylates.
3. Polyesters.
4. Urethanes.
h. Proprietary materials: high waste.
Magnesium-phosphate cements are a blend of materials that react with water to form a rapid-hardening
concrete. This cement is available in one-component
systems, in which both are in powder form and water
is added to form the concrete. Also available is a twocomponent system in which a magnesium powder can
be combined with phosphate in an aqueous solution.
These cements can be extended with aggregate, and
several manufacturers produce both hot- and coldweather formulations.
High-alumina cements have monocalcium aluminate
as their main component ,(CaO•Al2O3). The lime
and alumina make up about 80 percent of the cement
in roughly equal parts. While this cement achieves a
high, early strength, it has a relatively long initial set
time, followed quickly by the final set. This cement is
commonly used in high-temperature (refractory) applications.
Regulated-set Portland cements are a mixture of
a Portland cement and calcium fluoroaluminate
(C11A7•CaF2). This cement provides high, early
Page 53
strengths and rapid set times that can be regulated with
admixtures.
Gypsum cement has calcium sulfate hemihydrate
(CaSO4•1/2 H2O) as its main ingredient. This cement
has a very fast initial set and high early strength. The
durability, abrasion resistance, and fuel resistance of
this cement are low.
Special blended cements are proprietary materials that
contain Portland cement and other materials. Among
the available rapid-hardening cements, these blended
cements are generally the least expensive and are simple to mix.
gregate can also provide greater economy, decreased
thermal movement and, often, increased strength. The
maximum size of the aggregate will vary with the size
and depth of the area to be repaired. Generally, clean,
coarse aggregate is all that is required.
Summary
This chapter introduced background information surrounding the use of repair materials. Next, the repair
parameters and the evaluation parameters were discussed, and we then were introduced to concrete materials and the material types.
Type III Portland cement, with accelerating admixtures, is a mixture of rapid-hardening cement and
accelerating admixtures. The admixtures commonly
used include gypsum, calcium chloride, calcium nitrate, and various carbonates. These cements achieve
high early strengths and good durability, although they
have relatively high heat of hydration and shrinkage.
Polymer cements are organic in nature and are produced by combining two or three liquid components
during mixing. These monomer components can be
polymerized with the addition of an aggregate into a
polymer concrete. The polymerized monomer (synthetic resin) replaces the hydraulic cement as the
bonding agent for the aggregates. Types of polymer
cements include epoxies, methacrylates, polyesters,
and urethanes. Polymer cements are good for expedient construction in that they are very versatile in set
time and strength gain, have high adhesion properties
and low shrinkage, and are relatively durable. However, they are generally expensive, relatively difficult
to mix, have thermal properties different from conventional PCC, and can be a safety hazard for workmen.
Proprietary materials cover a wide range of fast-setting materials that do not fit into one of the previously
listed categories. One group of this type of material
contains at least some amount of waste material, usually fly ash.
Aggregate
The rapid hardening repair materials used for concrete pavement in most cases will use aggregate to
provide the necessary volume of repair material. AgPage 54
Chapter Two:
Hot Mix Asphalt
Pavements and Testing
Overview
•
•
•
•
•
•
•
•
Hot-Mix Asphalt (HMA) Pavement
Triaxial Compression Testing
Test Pavement
Materials
Placement of Test Section
Traffic
Performance
Conclusions
Learning Objectives
• Identify the material types associated with
Hot-Mix Asphalt Pavement
• Understand the importance of strength and
workability of the material being used
• Comprehend triaxial compression testing,
the equipment needed and the testing protocol.
• Understand the procedures and materials
used to test pavement.
• Compare and contrast rigid repair materials
and asphalt repair materials
Hot-Mix Asphalt (HMA) Pavement
Material Types
Hot-mix asphalt is used to construct pavements because heating the asphalt binder and aggregates prior
to mixing is the most economical method to get proper
coating and achieve the highest degree of compaction.
Producing the same material for repairs requires a substantial investment in equipment and material when
satisfactory HMA cannot be purchased locally. The
requirement for rapid repairs, coupled with the need
for a reduced logistics footprint, increases the benefit
of using a material that does not require heating.
The major limitation of using cold asphalt mixtures is
they cannot be compacted to achieve the level of compaction of mixtures that are heated. However, with
available additives, cold patch materials have been developed that are capable of durability that approaches
that of HMA. The following is a list of available types
of asphalt binder materials:
a. Cutback
b. Emulsion
c. Proprietary products
Cutback asphalts have historically been used as the
binder for cold-mix asphalt patches. Cutbacks can
be combined with well-graded blends of aggregates
to produce dense asphalt pavement patches. The cutbacks used can be classified by type as either medium curing (MC) or slow curing (SC), as defined in
ASTM D 2027 and D 2026, respectively. The particular grades of each type recommended for applications
of immediate use in repairs include MC-250, MC-800,
and SC-800. The same grades are recommended for
applications of stockpiling with the addition of SC250 (Asphalt Institute 1997). The shelf life of a cutback is practically unlimited, provided it is kept in a
sealed container.
Emulsified asphalts are widely used in the repair of
asphalt pavements. In an emulsion, the asphalt binder
is suspended in an aqueous solution. This is an economical and environmentally acceptable method of
obtaining asphalt cement in a workable consistency
at ambient temperatures. A limitation of emulsions is
the relatively short time they take to break and cure.
Therefore, only slow-setting emulsions should be used
for cold mixes, and they should be used immediately
and not stockpiled. This would include grades SS-1,
Page 55
SS-1h, CSS-1, and CSS-1h, where the “C” stands for
cationic, and the “h” indicates an emulsion made from
harder (higher viscosity) asphalt cement (Asphalt Institute 1997). Depending upon storage conditions, the
shelf life of a typical asphalt cement emulsion is limited to between six months and one year. There are
colloid asphalt emulsions that are more stable and will
have a potential shelf life of several years.
Aggregate
Proprietary product manufacturers generally start with
a cutback or an emulsion and then add some type of
antistripping agent, polymer, or fiber. These materials are added to improve the strength, bonding, and
durability of the repair material. Proprietary materials are usually available in ready-to-apply containers,
sometimes varying from small bags or buckets to large
containers. Some proprietary material manufacturers
also sell the binder itself, which can be combined with
suitable aggregates in the area it is to be applied.
The grading of the aggregate can have an effect on the
performance of the cold patch material. Dense-graded
aggregates are used to provide a stable, low void, and
a relatively waterproof pavement structure. Open-graded aggregates, when properly confined, can be stable
and will generally be more porous than dense-graded
aggregate structures. Compared with dense-graded asphalt mixtures, open-graded asphalt mixtures are normally more workable when temperatures are at or below freezing. In most instances, a dense-graded asphalt
mixture would perform well at warm and hot temperatures, but an open-graded asphalt mixture is required
for satisfactory workability at freezing temperatures.
The materials used for repair of asphalt pavement generally include aggregate as part of the mixture. The size of
the aggregate will vary with the depth of the pavement
to be repaired. Most repair materials contain aggregates
that have a maximum aggregate particle size of less than
12.5 mm (0.5 in.). This allows the patching material to
be placed in thin layers of about 25 mm (1 in.).
A few products are available for repair of HMA pavements do not contain any asphalt binder material. These
materials are generally rigid materials that have low
enough modulus values to provide some compatibility with the surrounding HMA pavement. Non-asphalt
binders are not susceptible to the large changes in
modulus values that occur with asphalt binders during
changes in temperature. The modulus value of a HMA
pavement can decrease by a factor of 10 as temperatures
change from near freezing to 60 °C (140 °F) or above.
Asphalt Materials Evaluation
Using the review of asphalt repair materials, 12 products were eventually obtained. Samples of these products were either provided by the manufacturers or purchased for this study. The products that came in a bag
or pail were prepackaged in that form, but products
that were not prepackaged came in buckets from various points of manufacture from around the country.
Table 2 Asphalt Repair Materials Evaluated in Laboratory
Material
Container
Binder Type
Type of Gradation1
Cold Patch (Cold Weather)
18.9-L (5-gal) buckets
Cutback
Open
Cold Patch (Hot Weather)
18.9-L (5-gal buckets
Cutback
Open
DuraPave
Emulsion
Dense
ENVIROPATCH
Inverted emulsion
Open
EZ Pave
Emulsion
Dense
EZ Street
15.9-kg (35-lb) bags
Cutback
Dense
Instant Road Repair
22.7-kg (50-lb) pails
Cutback
Dense
Optimix
Cutback
Open
Perma-Patch
Cutback
Open
QPR-2000
Cutback
Open
Sylcrete-EV
Cutback
Open
UPM (Spring & Fall Grade)
Cutback
Open
2
1
2
The term “dense graded” could also be considered as well graded; the aggregate is spread over a series of sieve sizes.
EZ Pave is a cold-mix paving mixture and not a patching material.
Page 56
Roadway Pavement
Table 3
Asphalt Repair Materials Test Plan
Material Property
Test Method
Test Standard - ASTM
Workability
Workability test
D 6704
Marshall
D 1559
Triaxial
Detailed in text (page 19)
Strength
Cohesion
Durability
Adhesion
Binder content
Extraction
D 2172
Max. specific gravity
Theor. max. density
D 2041
Bulk specific gravity
Marshall sample
D 2726
Penetration
D5
Viscosity
D 2171
Recovered binder
Table 4a
Gradation, Cohesion, and Adhesion Properties of Test Materials
Cold Patch Cold
Control (Cold
Patch (Hot
Weather)
Weather)
Material
DuraPave
ENVIROPATCH EZ Pave
19.0 (3/4)
EZ
Street
97.5
12.5 (1/2)
100
100
100
75.1
9.5 (3/8)
86
99.7
100
98.3
59.8
100
4.75 (No. 4)
66
42.9
54.7
100
42.8
36.9
78.7
2.36 (No. 8)
53
9.2
13.3
94.3
8.6
22.8
28.1
1.18 (No. 16)
41
8.2
11.6
72.7
1.3
15.4
17.0
0.6 (No. 30)
31
7.7
10.7
51.6
1.2
11.5
12.2
0.3 (No. 50)
21
5.9
8.0
31.4
1.1
9.0
9.3
0.15 (No. 100)
13
2.5
3.6
19.1
1.0
7.2
5.7
0.075 (No. 200)
4.5
1.4
2.67
12.6
0.96
5.9
3.7
Binder content
5.2
3.0
3.9
6.05
2.3
4.24
4.19
Cohesion
94.3
64.5
---
44.5
62.1
54.3
96.7
93.6
99.4
---
47.3
87.7
67.8
96.9
91.9
98.4
---
---
69.9
69.5
95.9
93
87
---
46
73
64
97
0
5
---
120
11 (C)
33
29
0
7
---
120
1
4
3
7
---
---
2
14
---
6
---
120
5
17
16
1
Average
Adhesion, sec
2
0
Average
0
3
Reported value is retained percentage (original mass divided by final mass).
Time to separation of cold patch material and type of failure, which was adhesive in all cases except that marked Cohesive (C).
3
Specimens fell apart during extrusion from the mold.
1
2
Roadway Pavement
Page 57
Table 4b
Gradation, Cohesion, and Adhesion Properties of Test Materials
Material
Instant RR
Optimix
Perma-Patch QPR-2000
19.0 (3/4)
Sylcrete-EV
UPM
100
12.5 (1/2)
100
100
9.5 (3/8)
99.7
96.2
100
100
94.5
100
4.75 (No. 4)
69.7
34.9
84.8
39.7
45.6
98.1
2.36 (No. 8)
33.4
9.1
21.8
5.4
17.9
27.7
1.18 (No. 16)
20.9
5.4
6.9
3.7
9.6
7.6
0.6 (No. 30)
14.2
4.0
4.0
2.9
6.9
4.9
0.3 (No. 50)
10.2
3.4
3.5
2.3
5.8
4.1
0.15 (No. 100)
7.2
3.0
3.3
1.6
5.2
3.4
0.075 (No. 200)
6.1
2.8
3.2
1.2
4.8
2.9
Binder content
4.9
3.8
2.8
3.0
4.0
4.04
Cohesion
100
100
100
93.1
100
100
100
100
99.9
94.0
100
99.9
100
100
99.7
91.8
100
100
Average
100
100
100
93
100
100
Adhesion2
16
2
3
3
8
6
4
3
14
2
3
5
4
4
12
2
3
2
3
10
2
5
4
1
Average
1
2
99.4
Reported value is retained percentage (original mass divided by final mass).
Time to separation of cold patch material; type of failure was adhesive in all cases.
Table 5
Binder Properties of Recovered Asphalt1
Cold-Mix Material
Penetration, 0.1 mm
Viscosity,2 cSt
Cold Patch (Cold Weather)
56
683
Cold Patch (Hot Weather)
7
---3
DuraPave
6
180,0003
ENVIROPATCH
6
5,622
EZ Pave
13
9,012
EZ Street
24
2,894
Instant Road Repair
40
345
Optimix
20
1,270
Perma-Patch
25
4,153
QPR-2000
12
3,000
Sylcrete EV
27
675
UPM
27
1,250
ASTM D 2172 (Test Method A).
2
ASTM D 2170 (Kinematic Viscosity at 135 °C (275 °F)). Values expressed in centistokes (cSt).
3
Very stiff; would not flow in largest tube.
1
Page 58
Roadway Pavement
Workability
Workability, regarding cold-mix materials, can be considered as the amount of
effort required to properly construct a repair with the mixture into the pavement. This repair procedure includes placing the material in the void and compacting it to the desired density. Various methods of defining workability have
been used, including the subjective estimate of effort involved in penetrating
a stockpile of mixture with a shovel or other object, or using conventional
asphalt mixture test equipment to produce a parameter relating to workability.
Strength
The strength of a mixture is important because of the traffic loads that it may
be required to carry. The strength of a mixture can be measured in many ways,
with Marshall Stability being the most common for asphalt paving materials.
Another method of measuring the strength and even the workability of the
various materials is to observe their behavior during a triaxial test. Both unconfined and confined tests were conducted.
Table 6
Results of Workability Testing
Material
Cold Patch
(Cold Weather)
Cold Patch
(Hot Weather
DuraPave
ENVIROPATCH
EZ Pave
2
EZ street
Instant Road
Repair
Optimix
Perma-Patch
Roadway Pavement
Spec. Specimen
No.
Mass, kg (lb)
Workability,
N (psi)
1
2098.7 (5.62)
1.50 (338)
2
2119.8 (5.68)
1.83 (411)
3
2071.1 (5.55)
1.57 (353)
1
1935.9 (5.19)
2.08 (469)
2
1913.0 (5.13)
1.89 (425)
3
1956.8 (5.24)
2.21 (497)
1
1723.7 (4.62)
6.15 (1382)
2
1734.8 (4.65)
6.34 (1426)
3
1741.4 (4.67)
6.86 (1542)
1
2021.1 (5.41)
2.97 (668)
2
2043.0 (5.47)
3.48 (782)
3
2066.8 (5.54)
3.60 (810)
1
1995.8 (5.35)
11.83 (2659)
2
2046.9 (5.48)
9.19 (2066)
3
1971.2 (5.28)
9.77 (2196)
1
1707.5 (4.57)
2.34 ( 526)
2
1727.8 (4.63)
2.53(569)
3
1651.8 (4.43)
1.96 (440)
1
1836.6 (4.92)
3.67 (824)
2
1842.2 (4.94)
4.17 (938)
3
1738.0 (4.66)
5.25 (1181)
1
1885.0(5.05)
6.41 (1140)
2
1813.3(4.86)
6.02(1354)
3
1915.6(5.13)
5.96(1339)
1
2046.2(5.48)
1.24(279)
2
1976.2(5.29)
1.24(279)
3
1999.0(5.36)
1.44(323)
Avg.
Workability
Temp. 1
Range, °C
Remarks
1.63 (367)
-22 to -1
Sticky, difficult
to compact
2.06 (464)
-22 to -1
Sticky, difficult
to compact
6.45 (1450)
-1 to 21
Sandlike,
dry;compacts
easily
3.35 (754)
-12 to 10
Compacts
somewhat
easily
10.26 (2307)
10 to 32
Compacts
easily
2.27 (511)
-12 to 10
Sticky; difficult
to compact
4.36 (981)
-12 to 10
Stiff at ambient
temperature
6.13 (1378)
-1 to 21
Excess water in
container
1.31(294)
-22 to -1
Compacts
easily
Page 59
Spec. Specimen
No.
Mass, kg (lb)
Workability,
N (psi)
1
1844.2(4.94)
2.21(497)
2
1835.1(4.92)
3.16(711)
3
1852.7(4.96)
2.97(668)
1
2100.0(5.63)
4.87(1095)
2
1989.3(5.33)
6.02(1354)
3
2089.1(5.60)
6.60(1484)
1
UPM (Spring and
2
fall Grade)
3
1907.4(5.11)
3.41(768)
1954.2(5.24)
3.41(768)
1843.7(4.94)
3.54(796)
Material
QPR-2000
Sylcrete-EV
Avg.
Workability
Temp. 1
Range, °C
Remarks
2.78(626)
-12 to 10
Sticky:difficult
to compact
5.83(1311)
-1 to 21
Compacts
easily
3.46(777)
-12 to 10
Compacts
easily
Workability temperature ranges from ASTM D 6704
2
Cold-mix paving material; not manufactured as a cold patch material, but tested for workability.
1
The specimens for the Marshall testing were not compacted using a Marshall
compaction hammer but were instead compacted using the Corps of Engineers Gyratory Testing Machine (GTM). The GTM can compact to densities
equivalent to those obtained with the Marshall hammer. The specimens were
compacted to achieve a density equivalent to a 75-blow Marshall compaction.
In order to be able to compact the mixtures for Marshall specimens, the specimens were cured and heated prior to compaction. The curing was required
because these cold mixtures use either a cutback or an emulsified binder to
provide workability at ambient temperatures. The method used was to place
the mixture in a forced draft oven at 135 °C (275 °F) overnight (14 to 18 hour)
and compact at that temperature. Specimens compacted under these conditions
should represent the condition of the mixtures after being in place for several
months. Table 7 provides the results of the Marshall testing conducted on the
materials, including stability and flow values.
Table 7
GTM Compaction and Marshall Test Results
Material
Control
Specimen
No.
Specific
1
Gravity
Theor.
Maximum
Specific
Gravity
1.0
2
9.56 (2150)
12.5
1.0
3
9.99 (2245)
12.5
1.0
11.16 (2508)
13
1.0
5.07 (1140)
8.5
1.0
2.308
2.528
2
5.43 (1220)
9.0
1.0
3
5.12 (1150)
9.0
1.0
5.20 (1170)
9
1.0
1
8.83 (1985)
14.5
1.0
2
13.28 (2985) 14.5
1.0
13.32 (2995) 14.0
1.0
11.81 (2655)
1.0
Average
2.274
2.533
3
Average
Page 60
Gyratory
Stability
Index (GSI)
13.92 (3130) 13.0
1
DuraPave
Flow
0.25-mm
(0.01-in.)
1
Average
Cold Patch
(Cold Weather)
Stability
kN (lbf)
2.134
2.465
14
Roadway Pavement
Material
ENVIROPATCH
Flow
0.25-mm
(0.01-in.)
Gyratory
Stability
Index (GSI)
1
3.25 (730)
7.0
1.0
2
6.02 (1354)
6.5
1.0
6.92 (1555)
7.5
1.0
5.50 (1213)
7
1.0
1
7.30 (1640)
12.0
1.0
2
8.38 (1885)
13.0
1.0
3
7.98 (1795)
10.0
1.0
7.89 (1773)
12
1.0
Average
EZ Street
2.491
9.67 (2175)
11.5
1.0
9.45 (2125)
10.5
1.0
3
11.45 (2575)
11.0
1.0
10.20 (2292) 11
1.0
1
5.94 (1335)
15
1.0
2
6.46 (1452)
13
1.0
2.040
2.393
3
6.61 (1485)
12
1.0
6.33 (1424)
13
1.0
1
7.94 (1785)
12.0
1.0
2
6.52 (1465)
12.0
1.0
3
8.19 (1841)
12.5
1.0
7.55 (1697)
12
1.0
Average
Perma-Patch
2.312
2.679
1
Average
Optimix
2.347
2
Average
Instant Road
Repair
Specific
1
Gravity
3
Average
EZ Pave
Theor.
Maximum
Specific
Gravity
Stability
kN (lbf)
Specimen
No.
2.201
2.245
2.461
2.500
1
4.58 (1030)
9.0
1.0
2
5.34 (1200)
9.0
1.0
3
5.12 (1150)
8.5
1.0
5.01 (1127)
9
1.0
5.58 (1255)
11.0
1.0
Average
2.278
2.693
1
QPR-2000
2
6.04 (1385)
7.0
1.0
3
5.85 (1315)
11.5
1.0
5.86 (1318)
10
1.0
1
4.92 (1105)
13.0
1.0
2
5.93 (1332)
12.5
1.0
3
6.15 (1382)
12.5
1.0
Average
Sylcrete-EV
Average
UPM
2.253
2.549
5.66 (1273)
13
1.0
1
7.92 (1780)
10.0
1.0
2
4.40 (990)
9.5
1.0
3
6.14 (1380)
10.0
1.0
6.15 (1383)
10
1.0
Average
1
2.229
2.139
2.545
2.600
Results shown are from the evaluation of one specimen, except where other results are given.
Roadway Pavement
Page 61
Triaxial Compression Testing
One of the most common failures in asphalt pavements is caused by permanent deformation (rutting) under high traffic volumes and extreme loading
conditions. To better understand the mechanism of the load carrying process
,through which asphalt mixtures undergo gradual permanent deformation that
leads to failure, the fundamental response parameters of the material need to
be obtained. One method to obtain these fundamental parameters is to conduct
triaxial compression testing. Triaxial compression tests have also been used
to evaluate the workability of cold patch mixtures. One study found the angle
of internal friction remained constant, but the cohesion value (y-intercept) increased with increased aging of the material (Estakhri and Button 1997).
In triaxial compression testing, an axial load is applied to a cylindrical specimen, along with a constant confining stress applied to all sides of the specimens. The axial stress-resistant properties of a material tested triaxially are
derived from the relation between the testing load and the confining stress. In
the triaxial compression test method, the stress acting on an asphalt mixture
specimen simulates the state of stress in flexible pavements while carrying
traffic loading.
Equipment
The triaxial device used during testing was a full-feedback instrumented system. The following parameters are provided during testing: axial strain, horizontal strain, axial stress, and confining pressure. The confinement chamber is
designed to accommodate Superpave gyratory compacted specimens (AASHTO 2002) up to 150 mm (6 in.) in diameter and 150 mm (6 in.) in height.
Testing protocol
Six cold patch materials were triaxially compression tested (Table 8). The test
specimens were temperature conditioned for 24 hours at 10 ºC (50 °F) before
testing. This temperature was selected to provide test conditions that would
reasonably represent the pavement temperature during trafficking of the repair test sections. Actual trafficking temperatures, as detailed in the following chapter, were somewhat in excess of this value. However, several repair
materials could not be tested at temperatures exceeding 10 ºC (50 °F). Four
replicate 100- by 150-mm (4- by 6-in.) specimens were produced for each cold
patch material to be tested. Confining stresses of 0.0, 138.0, and 276.0 kPa (0,
20, and 40 psi) were used for the triaxial compression testing of the mixtures.
A BISAR layered elastic analysis program was used to estimate the confining
stresses developed in the pavement section, as detailed in Table 8. The approximate confining (horizontal) stresses of the asphalt layer were arbitrarily
calculated at depths of 25 mm (1 in.), 63.5 mm (2.5 in.), and 100 mm (4 in.),
as shown in Table 9. The total asphalt patches were placed to depths between
100 and 113 mm (4 to 4.5 in.). One test was conducted at the confining stresses
of 0.0 kPa (0 psi) and 138.0 kPa (20 psi), and two tests were conducted at a
confining stress of 276.0 kPa (40 psi). Field testing was the primary means
used for evaluating the performance of the asphalt patching materials. A minimum number of triaxial compression tests were conducted to verify what was
observed in the field.
Page 62
Roadway Pavement
Table 8
BISAR Input Parameters
Young’s Modulus
Thickness
mm
in.
kPa
psi
Poisson’s
Ratio
Asphalt
1,380,000
200,000
0.40
100
4
Base
689,475
100,000
0.35
150
6
Subgrade
68,950
10,000
0.30
N/A
N/A
Layer
Table 9
BISAR Analysis Results
Asphalt Depth
Confining Stress
mm
in
kPa
psi
25.4
1.00
276
40
63.5
2.50
138
20
100
4.00
0
0
The results of the triaxial compression tests for the cold patch asphalt specimens
are provided in Table 10. The listed parameters include; confining stress, maximum compressive stress, and corresponding axial and horizontal (radial) strains.
The specimens for the triaxial compression testing were not compacted using a
Marshall compaction hammer but were instead compacted using the Superpave
Gyratory Compactor (SGC) (AASHTO 2002). The SGC can compact to densities equivalent to those obtained with the Marshall hammer. The specimens were
compacted to achieve a density equivalent to a 50-blow Marshall compaction.
The three confining pressures were used to define the stress envelope. A
comparison was also made between the confined and unconfined compressive strength. This was done to analyze which materials show an increase in
strength when confined. Some asphalt mixtures that exhibit low unconfined
compressive strength and large volume changes may exhibit increased compressive strength and a reduction in volume change when confined.
Durability
The durability of a repaired area depends on the quality of both the mixture and
the repair. This assumes the amount of binder present in the mixture is correct
and the repair is properly placed and compacted. The durability of the repair
mixtures was investigated through three parameters, compatibility, adhesion,
and cohesion. One measure of mixture quality is the compatibility between
binder and aggregate. If they are not compatible, the binder will not adhere
to the aggregate (called stripping), and the repair will fail, usually through
raveling. The compatibility between binder and aggregate is measured by mixing these materials together, exposing them to moisture, and then observing
if there is a decrease in performance. This performance can be measured by
either visual means, such as ASTM D 1664, or through comparisons of mechanical tests.
Cohesion is the property of a material which is able to adhere to itself better
than it adheres to materials around it. This means a cold-mix material will not
Roadway Pavement
Page 63
be displaced or removed from the patch by traffic but will remain cemented to
the patch itself. The greater cohesiveness of a mixture, the better it will perform, provided it is still sufficiently workable. The test method used for cohesion testing was developed from AASHTO TP-44-94 (AASHTO 1996), with
the following detials. Place about 800 g of cold mix into a suitable pan and then
place it into a refrigerator at 4 ± 1 °C (39 ± 2 °F). After the mix has been in the
refrigerator for about 2 hours, remove it and immediately compact a Marshall
specimen with five blows, each side. Then place the compacted specimen, still
in the Marshall mold, back into the refrigerator for 4 to 6 hours. Leave the
specimen in the mold until it is time to remove it for testing. The test procedure itself involves placing a compacted cold-mix specimen within a 305-mm
(12-in.)-diameter, 25.4-mm (1-in.) opening metal sieve. A lid is placed on the
sieve, and both the specimen and the sieve are stood on edge and rolled back
and forth. The sieve is rolled back and forth 20 times, taking approximately one
second for each of the 20 passes.
Table 10 Repair Materials Evaluated by Triaxial Compression Test
Material
Type of
Gradation
DuraPave
Dense
EZ Pave
Instant Road
Repair
Perma-Patch
QPR-2000
UPM
Page 64
Dense
Dense
Open
Open
Open
Maximum
Compressive
Stress
Confining
Stress
KPa
psi
KPa
psi
Axial
Strain
mm/mm
0
0
1940
281
2.71 x 10
1.90 x 10
138
20
2000
290
1.68 x 10
6.01 x 10
276
40
1890
274
1.07 x 10
276
40
1950
283
1.20 x 10
0
0
2000
290
5.29 x 10
1.47 x 10
138
20
1950
283
4.49 x 10
4.38 x 10
276
40
1890
274
3.25 x 10
276
40
1960
284
3.88 x 10
0
(Damaged)
0
N/A
(Damaged)
N/A
N/A
138
20
1400
203
2.03 x 10
1.70 x 10
276
40
1920
278
1.59 x 10
1.24 x 10
276
40
1800
261
1.77 x 10
0
0
340
49
1.04 x 10
138
20
1320
191
3.60 x 10
276
40
1900
276
(Damaged)
-2
-2
-2
-2
-3
-3
-3
-3
Radial
Strain
mm/mm
2.07 x 10
3.40 x 10
2.18 x 10
5.93 x 10
-2
-3
-3
-3
-3
-4
-4
-4
N/A
-2
-2
-2
-2
1.12 x 10
-2
-2
-2
8.66 x 10
-3
-2
2.37 x 10
-2
276
3.27 x 10-2
1.93 x 10
40
N/A
(Damaged)
N/A
N/A
N/A
0
0
200
29
1.15 x 10
138
20
1120
162
2.84 x 10
276
40
1740
252
2.68 x 10
276
(Damaged)
40
N/A
(Damaged)
N/A
N/A
0
0
260
38
1.93 x 10
138
20
1260
183
2.60 x 10
276
40
1860
270
6.55 x 10
276
40
1850
268
3.14 x 10
-2
-2
-2
1.26 x 10
1.96 x 10
1.39 x 10
-2
-2
-2
-2
N/A
-2
-2
-2
-2
2.44 x 10
1.73 x 10
1.03 x 10
1.76 x 10
-2
-2
-2
-2
Roadway Pavement
Adhesion is the property of a material to adhere onto
the surface upon which it is placed. This means the
material stays in the hole and adheres to the edge of
the hole in which it was placed. The edges of the repairs are usually the weakest areas of the repair, in regard to opening and allowing water to penetrate into
the substrata. The adhesion test used was similar to
that detailed by Prowell and Franklin (1996) with several changes. The following details the test procedure
used to determine adhesion
The cold mix for each test specimen was oven-aged
by placing about 750 g (1.65 lb) in a forced-draft oven
at 60 °C (140 °F) for four hours. This oven-aged cold
mix was then cooled to room temperature prior to
compaction. The Marshall cores, whose construction
was previously described in the section Strength, were
used for the adhesion test prior to Marshall testing.
One face of each of the GTM-compacted cores, used
for the adhesion test, was thin-cut with a concrete saw
to expose an aggregate face, on which the oven-aged
mixture was compacted. The surface was cleaned and
completely dried, and slightly heated to help to get it
back within the 150-mm (6-in.)-high GTM mold, with
the cut face on the upper side. The oven-aged mix was
placed on top of the Marshall core, within the GTM
mold and compacted with 10 blows of the Marshall
hammer. After compaction, the oven-aged mix and
the underlying Marshall core were extruded and then
turned upside down then observed to determine the
time it took for the oven-aged mix to come loose from
the Marshall core. The time until separation was recorded up to two minutes. While recording the time
until failure, it was also noted whether it was an adhesion or cohesion failure.
Test Pavement
An HMA pavement section on the Engineer Research
and Development Center-Vicksburg was selected as a
site on which to place several repair materials. The site
selected was part of a previously existing test road.
The test items were placed in an area of the road which
contained 100 mm (4 in.) of HMA over a 150-mm (6in.) crushed stone base. The center section of this area
of the road was selected as the location for placing the
test items. The items were all placed in a line down
the center of the roadway section. The individual test
Roadway Pavement
items were about 0.5 m (20 in.) wide and 0.9 m (36
in.) long. The holes for these items were cut with a
dry-cut saw through the HMA and then pried out; the
removed material was wasted.
Materials
Cementitious
Four rigid patch materials were placed in the test
section: ABC Cement, alkali-activated slag concrete
(AASC), PaveMend 15, and PaveMend 30.
Asphalt
Five cold patch materials were placed in the test section: DuraPave, Instant Road Repair, Optimix, QPR,
and UPM. A control mixture consisting of an MC-800
binder and a well-graded blend of aggregates, as described in Tables 4a and 4b, was also placed (Figure
9). This control mixture was used to represent a typical cold mix and was designed to meet the requirements of UFGS 02742 (UFGS 1997). Information for
the control mixture was also obtained from the Asphalt
Institute’s publication MS-16 (Asphalt Institute 1997)
and the manual UFC 3-250-03 (UFC 2001).
Placement of Test Section
The placement of the materials occurred over a period
of two days. The weather was hot, with high temperatures of about 35 °C (95 °F) and overnight lows of
about 22 °C (72 °F). The days were generally sunny,
and no precipitation fell during the placement. The
PaveMend 15 and the ABC Cement were placed on
the first day; the PaveMend 30 and the AASC were
placed the second day. Components for the PaveMend
15 were allowed to sit in the sun for a few hours prior
to placement. These components were mixed using a
high-speed vertical mortar mixer and then dumped directly into the hole. The hole was filled in two layers
using two separate batches of the material. The PaveMend 15 set very quickly, and only a quick troweling
of the surface was possible before it hardened. PaveMend is self-leveling, and finishing may not be necessary. The ABC Cement was mixed in a portable drumvane mixer. After mixing, the material was dumped
directly into a hole and screenedd with a board. The
mixture was consolidated using a small electric vibraPage 65
tor, then floated and finished. The next day, the components for the PaveMend
30 were kept at room temperature until right before mixing. The components
were mixed directly in the bucket the material was shipped in, as given in the
manufacturer’s instructions, using a blade mixer and an electric drill. After
mixing, the material was poured into a hole, and other buckets were mixed and
added until the hole was full. Both PaveMend products are self-leveling and
therefore will form a level surface, which should be considered when there is a
slope on the pavement surface. The fourth and last product, placed on the second day, was an AASC. This concrete mixture was developed and patented by
the Corps of Engineers for rapid pavement repair. It was mixed in the portable
drum-vane mixer, poured into the hole, screed with a board, vibrated, floated,
and finished.
The proprietary cold-mix products were placed over the same two-day time
period as the asphalt materials. The cold-mix materials were placed in the holes
in two lifts. The first lift was compacted initially with several coverage’s of a
tamping compactor with a 125-mm (5-in.)-diameter circular head. The remaining compaction of the first lift was accomplished with about 12 to 15 passes
using a vibratory plate compactor. The second lift was compacted in the same
manner except only the plate compactor was used. With the exception of the
control mixture that was placed in only a dry hole, each of the other mixtures
were placed in both dry and wet holes. To wet the holes, enough water was
added to saturate the hole and leave somewhere between 12.5 and 25 mm (0.5
to 1 in.) of water in the hole prior to the introduction of the cold-mix material.
Materials that the manufacturers said could be placed in wetted holes and/or
displace free water worked very well.. The dry-hole patches were placed on the
first day, and the wet-hole patches were placed on the second day. The control
cold mix was spread out and allowed to warm in the sunshine prior to placement. This allowed for the larger chunks of the patch material to be broken up,
during placement.
Traffic
The vehicle used to traffic the repaired areas was an Oshkosh, PQT. This dualaxle truck was loaded with five tons of payload, and the super-single tires were
14.00R20 XZL Michelin tires. The specifications on the truck and its tires are
given in Table 11. The performance of the patches under traffic is summarized
in Table 12. The holes patched on the first day were given four passes with
traffic on the same day. The holes patched the second day and those patched
the previous day were given six passes. The following day (the third day) all
patched holes, both asphalt and cementitious, were eventually given a total of
70 passes with all three tires on the driver’s side of the truck. The last 10 passes
were applied after the asphalt patch material in holes 2, 3, 5, 8, 9, and 10 had
been reworked or leveled. The mixture in these holes had to be reworked because, after trafficking, the material upheaved above the level of the surrounding pavement. The passes were applied down the middle of the patched areas.,
as much as possible.
Page 66
Roadway Pavement
Table 11 Information on Truck Used for Trafficking
Front Axle
Middle Axle
Vehicle type
Rear Axle
Oshkosh, PQT (Flatbed Truck)
Mass per axle, kg (lb)
4,773 (12,789)
1
4,914 (13,165)
4,902 (13,133)
Tire characteristic
Tire ID
14.00R20 XZL, Michelin
Tire width, mm (in.)
336 (13.2)
Front Axle
Middle Axle
Tire pressure,2 kPa (psi)
283 (41)
393 (57)
393 (57)
Contact area,2 sq cm (sq in.)
2394 (371)
2316 (359)
2303 (357)
Rear Axle
Vehicle had 5 tons of ballast on the truck bed.
2
Values determined with 425/95 R20 XZL Michelin tire.
1
Table 12 Effect of Traffic on Patching Materials
Location No. Material
Wetted
1
DuraPave
2
Depth Change with Number of Passes, mm (in.)
Initial
Final1
Effective
Final1,2
Y
4 (1/4)
---3
---3
QPR
Y
2.5 (3/32)
9.5 (3/8)
4 (5/32)
3
UPM
Y
3 (1/8)
12.5 (1/2)
6 (1/4)
4
Instant Road
Repair
Y
6 (1/4)
12.5 (1/2)
3 (1/8)
5
Optimix
Y
3 (1/8)
9.5 (3/8)
3 (1/8)
6
Instant Road
Repair
N
8 (5/16)
16 (5/8)
5 (3/16)
7
Control
N
9.5 (3/8)
17.5 (11/16)
5 (3/16)
8
Optimix
N
- 1.5 (-1/16)
9
QPR
10
UPM
11
9.5 (3/8)
11 (7/16)
N
-1.5 (-1/16)
4
9.5 (3/8)
11 (7/16)
N
-1.5 (-1/16)4
9.5 (3/8)
11 (7/16)
DuraPave
N
8 (5/16)
19 (3/4)
8 (5/16)
12
PaveMend 15
N
---
---5
---
13
ABC Cement
N
---
---
5
---
14
PaveMend 30
N
---
---
5
---
15
AASC
N
---
---5
---
4
After 60 passes, longitudinal measurement at center of repair.
Effective final height includes reduction for 3 mm (1/8 in.) rutting of surrounding HMA pavement.
3
Placed in water, against manufacturer’s recommendations, left to dry for 2 days, the patch could be trafficked with only
minor movement.
4
Negative number indicates the elevation was above the level of the pavement.
5
Repair showed no distress from traffic; however, there was some rutting of HMA between patches, and patch rocked or
moved under traffic.
1
2
Performance
The overall performance of the rigid repairs was very good. The rigid patches showed no distress after traffic. Due to of their relatively high stiffness, or modulus value, compared with
the surrounding asphalt pavement, the rigid patches started rocking and moving under traffic,
in relation to the surrounding asphalt pavement. The severity of this type of problem would
Roadway Pavement
Page 67
probably increase with increasing temperatures, and decrease with decreasing temperatures, because of changes
in the surrounding HMA and the underlying base course.
The HMA experienced some rutting between the rigid
patches, which was probably a major contributor to the
rocking of the rigid patches.
The overall performance of each of the asphalt materials placed in the test section was similar. Each material
experienced some additional compaction under traffic, as
evidenced by the slight rutting that occurred. Of note is
the fact that the surrounding asphalt pavement also rutted
under the traffic applied. This rut depth was about 3 mm
(1/8 in.) throughout the trafficked area. The depth of ruts,
as given in Table 12, varied from 3 to 11 mm (1/8 to 7/16
in.). In all cases, the depth of the rut was somewhat larger when the patch material was not restrained within the
patch. In other words, the amount of rutting was greater
when the hole was overfilled. All the materials exhibited
a degree of cohesion, especially the mixtures in holes 2, 3,
5, 8, 9, and 10, as these mixtures were reworked and then
trafficked without first being recompacted, and there was
no noticeable pickup by the tires. Except for the control
mixture, after compaction and numerous passes of traffic, any of the patch materials could easily be scarified,
leveled, and recompacted without pickup on the wheels.
The five proprietary products were all relatively easy to
handle, place, and compact. The manufacturer of DuraPave stated the product could be placed in a wet hole,
but did not recommend it for displacing water. The excess water acted to prevent compaction and resulted in an
unstable mixture that stabilized on its own after drying
for a few days, and was then able to withstand traffic.
The control mixture was somewhat stiffer than the other
products; however, using solar heating by spreading it
out in the sun for about one half hour made it sufficiently
easy to place and compact. Because it was a conventional cutback cold mix, the control mixture was placed only
as a dry patch. Table 13 gives a subjective estimate of the
workability and rate of curing of the mixtures under field
conditions.
Table 13
Estimation of Field Workability of Cold Mixtures
Location No.
Material
Workability At Given Time After
Placement1
Initial
3
Weeks,
Warm2
6 Weeks
Warm2 Cool2
1
DuraPave
5
3.5
2
1
2
QPR
5
5
3.5
0
3
UPM
5
5
3.5
0
4
Instant Road Repair
5
1
0.5
0
5
Optimix
5
4
2
0
6
Instant Road Repair
5
1.5
0.5
0
7
Control
4
0
0
0
8
Optimix
5
4
2
0
9
QPR
5
5
3.5
0
10
UPM
5
5
3.5
0
5
3.5
2
0.5
11 DuraPave
Field workability was based on subjective evaluation of ability of mixture to be worked or penetrated
with a sharp spade. Scale based from 0 to 5, with 0 being difficult to mark and 5 being relatively
easy to penetrate and move.
Warm
refers to a sunny day with temperatures of about 32 °C (90 °F), while cool refers to a cloudy
day with temperatures of about 24 °C (75 °F).
1 2
Page 68
Roadway Pavement
Conclusions
Summary
Rigid Repair Materials
We were introduced to HMA pavements in the beginning of this chapter. Triaxial compression testing was
discussed at length, along with the equipment and
protocol. Test pavement was presented next, with discussion of the materials tested and placement of the
test sections addressed. The topic of traffic was introduced and we examined how well the materials held
up against it. The course concluded with a brief discussion of the contrast and comparison of rigid repair
materials and asphalt repair materials.
Trafficking on the areas repaired with the rigid materials had no effect on the repair materials themselves;
however, after several passes, the HMA between the
patched areas started to rut and, eventually, the repairs
started to move or rock under the wheels of the vehicle.
This type of failure of rigid repair materials used in flexible pavement surfaces is typical due to the movement
of the flexible material in relation to the rigid pavement.
Asphalt Repair Materials
The asphalt cold-mix repair products that were investigated in the laboratory and in the field all performed
well. The workability test values of these products
showed that they would all be considered workable,
down to the freezing point of water, and slightly below. Aggregate gradation probably has a greater effect on workability than the grade of the binder. The
Marshall Stability test is probably not a good indicator of performance as it is not an appropriate test for
open-graded mixtures. The materials tested showed
good cohesive and adhesive properties. The control
mixture did not show adhesive properties, indicating
a tack coat would be required to achieve adhesion in
the repaired area.
The cold-mix materials were all easier to apply in field
repair test sections when compared with the control mixture. The repair materials were able to carry the applied
load without excessive displacement. When material was
displaced, it could easily be re-leveled, and trafficking
continued without any loss of material. The four coldmix products which advertised application to a wet pavement performed very well and did not show any difference in performance between the dry and watered holes.
Roadway Pavement
Page 69
Roadway Pavement
Student Assessment
Final Exam
1. _______________can be defined as the means
to gain and maintain freedom of maneuver and
force protection within a theater of operations.
a. Maneuver support
b. Concrete support
c. Material support
d. Asphalt support
2. Concrete repair materials are formed through
which type of process?
a. Thermal
b. Matrix
c. Chemical
3. _________ were established to provide a
framework for comparison of the various
asphalt repair materials.
a. Field Placement Parameters
b. Stock Placement Parameters
c. Asphalt Tests
d. Concrete Tests
4. Magnesium-phosphate cements are a blend of
materials that react with ________ to form a
rapid-hardening concrete.
a. Heat
b. Air
c. Water
d. Light
5. Polymer cements are_________ in nature.
a. Organic
b. Inorganic
c. Both
d. Neither
Page 70
6. What is the most economical method for
constructing pavement?
a. Cold asphalt mix
b. Hot mix asphalt
c. Wet mix asphalt
d. Dry mix asphalt
7. Depending on storage conditions, what is the
shelf life of a typical asphalt cement emulsion?
a. Six months to one year
b. one to two years
c. 0 months to three months
d. three months to six months
8. With regard to cold-mix materials, _________
can be considered as the amount of effort
required to properly construct a repair with
the mixture into the pavement.
a. Strength
b. Pave-ability
c. Workability
d. Patch Work
9. In____________, an axial load is applied to a
cylindrical specimen, along with a constant
confining stress applied to all sides of the
specimens.
a. Triaxial Compression Testing
b. Marshall Stability
c. Triaxial Load Test
d. Marshall Traffic Test
10. _________ is the property of a material which
is able to adhere to itself better than it adheres
to materials around it.
a. Cohesion
b. Adhesion
c. Durability
d. Strength
Roadway Pavement
Building Safe Structures
in Flood Zones
Course Description
This course is designed to fulfill professional development credits for professional engineers. It will examine
the building of safe flood zone structures to ensure that those built on fill in or near special flood hazard areas are
reasonably safe from flooding, in accordance with the national flood insurance program. Unless otherwise noted,
the information in this course was taken from the Federal Emergency Management Agency (FEMA).
Chapters
• Chapter One: Building Safe Flood Zone Structures
• Chapter Two: Flood Insurance Coverage For Basements
Learning Objectives
• Define pertinent terms
• Understand and identify the NFIP Regulations
• Recognize the administrative options for community permitting
• Comprehend soil mechanics to be able to properly place fill
• Determine when and how a stem wall foundation can be used
• Understand the reasons for and against using a basement foundation
• Grasp why the NFIP has only limited coverage for basement flooding
• Recognize the different flood risks due to foundation types
• Understand the simplified approach to constructing a basement
• Describe the engineered basement option
• Identify the NFIP and what it does
Page 71
Chapter One:
Building Safe Flood
Zone Structures
Overview
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Introduction
Definitions
NFIP Regulations
Warning
Floodways, V Zones, and Alluvial Fan Flood
Hazard Areas
More Restrictive State and Local Requirements
Professional Certification
Administrative Options for Community Permitting
Placement of Fill
Loss of Storage and Conveyance
Risk of Flood Damage in Areas Adjacent
to the SFHA
Building on Land Removed From the SFHA
by the Placement of Fill
Freeboard
Non-Basement Foundations
Basement Foundations
Warning
Summary
Learning Objectives
• Define pertinent terms given at the beginning
of the course
• Understand and identify the NFIP Regulations
• Recognize the administrative options for
community permitting
• Comprehend soil mechanics to be able to properly
place fill
• Realize when and how a stem wall foundation
can be used
• Understand the reasons for and against using
a basement foundation
Page 72
Introduction
For the purpose of administering the National Flood
Insurance Program (NFIP), FEMA identifies and maps
flood hazard areas nationwide by conducting flood
hazard studies and publishing Flood Insurance Rate
Maps (FIRMs). These flood hazard areas, referred to
as Special Flood Hazard Areas (SFHAs), are based on
a flood having a 1-percent probability of being equaled
or exceeded in any given year (also referred to as the
100-year flood or Base Flood).
Structures within the SFHA in a community participating
in the NFIP are subject to floodplain management regulations that impact building standards and are designed
to minimize flood risk. For example, Title 44, Part 60,
Section 3(c)(2) of the Code of Federal Regulations—abbreviated as 44 CFR 60.3(c)(2)—requires that the lowest
floor of a residential structure, including basement, built
within the SFHA be at or above the Base Flood Elevation (BFE). In addition, flood insurance must be purchased for these structures if they are used as collateral
to secure a loan provided by a federally regulated lender.
Flood insurance coverage may be purchased for all eligible structures within a participating community. Insurance rates for structures located within the SFHA differ
from the rates for structures located outside the SFHA.
When permitted under applicable Federal, state, and
local laws, ordinances, and regulations, earthen fill is
sometimes placed in an SFHA to reduce flood risk to
the filled area. Under certain conditions, when engineered earthen fill is placed within an SFHA to raise
the surface of the ground to or above the BFE, a request may be submitted to FEMA to revise the FIRM
to indicate the filled land is outside of the SFHA.
When such revisions are warranted, FEMA usually revises the FIRM by issuing a Letter of Map Revision
based on fill (LOMR-F). After FEMA has revised the
FIRM to show the filled land is outside the SFHA, the
community is no longer required to apply the minimum NFIP floodplain management standards to any
structures built on the land and the mandatory flood
insurance purchase requirements no longer apply. It
is worth noting that states and local communities may
have floodplain regulations that are more restrictive
than the minimum requirements of the NFIP and may
continue to enforce some or all of their floodplain
management requirements in areas outside the SFHA.
Although a structure built on a site that has been elevated by the placement of fill may be removed by
FEMA from the SFHA, the structure may still be
subject to damage during the Base Flood and highermagnitude floods. Constructing the entire structure at
or above the level of the BFE will minimize the flood
risk from the Base Flood, and is therefore the most
prudent approach to constructing on fill. Conversely,
a structure with a basement (sub-grade area) adjacent
to or near the floodplain may well be impacted by subsurface flooding brought on by surface flooding.
This course provides guidance on the construction of
buildings on land elevated above the BFE through the
placement of fill. Several methods of construction are
discussed, and the most prudent—those that result in
the entire building being above the BFE—are recommended.
In some areas of the country, basements are a standard construction feature. Individuals may wish to
construct basements on land after it has been removed
from the floodplain by a FEMA revision. Buildings
with basements built in filled areas are at an added risk
of flooding when compared to buildings on other types
of foundations. However, there are two major ways to
minimize this additional risk from subsurface flooding. First, the building should be located away from
the edge of the fill closest to the flooding source. Second, the higher the basement floor is elevated, the less
the risk associated. This technical bulletin provides
guidance on how to determine these buildings will be
reasonably safe from flooding during the occurrence
of the Base Flood and larger floods. To be reasonably
safe from flooding during the Base Flood condition,
the basement must:
• Be dry, not have any water in it
• Be structurally sound, not have loads that either
exceed the structural capacity of walls or floors or
cause unacceptable deflections.
In practice, this means soils around the basement must
have low permeability to minimize or stop water infiltration to the basement wall and floors. Any water that
does permeate to the basement must be removed by a
drainage layer on the outside (soil side) of the basement.
In addition, the foundation walls and floor slab must be
designed and constructed for any increased loads that
may occur during the Base Flood condition.
Definitions
Base Flood – The flood that has a one percent probability of being equaled or exceeded in any given year
(also referred to as the 100-year flood).
Basement – Any area of a building having its floor
subgrade below ground level on all sides.
Community – Any state, area or political subdivision
thereof, or any Indian tribe or authorized tribal organization, or Alaska Native village or authorized native
organization, which has the authority to adopt and enforce floodplain management regulations for the areas
within its jurisdiction.
Federal Emergency Management Agency (FEMA)
– The independent Federal agency that, in addition to
carrying out other activities, administers the NFIP.
Federal Insurance Administration (FIA) – The
component of FEMA directly responsible for administering the flood insurance aspects of the NFIP.
Flood Insurance Rate Map (FIRM) – The insurance
and floodplain management map issued by FEMA that
identifies, on the basis of detailed or approximate analysis, areas of 100-year flood hazard in a community.
Flood prone area – Any land area susceptible to being inundated by flood water from any source.
Mitigation Directorate – The component of FEMA
directly responsible for administering the flood hazard
identification and floodplain management aspects of
the NFIP.
New construction/structure – For floodplain management purposes, new construction means structures
for which the start of construction commences on or
after the effective date of a floodplain management
regulation adopted by a community and includes subsequent improvements to the structure. For flood insurance purposes, these structures are often referred to
as “post-FIRM” structures.
Special Flood Hazard Area (SFHA) – Area subject
to inundation by the base flood, designated Zone A,
A1-30, AE, AH, AO, V, V1-V30, or VE.
Page 73
NFIP Regulations
Part of a community’s application to participate in the
NFIP must include “a commitment to recognize and
duly evaluate flood hazards in all official actions in the
areas having special flood hazards and to take other
such official actions reasonably necessary to carry out
the objectives of the program” [44 CFR 59.22 (a)(8)].
NFIP regulations at 44 CFR 60 include Subpart A: Requirements for Flood Plain Management Regulations.
Each community participating in the NFIP adopts a
floodplain management ordinance that meets or exceeds the minimum requirements listed in 44 CFR 60.
Subpart A establishes specific criteria for determining
the adequacy of a community’s floodplain management regulations. The overriding purpose of the floodplain management regulations is to ensure participating communities take into account flood hazards, to
the extent that they are known, in all official actions
relating to land management and use. One of the minimum requirements established by the regulations is
set forth at 44 CFR 60.3 (a)(3), which states that, for
all proposed construction or other development within
a participating community, the community must “Review all permit applications to determine whether the
proposed building sites will be reasonably safe from
flooding.” 44 CFR 59.1 defines “development” as:
“…Any manmade change to improved or unimproved real estate, including but not limited to
buildings or other structures, mining, dredging,
filling, grading, paving, excavation or drilling operation or storage of equipment or materials.”
Warning
Construction of a residential building in an identified
SFHA with a lowest floor below the BFE is a violation
of the floodplain management requirements set forth
at 44 CFR 60.3(c)(2), unless the community has obtained an exception to NFIP requirements from FEMA
and has approved procedures in place.
By issuance of this Technical Bulletin, FEMA is noting that residual flood hazards may exist in areas elevated above the BFE by the placement of engineered
earthen fill. Residual risks in these areas include subPage 74
surface flood conditions and flooding from events
that exceed the base flood. This bulletin is intended
to guide local floodplain management officials in determining whether structures placed in filled areas are
reasonably safe from flooding. FEMA will require the
jurisdiction having authority for floodplain management determine an area is reasonably safe from flooding before removing it from the SFHA.
Floodways, V Zones, and Alluvial Fan
Flood Hazard Areas
This course does not apply to the following:
Construction in the floodway
The NFIP prohibits encroachments into the floodway
that would cause increases in flood stage.
Construction in SFHAs designated Zone V,
VE, or V1-V30 on FIRMs
The NFIP prohibits the use of structural fill for support of buildings in V zones. Buildings constructed in
a V zone must be constructed on an open foundation
consisting of piles, piers, or pots and must be elevated so the bottom of the lowest horizontal structural
member is at or above the BFE. In addition, this bulletin strongly recommends structural fill not be used
to elevate buildings constructed in A zones in coastal
areas. Detailed guidance concerning proper construction methods for buildings in coastal areas is presented
in FEMA’s Coastal Construction Manual (FEMA 55)
and in NFIP Technical Bulletin 5, Free-of-Obstruction
Requirements.
Construction in SFHAs subject to alluvial fan flooding
(designated Zone A0 with depths and velocities shown
on FIRMs). The NFIP will not remove land from the
floodplain based on the placement of fill in alluvial fan
flood hazard areas.
More Restrictive State and
Local Requirements
NFIP Technical Bulletins provide guidance on the
minimum requirements of the NFIP regulations. State
or local requirements that exceed those of the NFIP
take precedence. Design professionals should contact
community officials to determine whether more restrictive state or local regulations apply to the building
or site in question. All applicable standards of the state
or local building code must be met for any building in
a flood hazard area.
Professional Certification
As required by state and local floodplain management
ordinances, a proposed development must be determined to be reasonably safe from flooding. The official having the authority to make this determination
should require all appropriate information for making
the determination. This may include a certification
by a qualified design professional that indicates the
land or structures to be removed from the SFHA are
reasonably safe from flooding, according to the criteria described in this technical bulletin. Such a professional certification may come from a professional
engineer, professional geologist, professional soil scientist, or other design professional qualified to make
such evaluations. A sample of such a certification is
shown in Figure 1.
are encouraged to establish procedures that alert them
to potential future development of a filled area. These
procedures should allow for the evaluation of future
development and a means to determine whether it will
be reasonably safe from flooding. The following are
examples of such procedures:
• Require building sites to be identified on final subdivision plats and evaluate those building sites
against the standards described in this Technical
Bulletin.
• Require grading plans as a condition of issuing fill
permits and require those grading plans to include
building sites, and evaluate those building sites
based on this Technical Bulletin.
• Require buffer zones or setback zones around the
perimeter of fill pads or at the edge of the floodplain, and establish construction requirements
within these buffer zones to ensure buildings are
safe from residual risk.
• Require as a condition of final subdivision plat approval that the developer agree that no basements
will be built in any flood areas.
• Adopt or have regulations that control development
of areas immediately adjacent to floodplains that
would ensure any construction is reasonably safe
from flooding. For example, under the Minnesota
State Building Code, communities designate areas
outside of the floodplain as “Secondary Flood Hazard Areas” where building officials evaluate plans for
basements and can require modifications to the basement if an official believes there is a residual risk.
• When issuing a permit for the placement of fill only
in the SFHA, stipulate that no buildings will be built
on the site without a subsequent building permit.
Placement of Fill
Figure 1. Sample of professional certification form
Administrative Options for
Community Permitting
Communities may choose a variety of administrative
procedures to assist them in gathering information that
can be used to determine whether a proposed development is reasonably safe from flooding. Communities
Properly placing fill requires an understanding of soil
mechanics, local site conditions, the specific characteristics of the soils being placed, the methods used to place
and compact the fill, and soil testing procedures. Standard engineering and soil mechanics texts cover these
subjects in detail. The performance of these filled areas
should consider, but is not limited to, the following:
• The consolidation of the fill layers and any underlying layers
• The effect of this consolidation on either excessive
settlement or differential settlement
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• How the permeability of the soils affects water infiltration on any structures built on the site
Loss of Storage and Conveyance
The placement of fill in the SFHA can result in an increase in the BFE by reducing the ability to convey and
store flood waters. This can result in increased flood
damage to both upstream and downstream properties.
To prevent these possible results, some communities
prohibit fill, require compensatory storage for filled
areas, and/or identify a more restrictive floodway.
Risk of Flood Damage in Areas
Adjacent to the SFHA
Areas adjacent to the SFHA may have residual risks of
flood damage similar to those in areas removed from
the SFHA through the placement of fill. Both areas
are subject to residual risk from subsurface water related to flooding and from floods greater than the Base
Flood. Methods of construction discussed in this bulletin should also be used in these areas.
Building on Land Removed From
the SFHA by the Placement of Fill
The safest methods of constructing a building on filled
land removed from the SFHA are those that result in the
entire structure being above the BFE. Methods that place
the lowest floor of the building at, rather than above, the
BFE are at greater flood risk, and methods that result
in the lowest floor (including a basement floor) below
the BFE have the highest flood risk of all. Placement of
the lowest floor of these structures below the BFE, even
through they are outside the SFHA, will result in an increased threat from subsurface flooding and magnified
damages from flooding that exceeds the BFE.
Design and Construction). When fill is used to protect
buildings from the Base Flood, the community should
consider whether freeboard should be required. This
consideration should include whether better information exists or conditions have changed (from when the
BFE was originally established) that indicate the BFE
may be higher than originally expected. One example
of when the BFE may be higher is when a culvert or
bridge is blocked by debris. Flood modeling assumes
an open channel or culvert. Even when the BFE is not
expected to be higher, freeboard may be appropriate
to provide increased protection from flood events less
frequent than the Base Flood or to account for future
changes that may increase the BFE.
The foundation types for buildings outside the SFHA
described in the following sections are listed in order
of their increasing risk of flood damage.
Non-Basement Foundations
Non-basement foundations consist primarily of stem
wall, crawlspace, and slab-on-grade foundations.
Stem Wall Foundation
A stem wall foundation can be used to raise the lowest floor above the surrounding grade. After the stem
walls have been constructed and extended to the desired
elevation, the area enclosed by the stem walls is filled
with engineered compacted fill and a slab is poured on
top (see Figure 2). Through the placement of additional
fill, the site may be elevated above the BFE. This approach provides freeboard—an additional amount of
elevation that helps protect against subsurface flooding
and floods that exceed the Base Flood. Constructing a
stem wall foundation and placing this additional fill on
the site provide the highest level of flood protection.
Freeboard
Freeboard is an additional height used as a factor of
safety in determining the elevation of a structure, or
flood proofing, to compensate for factors that may increase the flood height (ASCE 24-98, Flood Resistant
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Figure 2. Structure on a stem wall foundation. The lowest
is raised above the BFE. The space encloses by the stem
walls is filled with engineered compacted fill.
Crawlspace Foundation
Constructing a crawlspace beneath the first floor will
raise the lowest floor of the structure above the surrounding grade (see Figure 3). Openings in the foundation walls are recommended. If flooding reaches the
building, the openings allow flood waters to enter the
area below the lowest floor and equalize the hydrostatic pressure on the foundation walls (see NFIP Technical Bulletin 1, Openings In Foundation Walls).
The crawlspace alternative is less preferable than stem
wall construction, which does not result in an enclosed
area under the first floor and therefore requires no
flood openings. Placing additional fill to a level above
the BFE provides freeboard that helps protect against
subsurface flooding and floods that exceed the Base
Flood. Constructing a crawlspace foundation and
placing additional fill on the site provide increased
flood protection.
Figure 3. Structure on a crawlspace foundation. The lowest
floor is raised above the BFE. Openings in the foundation
walls allow water from floods higher than the fill elevation
to enter the crawlspace and equalize the pressure on
foundation walls.
Slab-On-Grade Foundation
This method normally provides less flood protection
than crawlspace construction because it does not elevate the house above the adjacent grade (see Figure
4). As a result, the lowest floor of the house can be
as low as the BFE and would be inundated by any
flood greater than the BFE. Placing additional engineered fill beneath the building to a level above the
BFE would provide freeboard and therefore increased
flood protection.
Figure 4. Structure on a slab-on-grade foundation. The
lowest floor is typically slightly higher than the surronding
grade.
Basement Foundations
Although basements are a desired feature in some areas of the United States, NFIP minimum requirements
generally do not allow their construction in the SFHA,
because of the increased risk of flood damages. The
only instances where this is not the case is buildings
for which FEMA has granted a special exemption to
allow flood proofed basements. However, once land
is removed from the SFHA through a map revision,
these NFIP minimum requirements no longer apply.
As a result, builders and property owners who build
on land removed from the SFHA sometimes elect to
install basements, which are at a higher risk of flood
damage than the foundation types described previously.
Constructing a basement on such land is not recommended, because the basement (i.e., lowest) floor and
portions of the basement walls may be subjected to
subsurface flooding. The basement may therefore be
subject to seepage and lateral hydrostatic and uplift
pressure caused by high groundwater levels associated
with flooding in surrounding areas. Additionally, when
flooding exceeds the BFE, the basement area may be
totally inundated with floodwater. When builders and
homeowners decide to accept the additional risk associated with basement construction on filled land, they
need to ensure that the basement and the rest of the
house are reasonably safe from flooding.
Warning
In filled areas adjacent to floodplains, floods can still
greatly influence the groundwater at the filled site.
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High groundwater at a site with a basement can result
in water infiltrating the basement or greatly increased
hydrostatic pressures on the walls and basement slab
that can cause failure or permanent deformation. Even
when floods have not reached houses with basements,
FEMA has seen numerous examples of flooded basements, bowed basement floors, and collapsed basement walls that have resulted from the effects of high
groundwater caused by flooding. In addition, the collapse of flooded basements has also occurred when
water is rapidly pumped from basements surrounded
by saturated soils whose pressure exceeds the capacity
of the basement walls.
Summary
In this chapter, we were introduced to the NFIP and
FEMA, who conduct nationwide flood hazard studies. Next, we studied the definitions of terms used in
this course. The NFIP regulations were discussed at
length, and local requirements and personal certifications were touched on briefly. To end the chapter, nonbasement foundations were examined.
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Chapter Two:
Flood Insurance
Coverage for Basements
Overview
•
•
•
•
•
•
•
•
•
•
Introduction
Flood Risk by Foundation Type
Basement Construction Guidance
Structural Design
Simplified Approach
Engineered Basement Option
Buildings in Existing Filled Areas
The NFIP
Technical Bulletins
Summary
Learning Objectives
• Understand why the NFIP has only limited
coverage for basement flooding
• Recognize the different flood risks due to
foundation types
• Comprehend the simplified approach to
constructing a basement
• Be able to describe the engineered basement
option
• Identify the NFIP and what it does
Introduction
It is extremely important to note that the NFIP offers
only limited coverage for basement flooding. First,
in order for a claim to be paid, there must be a general condition of overland flooding where floodwaters
come in contact with the structure. Secondly, the NFIP
does not provide coverage for finished nonstructural
elements, such as paneling and linoleum, in basement
areas. Contents coverage is restricted to a limited
number of items listed in the flood insurance policy.
Contact a local insurance agent for more information.
Four basement construction methods are described below, in increasing order of flood risk.
Basement Foundation with Lowest Floor
at or Above BFE
Placing the lowest floor of the basement at or above
the BFE has the effect of eliminating flood induced
damage up to the BFE (see Figure 5). In general, the
higher the basement floor is above the BFE the lower
the risk of damage from seepage and hydrostatic pressure caused by flood-related groundwater. Where possible, the basement should be built with its floor at or
above the BFE. An added benefit is that floods that exceed the BFE will cause significantly less damage to a
structure with this type of basement than to structures
with basements whose floors are at greater depths.
Figure 5. Basement foundation with lowest above the BFE.
Damage from floods below the BFE is eliminated.
Basement Foundation in Fill Placed Above BFE
Placing fill to a level higher than the BFE has the effect of reducing the depth of the basement floor below
the BFE (see Figure 6). It is recommended that fill be
placed to a level at least 1 foot above the BFE. In general, the higher the basement floor the lower the risk of
damage from seepage and hydrostatic pressure caused
by flood-related groundwater. Where possible, enough
Page 79
fill should be properly placed so the lowest grade adjacent to the structure is raised to an elevation greater
than the BFE. An added benefit of fill placed above the
BFE is that it helps protect the building from floods
greater than the Base Flood. These floods are less likely to reach the structure.
Figure 6. Basement foundation in fill placed above the
BFE. The depth of the basement floor below the BFE is
less than when no fill is placed.
Basement Foundation with Lowest Opening
above BFE
In the event the lowest floor is not elevated to or above
the BFE and fill is not placed to a level above the BFE,
the next best method of reducing flood risk is to place
the lowest opening into the basement (e.g., window
well) at a level higher than the BFE (see Figure 7).
This will reduce the chances of surface flooding entering and inundating the basement. However, the
basement walls and floor slab will still be subjected
to hydrostatic pressure, with the potential for damage
and seepage into the basement. In addition, the abovegrade basement walls will be exposed to water from
floods greater than the Base Flood. For this reason, the
lowest opening in the basement walls should be above
the BFE, as shown in Figure 7.
Basement Foundation with Lowest Opening
at BFE
This is the least preferable condition of all because
it results in the highest flood risk and is not recommended (see Figure 8). The lack of fill above the BFE,
coupled with the lowest floor being below BFE and
lowest opening at the BFE, exposes the basement to
flooding from both subsurface flooding and any flood
greater than the Base Flood.
Figure 8. Basement foundation with lowest opening at the
BFE. The basement is exposed to flooding from any flood
greater than the Base Flood.
Flood Risk by Foundation Type
Table 1 summarizes the foundation construction methods described in this bulletin and ranks them in order of
increasing flood risk—the safest foundation types appear
near the top; the less safe foundation types appear near
the bottom. The foundation construction methods that
result in a building that is reasonably safe from flooding are shown in the dark gray area of the table. If the
basement construction methods shown in the light gray
area are used, the requirements described in the following sections of this bulletin must be met in order for the
building to be considered reasonably safe from flooding.
Figure 7. Basement foundation with lowest opening above
the BFE. Surface flooding is less likely to enter and inundate
the basement.
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Table 1 Flood Risk by Foundation Construction
Method
structural design, it is recommended the full hydrostatic
pressures be assumed unrelieved by the drainage system. Foundation walls that have not been designed for
hydrostatic pressures, such as un-reinforced masonry or
pressure-treated wood wall systems, should not be used.
Simplified Approach
Design Requirements
Reasonably Safe From Flooding
ollow Guidance in This Bulletin To Ensure That
F
Building Is Reasonably Safe From Flooding
Basement Construction Guidance
For those who have chosen to accept the additional
risk associated with basement construction below the
Base Flood on filled land that has been removed from
the SFHA, this bulletin provides technical guidance
about measures that can be taken to protect basements
and meet the requirement that buildings be made reasonably safe from flooding. A simplified approach,
including the requirements that must be met for its
use, is presented first. For buildings that do not meet
the criteria for the simplified approach, this bulletin
provides technical guidance for the development of an
engineering design tailored to the site conditions.
Structural Design
Design of foundation elements is addressed in model
building codes. This technical bulletin does not address
the structural design of basement walls or foundations.
Floors and slabs should be designed for the hydrostatic
pressures that can occur from the Base Flood. For the
If, for a building and building site, all the requirements
listed below are met (see Figure 10), the building is
reasonably safe from flooding. If all of these requirements are not met, the more detailed analysis described under Engineered Basement Option, on page
19 of this bulletin, should be performed to determine
whether the building is reasonably safe from flooding.
• The ground surface around the building and within a defined setback distance from the edge of the
SFHA (see next item) must be at or above the BFE.
• The setback is the distance from the edge of the
SFHA to the nearest wall of the basement. The
minimum allowable setback distance is 20 feet.
• The ground around the building must be compacted
fill; the fill material—or soil of similar classification
and degree of permeability—must extend to at least
5 feet below the bottom of the basement floor slab.
• The fill material must be compacted to at least 95
percent of Standard Laboratory
• Maximum Dry Density (Standard Proctor), according to ASTM Standard D-698. Fill soils must
be fine-grained soils of low permeability, such as
those classified as CH, CL, SC, or ML according
to ASTM Standard D-2487, Classification of Soils
for Engineering Purposes. See Table 1804.2 in the
2000 International Building Code (IBC) for descriptions of these soil types.
• The fill material must be homogeneous and isotropic; that is, the soil must be all of one material, and
the engineering properties must be the same in all
directions.
• The elevation of the basement floor should be no
more than 5 feet below the BFE.
• There must be a granular drainage layer beneath
the floor slab, and a ¼-horsepower sump pump
with a backup power supply must be provided to
remove the seepage flow. The pump must be rated
at four times the estimated seepage rate and must
discharge above the BFE and away from the buildPage 81
ing. This arrangement is essential to prevent flooding of the basement or uplift of the floor under the
effect of the seepage pressure.
• The drainage system must be equipped with a positive means of preventing backflow.
• Model building codes (such as the 2000 International Residential Code) also address foundation
drainage (IRC Section R405) and foundation walls
(IRC Section R404). Model building codes generally allow foundation drains to discharge through
either mechanical means or gravity drains. In addition, there is often an exception to the requirement
for drainage systems in well-drained soils. However, in or near floodplains, well-drained soils can, in
fact, help convey groundwater towards the building foundation. Therefore, this exception should
not apply in or near floodplains.
• In some cases in or near floodplains, even with standard drainage systems, hydrostatic pressures from
groundwater against the basement can result. When
a standard drainage system is unable to eliminate hydrostatic pressure on the foundation, model building
codes, including the 2000 International Residential
Code (IRC Section R404.1.3), require the foundation
be designed in accordance with accepted engineering practice. The simplified approach contained
in this Technical Bulletin assumes no hydrostatic pressure on the foundation and should be
used only when a standard drainage system, discharged by a sump pump that is equipped with
backup power and that discharges above BFE, is
employed. For other drainage systems, the designer
should use the engineered basement option presented on page 19 of this bulletin and other appropriate
building code requirements.
Figure 10. Requirements for use of the simplified
approach to basement construction.
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Technical Background for the Simplified
Approach
The simplified approach is based on the following
conditions:
1. The area of the footprint of the basement is less
than or equal to 1,200 square feet.
2. The soil is saturated; therefore, there is no time
lag in the development of the seepage pattern with
a change in flood water level. The groundwater
table in floodplains is typically very shallow, and
fine grained soils have a substantial potential for
maintaining saturation above the water table by
capillary rise.
3. The tailwater level is at the elevation of the BFE.
For this bulletin, “tailwater” is defined as the
groundwater level beyond the structure, on the
side away from the flood water surface. This is
a reasonably conservative assumption because
the flood would raise the groundwater level in the
general area. In some cases, the tailwater level
can be higher than the flood level because there
is higher ground, as a valley wall, that feeds the
groundwater into the floodplain soils.
4. The effective elevation of the base of the seepage flow zone can be defined (see Figure 11). This
elevation is needed to permit calculation of the
quantity of seepage flow. If the base elevation is
not known, its depth below the base of the floor
slab can be conservatively approximated as onehalf of the building width most nearly perpendicular to the shoreline of the flood water. This would
approximate the boundary effects of the three-dimensional seepage flow, in that it would represent
the flow coming in from all sides and meeting in
the center beneath the floor slab. This approach
assumes a constant soil type and density over the
flow zone. If the site has stratified soil layers, the
engineered basement option should be used (see
page 19 of this bulletin).
5. The quantity of seepage flow can be calculated by
a simplified method based on Dupuit’s assumption
that equipotential lines are vertical. (The Dupuit
method uses Darcy’s law with specific physical
characteristics. A more detailed description can be
found in the first two references listed under “Further Information,” on page 23 of this bulletin.) The
elements of the method are presented in Figure 11.
The entry surface, with hydraulic head “a,” is a vertical line extending downward from the edge of the
flood surface. The exit surface, with hydraulic head
“b,” is a vertical line extending downward from
the side of the structure closest to the flood water’s
edge. The length of the flow path, “L,” is the setback distance. Flow is assumed to be horizontal,
and the horizontal coefficient of permeability is
the effective permeability. For simplicity, the small
inclined entry zone at the river bank and the exit
zone below the basement floor are ignored. This is
a reasonably conservative measure. The phreatic
line, or the line below which the seepage flow occurs under positive pressure, extends from the edge
of the flood water to the elevation of the bottom of
the basement floor slab. If the exit zone below the
basement floor were included, the hydraulic head
at “b” would be higher. As shown in Figure 11, the
phreatic line is not a straight line, but within the
limits of the assumed boundary values, it is close
to a straight line.
To obtain Q, the total seepage flow, in cubic feet per
second, q must be multiplied by the length around
the periphery of the four sides of the structure. This
is a simplifying approach that obviates the need for a
three-dimensional flow net calculation and is reasonably conservative.
It should be noted that the soil permeability does not
affect the geometry of the seepage zone or the geometry of the phreatic line. The permeability does have a
significant effect on the quantity of seepage that must
be collected and discharged by the drainage layer and
the sump pump. The calculation of the quantity Q
provides a basis for the selection of a sump pump of
adequate capacity. To allow for possible errors in the
estimation of the soil permeability, the pump should
have a capacity of at least four times the calculated
value of Q. As noted in the requirements section, a
standard sump pump of ¼ horsepower or greater will
generally satisfy the requirements of seepage removal
for the conditions described above.
Engineered Basement Option
If the requirements specified for the simplified approach are not met, a licensed soils engineer or geologist should perform a detailed engineering analysis
to determine whether the structure will be reasonably
safe from flooding. The analysis should consider, but
is not limited to, the issues described in the following
sections.
Depth, Soil Type, and Stratification
of Subsurface Soils
Figure 11. Method for calculation of seepage flow.
The Dupuit equation for the quantity of seepage flow is:
q = k(a2 – b2)/2L
Where: • q—is the flow in cubic feet per second for a 1-foot
width of seepage zone
• k—is the soil permeability in feet per second (fps)
(maximum value of k is 1x10-3 fps)
• a and b—are hydraulic heads in feet (a < b + 5)
• L—is the length of the flow zone in feet (L > 20
feet)
The depth, soil type, and stratification of the subsurface
soils may be complex. Four potential generalized scenarios are shown in Figures 12 and 13. Figure 12 shows
two cases of homogeneous soil. The depth of penetration of the basement and the depth of the flow zone are
not limited to the assumptions on which the simplified
approach is based. Case I represents a foundation consisting of clayey soils, either fill or natural deposits or
a combination, which are more or less homogeneous
because they have similar engineering properties. If
an adequate setback distance is provided, the seepage
quantity would be relatively low, and uplift pressure
beneath the slab could be controlled by an appropriately sized sump pump because of low permeability.
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Case II represents a foundation consisting of sandy
soils, either fill or natural soil deposits or a combination, which are more or less homogeneous because
they have similar engineering properties. The seepage quantity would be fairly large, and more attention
would have to be given to the setback distance and
to the provision of an adequately sized sump pump
to prevent excessive uplift pressure beneath the floor
slab because of high permeability.
Figure 13 shows two simple cases of stratified soils,
with impervious clays overlying pervious sands. This
is a common occurrence in natural floodplain deposits. In Case III, the contact between the two soil strata
is at some distance below the basement floor. This
case would involve a moderate quantity of seepage,
depending on the thickness, indicated as d, of the impervious stratum below the basement floor. There is
also a potential for excessive uplift pressure beneath
the floor, at the level of the bottom of the clay stratum.
If d is equal to h, the net hydraulic head between the
flood level and the floor level, the safety factor against
uplift would be approximately 1.0. If d is less than h,
there would be excessive uplift, with a safety factor
equal to less than 1.0.
Figure 12. Case I and Case II – homogeneous soil.
Case IV shows impervious soils overlying pervious
soils, with the contact between the soil strata at some
distance above the basement floor. This case would
involve a large quantity of seepage and potential for
excessive uplift beneath the basement floor.
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Geotechnical Investigations
Geotechnical investigations must be made for cases
that do not conform to the assumptions on which the
simplified approach is based. Information that is needed to permit an adequate engineering analysis includes
the following:
• The BFE, which is to be used as the design flood
water surface for calculating expected seepage.
Figure 13. Case III and Case IV – stratified soils.
• The elevation of the bottom of the basement floor.
This can be adjusted as needed to achieve more
suitable conditions.
• The setback distance of the basement wall from
the edge of the flood water. This can be adjusted to
achieve more suitable seepage control or to accommodate available space restraints.
• The elevation of the groundwater table and its seasonal variations. A high water table would cause
problems with groundwater control during construction of a basement, even without a flood event.
• The stratification of the subsurface materials, for
both natural and fill soils. In general, borings should
be drilled to a depth below the bottom of the floor
slab that is at least two times as great as the depth
of the bottom of the floor slab below the BFE.
• The engineering classification of the soils, for
both natural and fill soils. This must be done in
accordance with ASTM D2487, Classification of
Soils for Engineering Purposes. This is the Unified Soil Classification System that is universally
used throughout the United States. Local or county
agricultural soil survey maps should not be used,
because they do not give specific information about
location and depth of soils, and their designations
are not pertinent to civil engineering use.
• Subsurface conditions landward from the structure.
This includes information about the location of the
water table, whether it is higher or lower than the
flood level, and information about any penetrations of the soil, such as ponds. Attention should be
given to the possibility that higher ground, such as
valley walls, could contribute to the groundwater
level in the floodplain, either perennially or during
periods of heavy rain.
• Information about any penetrations through the
basement walls below the BFE, such as utility lines
and other openings.
• Analysis of seepage quantity. The analysis can
be made by the conservative simplified method
described in Item 5 in the section titled Technical Background for the Simplified Approach (illustrated in Figure 11), or by the construction of a
flow net that takes into account all of the boundary
conditions more rigorously. A flow net may be required to permit analysis of uplift pressures. Uplift
pressures may be more significant in laminated or
stratified soil deposits.
Buildings in Existing Filled Areas
In evaluating buildings in existing filled areas, the
two approaches already described—the simplified approach or the engineered basement option—can be
used. If the simplified approach is used, all the requirements for the use of this approach must be met. Some
possible means for evaluating whether these requirements are met include soil tests and investigations,
including soil borings and hand augers; field records
from the time the fill was placed; and soil surveys. If
the requirements for the simplified approach are not
met, a licensed soils engineer or geologist should
perform a more detailed engineering analysis as described under Engineered Basement Option on page
19. More extensive soil investigations and testing may
be required to complete the analysis.
The NFIP
The NFIP was created by Congress in 1968 to provide
federally backed flood insurance coverage, because
flood coverage was generally unavailable from private insurance companies. The NFIP is also intended to reduce future flood losses by identifying flood
prone areas and ensuring new development in these
areas is adequately protected from flood damage. The
NFIP is based on an agreement between the Federal
government and participating communities that have
been identified as flood prone. FEMA, through the
Federal Insurance Administration (FIA), makes flood
insurance available to the residents of a participating
community, provided the community adopts and enforces adequate floodplain management regulations
that meet the minimum NFIP requirements. The NFIP
encourages communities to adopt floodplain management ordinances that exceed the minimum NFIP criteria set forth in Part 60 of the NFIP Floodplain Management Regulations (44 CFR 60). Included in the
NFIP requirements, found under Title 44 of the U.S.
Code of Federal Regulations, are minimum building
design and construction standards for buildings located in SFHAs. Through their floodplain management
ordinances or laws, communities adopt the NFIP performance standards for new, substantially improved,
and substantially damaged buildings in flood prone
areas identified on FEMA’s FIRMs.
Technical Bulletins
This publication is one of a series of Technical Bulletins that FEMA has produced to provide guidance
concerning the building performance standards of the
NFIP. These standards are contained in 44 CFR 60.3.
The bulletins are intended for use primarily by state
and local officials responsible for interpreting and
enforcing NFIP regulations, and by members of the
development community, such as design professionals
and builders. New bulletins, as well as updates of existing bulletins, are issued periodically, as necessary.
The bulletins do not create regulations; rather they
provide specific guidance for conforming with the
Page 85
minimum requirements of existing NFIP regulations.
Users of the Technical Bulletins who need additional
guidance concerning NFIP regulatory requirements
should contact the Mitigation Division of the appropriate FEMA regional office or the local floodplain
administrator. NFIP Technical Bulletin 0, the User’s
Guide to Technical Bulletins, lists the bulletins issued
to date, provides a key word/subject index for the entire series, and lists addresses and telephone numbers
for FEMA’s 10 Regional Offices.
Summary
In this chapter, we studied basement foundations at or
above the BFE. Nex,t we talked briefly about the flood
risk associated with each foundation type, followed by
a discussion of structural design, focusing on several
methods in detail. Finally, engineered basement options were explored, along with the geotechnical investigations made for cases that don’t conform to the
assumptions of the simplified approach.
Page 86
Student Assessment
Final Exam
1. What does NFIP stand for?
a. National Flood Insurance Program
b. National Floor Insurance Plan
c. National Flood Institute Program
d. Non-Flood Insurance Program
2. The safest method of constructing a building
on filled land removed from the SFHA is to
build it at or above:
a. The BFE
b. The FER
c. The BAR
d. The BBC
3. SFHA stands for:
a. Special Flood Hazard Area
b. Special Force Home Area
c. Special Flood Home Association
d. Special Flood Hazard Association
4. As required by state and local floodplain
management ordinances, a proposed
development must be determined to be:
a. Cost efficient
b. Level
c. Reasonably safe from flooding
5. The NFIP offers which of the following for
basement flooding?
a. Limited coverage
b. Full Coverage
c. No Coverage
d. Liability
6. In general, the __________the basement
floor is above the BFE, the ________the risk
of damage from seepage and hydrostatic
pressure caused by flood-related groundwater.
a. Lower; higher
b. Higher; lower
c. Shorter; longer
d. Longer; shorter
7. How far should fill be placed above the BFE?
a. 3 feet
b. 5 feet
c. 1 foot
d. 2 feet
8. Floors and slabs should be designed for the
________ pressures that can occur from the
Base Flood.
a. Elevated
b. Decreased
c. Hypostatic
d. Hydrostatic
9. What is the minimal allowable setback
distance from the edge of the SFHA to the
nearest wall of the basement?
a. 10 feet
b. 20 feet
c. 15 feet
d. 30 feet
10. The fill material being used must be:
a. Homogeneous
b. Isotropic
c. Both
d. Neither
Page 87
Heavy Loads
Course Description
This course is designed to fulfill continuing education requirements for Professional Engineers. This course will
cover the criteria for the design of concrete floor slabs on grade in buildings for heavy loads. Theoretical concepts, practical applications, basis of design and design procedures for heavy loads will be discussed and studied
throughout this course to give better understanding to the student. All of the information, unless otherwise noted
was taken from the list of references found at the end of this course.
Chapters
•
•
•
•
Chapter One: Basis of Floor Slab on Grade Design
Chapter Two: Determination of Floor Slab Requirements
Chapter Three: Site Investigation
Chapter Four: Design Procedure
Learning Objectives
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Describe how stresses control structural designs of concrete
Discuss the general classifications for vehicle-imposed loads
Distinguish positive and negative bending moments limits for stationary live loads
Discuss the forklift truck traffic in terms of maximum axle load
Learn the six categories of forklift trucks
Study the adequate thickness to carry wall loads using tables
Learn what existing conditions must be investigated for a site
Understand the importance of subgrade conditions to provide maximum support
Examine the environmental conditions that affect site conditions
Study concrete strength to provide high wear resistance
Study traffic loads in terms of equivalent operations of basic axle loading
Examine the design procedures for stabilizing foundations
Identify the portions of the subgrade and what is needed to improve the foundation
Distinguish the advantages of using steel reinforcement
Learn the design procedure for reinforced concrete floor slabs
Discuss the three general joint types and their purpose
Understand joint spacing throughout any paved area
Examine design examples for better understanding
Page 88
Heavy Loads
Chapter One:
Basis of Floor Slab
on Grade Design
Overview
•
•
•
•
•
•
•
Introduction
Stresses
Vehicle-Imposed Loads
Stationary Live Loads
Wall Loads
Design Examples
Summary
Learning Objectives
• Describe how stresses control structural designs
of concrete
• Discuss the general classifications
for vehicle-imposed loads
• Distinguish positive and negative bending
moments limits for stationary live loads
• Study the thickness of wall loads developed
by Staab
Introduction
This course describes the criteria for the design of
concrete floor slabs on grade in building for heavy
loads and is applicable to all elements responsible for
military construction. Heavy loads in buildings such
as a warehouse include moving loads, stationary live
loads, and wall loads.
Here are a few helpful definitions for better understanding:
Slab on grade- Concrete slab supported directly on
foundation soil.
Light loads- Loads which consist of (comparable)
forklift axle load of 5 kips or less and stationary live
loads less than 400 pounds per square foot.
Heavy loads- Loads which consist of any one of the
following: moving live loads exceeding a forklift axle
load of 5 kips, stationary live loads exceeding 400
pounds per square foot, and concentrated wall loads
exceeding 600 pounds per linear foot.
Wall load- Concentrated loads imposed by walls or
partitions.
Dead load- All the materials composing the permanent structure, including permanent wall loads and all
equipment that is fixed in position.
Live load- Loads imposed by the use and occupancy
of the structure.
• Moving live load- Loads imposed by vehicular
traffic such as forklift trucks.
• Stationary live load- Loads imposed by movable
items, such as stored materials.
Vibratory Loads- Dynamic and/or oscillatory loading
of significant magnitude.
Design load- The effects of stationary live, dead, and
wall loads and moving live loads. Dead loads of floor
slabs on grade are ignored.
Special soils- Soils which exhibit undesirable properties for construction uses such as high compressibility
or swell potential.
Heavy Loads
Page 89
Nonreinforced slab- Concrete slab resting on grade
containing minimal distributed steel, usually of welded wire fabric (WWF), for the purpose of limiting
crack width due to shrinkage and temperature change.
Reinforced slab- Concrete slab resting on grade containing steel reinforcement, which consists of either a
welded wire fabric or deformed reinforcing steel bars.
For your thought:
Concrete floor slabs on grade are subjected to a variety
of loads and loading conditions. The design procedure
includes determining slab thickness based on moving
live loads, then checking adequacy of slab thickness
for stationary live load. The design procedure separately includes determining thickness of slab under
wall load. The entire design procedure is based on a
working stress concept. Stresses induced by temperature gradients and other environmental effects are taken into account by the assignment of working stresses.
Working stresses have been established empirically
based on experience gained in roadway and airfield
pavement performance data.
Stresses
The structural design of a concrete floor slab on grade
is primarily controlled by the stresses caused by moving live loads, and in some cases, the stationary loads.
Stresses in floor slabs on grade resulting from vehicular loads include:
• function of floor slab thickness
• vehicle weight and weight distribution
• vehicle wheel or track configuration
• modulus of elasticity and Poisson’s ratio of concrete
• modulus of subgrade reaction of supporting material
The volume of traffic during the design life is important for fatigue considerations. The floor slab design
procedure presented erations. The floor slab design
procedure presented herein is based on limiting the
critical tensile stresses produced within the slab by the
vehicle loading. Correlation studies between theory,
small-scale model studies, and full-scale accelerated
traffic test have shown maximum tensile stresses in
floor slabs will occur when vehicle wheels are tangent
to a free edge. Stresses for the condition of the vehicle
wheels tangent to an interior joint, where the two are
Page 90
tied together, are less serve than a free edge because
of the load transfer across the two adjacent slabs. In
the case of floor slabs, the design can be based on the
control of stress at interior joints. Exceptions to this
assumption for interior joint loading occur when a
wheel is placed at the edge of doorways or near free
edge at a wall.
Vehicle-Imposed Loads
For determining floor slab design requirements, military vehicles have been divided into three general
classifications: forklift trucks, other pneumatic and
solid tired vehicles, and tracked vehicles. The relative
severity of any given load within any of the three classifications is determined by establishing a relationship
between the load in question and a standard loading.
Floor slab design requirements are then established in
terms of the standard load. Other stresses, such as restraint stresses resulting from thermal expansion and
contraction of the concrete slab and warping stresses
resulting from moisture and temperature gradients
within the slab, due to their cyclic nature, will, at
times, be added to the moving live load stresses. Provision for these stresses that are not induced by wheel
loads is made by safety factors developed empirically
from full scale accelerated traffic tests and from the
observed performance of pavements under actual service conditions.
The maximum allowable stationary live load is limited by both the positive bending moment stress under
the load and the negative bending moment stresses occurring at some distance from the load.
Positive Bending Moments
Stresses due to positive bending moment are relatively
simple to compute by using Westergaard’s analysis1 of
elastically support plates. An appropriate safety factor
is applied to determine allowable stresses due to these
1 Westergaards analysis is actually for plates on a liquid
foundation, sometimes called a Winkler foundation. There
is a distinct difference between the structural behavior of
plates on a liquid and on an elastic foundation. In many
textbooks, the term “beam on elastic foundation” is actually
“beam on liquid foundation.”
Heavy Loads
loads, because environmentally imposed stresses must
also be accounted for when considering stationary
loads.
Negative Bending Moments
The effect of negative bending stress is somewhat more
difficult to determine. A slab on an elastic subgrade
will deform under loading, somewhat like a damped
sine curve, in which the amplitude or deformation of
successive cycles at a distance from the loading position decreases asymptotically to zero. Thus, there exists some critical aisle width where the damped sine
curve from parallel loading areas is in phase and additive. In this situation, the negative bending moment
stresses will become significant and must be considered. Therefore, allowable stationary live loads were
established to include the effects of negative moment
bending stresses. These calculations are reflected in
the tabulated values of allowable stationary live loads.
Summary
This chapter discussed how structural design is primarily controlled by the different stresses caused by
moving live loads. We learned the three classifications of military vehicles. We also studied the positive
and negative bending moments that limit stationary
live loads, then touched briefly on wall loads and the
design table for determining thickness developed by
Staab.
Wall Loads
There are situations where a wall is placed on a new
thickened slab or on an existing concrete floor slab on
grade. Walls weigh from several hundred to several
thousand pounds per linear foot. The design table used
for determining thicknesses required under walls is
developed by Staab and is based on the theory of a
beam on a liquid foundation subjected to concentrated
loads. Three loading conditions are considered: loads
at the center of the slab, loads at a joint, and loads at
the edge of the slab. The widths of thickened slabs are
developed together with the recommended transitions.
Heavy Loads
Page 91
Chapter Two:
Determination of Floor
Slab Requirements
Overview
•
•
•
•
•
Vehicular Loads
Traffic Distribution
Wall Loads
Unusual Loads
Summary
Learning Objectives
• Discuss the forklift truck traffic in terms
of maximum axle load
• Learn the six categories of forklift trucks
• Study the adequate thickness to carry wall loads
using tables
Vehicular Loads
The following traffic data is required to determine the
floor slab thickness requirements:
• Types of vehicle
• Traffic volume by vehicle type
• Wheel loads, including maximum single-axle, and
tandem-axle loading for trucks, forklift trucks, and
tracked vehicles
• The average daily volume of traffic (ADV) which,
in turn, determine the total traffic volume anticipated during the design life of the floor slab
For floor slabs, the magnitude of the axle load is of far
greater importance than the gross weight. Axle spacings generally are large enough so there is little or no
interaction between axles. Forklift truck traffic is expressed in terms of maximum axle load. Under maximum load conditions, weight carried by the drive axle
of a forklift truck is normally 87 to 94 percent of the
total gross weight of the loaded vehicle.
For tracked vehicles, the gross weight is evenly divided between two tracks, and the severity of the load can
easily be express in terms of gross weight. For moving live loads, axle loading is far more important than
the number of load repetitions. Full-scale experiments
have shown changes as little as 10 percent in the magnitude of axle loading are equivalent to changes of 300
to 400 percent in the number of load repetitions.
Traffic Distribution
To aid in evaluating traffic for the purposes of floor
slab design, typical forklift trucks have been divided
into six categories as follows:
Page 92
Forklift Truck
Category
Forklift Truck
Maximum
Axle Load, kips
Maximum Load
Capacity, kips
I
5 to 10
2 to 4
II
10 to 15
4 to 6
III
15 to 25
6 to10
IV
25 to 36
10 to 16
V
36 to 43
16 to 20
VI
43 to 120
20 to 52
Heavy Loads
When forklift trucks have axle loads less than 5
kips and the stationary live loads are less than 400
pounds per square foot, the floor slab should be designed in accordance with TM 5-809-2/AFM 88-3,
Chap.2. Vehicles other than forklift trucks, such as
conventional trucks, shall be evaluated by considering each axle as one forklift truck axle of approximate weight. For example, a three-axle truck with
axle loads of 6, 14, and 14 kips will be considered
as three forklift truck axles, one in Category I and
two in Category II. Tracked vehicles are categorized as follows:
Forklift Truck
Category
Tracked Vehicles Maximum
Gross Weight, kips
I
Less than 40
II
40 to 60
III
60 to 90
IV
90 to 120
The above equation may be used to find allowable
loads for combinations of values of s, h, and k not
given in Table 2-1. Further safety may be obtained by
reducing allowable extreme fiber stress to a smaller
percentage of the concrete flexible strength have been
presented by Grieb and Werner, Waddell and Hammitt
(see Biblio). The selection of the modulus of subgrade
reaction for use is shown in Table 2-1. The design
should be examined for the possibility of differential
settlements, which could result from nonuniform subgrade support. Also, consideration of the effects of
long-term overall settlement for stationary live loads
may be necessary for compressible soils.
Table 2-1. Maximum allowable stationary live load
Categories for tracked vehicles may be substituted for
the same category for forklift trucks.
Floor slabs on grade should have adequate structural
live loads. Since floor slabs are designed for moving
live loads, the design should be checked for stationary live loading conditions. Table 2-1 lists values for
maximum stationary live loads on floor slabs. For very
heavy stationary live loads, the floor slab thicknesses
listed in Table 2-1 will control the design. Table 2-1
was prepared using the equation
w = 257.876 s
kh
E
(eq 3-1)
Where:
• w= the maximum allowable distributed stationary
live load, pounds per square foot
• s= the allowable extreme fiber stress in tension excluding shrinkage stress and is assumed to be equal
to one-half the normal 28-day concrete flexural
strength, pounds per square inch
• k= the modulus of subgrade reaction, pounds per
cubic inch
• h= the slab thickness, inches
• E= the modulus of elasticity for the slab (assumed
to equal 4.0 x 106 pounds per square inch)
Heavy Loads
Slab
Thickness
Stationary Live Load w in lb/ft2
These Flexural Strengths of Concrete
inches h
550 lb
in2
600 lb
in2
650 lb
in2
700 lb
in2
6
868
947
1,026
1,105
7
938
1,023
1,109
1,194
8
1,003
1,094
1,185
1,276
9
1,064
1,160
1,257
1,354
10
1,121
1,223
1,325
1,427
11
1,176
1,283
1,390
1,497
12
1,228
1,340
1,452
1,563
14
1,326
1,447
1,568
1,689
16
1,418
1,547
1,676
1,805
18
1,504
1,641
1,778
1,915
20
1,586
1,730
1,874
2,018
Note: Stationary live loads tabulated above are based
on modulus of subgrade reaction (k) of 100 lb/in2.
Maximum allowable stationary live loads for other
moduli of subgrade reaction will be computed by multiplying the above-tabulated loads by a constant facPage 93
tor. Constants for other subgrade moduli are tabulated
below.
Modulus of
25
Subgrade reaction k
Constant Factor
0.5
50
100
200
300
0.7 1.0
1.4
1.7
Thickness of
Thickened
Floor
Thickness
For other modulus of subgrade reaction values the
constant values may be found from 5 100 / k .
Wall Loads
Floor slabs on grade should have adequate thickness
to carry wall loads. Tables 2-2 and 2-3 show the minimum thickness of thickened slabs for various wall
loads. The equations used to compute these values are
included in appendix A. When slab thickness required
for wall loads exceeds that required for moving live
loads, the slab will be thickened in accordance with
figure 2-1. The safety factor for the design was considered by using a reduced allowable tensile stress of
concrete, Ot. Which was computed using the equation
Ot= 1.6 f ' c ' , where f ' c is the ultimate compressive
strength of the concrete. If wall loads exceed the tabulated values shown in Table 2-2, separate wall footings
are suggested. Figure 2-4 shows the widths of thickened slabs when the interior wall loads are near the
slab center. A recommended transition is also shown.
The thickened slab width is determined by the same
theory as the wall loads. The slab under the wall is
widened to the point where the stress in the thinner
slab section does not exceed the allowable tensile
stress of 1.6 f ' c ' . Figure 2-2 shows a slab loaded
near a keyed or doweled edge. Figure 2-3 shows a
recommended slab thickening for a slab loaded near
a free edge. The width of the thickened edge varies
depending upon the width of the wall.
Slab Line Load Capacity, P,
(lb/lin ft) Flexural Strengtha
of Concrete (lb/in2)
Slab, te, (inches)
550
600
650
700
4
425
455
485
510
5
565
600
640
675
6
710
755
805
850
7
860
920
975
1,030
8
1,015
1,080
1,150
1,215
9
1,175
1,255
1,330
1,410
10
1,340
1,430
1,520
1,605
14
1,326
1,447
1,568
1,689
16
1,418
1,547
1,676
1,805
18
1,504
1,641
1,778
1,915
20
1,586
1,730
1,874
2,018
Note:The allowable wall loads are based on a modulus of subgrade reaction (k) of 100 pounds per cubic
inch. The thickness of the thickened slab will be computed by multiplying the above thicknesses by a constant factor. Constants for other subgrade moduli are
tabulated below.
Modulus of
25
50
100
200
300
Subgrade reaction k
Constant Factor
1.3
1.1
1.0
0.9
0.8
For other modulus of subgrade reaction values the
constant values may be found from 5 100 / k .
For this application the flexural strength of concrete
was assumed equal to 9 f ' c where f ' c is the specified compressive strength of concrete (lb/in2).
a
Table 2-2. Minimum thickness of thickened floor slab for
wall load near center of slab or near keyed or doweled joint
Page 94
Heavy Loads
Table 2-3. Maximum allowable wall load near free edge
Thickness of
Thickened
Slab Line Load Capacity, P,
(lb/lin ft) Flexural Strengtha
of Concrete (lb/in2)
Slab, te, (inches)
500
600
650
700
4
330
355
375
395
5
435
465
495
525
6
550
585
620
660
7
665
710
755
800
8
785
840
890
945
9
910
975
1,035
1,090
10
1,040
1,110
1,180
1,245
Figure 2-4 Widths of thickened slabs and slab edge conditions under wall loads
Heavy Loads
Unusual Loads
Information regarding floor slab requirements for special purpose ordnance, engineer, or transport vehicles
producing loads significantly greater than those defined herein should be requested from Headquarters,
Department of the Army (HQDA) (DAEN-ECE-G)
Washington, DC 20314-1000 or Headquarters, Air
Force Engineering and Services Center (DEMP), Tyndall MB, Fla. 32403.
Summary
Chapter Two explains the traffic data used to determine the floor slab thickness and the importance of
the magnitude of the axle, load rather than the gross
weight. We learned the six categories of forklift trucks
for the purpose of floor slab design. The equation was
provided for finding the maximum allowable stationary live loads and explained in table 2-1. We also studied the equation for the safety factor of the design by
using a reduced allowable tensile stress. In conclusion
of the chapter ,charts and figures were provided for
examples for better understanding.
Page 95
Chapter Three:
Site Investigation
Overview
•
•
•
•
•
Introduction
Subgrade Conditions
Environmental Conditions
Concrete Strength
Summary
Learning Objectives
• Learn what existing conditions must be
investigated for a site
• Understand the importance of subgrade conditions
to provide maximum support
• Examine the environmental conditions that affect
site conditions
• Study concrete strength to provide high wear
resistance
Introduction
Once the floor slab load capacity requirements have
been established, an investigation of the existing conditions at the site must be made. Conditions to be considered include an investigation of the subgrade, climatic conditions, the need for and availability of base
course materials, and the concrete strength properties
likely to be encountered in the locale.
Subgrade Conditions
Importance of Subgrade Conditions
The subgrade provides a foundation for supporting the
floor slab and base courses. As a result, the required
floor slab thickness and the performance obtained
from the floor slab during its design life will depend,
in a large part, on the uniformity and bearing capacity
of the subgrade. It is desirable, if economically feasible, to thoroughly investigate the subgrade to assess
the maximum support potential for the particular subgrade. In unheated structures, the possibility of frost
heave emphasizes the importance of uniformity of soil
conditions under the floor slab.
Initial Investigation
Preliminary investigations of subgrade conditions at
the site of proposed construction should be performed
to determine the engineering characteristics of the subgrade soils and the extent of any peculiarities of the
proposed site. The general suitability of the subgrade
soils is to be based on classification of the soil, moisture density relationships, expansive characteristics,
susceptibility to pumping, and susceptibility to detrimental frost action. A careful study of the service history of existing floor slabs on similar subgrade materials in the locality of the proposed site should be made.
Factors such as ground water, surface infiltration, soil
capillarity, topography, rainfall, drainage conditions,
and the seasonal change of such factors also may affect the support rendered by the subgrade.
Exploration and Classification
If field reconnaissance and analysis of existing subsurface information are insufficient to provide the necessary data for floor-slab design, an exploration program
should be initiated according to provisions of TM 5-81
8-1/ AFM 88-3, Chap. 7. All soils should be classiPage 96
Heavy Loads
fied in accordance with MIL-STD-619. Sufficient investigations should be performed at the proposed site
to facilitate the classification of all soils that will be
used or removed during construction; other pertinent
descriptive information should also be included.
Performance Data
For the design of rigid floor slabs in areas where
no previous experience regarding floor slab performance is available, the modulus of subgrade reaction
k to be used for design purposes is determined by the
field plate-bearing test. A description of the procedure to be followed for this test and the method for
evaluating test results are given in MIL-STD-621.
Where performance data from existing floor slabs on
grade are available, adequate values for k usually can
be estimated on the basis of soil type, drainage conditions, and frost conditions that prevail at the proposed site. Table 3-1 lists typical values of modulus
subgrade reaction for various soil types and moisture
contents. Values shown may be increased slightly if
the density is greater than 95 percent maximum CE
55 density, except a maximum of 500 pounds per cubic inch will be used for design. These values should
be considered as a guide only, and their use in lieu of
Table 3-1. Typical values
of modulus of subgrade reaction
the field plate bearing test is left to the discretion of
the engineer. The fact that the materials are shown in
the table does not indicate suitability for use. Suitability must be determined for the particular job conditions.
Environmental Conditions
Freezing and Thawing
Special additional design consideration and measures
are necessary where freezing and thawing may occur
in underlying soils. The effects of such occurrences, which are termed “frost action,” include surface
heaving during freezing and loss of bearing capacity
upon thawing. Detrimental frost action is the result of
the development and/or thawing of segregated ice in
underlying soils. Potential difficulties from frost action exist whenever a source of water is available to
a frost-susceptible soil, which is subject to subfreezing temperatures during a portion of the year. Conditions necessary for the development of ice segregation
process and the detrimental effects of frost action are
given in TM 5-818-2/AFM 88-6, Chap. 4.
Modulus of Subgrade Reaction, k, in lb/in2 For Moisture Contents of
1
5
9
13
17
21
25
to
to
to
to
to
to
to
Over
Types of Materials
4%
8%
12%
16%
20%
24%
28%
29%
Silts and clays
Liquids limit > 50 (OH, CH, MH)
--
175
150
125
100
75
50
25
Silts and clays
Liquids limit < 50 (OL, CL, ML)
--
200
175
150
125
100
75
50
Silty and clayey Sand
(SM & SC)
300
250
225
200
150
--
--
--
Gravelly sands
(SW & SP)
300+
300
250
--
--
--
--
--
Silty and clayey Gravels
(GM & GC)
300+
300+
300
250
--
--
--
--
Gravel and sandy Gravels
(GW & GP)
300+
300+
--
--
--
--
--
--
Note: k values shown are typical for materials having dry densities equal to 90 to 95 percent of the maximum CE 55
density. For materials having dry densities less than 90 percent of maximum CE 55 density, values should be reduced by
50 lb/in2, except that a k of 25 lb/in2 will be the minimum used for design.
Page 97
Heavy Loads
Cold Storage Facilities
A somewhat different problem is encountered in cold
storage facilities where a structure in contact with
the ground is maintained at subfreezing temperature.
Thus, frost action under such structures is a longterm rather than a seasonal phenomenon, and deep
frost penetration will eventually result, even in areas where subfreezing ground temperatures are not
naturally experienced, unless insulation or provisions for circulation of warm air beneath the slab are
provided in design. Recommended as a reference is
American Society of Heating, Refrigerating, and AirConditioning Engineering ASHRAE Handbook and
Product Directory, Equipment, and Applications. It
should be kept in mind that insulation may merely
slow frost penetration. It does not prevent heat flow.
Permafrost
Since construction alters the existing thermal regime
in the ground, an additional problem is encountered
in regions where heat flow from the facility may result in the progressive thawing of perennially frozen
ground (permafrost). Thermal degradation of permafrost, which contains masses of ice, will result in
subsidence ,as well as reduction in bearing capacity.
Both may be severe. The most widely employed,
effective, and economical means of maintaining a
stable thermal regime in permafrost under slabs-ongrade is by means of a ventilated foundation. Provision is made for ducted circulation of cold winter air
between the insulated floor and underlying ground.
The air circulation serves to carry away the heat
both from the foundation and the overlying building,
freezing back the upper layers of soil which were
thawed the preceding summer. The characteristics of
permafrost and engineering principles in permafrost
regions are described in TM-5-852-1/AFM 88-19,
Chap. 1, and TM 5-852-4.
Concrete Strength
General
For a given water-cement ratio, the concrete strength
likely to be obtained in a given locale depends primarily on the aggregate sources available. Maximum
particle size and quality of coarse aggregate will have
a pronounced effect on concrete strength, as will the
gradation of the blended coarse and fine aggregate.
In general, aggregates of the bank run in variety, as
opposed to crushed aggregates, which will produce a
lower-strength concrete due to particle shape. Specified concrete strength should be sufficient to provide
high wear resistance properties, constructability, and
a reasonably high flexural stress to attain the greatest
economy in the design. A study should be made of the
strengths likely to be encountered, since specifying an
unusually high-strength concrete mix may result in a
higher material cost for the project.
Traffic Types
The minimum concrete compressive strength for
floors subjected to pneumatic tired traffic will be
4,000 pounds per square inch; for floors subjected to
abrasive traffic such as steel wheels, the minimum
concrete compressive strength will be 5,000 pounds
per square inch.
Summary
In this chapter we examined the conditions that need
to be considered when investigating a site. These conditions included subgrade, climatic conditions, the
availability of base course materials, and the concrete
strength properties as discussed.
Applicable Technical Manuals
Where freezing and/or thawing may occur in underlying soils, slab design will be in accourdance, as applicable, with TM 5-818-2/AFM 88-6, Chap. 4 and
TM 5-852-4. Thermal computation procedures are
detailed in TM 5-852-6/ AFM 88-19, Chap. 6.
Page 98
Heavy Loads
Chapter Four:
Design Procedure
Overview
•
•
•
•
•
•
•
•
•
Introduction
Floor Slab Types
Subgrade
Steel Reinforcement
Reinforced Design
Joint Types and Usage
Floor Slab Geometry
Fiber Reinforced Design
Summary
Learning Objectives
• Study traffic loads in terms of equivalent operations of basic axle loading
• Examine the design procedures for stabilizing
foundations
• Identify the portions of the subgrade and what is
needed to improve the foundation
• Distinguish the advantages of using steel reinforcement
• Learn the design procedure for reinforced concrete
floor slabs
• Discuss the three general joint types and their purpose
• Understand joint spacing throughout any paved area
• Examine design examples for better understanding
Heavy Loads
Introduction
Once the floor-slab design requirements have been
established, i.e., the type of loadings, including wall
loads, and both stationary live and moving live loads,
the requirements are translated into meaningful design data. These design data are then compared with
the existing condition data, and a floor slab design is
evolved. The design procedure covers subgrade conditions, steel reinforcing, and various details such as
jointing.
Floor Slab Loads
Traffic Loadings
In order to satisfy requirements of different types of
vehicles and traffic volumes, all Category I, II, and
III traffic has been expressed in terms of equivalent
operations of a basic axle loading. The basic loading was assumed to be an 18,000- pound single-axle load, with two sets of dual wheels spaced 58-1/2
inches apart, with 13-1/2 inches between dual wheels.
It should be noted the basic loading was arbitrarily selected to prove a reasonable spread in the loadings and
traffic volumes likely to be encountered under normal
conditions. A design index (DI) was devised, which
expresses varying axle loads and traffic volume, in
terms of relative severity. The DI ranges from 1 to 10,
with the higher number indicating a more severe design requirement. The basic loading described above
was used to assign and rank the DI’s. More information concerning the DI can be found in TM 5-822-6/
AFM 88-7, Chap. 1. Table 4-1 shows the DI’s for various traffic volumes. Thickness requirements for floor
slabs, which contain only temperature reinforcement
for the ten DI’s, are shown in figure 4-1. The floorslab thickness requirements are a function of concrete
strength, subgrade modulus and DI. Larger forklifts,
having axle loads greater than 25 kips, are treated
separately. The required slab thickness for pavements
designed for these loads are not significantly affected
by vehicles having axle loads less than 25 kips (trucks,
cars, buses, and small forklifts). These light loads are,
therefore, ignored in determining requirements for
pavements carrying axle loads greater than 25 kips.
The thickness requirements for these loads are shown
in figure 4-2.
Page 99
Table 4-1. Traffic categories for design index
Figure 4-2. Design curves for concrete floor slabs for
heavy forklifts
Maximum
Operations
Per day
Over 25
Years
Load
Design
Index
50
10-kip axle-load forklift truck
4
250
10
5
250
100
7
250
5
8
Figure 4-1. Design curves for concrete floor slabs by design index
Stationary live loads are expressed in terms of maximum allowable pounds per square foot. These loadings are given in Table 2-1. The method used to determine the allowable loads is based on the concrete
flexural strength, the slab thickness, and the modulus
of subgrade reaction. Entering Table 2-1 with the flexural strength and the slab thickness, the allowable stationary live load can be selected. Based on the modulus of subgrade reaction, the load is adjusted using the
constant factor given in the note (Table 2-1).
Wall Loads
Stationary-partition loads are expressed in terms of
pounds per linear foot. These loadings are given in
Table 2-2. The method used to determine thickness,
tc , of the thickened floor slab is based on the concrete flexural strength, the load, and the modulus of
subgrade reaction. Entering Table 2-2 with the flexural strength of the concrete and the load, the concrete
thickness is selected, based on a modulus of subgrade
reaction of 100 pci. The thickness is adjusted using the
constant factor given in the note (Table 2-2), for other
subgrade module.
Design Procedures for Stabilized Foundations
Soil stabilization or modification. Soils that have been
treated with additives, such as cement, lime, fly ash, or
bitumen, are considered to be either stabilized or modified. A stabilized soil is one that shows improvement
in load-carrying capability and durability characteristics. A modified soil is one that shows improvement
in its construction characteristics, but which does not
show an increase in the strength of the soil sufficiently
to qualify as a stabilized soil. The principal benefits
Page 100
Heavy Loads
of soil modification or stabilization include a stable,
all-weather construction platform and a reduction of
rigid pavement thickness requirements when applicable, swell potential, and susceptibility to pumping and
strength loss due to moisture.
Requirements. The design of the stabilized or modified
layers will follow TM 5-822- 4, and TM 5-818-2/AFM
88-6, Chap. 4. To qualify as a stabilized layer, the stabilized material must meet the unconfined compressive
strength and durability requirements in TM 5-882-4;
otherwise, the layer is considered to be modified.
Thickness design. The thickness requirements for a
rigid pavement on a modified soil foundation will be
designed as if the layer is unbounded using the k value
measured on top of the modified soil layer. For stabilized soil layers, the treated layer will be considered to
be a low-strength base pavement and the thickness determined using the following modified partially bonded rigid overlay pavement design in equation 4-1:
(eq 4-1)
Where:
• ho = thickness of rigid pavements overlay required
over the stabilized layer, inches
• h = thickness of rigid pavement from design chart
(fig. 4.1) based on k value of unbound material,
inches
• Ef = flexural modules of elasticity (as determine by
ASTM C 78)
• hs = thickness of stabilized layer, inches
Subgrade
Compaction
Compaction improves stability of most subgrade soils
and provides a more uniform foundation for the floor
slabs or base course. Method 100 of MLD-STD-621,
Compaction Efforts CE 55, should be used to determine the compaction characteristics of the subgrade
soils. During construction, prolonged exposure of the
subgrade to the atmosphere may allow over wetting or
and drying, therefore, should not be allowed.
Heavy Loads
Cut Sections
With the exception of areas of special soil, the top 6
inches of subgrade in cut sections should be scarified
and moistened to approximately optimum moisture
content, then compacted. Cohesive subgrade soils
should be compacted to minimum of 90 percents of
CE 55 maximum density and cohesionless soils to a
minimum percent of CE 55 maximum density.
Fill Sections
With the exception of fill composed of special soils, all
fills composed of cohesive materials should be compacted to minimum of 90 percent of CE55 maximum,
and all fills composed to a minimum of cohesionless
materials should be compacted to a minimum of 95
percent of CE 55 maximum density. Some adjustment
for compaction requirements may be necessary for
fills of expansive soils.
Cut-to-Fill Sections
When rigid floor slab is located partially on a fill area
and partially on a cut area, the compaction requirements set forth in the preceding paragraphs should be
followed. The depth of subgrade compaction in the
cur area should be increased to 12 inches.
Non-Uniformity
Where it is not possible to create uniform subgrade
conditions by the methods described herein, the slab
design can be varied throughout the project to maximize economy. Concrete flexural strength, percent reinforcing steel, and slab thickness can all be adjusted
to provide a design which is balanced in terms of service life. The specific combinations to be used will
depend upon local conditions and costs. Selection of
design alternatives is left to the discretion of the design engineer.
Special Soils
Although compaction increases the stability and
strength of most soils, some soil types show a marked
decrease in stability when scarified, worked, and
rolled. Also, there are some solids that shrink excessively during dry periods and expand excessively
when allowed to absorb moisture. In general, these are
inorganic clays of relatively high plasticity, usually
classified as CH soils. Special types of soils are discussed in TM 5-825-2/AFM 88-6, Chap. 2. TM 5-8181/AFM 88-3 Chap. 7, and TM 5-818-7.
Page 101
Back Filling
Special care should be exercised in the backfill area
around walls and columns to ensure compliance with
compaction requirements as outlined in the above paragraphs. Backfilling around walls and columns should
be performed with pneumatic tampers, gasoline-powered tampers, and other mechanized hand-operated devices. Soil moisture content and lift thickness should
be carefully controlled to ensure compaction requirements are met though the full depth of the backfill.
Treatment of Unsuitable Materials
Soils designated as unsatisfactory for subgrade use
MIL-STD-619 should be removed and replaced. The
depth to which such undesirable soils should be removed depends on the soil type, drainage conditions,
type of material stored, magnitude of tolerable differential settlement, and depth of freezing-temperature
penetration. The depth of removal and replacement
should be determined by the engineer on the basis of
judgment and previous experience, with due consideration of the traffic to be served as well as the costs
involved. In some instances, unsatisfactory or undesirable soils may be improved economically by stabilization with such materials as cement, fly ash, lime, or
certain chemical additives whereby the characteristics
of the composite material become suitable for use as
subgrade. Criteria for soil stabilization are given in
TM 5-822-4. Subgrade stabilization, however, should
not be attempted unless the cost reflects corresponding
savings in base course, floor slabs, or drainage facilities construction, and is approved by HQDA (DAENECE-G) Washington, DC 2031 4-1 000 or Headquarters, Air Force Engineering Services Center (DEMP),
Tyndall AFB, Fla. 32403 5-4.
Base courses
Requirements
Base courses may be required under rigid floor slabs
to provide protection against detrimental frost action, drainage, a suitable working platform for the
construction operation during adverse weather conditions, and additional support to the floor slab. In any of
the abovementioned applications for base courses, an
economic study is required to determine base course
requirements in floor-slab design. The economic study
will typically include costs of base course materials
Page 102
such as hauling and required floor-slab thickness with
and without base course.
Consideration should also be given to the use of the
floor slab, i.e., what material is to be stored and what
operations are likely to occur on the floor slab. These
considerations will also have an impact on whether to
include a base course.
Compaction
Where base courses are used, the base-course materials should be compacted in accordance with the criteria given above. With this in mind, compaction of
thin base courses placed on yielding subgrades to high
densities is difficult.
Drainage
Adverse moisture conditions resulting from high water
table and subsoils subject to capillary action may cause
damage to floor covering and stored material. If the subgrade soils provide for movement of water by capillary
flow (CH, CL, MH, and ML types) and the ground-water table is less than 5 feet from the final grade, a minimum thickness of 6 inches of free-draining base course
will be required. Base courses for drainage will not be
required under conditions of deep ground-water table.
Positive drainage is to be provided to ensure against
water being trapped beneath the pavement. The floor
should be protected against the migration of water vapor through the slab and joints. Water vapor damage is
to be prevented by an impermeable membrane, placed
on the subgrade prior to concrete placement. Such vapor barriers shall be installed, even in conjunction with
base courses, if moisture susceptible floor coverings or
conduits are present.
See TM 5-809-2/AFM 88-3, Chap. 2 for embedment
of conduits.
Materials
If conditions indicate a base course is desirable, a
thorough investigation should be made to determine
the source, quantity, and characteristics of the available materials. A study should be made to determine
the most economical thickness of material for a base
course that will meet the requirements. The base
course may consist of natural materials, processed
materials, or stabilized materials as defined in TM
Heavy Loads
5-882-4. The material selected should be the one that
best accomplishes the intended purpose of the base
course. In general, the base course material should be
well-graded high-stability material. TM 5-822-6/AFM
88-7, Chap. 1 and TM 5-818-2/AFM 88-6, Chap. 4
provided requirement for base courses for additional
support and frost action. If the base course is for drainage, the maximum particle size shall be 1-1/2 inches,
and no particles shall be smaller than the No. 4 sieve
size. If a free-draining, open graded sub-base is used, a
filter layer may be placed under the base course to prevent pumping action and subgrade intrusion. Coarse
aggregate shall have a percentage of wear by the Los
Angeles abrasion test of not more than 50. Uniform,
high-quality materials shall be used. Weakly cemented
rocks and most shale should not be used; an exception
would be baked shales occurring adjacent to intrusive
dikes. The frost susceptibility criterion listed previously in Chapter 3 is also applicable to base course
materials. Durability will be checked if the base aggregate will be exposed to frost. Aggregates that break
down excessively when subjected to freeze-thaw cycles will not be used.
Economic Considerations
For the general case, reinforced, rigid pavements will
not be economically competitive with nonreinforced
rigid pavements of equal load-carrying capacity, even
though a reduction in pavement thickness is possible.
Alternate bids, however, should be invited if reasonable doubt exists on this point.
Nonreinforced Slabs
On otherwise nonreinforced floor slabs, steel reinforcement should be used for the conditions below
Odd-Shaped Slabs Odd-shaped slabs should be reinforced using a minimum of 0.06 percent steel, in
directions normal to each other over the entire area
of the slab. An odd-shaped slab ,is considered to be
one in which the longer dimension exceeds the shorter
dimension by more than 25 percent, or a slab which
essentially is neither square nor rectangular. Figure
4.3 presents an example of reinforcement required in
odd-shaped slabs.
Figure 4-3. Reinforcement for odd-shaped slabs
Steel Reinforcement
Under certain conditions, concrete pavement slabs
may be reinforced with welded wire fabric or deformed
bar mats, arranged in a square or rectangular grid. The
advantages in using steel reinforcement include: (a)
a reduction in the required slab thickness usually is
permissible; (b) wider spacing between transverse
contraction joints may be used; (c) the width of crack
opening is controlled with the result that load transmission is maintained at a high level at these points,
and objectable material is prevented from infiltrating
the cracks; and (d) differential settlement due to nonuniform support or frost heave is reduced materially.
Guidance relative to the use of reinforced pavement is
discussed in the following paragraphs.
Subgrade Conditions
Reinforcement may be used to control cracking in rigid pavements found on subgrades, where differential
vertical movement is definite potential (for example,
foundations with definite or borderline frost susceptibility that cannot feasibly be made to conform to conventional frost design requirements.)
Heavy Loads
Mismatched Joints
A partial reinforcement of slab is required where the joint
patterns of abutting or adjacent floor slabs do not match,
and when the pavements are not positively separated by
Page 103
an expansion of slip-type joint. The floor slab directly
opposite the mismatched joint should be reinforced with
a minimum of 0.06 percent of steel in directions normal to each other for a distance of 3 feet back from the
juncture, and for the full width or length of the slab in a
direction normal to the mismatched joint. Mismatched
joints normally will occur at intersections of floor slabs
or between regular floor slab and fillet areas. (Fig 4-3).
Other Uses
Reinforced and continuously reinforced floor slabs may
be considered for reasons other than those describe
above, provided a report containing a justification of
the need for reinforcement is prepared and submit for
approval to HQDA (DAEN-ECE-G), Washington, DC
20314-1 000, or Headquarters, Air Force Engineering
and Services Center (DEMP), Tyndall AFB, Fla. 32403.
Reinforced Design
Thickness Design on Unbonded Base or Subbase
The design procedure for reinforced concrete floor slabs
uses the principle of allowing a reduction in the required
thickness of nonreinforced concrete floor slab, due to the
presence of the steel reinforcing. The design procedure
has been developed empirically from a limited number
of prototype test pavements subjected to accelerated traffic testing. Although it is anticipated some cracking will
occur in the floor slab under the design traffic loadings,
the steel reinforcing will hold the cracks tightly closed.
The reinforcing will prevent spalling or faulting at the
cracks and provide a serviceable floor slab during the anticipated design life.
then used to enter the nomogram in Figure 4-4. A straight
line is then drawn from the value of h to the value selected for the thickness of reinforced floor slab, hr, and
extended to the required percentage of reinforcing steel,
S, or drawn from the value h to the value selected for
the percentage of reinforcing steel, and extended to the
thickness, hr. The thickness, hr, will always be equal to
or less than the thickness, h. It should be noted that the
S value indicated in Figure 4-4 is the percentage to be
used in the longitudinal direction only, for normal designs, the percentage of non- reinforcing steel used in the
transverse direction only.
For normal designs, the percentage of non reinforcing
steel used in the transverse direction will be one-half of
that to be used in the longitudinal direction. Once the hr
and S values have been determined, the maximum allowable slab length L is obtained from the intersection
of the straight line and the scale of L. Provision also is
made in the nomograph for adjusting L on the basis of
the yield strength fs of the reinforcing steel. Difficulties
may be encountered in sealing joints between very long
slabs, due to large volumetric changes caused by temperature changes.
Figure 4-4. Design thickness for reinforced floor slabs
Essentially, the design method consists of determining
the percentage of steel required, the thickness of the reinforced floor slab, and the maximum allowable length
of the slabs. Figure 4-4 presents a graphic solution for
the design of reinforced floor slabs. Since the thickness
of the reinforced floor slab is a function of the percentage of steel reinforcing, the designer may determine the
required percentage of steel for a predetermined thickness of floor slab or determine the required thickness of
floor slab for a predetermined percentage of steel. In either case, it is necessary first to determine the required
thickness of nonreinforced floor slab by the method outlined previously for non reinforced floor slabs. The exact
thickness (to the nearest 1/10 inch) of the floor slab, h, is
Page 104
Heavy Loads
Thickness Design on Stabilized Base
or Subgrade
To determine the thickness requirements for reinforced concrete floors slabs on a stabilized foundation, it is first necessary to determine the thickness
of nonreinforced concrete floor slab required for
the design conditions. Figure 4-4 is entered with the
values h, hr, and S.
Limitations
The design criteria for reinforced concrete floor
slabs on grade are subject to the following limitations:
• No reduction in the required thickness of non
-reinforced floor slabs should be allowed for percentages of steel less than 0.05 percent.
• No further reduction in the required thickness
of non-reinforced floor slabs should be allowed
over that indicated in Figure 4-4 for 0.50 percent
steel, regardless of the percentage of steel used.
• The maximum length L of reinforced floor slabs
should not exceed 75 feet, regardless of the percentage of steel, yield strength of the steel, or
thickness of the pavement.
• The minimum thickness of reinforced floor slabs
should be 6 inches.
Reinforcing Steel
Type
The reinforcing steel for floor slabs may be either
deformed bars or welded wire fabric. Specifications
for both types of reinforcement at given in TM
5-825-3/AFM 88-6, Chap. 3.
Placement
Placement of the reinforcing steel in floor slabs
should follow the criteria given in TM 5-825-3,
AFM 88-6 Chap. 3. In addition, the following criteria regarding the maximum spacing of the reinforcement should be observed. For welded wire fabric, the maximum spacing of the longitudinal wires
and transverse wires should not exceed 6 inches and
12 inches, respectively; for bar mats, the maximum
spacing of the longitudinal bars and the transverse
bars should not exceed 15 inches and 30 inches, respectively.
Heavy Loads
Joint Types and Usage
Joints are provided to; permit contraction and expansion of the concrete, resulting from temperature
and moisture changes; to relieve warping and curling stresses due to temperature and moisture differentials; to prevent unsightly, irregular breaking of
the floor slab; as a construction expedient, to separate sections or strips of concrete placed at different
times; and to isolate the floor slab from other building components. The three general types of joints
are contraction, construction, and isolation. A typical floor-slab joint layout is shown in figure 5-5.
Figure 4-5. Typical floor slab joint layout
Contraction Joints
Weakened-plane contraction joints are provided to
control cracking in the concrete, to limit curling or
warping stresses resulting from drying shrinkage and
contraction and from temperature and moisture gradients in the slab, respectively. Shrinkage and contraction of the concrete causes slight cracking and separation of the slabs at the weakened planes, which will
provide some relief from tensile forces, resulting from
foundation restraint and compressive forces, caused
by subsequent expansion. Contraction joints will be
required transversely and may required longitudinally,
depending upon slab thickness and spacing of construction joints. Contraction joints for reinforced and
non reinforced floor slabs are shown in Figures 4-6 and
4-7, respectively. Instructions regarding the use of saw
cuts or preformed inserts to form the weakened plane
are contained in TM 5-822-7/AFM 88-6, Chap. 8.
Page 105
Figure 4-6. Contraction joints for reinforced and non reinforced floor slabs
Figure 4-7. Joint sealant details
cause the concrete to crack under the tensile stresses, resulting from the shrinkage and contraction of
the concrete as it cures. Experience, supported by
analysis, indicates this depth should be at least ¼ of
the slab thickness for floor slabs 12 inches or less,
3 inches for pavements greater than 18 inches in
thickness. In no case will the depth of the groove
be less than the maximum nominal size of aggregate used. Saw cut contraction joints for steel-fiber
reinforced concrete should be cut a minimum of 1/3
of the slab thickness. Concrete placement conditions may influence the fracturing of the concrete
and dictate the depth of groove required. For example, concrete placed early in the day, when the
air temperature is rising, may experience expansion
rather than contraction during the early life of the
concrete, with subsequent contraction occurring
several hours later as the air temperature drops.
The concrete may have attained sufficient strength
before the contraction occurs, so that each successive weakened plane does not result in fracturing
of the concrete. As a result, excessive opening may
result where fracturing does occur. To prevent this,
the depth of the groove will be increased to ensure
the fracturing and proper functions of each of the
scheduled joints.
Width and Depth of Sealant Reservoir
The width and depth of the sealant reservoir for the
weakened plane groove will conform to dimensions
shown in Figure 4-8. The dimensions of the sealant
reservoir are critical to satisfactory performance of
the joint sealing materials.
Figure 4-8. Contraction joint details
Width and Depth of Weakened Plane Groove
The width of the weakened plane groove will be
a minimum of 1/8 inch and a maximum equal to
the width of the sealant reservoir. The depth of the
weakened plane groove must be great enough to
Page 106
Heavy Loads
Spacing of Transverse Contraction Joints
Transverse contraction joints will be constructed
across each paving lane, perpendicular to the center
line. The joint spacing will be uniform throughout
any major paved area, and each joint will be straight
and continuous from edge to edge of the paving
lane, and across all paving lanes for the full width
of the paved area. Staggering of joints in adjacent
paving lanes can lead to sympatric cracking and
will not be permitted unless reinforcement is used.
The maximum spacing of transverse joints that will
effectively control cracking will vary appreciably
depending on pavement thickness, thermal coefficient, and other characteristics of the aggregate and
concrete, climatic conditions, and foundation restraint. It is impractical to establish limits on joint
spacing that are suitable for all conditions without,
making them unduly restrictive. For best slab performance, the number of joints should be kept to a
minimum by using the greatest joint spacing that
will satisfactorily control cracking. Experience has
shown, however, that oblong slabs, especially in
thin slabs, tend to crack into smaller slabs of nearly
equal dimensions under traffic. Therefore, it is desirable, insofar as practical, to keep the length and
width dimensions as nearly equal as possible. In no
case should the length dimension (in the direction
of paving) exceed the width dimension more than
25 percent.
Non Reinforced Slabs
The joint spacings in Table 4-2 have given satisfactory control of transverse cracking, in most instances, and may be used as a guide, subject to modification, based on available information regarding the
performance of existing pavements in the vicinity
or unusual properties of the concrete. The maximum size of a slab panel bounded by contraction
joints should be 600 square feet, in accordance with
TM 5-809-2/AFM 88-3, Chap. 2. Under certain
climatic conditions, joint spacings different from
those in Table 4-2, may be satisfactory. Where it is
desired to change the joint spacing, a request will
be submitted to HQDA (DAEN-ECE-G), Washington, DC 20314-1 000, or Headquarters, Air Force
Engineering and Services Center (DEMP), Tyndall
AFB, Fla. 32403, outlining the local conditions that
indicate that the proposed change in joint spacing is
desirable.
Heavy Loads
Table 4-2. Recommended spacing of transverse contraction joints
Slab Thickness
in.
Joint Spacing
ft.
4-6*
Up to 12.5
6-9
12.5-15.0
9-12
15.0-20.0
>12
20.0-25.0
*This thickness is allowed for steel-fiber reinforced
concrete only.
Reinforced Slabs
Transverse contraction joints in reinforced concrete
slabs should not be constructed at intervals of less
than 25 feet, nor more than 75 feet. Maximum allowable slab width and length may be determined from
the equation
(eq 4-2)
Where
• L = the maximum length (or width), feet
• hr = the reinforced slab thickness, inches
• fs = the steel yield strength, pounds per square inch
• S = steel reinforcing ration, percentage
Allowable slab dimensions can be determined directly from Figure 4-4 for yield strength of 60,000
pounds per square inch. Selection of final spacing
should be based on local conditions. Where only
a portion of the slabs are reinforced, joint spacing
should be a maximum commensurate with the unreinforced slab configurations.
Spacing of Longitudinal Contraction Joints
Contraction joints will be placed along the centerline of paving lanes that have a width greater than
the indicated maximum spacing of transverse contraction joints in Table 4-2. These joints may also be
required in the longitudinal direction for overlays,
regardless of overlay thickness, to match joints existing in the base pavement, unless a bond breaking
medium is used between the overlay and base slab
or the overlay slab is reinforced.
Page 107
Doweled and Tied Contraction Joints
• Dowels are required in transverse contraction
joints.
• For nonreinforced slabs, deformed tie bars,
which are 5/8 inch in diameter, 30 inches long,
and space on 30-inch centers, will be required in
longitudinal contraction joints that fall 15 feet or
less from the free edge of paved areas, greater
than 100 feet in width, to prevent cumulative
opening of these joints.
Construction Joints
Construction joints are provided to separate areas of
concrete placed at different times. They may be required in both the longitudinal and transverse directions. The spacing of construction joints will depend
largely on the size and shape of the floor slab that
is being placed and the equipment used to place it.
Joint spacing will also be affected by column spacing and bay sizes. Longitudinal construction joints,
generally spaced 20 to 25 feet apart but may reach
50 feet apart, depending on construction equipment
capability, will be provided to separate successively
placed paving lanes. Transverse construction joints
will be installed when it is necessary to stop concrete placement within a paving lane for a sufficient
time for the concrete to start to set. All transverse
construction joints will be located in place of other
regularly spaced transverse joints (contraction or
isolation types). There are several types of construction joints available for use, as shown in Figures 4-9, 4-10, and 4-11 and as described below.
The selection of the type of construction joint will
depend on such factors as the concrete placement
procedure and foundation conditions.
Figure 4-10. Keyed construction joints for concrete floor
slabs
Figure 4-11. Doorway slab design for vehicular traffic
Figure 4-9. Doweled construction joints for concrete floor
slabs
Doweled Butt Joint
The doweled butt joints considered to be the best joint
for providing load transfer and maintaining slab alignment. It is a desirable joint for the most adverse conditions, such as heavy loading, high traffic intensity,
and lower strength foundations. However, because the
alignment and placement of the dowel bars are critical
to satisfactory performance, this type of joint is difficult
to construct, especially for slip-formed concrete. However, the doweled butt joint is required for all transverse
Page 108
Heavy Loads
construction joints in nonreinforced pavements. Doweled construction joints are shown in Figure 4-9.
Keyed Joint
The keyed joint is the most economical method, from a
construction standpoint, of providing load transfer in the
joint. It has been demonstrated that the key or keyway
can be satisfactorily constructed using either formed
or slip-formed methods. Experience has proved the required dimensions of the joint can best be maintained
by forming, or slip-forming, the keyway rather than the
key. The dimensions and location of the key (Figure
4-10) are critical to its performance. The structural adequacy of keyed construction joints in rigid floor slabs,
however, can be impaired seriously by such factors as
small changes in the dimensions of the key and positioning the key other than at the mid-depth of the slab.
Exceeding the design values for the key dimensions
producing an oversize key, this can result in failure of
either the top or bottom edge of the female side, or the
joint. Similarly, construction of an under size key can
result in shearing off the key. Keyed joints should not
be used in floor slabs 8 inches or less in thickness, except where tie bars are used. Tie bars in the keyed joint
will limit opening of the joint and provide some shear
transfer that will improve the performance of the keyed
joints. However, tied joints in floor widths of more than
75 feet can result in excessive stresses and cracking in
the concrete during contraction. When a longitudinal
construction joint is used at the center of a floor, two
paving lanes wide, a keyed joint with tie bars should
be used. When a keyed longitudinal structure joint is
used at the center of a floor four or more paving lanes in
width, dowel should be used.
medium strength foundations. The thickened-edge joint
may be used at free edges of paved areas to accommodate future expansion of the facility or where wheel
loadings may track the edge of the pavement. All floor
slabs accommodating vehicular traffic will be thick at
doorways to have an edge thickness of 1.25 times the
design thickness as shown in Figure 4-11. The use of
the type joint is contingent upon adequate base-course
drainage.
Isolation Joints
Isolation joints are provided to prevent load transfer and
allow for differential settlement between the floor slab
and other building components. Isolation joints also
allow for some horizontal movement. Isolation joints
should be placed at locations where slabs abutt walls or
their foundations and around columns, column foundations, and other foundations that carry permanent dead
load, other than stored material. Isolation joints are provided by placing 30-pounds asphalt, coal-tar saturated
felt, or equivalent material between the floor slab and
the building’s structural components before the floor is
placed. Such sheets should be placed or fastened to the
buildings components to prevent any bonding or direct
contact between the floor slab and the building component. This requires the sheets have a height equal to the
floor slab thickness and be placed at the same elevation
as the floor slab, as shown in Figure 4-12.
Figure 4-12 Isolation joints
Thickened-Edge Joint
Thickened-edge-type joints may be used instead of other types of joint employing load-transfer devices. When
the thickened-edge joint is constructed, the thickness
of the concrete at the edge is increased to 125 percent
of the design thickness. The thickness is then reduced
by tapering from the free-edge thickness to the design
thickness, at a distance of 5 feet from the longitudinal
edge. The thickened-edge butt joint is considered adequate for the load-induced concrete stresses. However, the inclusion of a key in the thickened-edge joint
provides some degree of load transfer in the joint and
helps maintain slab alignment; although not required,
it is recommended for pavement constructed on low to
Heavy Loads
Special Joints and Junctures
Situations will develop where special joints, or variations of the more standard type joints, will be needed
to accommodate the movements that will occur and
to provide a satisfactory operational surface. Some of
these special joints or junctures are discussed below.
Page 109
Slip-Type Joint
At the juncture of two pavement facilities, expansion
and contraction of the concrete may result in movements that occur in different directions. Such movements may create detrimental stresses within the
concrete, unless provision is made to allow the movements to occur. At such junctures, a thickened-edge
slip joint shall be used to permit the horizontal slippage to occur.
The design of the thickened-edge slip joint will be
similar to the thickened-edge construction joint (Figure 4-13). The bond-breaking medium will be either
a heavy coating of bituminous material, not less than
1/16 inch thick, when joints match, or a normal nonextruding type expansion joint material not less than
¼ inch thick, when joints do not match. The 1/16 inch
bituminous coating may be either a low penetration
(60-70 grade asphalt) or clay type asphalt base emulsion to the used for roof coating (Military Specification MIL-R-3472) and will be applied to the face of
the joint by hand brushing or spraying.
Figure 4-13. Thickened-edge joints
Special Joint between New and Existing Floors
A special thickened-edge joint design (Figure 4-13)
will be used at the juncture of new and existing floors
for the following conditions:
• When load-transfer devices (keyways or dowels)
or a thickened edge was not provided at the free
edge of the existing floor.
• When load-transfer devices or a thickened edge
was provided at the free edge of the existing floor,
but neither met the design requirements for the new
floor.
• For transverse contraction joints, when removing
and replacing slabs in an existing floor.
• For longitudinal construction joints, when removing and replacing slabs in an existing floor if the
existing load-transfer devices are damaged during
the slab removal.
• Any other location where it is necessary to provide
load transfer for the existing floor. The special joint
design may not be required if a new floor joins an
existing floor that is grossly inadequate to carry the
design load of the new floor, or if the existing floor
is in poor structural condition. If the existing floor
can carry a load that is 75 percent or less of the new
floor design load, special efforts may be omitted;
however, if omitted, accelerated failures in the existing floor may be experienced. Any load-transfer
devices in the existing floor should be used at the
juncture to provide as much support as possible to
the existing floor. The new floor will simply be designed with a thickened edge at the juncture. Drilling and grouting dowels in the existing floor for
edge support may be considered as an alternate to
the special joint; however, a thickened-edge design
will be used for the new floor at the juncture.
Doweled Joints
The primary function of the dowels in floor slabs is
that of a load-transfer device. As such, the dowels affect a reduction in the critical edge stress, directly proportional to the degree of load transfer achieved at the
joint. A secondary function of dowels is to maintain
the vertical alignment of adjacent slabs, thereby preventing faulting at the joint. Dowels are required at all
contraction joints in slabs that are 8 inches or greater
in thickness and for thinner slabs in concentrated traffic areas.
Page 110
Heavy Loads
Dowel Specifications
Figure 4-14. Joints in concrete floor slabs
Dowel diameter, length, and spacing should be in
accordance with the criteria presented in Table 4-3.
When dowels larger than 1 inch in diameter are required, an extra strength pipe may be used as an
alternate for solid bars. When an extra-strength pipe
is used for dowels, however, the pipe should be
filled with stiff mixtures of sand-asphalt or cement
mortar, or the ends of the pipe should be plugged.
If the ends of the pipe are plugged, plugs should
fit inside the pipe and be cut off flush with the end
of pipe so there will be no protruding material to
bond with the concrete and prevent free movement
of the pavement. All dowels should be straight,
smooth, and free from burrs at the ends. One-half
of each dowel should be painted and oiled or otherwise treated to prevent bonding with the concrete. A
schematic drawing of joint layout showing dowels
is given in Figure 4-14.
Pavement
Thickness
inches
Minimum
Dowel
Length
inches
Maximum
Dowel
Spacing
inches
Dowel
Diameter
and Type
Less than 8
16
12
¾ - inch bar
8 to and
including
11.5
16
12
1 – inch bar
15
1- to – 1¼ inch bar, or
1- inch extra
strength pipe
18
1 – to 1½
- inch bar,
or 1 – 1½
inch extra
strength pipe
16 to and
including
20.5
21 to and
including
25.5
Over 26
Heavy Loads
20
20
24
30
Normally, dowels should be located at the mid-depth
of the floor slab. A tolerance of one-half of the dowel
diameter, above or below mid-depth of the slab, may
be allowed in locating the dowels in contraction and
construction joints, where the allowance of such a tolerance will expedite construction.
Joint Sealing
Table 4-3 Dowel size and spacing
12 to and
including
15.5
Dowel Placements
18
2 – inch
bar, or 2 –
inch extra
strength pipe
18
3 – inch
bar, or 3 –
inch extra
strength pipe
All joints will be sealed with a suitable sealant to prevent infiltration of surface water and solid substances.
A jet-fuel resistant (JFR) sealant will be used in the
joints of floors where diesel fuel or other lubricants
may be spilled during the operation, parking, maintenance, and servicing of vehicles. Sealants that are not
fuel resistant will be used in joints of all other pavements. JFR sealants will conform to Federal Specifications SS-S-200 and SS-S-1614, and non JFR sealants
will conform to Federal Specifications 55-5-1401.
Performed sealants must have an uncompressed width
of not less than twice the width of the joint sealant
reservoir. The selection of a pourable or preformed
sealant should be based upon the economics involved.
Compression type preformed sealants are recommended when the joint spacing exceed 25 feet and are
required when joint spacings exceed 50 feet.
Special Provisions for Slip-form Paving
Provisions must be made for slip-form payers when
there is a change in longitudinal joint configuration.
The thickness may be varied without stopping the paving train, but the joint configuration cannot be varied
without modifying the side forms, which will normally require stopping the paver and installing a header.
The following requirements shall apply:
• The header may be set on either side of the transition slab with the transverse construction joint
Page 111
doweled as required. The dowel size and location
in the transverse construction joint should be commensurate with the thickness of the pavement at
the header.
• When there is a transition between a doweled longitudinal construction joint and keyed longitudinal
construction joint, the longitudinal construction
joint in the transition slab may be either keyed or
doweled. The size and location of the dowels or
keys in the transition slabs should be the same as
those in the pavement with the doweled or keyed
joint, respectively.
• When there is transition between two keyed joints
with different dimensions, the size and location of
the key in the transition slab should be based on the
thickness of the thinner pavement.
Floor Slab Geometry
Careful attention should be given to floor slab geometry to ensure proper drainage and satisfactory operations. For proper drainage of the floor slab surface into
floor drains, a fall of
inch per foot toward the floor
drain is recommended. For sustained operations, gasoline and LP gas operated forklift trucks can generally
negotiate a maximum slope of 20 percent (20 feet vertically for every 100 feet horizontally) satisfactorily.
Electric powered forklift trucks can perform sustained
operations on a maximum slope of 10 percent (10 feet
vertically for every 100 feet horizontally). The above
mentioned maximum slopes are based on a coefficient
of friction of 0.9 for the operating surface. The use
of sealants, waxes, etc., to reduce ducting will lower
the coefficient of friction considerably. In area where
these compounds are used and a tough broom finish
is not practical, reducing the slope of the ramp should
be considered. If the slope cannot be reduced, pressure sensitive abrasive tapes should be installed. The
abrasive tapes are of the type used on stairway treads
to produce a non-skid surface.
Fiber Reinforced Design
Basis of jointed fiber reinforced concrete floor slab design. The design of jointed fiber concrete (JFC) floor
slabs is based upon limiting the ratio of the concrete
flexural strength and the maximum tensile stress at
Page 112
the joint, with the load either parallel or normal to
the edge of the slab, to a value found to give satisfactory performance in full scale accelerated test tracks.
Because of the increased flexural strength and tenacity at cracks of the developing fibrous concrete, the
thickness can be significantly reduced; however, this
results in a more flexible structure, which causes an
increase vertical deflections as well as in potential for
densification and/or shear failures in the foundations,
pumping of the subgrade material, and joint deterioration. To project against these latter factors, a limiting vertical deflection criterion has been applied to the
thickness developed from the tensile stress criteria.
Uses
Although several types of fiber have been studied for
concrete reinforcement, most of the experience has
been with steel fibers and the design criteria presented
herein with steel fibrous concrete. Fibrous concrete is
relatively new material for pavement construction and
lacks a long time performance history. Because of this,
its use will require approval of HQDA (DAEN-ECEG), Washington, DC 20314-1 000, and/or Headquarters, Air Force Engineering Services Center (DEMP),
Tyndall AFB, Fla. 32403. The major uses to date have
been for thin resurfacing or strengthening overlays
where grade problems restrict the thickness of overlay
that can be used. The use of JFC floor slabs should be
based upon the economics involved.
Mixing Proportioning Considerations
The design mix proportioning of fibrous concrete will
be determined by a laboratory study. The following are
offered as guides and to establish limits where necessary for the use of the design criteria included herein.
• The criteria contained herein is based upon fibrous
concrete containing 1 to 2 percent by volume (100
pounds to 250 pounds) of steel fibers per cubic yard
of concrete, and fiber contents within this range are
recommended.
• Most experience to date has been with fibers from
1 to 1 ½ inches long, and for use of the criteria
contained herein, fiber lengths within this range are
recommended.
• For proper mixing, the maximum aspect ratio
(length to diameter or equivalent diameter) of fibers should be about 100.
• The large surface area to volume ratio of the steel
fibers requires an increase in the necessary paste
Heavy Loads
to ensure the fibers and aggregates are coated. To
accomplish this, cement contents of 750 to 900
pounds per cubic yard of concrete are recommended. The cement content may be all Portland cement
or a combination of Portland cement and up to 25
percent fly ash or other pozzolans.
• Maximum size coarse aggregates should fall between 3/8 and ¾ inch. The percent of coarse aggregate (of the total aggregate content) can vary
between 25 and 60 percent.
Figure 4-16. Design curves for fiber reinforced concrete
floor slabs for heavy forklifts
Thickness Determination
The required thickness of FJC floor slabs will be a function of the design concrete flexural strength R, modulus
of soil reaction k, the thickness hb and flexural modulus
of elasticity Efs, of stabilized material is used, the vehicle or axle gross load, the volume of traffic, the type of
traffic area, and the allowable vertical deflection. When
stabilized material is not used, the required thickness
hdf of JFC is determined directly from the appropriate
chart (Figure 4-15 and 4-16). If the base or subgrade is
stabilized meets the minimum strength requirements of
TM 5-822-4/AFM 88-17, Chap. 4, the stabilized layer
will be treated as a low strength base and the design
will be made the equation given in equation 4-2 above.
The resulting thickness, h or hdof, will be rounded up to
the nearest half or full inch. The rounded thickness, hdf
or hdof will then be checked for allowable deflection as
explained previously. The minimum thickness for JFC
floor slabs will be 4 inches.
Figure 4-15. Design curves for fiber reinforced concrete
floor slabs by design index
Heavy Loads
Allowable Deflection for JFC Pavement
The elastic deflection that JFC floor slabs experience
must be limited to prevent overstressing of the foundation material and thus premature failure of the pavement. Curves are provided (Figure 4-17) for the computation of the vertical elastic deflection that a slab
will experience when loaded. Use of the curves requires three different inputs: slab thickness, subgrade
modulus, and gross weight in determined above. The
computed vertical elastic deflection is then compared
with appropriate allowable deflections, determined
from Figure 4-18. Deflection needs to be checked for
axle loads less than 25 kips. If the computed deflection is less than the allowable deflection, the thickness
meets allowable deflection criteria and is acceptable.
If the computed deflection is larger than the allowable deflection, the thickness must be increased, or a
new design initiated with a different value for either
R or k. The process must be repeated until a thickness
based upon the limiting stress criterion will also have
a computed deflection equal to or less than the allowable value.
Page 113
Figure 4-17 Deflection curves for fiber
reinforced concrete floor slabs
Figure 4-18. Allowable deflection for jointed
fiber reinforced concrete floor slabs
Heavy Loads
Page 114
Design Examples
Example 1: Concrete slab thickness
for interior loads
a. Problem. The floor slab for a warehouse will be
designed based on the following information:
(2) One must check for adequacy of 7 inch slab for
stationary live load, w = 1 200 pounds per square foot.
Table 2-1 should be entered using 650 pounds per
square inch flexural strength concrete and 7 inch slab
thickness; allowable stationary live load is selected,
w =1,109 pounds per square inch. The w is adjusted
based on k = 150 pounds per cubic inch.
Traffic
Type of Traffic
Average Daily
Volume
Category
50
I
15
II
5-axle trucks
4 axles, 5 kips each
1 axle, 10 kios
15-kip forklift or trucks
Interior
1,200 pounds per square inch
1,400 pounds per linear foot
Material properties:
• Concrete flexural strength = 650 pounds per square
inch
• Modulus of subgrade reaction, k = 150 pounds per
cubic inch
b. Solution
(1) Floor slab thicknesses h should be determined by
using equivalent forklift truck axle load below.
Equivalent
Number Average Maximum
Forklift
Design
of
Daily
Operations
Truck Axle
Index
Axles
Volume
Per Day
Load, kips
5
4
50
200
4
10
1
50
50
4
15
1
15
15
7
Matching the axle loads and maximum operations per
day in Table 4-1, the design index for each axle-load
group is selected, as shown, in the far right column in
the above-mentioned table. Design index 7 is selected
for the design. From figure 4-1, using k = 150 pounds
per cubic inch and 650 pounds per square inch flexural
strength, slab thickness equal to 6.7 inches, and round
to 7 inches should be selected.
Heavy Loads
(3) Thickness, tc , of thickened floor slab supporting
interior wall weighing 1,400 pounds per linear c foot
should be determined by entering table 2-2 using 650
pounds per square inch flexural strength concrete and
wall load p = 1,400 pounds per linear foot. Thus, tc
equals 10 inches, and tc is adjusted based on k = 150
pounds per square inch.
TM 5-809-12/AFM 88-3, Chap. 15
from figure 2-1, minimum tc = h h1 ; tc = 7+ 2 = 9
inches.
Example 2: Thickened floor slab design
for exterior wall
a. Problem. The thickened concrete floor slab supporting an 8 inch exterior load bearing wall weighing
1,000 pounds per linear foot should be designed.
Floor slab data
• Thickness = 4 inches
• Flexural strength = 600 pounds per square inch
• Modulus of subgrade reaction k = 200 pounds per
cubic inch
b. Solution. Table 2-3 should be entered using 600
pounds per square inch and wall load, p = 1,000 pounds
per linear foot. Thickness t should be adjusted based
on modulus of subgrade reaction, k = 200 pounds per
cubic inch.
Page 115
Summary
For thickened slab configuration, see table 3-3.
Note: For other practical considerations, i.e., frost
line, erosion etc., the thickness, te, may be increased.
Example 3: Reinforced concrete slab
a. Problem. It is decided that the 7-inch floor slab
in Example 1 should be reduced to 6 inches by reinforcing the slab using 60,000-pounds per square inch
yield strength steel reinforcement. The percent reinforcement required for the 6-inch slab should be determined.
Chapter Four discussed a lot of information. We learned
once the floor slabs design requirements have been
established, the requirements are then translated into
meaningful design data. This design data is then compared with the existing condition data, and a floor slab
design is evolved. The design procedure covers subgrade conditions, steel reinforcing, and various details,
such as jointing. We then studied the types of joints
and each of their uses and purpose. We concluded with
some design examples for better understanding.
b. Solution. From figure 4-4, a straight line should
be drawn between h = 7 inches and h = 6 inches r and
extend line to S. This should read S = 0.13 percent.
Example 4: Concrete slab Thickness
for tracked vehicle
a. Problem. The floor slab thickness h should be determined for a tank repair shop. The largest tank has
a gross weight of 60 kips, Traffic is limited to 40 vehicles per day.
Material Properties:
• Concrete flexural strength = 700 pounds per square
inch
• Modulus of subgrade reaction, k = 100 pounds per
cubic inch
b. Solution. Based on 60 kips gross weight, equivalent forklift truck category II is selected from second
tabulation. From first tabulation in for category II,
forklift truck axle load is 10 to 15 kips. Table 4-1 is
entered using 15 kips. Loaded at a frequency of 100
operation per day, the design index is 7. Figure 4-1 is
entered using concrete flexure strength = 700 pounds
per square inch, k = 100 pounds per cubic inch and
DI = 7, slab thickness, h = 6.6 inches, or if rounded,
7 inches.
Page 116
Heavy Loads
References
Government Publications
General Services
Federal Specifications
SS-S-200E
Sealing Compounds, Two-Component, Elestomeris, Polymer Type, Jet-FuelResistant, Cold Applied
SS-S-1401 C
Sealing Compound, Hot-Applied, for Concrete and Asphalt Pavements.
SS-S-1 614P
Sealing Compound, Jet-Fuel-Resistant, Hot-Applied, One Component, for
Portland Cement and Tar Concrete Pavements.
Department of Defense
Military Standards
MIL-STD-619B
Unified Soil Classification System for Roads, Airfields, Embankments, and
Foundations.
MIL-STD-621A & Notices 1 & 2
Test Method for Pavement Subgrade, Subbase, and Base-Course Materials
Military Specifications
MIL-R-3472
Roof-Coating, Asphalt-Base Emulsion.
Departments of the Army and the Air Force
TM 5-809-2/AFM 88-3, Chap. 2
Concrete Structural Design for Buildings.
TM 5-818-1/AFM 88-3, Chap. 7
Soils and Geology Procedures for Foundation Design of Buildings and Other
Structures (Except Hydraulic Structures).
TM 5-818-2/AFM 88-6, Chap. 4
Pavement Design for Seasonal Frost Conditions.
TM 5-818-7
Foundations in Expansive Soil.
TM 5-822-4
Soil Stabilization for Pavements.
TM 5-822-6/AFM 88-7, Chap. 1
Engineering and Design: Rigid Pavements for Roads, Streets, Walks, and
Open Storage Areas.
TM 5-822-7/AFM 88-6, Chap. 8
Standard Practice for Concrete Pavements.
TM 5-825-3/AFM 88-6, Chap. 3
Rigid Pavements for Airfields Other Than Army.
TM 5-825-2/AFM 88-6, Chap. 2
Flexible Pavement Design for Airfields.
TM 5-825-1/AFM 88-19, Chap. 1
Arctic and Subarctic Construction: General Provisions.
TM 5-825-4
Arctic and Subarctic Construction: Building Foundations.
TM 5-809-12/AFM 88-3, Chap. 15
TM-5-825-6/AFM 88-19, Chap. 6
Arctic and Subarctic Construction: Calculation Methods for Determination of
Depths of Freeze and Thaw in Soils.
Non-government Publications
American Society for Testing and Materials (ASTM) 1916 Race St., Philadelphia, PA 19103
C 78
Heavy Loads
Standard Method of Test for Flexural Strength of Concrete (using simple beam
with Third-Point Loading).
Page 117
Appendix A
Equations for Computing the Allowable Wall Loads
Near Center of Slab or Near Keyed or Doweled Joints
TM 5-809-12/AFM 88-3, Chap.15
Equations for Computing the Allowable Wall Load Near a Free Edge
Page 118
Heavy Loads
Bibliography
ASHRAE Handbook and Product Directory, 1981, Equipment, 1979,
and Applications, 1982, American Society of Heating, Refrigerating, and
Air Conditioning Engineers.
Beams on Elastic Foundation, M. Hetenyi, University of Michigan Press,
Ann Arbor, Mich., 1946.
“Comparison of Splitting Tensile Strength of Concrete with Flexural and
Compressive Strength,” W. W. Grieb and G. Werner, American Society for
Testing and Materials, Proceedings, Vol 62, pp 972-995, 1962.
Concrete Construction Handbook, J. W. Waddell, McGraw-Hill, New York,
pp 6- 12, 1968.
“Concrete Strength Relationships,” G. M. Hammitt II, US Army Engineer
Waterways Experiment Station, Miscellaneous Paper S-74-30, Vicksburg,
Miss., 1974.
“Criteria for Selection and Design of Residential Slabs-on-Ground.”
National Academy of Sciences, Building Research Advisory Board, 1968.
“Development of a Design Manual for Concrete Floor Slabs on Grade,”J.
L. Rice, A. C. Eberhardt, and L. Varga, US Army Construction Engineering
Research Laboratory, Technical Report S-27, 1974.
“Minimum Concrete Strength for Pavements and Floor Slabs,” R. S.
Rollings, US Army Engineer Waterways Experiment Station, Miscellaneous
Paper GL-80-3, Vicksburg, Miss., 1980.
“Partition Loads on Slabs-on-Grade,” E. Staab, US Army Engineer Missouri
River Division, 1980.
Heavy Loads
Page 119
Heavy Loads
Student Assessment
Final Exam
1. Concrete slab supported directly on
foundation soil is called:
a. heavy loads.
b. moving live loads.
c. slab on grade.
d. light loads.
6. Which of the following is a factor that may
affect the support rendered by the subgrade:
a. ground water
b. rainfall
c. topography
d. all of the above
2. Which of the following is a loading
conditioning for wall loads:
a. loads at center of slab
b. loads at a joint
c. loads at the edge of the slab
d. all of the above
7. Aggregates of the bank run in variety, as
opposed to crushed aggregates, which will
produce a _______ strength concrete due to
particle shape.
a. lower
b. higher
c. maximum
d. minimum
3. Which of the following is required
to determine the floor slab thickness
requirements:
a. types of vehicle
b. traffic volume by vehicle type
c. wheel loads
d. all of the above
4. The width of a thickened edge varies
depending on:
a. the thickness of the wall.
b. the length of the wall.
c. the width of the wall.
d. the thinness of the wall.
5. The subgrade provides a _______ for
supporting the floor slab and base courses:
a. load
b. foundation
c. thickness
d. none of the above
Page 120
8. Which index was developed to express varying
axle loads and traffic volume in terms of
relative severity:
a. performance
b. action
c. load
d. design
9. With the exception of areas of special soil, the
top 6 inches of subgrade are called:
a. compaction.
b. cut sections.
c. fill sections.
d. cut-to-fill sections.
10. Isolation joints will allow for some ______
movement.
a. sway
b. contraction
c. horizontal
d. vertical
Heavy Loads
Designing Buildings to Mitigate
Terrorist Attacks
Course Description
The purpose of this course is to introduce concepts that can help building designers, owners, and state and local governments mitigate the threat of hazards resulting from terrorist attacks on new buildings. This primer specifically addresses
four high-population, private-sector building types:
1.
2.
3.
4.
Commercial office
Retail
Multi-family residential
Light industrial
However, many of the concepts presented here are applicable to other building types and/or existing buildings. The focus
is on explosive attack, but the text also addresses design strategies to mitigate the effects of chemical, biological, and radiological attacks. All of the information, unless otherwise noted was taken from the Federal Emergency Management Agency
(FEMA) Website.
Chapters
•
•
•
•
•
•
Chapter One: Weapons Effects and Building Damage and Design Approach
Chapter Two: Site Location and Architectural Layout
Chapter Three: Structural Protection
Chapter Four: Building Envelope and Mechanical and Electrical Systems
Chapter Five: Chemical, Biological, And Radiological Protection
Chapter Six: Occupancy Types and Cost Considerations
Learning Objectives
• Address types of terrorist threats including chemical, biological and radiological threats
• Understand an explosive attack and the ways it can be delivered
• Comprehend how an explosive force occurs and how damage levels are predicted
• Identify the goals of the design approach
• Understand the importance of site location and layout
• Explain the importance of a perimeter line and controlled access zones
• Comprehend what can be done architecturally to mitigate the effects of a terrorist bombing on a facility
• Define the three ways to approach the structural design of buildings to mitigate damage due to progressive collapse
• List and describe the desirable structural characteristics for a building
• Understand the structural measures that enhance a structure’s robustness
• Describe the levels of damage computed by means of analysis
• Identify ways to enhance roof, floor and interior and exterior structures
• Describe the importance of the exterior wall
• Identify the significance of window design, including the glass design and mullion design
Page 121
• Be able to discuss the design of doors, louvers and
other openings
• Discuss specific recommendations for mechanical
and electrical systems
• Understand options for emergency power systems
• Identify the emergency elevator system
• Discuss three types of airborne hazards
• Grasp the importance of air intake systems and return
air systems, and their locations
• Identify vulnerable internal areas where hazards may
be brought into a building
• Describe an HVAC system, how to zone an HVAC
system, and the benefits of using the system
• Understand the importance of a detection system
• Recognize multi-family residential building and the
types of occupants
• Discuss the unique features of commercial retail space
occupancy and the challenges associated with them
• Understand the two types of initial construction costs
• Describe the setting of priorities if the initial cost of
construction is too high
Chapter One:
Weapons Effects and
Building Damage and
Design Approach
Overview
•
•
•
•
•
•
•
•
•
•
Introduction
Overview of Possible Threats
Explosive Attacks
Description of Explosion Forces
Predicting Damage Levels
Damage Mechanisms
Correlation Between Damage And Injuries
Goals of the Design Approach
Security Principles
Summary
Learning Objectives
• Address types of terrorist threats including
chemical, biological and radiological threats
• Understand an explosive attack and the ways
it can be delivered
• Comprehend how an explosive force occurs
and how damage levels are predicted
• Identify the goals of the design approach
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Introduction
Designing security into a building requires a complex
series of tradeoffs. Security concerns need to be balanced with many other design constraints, such as accessibility, initial and life-cycle costs, natural hazard
mitigation, fire protection, energy efficiency, and aesthetics. Because the probability of attack is very small,
security measures should not interfere with daily operations of the building. On the other hand, because
the effects of attack can be catastrophic, it is prudent
to incorporate measures that may save lives and minimize business interruption in the unlikely event of
an attack. The measures should be as unobtrusive as
possible to provide an inviting, efficient environment
that does not attract undue attention from potential attackers. Security design needs to be part of an overall
multi-hazard approach to ensure it does not worsen the
behavior of the building in the event of a fire, earthquake, or hurricane, all of which are far more probable
hazards than terrorist attacks.
Because of the severity of the types of hazards discussed, the goals of security-oriented design are by
necessity modest. With regard to explosive attacks,
the focus is on a damage-limiting or damage-mitigating approach, rather than a blast-resistant approach.
The goal is to incorporate some reasonable measures
to enhance the safety of persons within the building
and facilitate rescue efforts in the unlikely event of
attack.
It is clear that building owners are becoming interested in considering manmade hazards for a variety of
reasons, including the desire to:
• attract more tenants or a particular type of tenant
• lower insurance premiums or obtain high-risk insurance
• reduce life-cycle costs for operational security
measures
• limit losses and business interruption
Overview of Possible Threats
This chapter addresses several types of terrorist
threats, which are listed below.
Explosive Threats:
• Vehicle weapon
• Hand-delivered weapon
Airborne Chemical, Biological, and Radiological
Threats:
• Large-scale, external, air-borne release
• External release targeting building
• Internal release
Although it is possible the dominant threat mode may
change in the future, bombings have historically been a
favorite tactic of terrorists. Ingredients for homemade
bombs are easily obtained on the open market, as are
the techniques for making bombs. Bombings are easy
and quick to execute. Finally, the dramatic component
of explosions, in terms of the sheer destruction they
cause, creates a media sensation that is highly effective in transmitting terrorists’ message to the public.
Explosive Attacks
From the standpoint of structural design, the vehicle
bomb is the most important consideration. Vehicle
bombs are able to deliver a sufficiently large quantity
of explosives to cause potentially devastating structural damage. Security design intended to limit or mitigate damage from a vehicle bomb assumes the bomb
is detonated at a so-called critical location (see Figure
2-1). The critical location is a function of the site, the
building layout, and the security measures in place.
For a vehicle bomb, the critical location is taken to be
at the closest point which a vehicle can approach, assuming all security measures are in place. This may be
a parking area directly beneath the occupied building,
the loading dock, the curb directly outside the facility,
or at a vehicle-access control gate where inspection
takes place, depending on the level of protection incorporated into the design.
Another explosive attack threat is the small bomb that
is hand delivered. Small weapons can cause the greatest damage when brought into vulnerable, unsecured
areas of the building interior, such as the building lobby, mail room, and retail spaces. Recent events around
the world make it clear there is an increased likelihood
bombs will be delivered by persons who are willing
to sacrifice their own lives. Hand carried explosives
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are typically on the order of five to ten pounds of TNT
equivalent. However, larger charge weights, in the 50
to 100 pounds TNT equivalent range, can be readily
carried in rolling cases. Mail bombs are typically less
than ten pounds of TNT equivalent.
In general, the largest credible explosive size is a function of the security measures in place. Each line of
security may be thought of as a sieve, reducing the
size of the weapon able to gain access. Therefore, the
largest weapons are considered in totally unsecured
public space (e.g., in a vehicle on the nearest public
street), and the smallest weapons are considered in the
most secured areas of the building (e.g., in a briefcase
smuggled past the screening station).
Two parameters define the design threat:
• The weapon size, measured in equivalent pounds
of TNT
• The standoff: the distance measured from the center of gravity of the charge to the component of
interest
The design weapon size is usually selected by the
owner in collaboration with security and protective
design consultants (i.e., engineers who specialize in
the design of structures to mitigate the effects of explosions). Although there are few unclassified sources
giving the sizes of weapons that have been used in
previous attacks throughout the world, security consultants have valuable information that may be used
to evaluate the range of charge weights that might be
reasonably considered for the intended occupancy. Security consultants draw upon the experience of other
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countries, such as Great Britain and Israel, where terrorist attacks have been more prevalent, as well as data
gathered by U.S. sources.
To put the weapon size into perspective, it should
be noted that thousands of deliberate explosions occur every year within the United States, but the vast
majority have weapon yielding less than five pounds.
The number of large-scale vehicle weapon attacks that
have used hundreds of pounds of TNT during the past
twenty years is very small by comparison.
The design vehicle weapon size will usually be
much smaller than the largest credible threat. The design weapon size is typically measured in hundreds
of pounds rather than thousands of pounds of TNT
equivalent. The decision is usually based on a tradeoff between the largest credible attack directed against
the building and the design constraints of the project.
Further, it is common for the design pressures and impulses to be less than the actual peak pressures and
impulses acting on the building. This is the approach
that the federal government has taken in their design
criteria for federally owned domestic office buildings.
There are several reasons for this choice:
1. The likely target is often not the building under
design, but a high risk building that is nearby. Historically, more building damage has been due to
collateral effects than direct attack.
2. It is difficult to quantify the risk of man-made hazards. However, qualitatively it may be stated that
the chance of a large-scale terrorist attack occurring is extremely low. A smaller explosive attack
is far more likely.
3. Providing a level of protection that is consistent
with standards adopted for federal office buildings
enhances opportunities for leasing to government
agencies in addition to providing a clear statement
to other potential tenants regarding the building’s
safety.
4. The added robustness inherent in designing for a
vehicle bomb of moderate size will improve the
performance of the building under all explosion
scenarios.
Description of Explosion Forces
An explosion is an extremely rapid release of energy
in the form of light, heat, sound, and a shock wave.
The shock wave consists of highly compressed air
that wave-reflects off the ground surface to produce
a hemispherical propagation of the wave that travels
outward from the source at supersonic velocities (see
Figure 2-1). As the shock wave expands, the incident
or over-pressures decrease. When it encounters a surface that is in line-of-sight of the explosion, the wave
is reflected, resulting in a tremendous amplification of
pressure. Unlike acoustical waves, which reflect with
an amplification factor of two, shock waves can reflect
with an amplification factor of up to thirteen, due to
the supersonic velocity of the shock wave at impact.
The magnitude of the reflection factor is a function of
the proximity of the explosion and the angle of incidence of the shock wave on the surface.
The pressures decay rapidly with time (i.e., exponentially), measured typically in thousandths of a second
(milliseconds). Diffraction effects, caused by building features such as re-entrant corners and overhangs
of the building, may act to confine the air blast and
prolong its duration. Late in the explosive event, the
shock wave becomes negative, followed by a partial
vacuum, which creates suction behind the shock wave
(see Figure 3-1). Immediately following the vacuum,
air rushes in, creating a powerful wind, or drag pressure, on all surfaces of the building. This wind picks
up and carries flying debris in the vicinity of the detonation. In an external explosion, a portion of the energy is also imparted to the ground, creating a crater
and generating a ground shock wave analogous to a
high-intensity, short-duration earthquake.
The peak pressure is a function of the weapon size or
yield, and the cube of the distance (see Figure 3-2).
For an explosive threat defined by its charge weight
and standoff, the peak incident and reflected pressures
of the shock wave and other useful parameters, such
as the incident, and reflected impulse, shock velocity,
and time of arrival are evaluated using charts available
in military handbooks.
Figure 3-1 Air-blast pressure time history
Figure 3-2 Plots showing pressure decay with distance
Predicting Damage Levels
The extent and severity of damage and injuries in an
explosive event cannot be predicted with perfect certainty. Past events show the specifics of the failure
sequence for an individual building, due to air blast
effects and debris, impact, significantly affect the
overall level of damage.
For instance, two adjacent columns of a building may
be roughly the same distance from the explosion. Only
one fails, though, as a result of being struck by a fragment in a particular way, which initiates collapse. The
other, by chance, is not struck and remains in place.
Similarly, glass failures may occur outside the predicted areas, due to air-blast diffraction effects, caused by
the arrangement of buildings and their heights in the
vicinity of the explosion. The details of the physical
setting surrounding a particular occupant may greatly
influence the level of injury incurred. The position of
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the person, seated or standing, facing towards or away
from the event as it happens, may result in injuries
ranging from minor to severe.
Despite these uncertainties, it is possible to calculate
the extent of damage and injuries to be expected in
an explosive event, based on the size of the explosion, distance from the event, and assumptions about
the construction of the building. Additionally, there is
strong evidence to support a relationship between injury patterns and structural damage patterns.
Damage Mechanisms
Damage due to the air-blast shock wave may be divided into direct air blast effects and progressive collapse.
Direct air-blast effects are damage caused by the highintensity pressures of the air blast close to the explosion. These may induce localized failure of exterior
walls, windows, roof systems, floor systems, and columns.
Progressive collapse refers to the spread of an initial
local failure from element to element, eventually resulting in a disproportionate extent of collapse relative to the zone of initial damage. Localized damage
due to direct air-blast effects may or may not progress,
depending on the design and construction of the building. To produce a progressive collapse, the weapon
must be in close proximity to a critical load-bearing
element. Progressive collapse can propagate vertically
upward or downward (e.g., Ronan Point1) from the
source of the explosion, and it can propagate laterally
from bay to bay as well.
The pressures that an explosion exerts on building surfaces may be several orders of magnitude greater than
the loads for which the building is designed. The shock
wave also acts in directions that the building may not
have been designed for, such as upward pressure on
the floor system. In terms of sequence of response, the
air blast first impinges on the exterior envelope of the
building. The pressure wave pushes on the exterior
walls and may cause wall failure and window breakage. As the shock wave continues to expand, it enters
the structure, pushing both upward on the ceilings and
downward on the floors (see Figure 4-1).
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Floor failure is common in large-scale, vehicle-delivered, explosive attacks, because floor slabs typically
have a large surface area for the pressure to act on and a
comparably small thickness. Floor failure is particularly
common for close-in and internal explosions. The loss of
a floor system increases the unbraced height of the supporting columns, which may lead to structural instability.
For hand-carried weapons that are brought into the
building and placed on the floor away from a primary
vertical load-bearing element, the response will be
more localized, with damage and injuries extending a
bay or two in each direction (see Figure 4-2). Although
the weapon is smaller, the air-blast effects are amplified due to multiple reflections from interior surfaces.
Typical damage types that may be expected include:
• localized failure of the floor system immediately
below the weapon
• damage and possible localized failure of the floor
system above the weapon
• damage and possible localized failure of nearby
concrete and masonry walls
• failure of nonstructural elements such as partition
walls, false ceilings, ductwork, window treatments
• flying debris generated by furniture, computer
equipment, and other contents
Figure 4-1 Schematic showing sequence of building
damage due to a vehicle weapon
More extensive damage, possibly leading to progressive collapse, may occur if the weapon is strategically
placed directly against a primary load-bearing element, such as a column.
In comparison to other hazards, such as earthquake or
wind, an explosive attack has several distinguishing
features, listed below.
• The intensity of the localized pressures acting on
building components can be several orders of magnitude greater than these natural hazards. It is not
uncommon for the peak pressure on the building
from a vehicle weapon parked along the curb to be
in excess of 100 psi. Major damage and failure of
building components is expected, even for relatively
small weapons in close proximity to the building.
• Explosive pressures decay extremely rapidly with
distance from the source. Pressures acting on the
building, particularly on the side facing the explosion, may vary significantly, causing a wide range
of damage types. As a result, air blasts tend to
cause more localized damage than other hazards
that have a more global effect.
• The duration of the event is very short, measured in
thousandths of a second (milliseconds). In terms of
timing, the building is engulfed by the shockwave,
and direct air-blast damage occurs within tens to
hundreds of milliseconds from the time of detonation, due to the supersonic velocity of the shock
wave and the nearly instantaneous response of the
structural elements. By comparison, earthquake
events last for seconds and wind loads may act on
the building for minutes or longer.
Figure 4-2 Schematics showing sequence of building
damage due to a package weapon
Correlation between
Damage and Injuries
Three types of building damage can lead to injuries
and possible fatalities. The most severe building response is collapse. In past incidents, collapse has
caused the most extensive fatalities. For the Oklahoma City bombing in 1995 (see Figure 4-3), nearly
90 percent of the building occupants who lost their
lives were in the collapsed portion of the Alfred P.
Murrah Federal Office Building. Many of the survivors in the collapsed region were on the lower
floors and had been trapped in void spaces under
concrete slabs.
Figure 4-3 Exterior view of Alfred P. Murrach Federal
Building collapse
Although the targeted building is at greatest risk of
collapse, other nearby buildings may also collapse.
For instance, in the Oklahoma City bombing, a total
of nine buildings collapsed. Most of these were unreinforced masonry structures that, fortunately, were
largely unoccupied at the time of the attack. In the
bombing of the U.S. embassy in Nairobi, Kenya in
1998, the collapse of the Uffundi building, a concrete
building adjacent to the embassy, caused hundreds of
fatalities.
For buildings that remain standing, the second most
severe type of injury-producing damage is flying debris generated by exterior cladding. Depending on the
severity of the incident, fatalities may occur as a result
of flying structural debris. Some examples of exterior
wall failure causing injuries are listed below.
• In the Oklahoma City bombing, several persons
lost their lives after being struck by structural debris generated by infill walls from the concrete
frame Water Resources building across the street
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from the Murrah building.
• In the Khobar Towers bombing in 1996 (see Figure
4-4), most of the 19 U.S. servicemen who lost their
lives were impacted by high velocity projectiles created by the failed exterior cladding on the wall that
faced the weapon. The building was an all-precast,
reinforced concrete structure with robust connections between the slabs and walls. The numerous
lines of vertical support, along with the ample lateral
stability provided by the “egg crate” configuration
of the structural system, prevented collapse.
Another example is the bombing of the U.S. embassy
in Nairobi, Kenya. The explosion occurred near one of
the major intersections of the city, which was heavily
populated at the time of the bombing, causing extensive glass lacerations to pedestrians. The ambassador,
who was attending a meeting at an office building
across from the embassy, sustained an eye injury as a
result of extensive window failure in the building.
Figure 4-4 Exterior view of Khobar Towers exterior wall
failure
Figure 4-5 Photography showing non-structural damage in
building impacted by blast
Table 4-1: Damage and Injuries due to Explosion Effects
A summary of the relationship between the type of
damage and the resulting injuries is given in Table 4-1.
In the bombing of the U.S. embassy in Dar es Salaam,
Tanzania in 1998, the exterior unreinforced masonry
infill wall of the concrete framed embassy building
blew inward. The massiveness of the construction
generated relatively low-velocity projectiles that
injured and partially buried occupants, but did not
cause fatalities.
Even if the building remains standing and no structural damage occurs, extensive injuries can occur due
to nonstructural damage (see Figure 4-5). Typically,
for large-scale incidents, these types of injuries occur
to persons who are in buildings that are within several blocks of the incident. Although these injuries
are often not life-threatening, many people can be
affected, which has an impact on the ability of local
medical resources to adequately respond. An example
of nonstructural damage causing injuries is the extensive glass lacerations that occurred in the Oklahoma
City Bombing within the Regency Towers apartment
building, which was approximately 500 feet from the
Murrah Building. In this incident, glass laceration injuries extended as far as 10 blocks from the bombing.
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Goals of the Design Approach
It is impractical to design a civilian structure to remain
undamaged from a large explosion. The protective
objectives are, therefore, related to the type of building and its function. For an office, retail, residential,
or light industrial building where the primary asset is
the occupants, the objective is to minimize loss of life.
Because of the severity of large scale explosion incidents, the goals are by necessity modest. Moreover, it
is recognized that the building will be unusable after
the event. This approach is considered a damage-limiting or damage-mitigating approach to design.
To save lives, the primary goals of the design professional are to reduce building damage and to prevent
progressive collapse of the building, at least until it
can be fully evacuated. A secondary goal is to maintain
emergency functions until evacuation is complete.
The design professional is able to reduce building damage by incorporating access controls that allow building security to keep large threats away from the building and to limit charge weights that can be brought into
the building.
Preventing the building from collapsing is the most important objective. Historically, the majority of fatalities
that occur in terrorist attacks directed against buildings are due to building collapse. Collapse prevention
begins with awareness by architects and engineers
that structural integrity against collapse is important
enough to be routinely considered in design. Features
to improve general structural resistance to collapse can
be incorporated into common buildings at affordable
cost. At a higher level, designing the building to prevent progressive collapse can be accomplished by the
alternate-path method (i.e., design for the building to
remain standing following the removal of specific elements) or by direct design of components for air-blast
loading.
Furthermore, building design may be optimized by
facilitating evacuation, rescue, and recovery efforts
through effective placement, structural design, and redundancy of emergency exits and critical mechanical/
electrical systems. Through effective structural design,
the overall damage levels may be reduced to make it
easier for occupants to get out and emergency responders to safely enter.
Beyond the issues of preventing collapse and facilitating evacuation/rescue, the objective is to reduce flying
debris generated by failed exterior walls, windows and
other components to limit severe injuries and risk of
fatalities. This may be accomplished through selection
of appropriate materials and use of capacity-design
methods to proportion elements and connections. A
well- designed system will provide predictable damage modes, selected to minimize injuries. Finally, good
anti-terrorist design is a multidisciplinary effort requiring the concerted efforts of the architect, structural engineer, security professional, and the other design team
members. It is also critical for security design to be
incorporated as early as possible in the design process
to ensure a cost-effective, attractive solution.
Security Principles
This section provides some fundamental security concepts that place physical security into the context of
overall facility security. The components of security
include deception, intelligence, operational protection, and structural hardening. These components are
interrelated (see Figure 5-1).
Figure 5-1 Components of security
Ideally, a potential terrorist attack is prevented or preempted through intelligence measures. If the attack is
attempted, physical security measures combine with
operational forces (e.g., surveillance, guards, and sensors) to provide layers of defense that delay and/or
thwart the attack. Deception may be used to make the
facility appear to be a more protected or lower-risk
facility than it actually is, thereby making it appear to
be a less attractive target. Deception can also be used
to misdirect the attacker to a portion of the facility that
is non-critical. As a last resort, structural hardening is
provided to save lives and facilitate evacuation and
rescue by preventing building collapse and limiting
flying debris.
Because of the interrelationship between physical and
operational security measures, it is imperative for the
owner and security professional to define early in the
design process what extent of operational security is
planned for various threat levels.
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If properly implemented, physical security measures
will contribute toward the goals listed below in prioritized order.
• Deterring attack. By making it more difficult to
implement some of the more obvious attack scenarios (such as a parked car in the street) or making
the target appear to be of low value in terms of the
amount of sensation that would be generated if it
were attacked, the would-be attacker may become
discouraged from targeting the building. On the
other hand, it may not be advantageous to make the
facility too obviously protected or not protected;
this may have the opposite of the intended effect
and provide an incentive to attack the building.
• Delaying the attack. If an attack is initiated, properly designed landscape or architectural features
can delay its execution by making it more difficult
for the attacker to reach the intended target. This
will give the security forces and authorities time
to mobilize and possibly stop the attack before it
is executed. This is done by creating a buffer zone
between the publicly accessible areas and the vital
areas of the facility by means of an obstacle course,
a serpentine path and/or a division of functions
within the facility. Alternatively, through effective
design, the attacker could be enticed to a non-critical part of the facility, thereby delaying the attack.
• Mitigating the effects of the attack. If these precautions are implemented and the attack still takes
place, structural protection efforts will serve to
control the extent and consequences of damage. In
the context of the overall security provided to the
building, structural protection is a last resort that
becomes effective only after all other efforts to stop
the attack have failed. In the event of an attack, the
benefits of enhancements to life-safety systems
may be realized in lives saved.
An effective way to implement these goals is to create
layers of security within the facility (see Figure 5-2).
The outermost layer is the perimeter of the facility.
Interior to this line is the approach zone to the facility, then the building exterior, and finally the building
interior. The interior of the building may be divided
into successively further protected zones, starting with
publicly accessible areas, such as the lobby and retail
space, to the more private areas of offices, and finally the vital functions, such as the control room and
emergency functions. The advantage of this approach
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is even if one line of protection is breached, the facility is not completely compromised. Having multiple
lines of defense provides redundancy to the security
system, adding robustness to the design. Also, by using this approach, not all of the focus is on the outer
layer of protection, which may lead to an unattractive,
fortress-like appearance.
Figure 5-2 Schematic showing line of defense against blast
To provide a reliable design, each ring must have a
uniform level of security provided along its entire
length; security is only as strong as the weakest link.
To have a balanced design, both physical and operational security measures need to be implemented in
the facility. Architects and engineers can contribute to
an effective physical security system, which augments
and facilitates the operational security functions. If security measures are left as an afterthought, expensive,
unattractive, and make-shift security posts are the inevitable result. For more information on security, refer
to FEMA 426 (Reference Manual to Mitigate Potential Terrorist Attacks in High-Occupancy Buildings).
Summary
Chapter Two:
Site Location and
Architectural Layout
In this chapter, we were introduced to the possible terrorist threats a building can receive. The most popular of
these threats is explosive attacks, which were discussed
at length. Next we examined ways to predict damage
levels and damage mechanisms used. The correlation of
damage level and injury was talked about as well. Final- Overview
ly, the goals of the design approach were touched upon
briefly.
• Site location and layout
• Checklist – Site and Layout Design Guidance
• Architectural
• Summary
Learning Objectives
• Understand the importance of site location and layout
• Explain the importance of a perimeter line and
controlled access zones
• Comprehend what can be done architecturally to
mitigate the effects of a terrorist bombing on a
facility
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Site Location and Layout
Because air-blast pressures decrease rapidly with
distance, one of the most effective means of protecting assets is to increase the distance between a
potential bomb and the assets to be protected. The
best way to do this is to provide a continuous line of
security along the perimeter of the facility, to protect
it from unscreened vehicles and to keep all vehicles
as far away from critical assets as possible.
This section discusses the perimeter and the approach to the building. For discussions about building shape and placement on the site, see Section 6.2,
Architectural.
Perimeter Line
The perimeter line of protection is the outermost line
that can be protected by facility security measures.
The perimeter needs to be designed to prevent carriers of large-scale weapons from gaining access to
the site. In design, it is assumed all large-scale explosive weapons (i.e., car bombs or truck bombs) are
outside this line of defense. This line is defended by
both physical and operational security methods.
It is recommended the perimeter line be located as
far as is practical from the building exterior. Many
times, vulnerable buildings are located in urban areas where site conditions are tight. In this case, the
options are obviously limited. Often, the perimeter
line can be pushed out to the edge of the sidewalk
by means of bollards, planters, and other obstacles.
To push this line even further outward, restricting or
eliminating parking along the curb can be arranged
with the local authorities, but this can be a difficult
and time consuming effort. In some cases, eliminating loading zones and closing streets/lanes may be
options.
Controlled Access Zones
Access control refers to points of controlled access
to the facility through the perimeter line. The controlled access check, or inspection points for vehicles, require architectural features or barriers to
maintain the defensible perimeter. Architects and
engineers can accommodate these security functions
by providing adequate design for these areas, which
makes it difficult for a vehicle to crash onto the site.
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Deterrence and delay are major attributes of the
perimeter security design that should be consistent
with the landscaping objectives, such as emphasizing the open nature characterizing high-population
buildings. Since it is impossible to thwart all possible threats, the objective is to make it difficult to
successfully execute the easiest attack scenarios
such as a car bomb detonated along the curb, or a vehicle jumping the curb and ramming into the building prior to detonation.
If space is available between the perimeter line and
the building exterior, much can be done to delay an
intruder. Examples include, terraced landscaping,
fountains, statues, staircases, circular driveways,
planters, trees, high-strength cables hidden in bushes and any number of other obstacles that make it
difficult to rapidly reach the building. Though individually these features may not be able to stop a
vehicle, in combination, they form a daunting obstacle course. Other ideas for implementing secure
landscaping features may be found in texts on Crime
Prevention through Environmental Design (CPTED). These concepts are useful for slowing down
traffic, improving surveillance, and site circulation.
On the sides of the building that are close to the curb
where landscaping solutions are limited, anti-ram
barriers, capable of stopping a vehicle on impact,
are recommended for high-risk buildings. Barrier
design methods are discussed in more detail below.
The location of access points should be oblique to
oncoming streets so that it is difficult for a vehicle to
gain enough velocity to break through these access
locations. If the site provides straight-on access to
the building, some mitigation options include concrete medians in the street to slow vehicles or, for
high-risk buildings, use of anti-ram barriers along
the curb capable of withstanding the impact of highvelocity vehicles.
Place parking as far as practical from the building.
Off-site parking is recommended for high-risk facilities vulnerable to terrorist attack. If onsite surface
parking or underground parking is provided, take
precautions, such as limiting access to these areas
only to the building occupants and/or having all vehicles inspected in areas close-in to the building. If
an underground area is used for a high-risk building,
the garage should be placed adjacent to the building,
under a plaza area, rather than directly underneath
the building. To the extent practical, limit the size of
vehicle that is able to enter the garage by imposing
physical barriers on vehicle height.
Physical Protective Barriers
There are two basic categories of perimeter anti-ram
barriers; passive (or fixed) and active (or operable).
Each is described below.
the bumper of the vehicle. The spacing of bollards is
based on several factors including ADA (Americans
with Disabilities Act) requirements, the minimum
width of a vehicle, and the number of bollards required to resist the impact. As a rule of thumb, the
center-to-center spacing should be between three
and five feet to be effective. The height of the bollard is to be at least as high as the bumper of the design threat vehicle, which is typically between two
and three feet.
Passive Barriers
Passive barriers are those that are fixed in place and
do not allow for vehicle entry. These are to be used
away from vehicle access points. The majority of
these are constructed in place.
For lower-risk buildings without straight-on vehicular access, it may be appropriate to install surfacemounted systems, such as planters, or to use landscaping features to deter an intrusion threat. An
example of a simple but effective landscaping solution is to install a deep permanent planter around the
building, with a wall that is as high as a car or truck
bumper.
Individual planters mounted on the sidewalk resist impact through inertia and friction between the
planter and the pavement. It can be expected that
the planter will move as a result of the impact. For a
successful design, the maximum displacement of the
planter should be less than the setback distance to
the building. The structure supporting the weight of
the planter must be considered prior to installation.
To further reduce displacement, the planter may be
placed several inches below the pavement surface.
A roughened, grouted interface surface will also improve performance.
The traditional anti-ram solution entails the use of
bollards (see Figure 6-1). Bollards are concretefilled steel pipes that are placed every few feet along
the curb of a sidewalk to prevent vehicle intrusion.
In order for them to resist the impact of a vehicle, the
bollard needs to be fully embedded into a concrete
strip foundation that is several feet deep. The height
of the bollard above ground should be higher than
Figure 6-1 Schematic of typical anti-ram bollard
An alternative to a bollard is a plinth wall, which
is a continuous knee wall constructed of reinforced
concrete with a buried foundation (see Figure 6-2).
The wall may be fashioned into a bench, a base for
a fence, or the wall of a planter. To be effective, the
height needs to be at least as high as the vehicle
bumper.
For effectiveness, the barriers need to be placed as
close to the curb as possible. However, the property
line of buildings often does not extend to the curb.
Therefore, to place barriers with foundations near
the curb, a permit is required by the local authorities, which can be difficult and time-consuming to
obtain. To avoid this, building owners are often
inclined to place bollards along the property line,
which significantly reduces the effectiveness of the
barrier system.
The foundation of the bollard and plinth wall system
can present challenges. There are sometimes vaults
or basements below the pavement that extend to the
property line, which often require special foundation details. Unless the foundation wall can sustain
the reaction forces, significant damage may occur.
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It is important that the installation of hydraulically operated systems be performed by a qualified contractor to
ensure a reliable system that will work properly in all
weather conditions.
Effectiveness of Anti-Ram Barriers
Figure 6-2 Schematic of typical anti-ram knee wall
Below-ground utilities, which frequently are close to
the pavement surface, can present additional problems. Their location may not be known with certainty,
and this often leads to difficulties during construction.
This also can be a strong deterrent to selecting barriers
with foundations as a solution. However, for high-risk
facilities, it is recommended these issues be resolved
during the design phase, so a reliable anti-ram barrier solution can be installed. For lower-risk buildings
without straight-on vehicular access, it may be more
appropriate to install surface-mounted systems, such
as planters, or to use landscaping features to deter an
intrusion threat. An example of a simple but effective
landscaping solution is to install a deep permanent
planter around the building with a wall that is at least
as high as a car or truck bumper.
Active Systems
At vehicular access points, active or operational anti-ram systems are required. There are off-the-shelf
products available that have been rated to resist various levels of car and truck impacts. Solutions include:
• crash beams
• crash gates
• surface-mounted plate systems
• retractable bollards
• rotating-wedge systems
The first three systems listed above generally have
lower impact ratings than the last two listed. Check
with the manufacturer to ensure the system has been
tested to meet the impact requirements for your project.
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The effectiveness of an anti-ram barrier is based on the
amount of energy it can absorb versus the amount of kinetic energy imparted by vehicle impact. The angle of
approach reduces this energy in non-head-on situations,
and the energy absorbed by the crushing of the vehicle
also reduces the energy imparted to the barriers. The kinetic energy imparted to the wall is one-half the product
of the vehicle mass and its impact velocity squared. Because the velocity term is squared, a change in velocity
affects the energy level more than a change in vehicle
weight. For this reason, it is important to review lines
of approach to define areas where a vehicle has a long,
straight road that can be used for picking up speed before
impact.
The vehicle weight used for the design of barriers typically ranges from 4000 lbs. for cars up to 15,000 lbs. for
trucks. Impact velocities typically range from 30 mph for
oblique impact areas (i.e., where the oncoming street is
parallel to the curb) up to 50 mph where there is straighton access (i.e., where the oncoming street is perpendicular to the curb).
The kinetic energy of the vehicle at impact is absorbed
by the barrier system. For fixed systems (like a concrete
bollard), the energy is absorbed through the deformational strain energy absorbed by the barrier, soil, and the
vehicle. For movable systems (like a surface-mounted
planter) energy is absorbed through shear friction against
the pavement and vehicle deformation.
Barrier effectiveness is ranked in terms of the amount
of displacement of the system due to impact. Standard
ratings defined by the federal government define the distance the vehicle travels before it is brought to rest. The
most effective systems stop the vehicles within three feet.
Moderately effective barriers stop the vehicle within 20
feet, and the least effective systems require up to 50 feet.
Checklist – Site and Layout Design Guidance
• Provide a continuous line of defense around the site,
as far from the building as practical.
• Place vehicular access points away from oncoming
streets.
• Limit the number of vehicular entrances through the
secured perimeter line.
• Use a series of landscape features to create an obstacle course between the building and the perimeter.
This approach is most effective if used in areas where
there is ample setback.
• Design planters for the design-level impact to displace the planter a distance less than the setback.
• Use anti-ram barriers along curbs, particularly on
sides of the building that have a small setback and in
areas where high-velocity impact is possible.
• Use operable anti-ram barriers at vehicular access
points. Select barriers rated to provide the desired
level of protection against the design impact.
Architectural
nate, practice is to create a large plaza area in front of the
building, but to leave little setback on the sides and rear
of the building. Though this practice may increase the
monumental character of the building, it also increases
the vulnerability of the other three sides.
The shape of the building can have a contributing effect
on the overall damage to the structure (see Figure 6-3).
Re-entrant corners and overhangs are likely to trap the
shock wave and amplify the effect of the air blast. Note
that large or gradual re-entrant corners have less effect
than small or sharp re-entrant corners and overhangs. The
reflected pressure on the surface of a circular building
is less intense than on a flat building. When curved surfaces are used, convex shapes are preferred over concave
shapes. Terraces that are treated as roof systems subject
to downward loads require careful framing and detailing
to limit internal damage to supporting beams.
There is much that can be done architecturally to mitigate the effects of a terrorist bombing on a facility. These
measures often cost nothing, or very, little if implemented early in the design process. It is recommended
that protective design and security consultants are used
as early as possible in the design process. They should
be involved in site selection, and their input should be
sought during programming and schematic design.
Building Exterior
This section discusses the building shape, placement, and
exterior ornamentation. For a discussion of exterior cladding, see Section 6.4, Building Envelope.
At the building exterior, the focus shifts from deterring
and delaying the attack, to mitigating the effects of an
explosion. The exterior envelope of the building is most
vulnerable to an exterior explosive threat because it is the
part of the building closest to the weapon, and it is typically built using brittle materials. It also is a critical line
of defense for protecting the occupants of the building.
The placement of the building on the site can have a
major impact on its vulnerability. Ideally, the building is
placed as far from the property lines as possible. This applies not only to the sides that are adjacent to streets, but
the sides that are adjacent to adjoining properties, since it
is not certain who will occupy the neighboring properties
during the life of the building. A common, but unfortuDesigning Buildings to Mitigate Terrorist Attacks
Figure 6-3 Schematics showing the effect of building shape
on air-blast impacts
Generally, simple geometries and minimal ornamentation (which may become flying debris during an explosion) are recommended, unless advanced structural
analysis techniques are used. If ornamentation is used, it
is preferable to use lightweight materials, such as timber
or plastic, which are less likely than brick, stone, or metal
to become lethal projectiles in the event of an explosion.
Soil can be highly effective in reducing the impact of a
major explosive attack. Bermed walls and buried roofPage 135
tops have been found to be highly effective for military
applications and can be effectively extended to conventional construction. This type of solution can also be effective in improving the energy efficiency of the building. Note that if this approach is taken, no parking can be
permitted over the building.
Interior courtyards or atriums are other concepts for
bringing light and a natural setting to the building, without adding vulnerable openings to the exterior.
Building Interior
In terms of functional layout, unsecured areas, such as the
lobby, loading dock, mail room, garage, and retail areas,
need to be separated from the secured areas of the building. Ideally, these unsecured areas are placed exterior to
the main building or along the edges of the building. For
example, a separate lobby pavilion or loading dock area
outside of the main footprint of the building (see Figure
6-4) provides enhanced protection against damage and
potential building collapse in the event of an explosion at
these locations. Similarly, placing parking areas outside
the main footprint of the building can be highly effective
in reducing the vulnerability to catastrophic collapse. If it
is not possible to place vulnerable areas outside the main
building footprint, they should be placed along the building exterior, and the building layout should be used to
create internal “hard lines”, or buffer zones. Secondary
stairwells, elevator shafts, corridors, and storage areas
should be located between public and secured areas.
When determining whether secured and unsecured areas are adjacent to one another, consider the layout
on each floor and the relationship between floors. Secured occupied or critical areas should not be placed
above or below unsecured areas.
Adequate queuing areas should be provided in front
of lobby inspection stations so visitors are not forced
to stand outside during bad weather conditions or in a
congested line inside a small lobby while waiting to
enter the secured areas. Occupied areas or emergency
functions should not be placed immediately adjacent
to the lobby, but should be separated by a buffer area
such as a storage area or corridor. The interior wall
area and exposed structural columns in unsecured lobby areas should be minimized.
Vehicular queuing and inspection stations need to be
accounted for in design of the loading docks and vehicle access points. These should be located outside
the building along the curb or farther away. A parking
lane may be used for this purpose.
Emergency functions and elevator shafts are to be
placed away from internal parking areas, loading
docks and other high-risk areas. In the 1993 World
Trade Center bombing incident, elevator shafts became chimneys, transmitting smoke and heat from the
explosion in the basement to all levels of the building.
This hindered evacuation and resulted in smoke inhalation injuries.
False ceilings, light fixtures, Venetian blinds, ductwork, air conditioners, and other nonstructural components may become flying debris in the event of an
explosion. Wherever possible it is recommended the
design be simplified to limit these hazards. Placing
heavy equipment, such as air conditioners, near the
floor rather than the ceiling is one idea for limiting this
hazard. Using fabric curtains or plastic vertical blinds
rather than metal Venetian blinds, and using exposed
ductwork as an architectural device are other ideas.
Mechanically attaching light fixtures to the slab above,
as is done in high seismic areas, is recommended.
Figure 6-4 Schematics showing an example approach for
improving the layout of adjacent unsecured and secured
areas
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Finally, the placement of furniture can have an effect
on injury levels. Desks, conference tables, and other
similar furniture should be placed as far from exterior
windows facing streets as practical. Desks with comDesigning Buildings to Mitigate Terrorist Attacks
puter monitors should be oriented away from the window to prevent injury due to impact of the monitor.
Checklist – Architectural
• Use simple geometries without sharp re-entrant
corners.
• Use lightweight nonstructural elements to reduce
flying debris hazards.
• Place the building on the site as far from the perimeter as practical.
• Place unsecured areas exterior to the main structure
or along the exterior of the building.
• Separate unsecured and secured areas horizontally
and vertically using buffer zones and/or hardening
of walls and floors.
• Provide sufficient queuing areas at lobby and delivery entrances.
• Limit nonstructural elements such as false ceilings
and metal blinds on the interior.
• Mechanically fasten light fixtures to the floor system above.
• Place desks and conference tables as far from exterior windows as practical.
• Orient desks with computer monitors to face away
from windows so the chair back faces the window.
Summary
We were first introduced, in this chapter, to the site
location and layout of a building. The perimeter line,
access zones, and barriers were discussed in detail.
Next, the architecture of the building was touched
upon. The building exterior and interior were discussed in detail.
Chapter Three:
Structural Protection
Overview
•
•
•
•
•
•
•
•
Introduction
Progressive Collapse
Building Structural Systems
Structural Layout
Direct Design Methods
Structural Elements
Checklist – Structural
Summary
Learning Objectives
• Define the three ways to approach the structural
design of buildings to mitigate damage due to
progressive collapse.
• List and describe the desirable structural
characteristics for a building
• Understand the structural measures to enhance
the robustness of the structure
• Describe the levels of damage computed by
means of analysis
• Identify ways to enhance roof, floor,
and interior and exterior structures
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Introduction
Given the evolving nature of the terrorist threat, it is
impossible to predict what threats may be of concern
during the lifetime of the building. It is, therefore,
prudent to provide protection against progressive
collapse, initiated by a localized structural failure, caused by an undefined threat. Because of the
catastrophic consequences of progressive collapse,
incorporating these measures into the overall building design should be given the highest priority when
considering structural design approaches for mitigating the effects of attacks.
Explicit design of secondary structural components
to mitigate the direct effects of air blast enhances life
safety by providing protection against localized failure, flying debris, and air blast entering the building.
It may also facilitate evacuation and rescue by limiting the overall damage level and improving access by
emergency personnel.
Specific issues related to structural protection measures are discussed separately in the sections below.
Progressive Collapse
ASCE-7 defines three ways to approach the structural
design of buildings to mitigate damage due to progressive collapse. Each is described below with an emphasis on how the method is applied in the situation where
explosive loads are the initiating cause of collapse.
Indirect Method: Consider incorporating general
structural integrity measures throughout the process
of structural system selection, layout of walls and columns, member proportioning, and detailing of connections to enhance overall structural robustness. In
lieu of calculations demonstrating the effects of explosions on buildings, one may use an implicit design
approach that incorporates measures to increase the
overall robustness of the structure. These measures are
discussed in the following sub-sections on structural
systems, structural layout, and structural elements.
This minimum standard is likely to be the primary
method used for design of the type of buildings that
are the focus of this primer.
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Alternate-Load-Path Method: Localize response by
designing the structure to carry loads by means of an
alternate load path, in the event of the loss of a primary load-bearing component. The alternate-load path
method has been selected by agencies, including the
General Services Administration (GSA), as the preferred approach for preventing progressive collapse.
This method provides a formal check of the capability of the structure to resist collapse, following the
removal of specific elements, such as a building column at the building perimeter. The method does not
require characterization of the explosive threat. The
structural engineer can usually perform the necessary
analysis, with or without guidance from a protective
design consultant. However, the analysis is likely to
benefit from advice of the protective design consultant
regarding element loss scenarios that should be considered in design.
Specific Local-Resistance Method: Explicitly design
critical vertical load-bearing building components to
resist the design-level explosive forces. Explosive
loads for a defined threat may be explicitly considered
in design by using nonlinear dynamic analysis methods. These are discussed in the subsection on direct
design methods, with additional information in the
subsection on structural elements. Blast-mitigating
structural design or hardening generally focuses on
the structural members on the lower floor levels that
are closest to defined stationary exterior vehicle weapon threats.
Useful references are provided at the end of this section that directly relate to progressive collapse prevention.
Building Structural Systems
In the selection of the structural system, consider
both the direct effects of air-blast and the potential for
progressive collapse, in the event a critical structural
component fails.
The characteristics of air-blast loading have been discussed previously. To resist the direct effects of air blast,
the structural characteristics listed below are desirable.
• Mass. Lightweight construction is unsuitable
for providing air-blast resistance. For example, a
building with steel deck (without concrete fill) roof
construction will have little air-blast resistance.
• Shear Capacity. Primary members and/or their
connections should ensure flexural capacity is
achieved prior to shear failure. Avoiding brittle
shear failure significantly increases the structure’s
ability to absorb energy.
• Capacity for Reversing Loads. Primary members
and their connections should resist upward pressure. Certain systems, such as pre-stressed concrete, may have little resistance to upward forces.
Seated connection systems for steel and pre-cast
concrete may also have little resistance to uplift.
The use of headed studs is recommended for affixing concrete fill over steel deck to beams for uplift
resistance.
Current testing programs are investigating the effectiveness of various conventional building systems. In
general, though, the level of protection that may be
achieved using these materials is lower than what is
achieved using well-designed, cast-in-place, reinforced concrete. The performance of a conventional
steel frame with concrete fill over metal deck depends
on the connection details. Pre-tensioned or post-tensioned construction provides little capacity for abnormal loading patterns and load reversals. The resistance
of load-bearing wall structures varies to a great extent.
More information about the response of these systems
is described in the subsection on structural elements
and in the section on exterior cladding in Section 6.4,
Exterior Envelope.
To reduce the risk of progressive collapse in the event
of the loss of structural elements, the following structural traits below should be incorporated:
• Redundancy. The incorporation of redundant load
paths in the vertical-load-carrying system helps ensure alternate load paths are available in the event
of failure of structural elements.
• Ties. An integrated system of ties in perpendicular directions, along the principal lines of structural
framing, can serve to redistribute loads during catastrophic events.
• Ductility. In a catastrophic event, members and
their connections may have to maintain their
strength while undergoing large deformations.
Structural Layout
Historically, the preferred material for explosion-mitigating construction is cast-in-place reinforced concrete. This is the material used for military bunkers,
and the military has performed extensive research
and testing of its performance. Reinforced concrete
has a number of attributes that make it the construction material of choice. It has significant mass, which
improves response to explosions, because the mass is
often mobilized only after the pressure wave is significantly diminished, reducing deformations. Members can be readily proportioned and reinforced for
ductile behavior. The construction is unparalleled in
its ability to achieve continuity between the members. Finally, concrete columns are less susceptible
to global buckling in the event of the loss of a floor
system.
To enhance the overall robustness of the structure, the
measures listed below are recommended.
• In frame structures, column spacing should be limited. Large column spacing decreases likelihood
the structure will be able to redistribute load in
event of column failure.
• The exterior bay is the most vulnerable to damage,
particularly for buildings that are close to public
streets. It is also less capable of redistributing loads
in the event of member loss, since two-way load
distribution is not possible. It is desirable to have
a shallow bay adjacent to the building exterior to
limit the extent of damage.
• Use of transfer girders is strongly discouraged.
Loss of a transfer girder or one of its supports can
destabilize a significant area of the building. Transfer girders are often found at the building exterior
to accommodate loading docks or generous entries,
increasing their vulnerability to air-blast effects. It
is highly desirable to add redundant transfer systems where transfer girders are required.
• In bearing-wall systems that rely primarily on interior cross-walls, interior longitudinal walls should
be periodically spaced to enhance stability and to
control the lateral progression of damage.
• In bearing-wall systems that rely on exterior walls,
perpendicular walls or substantial pilasters should
be provided at a regular spacing to control the
amount of wall that is likely to be affected.
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Direct Design Methods
The direct design approach (Figure 6-5) to be used
for the structural protective measures is to first design the building for conventional loads, then evaluate
the structure’s response to explosive loads and augment the design, if needed. Finally, the designer must
make sure all conventional load requirements are still
met. This approach ensures the design meets all the
requirements for gravity and natural hazards, in addition to air-blast effects. Take note that measures taken
to mitigate explosive loads may reduce the structure’s
performance under other types of loads, and therefore
an iterative approach may be needed. As an example,
increased mass generally increases the design forces
for seismic loads, whereas increased mass generally
improves performance under explosive loads. Careful
consideration between the protective design consultant and the structural engineer is needed to provide
an optimized design.
interpretation of the results for structural design details. Whenever possible, results are checked against
data from tests and experiments for similar structures
and loadings.
Charts are available that provide damage estimates
for various types of construction, as a function of
peak pressure and peak impulse, based on analysis or
empirical data. Military design handbooks typically
provide this type of design information.
Components such as beams, slabs, or walls can often
be modeled by an SDOF system and the governing
equation of motion solved by using numerical methods. There are also charts available in textbooks and
military handbooks for linearly decaying loads, which
provide the peak response and circumvent the need to
solve differential equations. These charts require only
knowledge of the fundamental period of the element,
its ultimate resistance force, the peak pressure applied to the element, and the equivalent linear decay
time to evaluate the peak displacement response of
the system. The design of the anchorage and supporting structural system can be evaluated by using the
ultimate flexural capacity obtained from the dynamic
analysis.
Figure 6-5 Direct design process flow chart
For SDOF systems, material behavior can be modeled
using idealized elastic, perfectly-plastic stress-deformation functions, based on actual structural support
conditions and strain-rate-enhanced material properties. The model properties selected provide the same
peak displacement and fundamental period as the
actual structural system in flexure. Furthermore, the
mass and the resistance functions are multiplied by
mass and load factors, which estimate the actual portion of the mass or load participating in the deflection
of the member along its span.
Non-linear dynamic analysis techniques are similar
to those currently used in advanced seismic analysis.
Analytical models range from handbook methods to
equivalent single-degree-of-freedom (SDOF) models to finite element (FE) representation. For SDOF
and FE methods, numerical computation requires
adequate resolution in space and time to account for
the high-intensity, short-duration loading and nonlinear response. Difficulties involve the selection of the
model and appropriate failure modes, and finally, the
For more complex elements, the engineer must resort
to finite-element numerical time integration techniques and/or explosive testing. The time and cost of
the analysis cannot be ignored when choosing design
procedures. Because the design process is a sequence
of iterations, the cost of analysis must be justified in
terms of benefits to the project and increased confidence in the reliability of the results. In some cases,
an SDOF approach will be used for the preliminary
design, and a more sophisticated approach using finite
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elements, and/or explosive testing may be used for the
final verification of the design.
A dynamic, nonlinear approach is more likely than
a static approach to provide a section that meets the
design constraints of the project. Elastic static calculations are likely to give overly conservative design
solutions if the peak pressure is considered without
the effect of load duration. By using dynamic calculations instead of static, we are able to account for the
very short duration of the loading. Because the peak
pressure levels are so high, it is important to account
for the short duration to properly model the structural
response. In addition, the inertial effect included in
dynamic computations greatly improves response.
This is because by the time the mass is mobilized; the
loading is greatly diminished, enhancing response.
Furthermore, by accepting that damage occurs, it is
possible to account for the energy absorbed by ductile
systems through plastic deformation. Finally, because
the loading is so rapid, it is possible to enhance the
material strength to account for strain-rate effects.
In dynamic nonlinear analysis, response is evaluated
by comparing the ductility (i.e., the peak displacement
divided by the elastic limit displacement) and/or support rotation (the angle between the support and the
point of peak deflection) to empirically established
maximum values that have been established by the
military through explosive testing. Note that these
values are typically based on limited testing and are
not well defined within the industry at this time. Maximum permissible values vary, depending on the material and the acceptable damage level.
Levels of damage computed by means of analysis may
be described by the terms minor, moderate, or major,
depending on the peak ductility, support rotation and
collateral effects. A brief description of each damage
level is given below.
Minor: Nonstructural failure of building elements,
such as windows, doors, cladding, and false ceilings.
Injuries may be expected, and fatalities are possible
but unlikely.
Moderate: Structural damage is confined to a localized area and is usually repairable. Structural failure
is limited to secondary structural members such as
beams, slabs, and non-load-bearing walls. However,
if the building has been designed for loss of primary
members, localized loss of columns may be accommodated. Injuries and possible fatalities are expected.
Major: Loss of primary structural components, such
as columns or transfer girders, precipitates loss of additional members that are adjacent to or above the lost
member. In this case, extensive fatalities are expected.
Building is usually not repairable.
Generally, moderate damage at the design threat level
is a reasonable design goal for new construction.
Structural Elements
Because the effects of direct explosion decay rapidly with
distance, the local response of structural components is
the dominant concern. General principles governing the
design of critical components are discussed below.
Exterior Frame
There are two primary considerations for the exterior
frame. The first is to design exterior columns to resist
the direct effects of the specified threats. The second
is to ensure the exterior frame has sufficient structural
integrity to accept localized failure without initiating
progressive collapse. The former is discussed in this
section, the latter in the sub-section on structural integrity. Exterior cladding and glazing issues are discussed in Section 6.4, Building Envelope.
Because columns do not have much surface area, airblast loads on columns tend to be mitigated by “cleartime effects”. This refers to the pressure wave washing
around these slender tall members, and consequently, the entire duration of the pressure wave does not
act upon them. On the other hand, the critical threat
is directly across from them, so they are loaded with
the peak reflected pressure, which is typically several
times larger than the incident or overpressure wave
that is propagating through the air.
For columns subjected to a vehicle weapon threat on
an adjacent street, buckling and shearing are the primary effects to be considered in analysis. If a very
large weapon is detonated close to a column, shattering of the concrete due to multiple tensile reflections
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within the concrete section can destroy its integrity.
Buckling is a concern if lateral support is lost due to
the failure of a supporting floor system. This is particularly important for buildings that are close to public
streets. In this case, exterior columns should be capable of spanning two or more stories without buckling.
Slender steel columns are at substantially greater risk
than are concrete columns.
Confinement of concrete using columns with closely
spaced closed ties or spiral reinforcing will improve
shear capacity, improve the performance of lap splices in the event of loss of concrete cover, and greatly
enhance column ductility. The potential benefit from
providing closely spaced closed ties in exterior concrete columns is very high, relative to the cost of the
added reinforcement.
For steel columns, splices should be placed as far
above grade level as practical. It is recommended that
splices at exterior columns that are not specifically designed to resist air-blast loads employ complete-penetration welded flanges. Welding details, materials, and
procedures should be selected to ensure toughness.
For a package weapon, column breach is a major consideration. Some suggestions for mitigating this concern are listed below.
• Do not use exposed columns that are fully or partially accessible from the building exterior. Arcade
columns should be avoided.
• Use an architectural covering that is at least six
inches from the structural member. This will make
it considerably more difficult to place a weapon
directly against the structure. Because explosive
pressures decay so rapidly, every inch of distance
will help to protect the column.
Load-bearing reinforced concrete wall construction
can provide a considerable level of protection if adequate reinforcement is provided to achieve ductile
behavior. This may be an appropriate solution for the
parts of the building that are closest to the secured perimeter line (within twenty feet). Masonry is a much
more brittle material that is capable of generating
highly hazardous flying debris in the event of an explosion. Its use is generally discouraged for new construction.
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Spandrel beams of limited depth generally do well
when subjected to air blast. In general, edge beams are
strongly encouraged at the perimeter of concrete slab
construction to afford frame action for redistribution
of vertical loads and to enhance the shear connection
of floors to columns.
Roof System
The primary loading on the roof is the downward airblast pressure. The exterior bay roof system on the
side(s) facing an exterior threat is the most critical. The
air-blast pressure on the interior bays is less intense, so
the roof there may require less hardening. Secondary
loads include upward pressure due to the air blast penetrating through openings and upward suction during
the negative loading phase. The upward pressure may
have an increased duration, due to multiple reflections
of the internal air-blast wave. It is conservative to consider the downward and upward loads separately.
The preferred system is cast-in-place reinforced concrete with beams in two directions. If this system is
used, beams should have continuous top and bottom
reinforcement with tension lap splices. Stirrups to
develop the bending capacity of the beams closely
spaced along the entire span are recommended.
Somewhat lower levels of protection are afforded by
conventional steel beam construction with a steel deck
and concrete fill slab. The performance of this system
can be enhanced by use of normal-weight concrete
fill, instead of lightweight fill, increasing the gauge of
welded wire fabric reinforcement and making the connection between the slab and beams with shear connector studs. Since it is anticipated the slab capacity
will exceed that of the supporting beams, beam end
connections should be capable of developing the ultimate flexural capacity of the beams to avoid brittle
failure. Beam-to-column connections should be capable of resisting upward as well as downward forces.
Pre-cast and pre-/post-tensioned systems are generally
considered less desirable, unless members and connections are capable of resisting upward forces generated
by rebound from the direct pressure, and/or the suction
from the negative pressure phase of the air blast.
Concrete flat slab/plate systems are also less desirable
because of the potential of shear failure at the columns.
When flat slab/plate systems are used, they should include features to enhance their punching shear resistance. Continuous bottom reinforcement should be
provided through columns in two directions to retain
the slab in the event that punching shear failure occurs. Edge beams should be provided at the building
exterior.
forcement. If pre-/post-tensioned systems are used,
continuous mild steel needs to be added to the top and
the bottom faces to provide the ductility needed to resist explosion loads.
Lightweight systems, such as un-topped steel deck
or wood frame construction, are considered to afford
minimal resistance to air-blast. These systems are
prone to failure, due to their low capacity for downward and uplift pressures.
Flat slab/plate systems are also less desirable because
of limited two-way action and the potential for shear
failure at the columns. When flat slab/plate systems
are employed, they should include features to enhance
their punching shear resistance, and continuous bottom reinforcement should be provided across columns
to resist progressive collapse. Edge beams should be
provided at the building exterior.
Floor System
Interior Columns
The floor system design should consider three possible scenarios: air blast loading, redistributing load
in the event of loss of a column or wall support below,
and the ability to arrest debris falling from the floor or
roof above.
For structures in which the interior is secured against
bombs of moderate size by package inspection, the
primary concern is the exterior bay framing. For buildings that are separated from a public street only by a
sidewalk, the uplift pressures from a vehicle weapon
may be significant enough to cause possible failure of
the exterior bay floors for several levels above ground.
Special concern exists in the case of vertical irregularities in the architectural system, either where the
exterior wall is set back from the floor above or where
the structure steps back to form terraces. The recommendations of Section 6.3.5.2 for roof systems apply
to these areas.
Structural hardening of floor systems above unsecured areas of the building, such as lobbies, loading
docks, garages, mailrooms, and retail spaces, should
be considered. In general, critical or heavily occupied
areas should not be placed underneath unsecured areas, since it is virtually impossible to prevent localized breach in conventional construction from package weapons placed on the floor.
Pre-cast panels are problematic because of their tendency to fail at the connections. Pre-/post-tensioned
systems tend to fail in a brittle manner if stressed much
beyond their elastic limit. These systems are also not
able to accept upward loads without additional reinDesigning Buildings to Mitigate Terrorist Attacks
Interior columns in unsecured areas are subject to
many of the same issues as exterior columns. If possible, columns should not be accessible within these
areas. If they are accessible, then obscure their location or impose a standoff to the structural component
through the use of cladding. Methods of hardening
columns (already discussed under Section 6.3.5.1, Exterior Frame) include using closely spaced ties, spiral reinforcement, and architectural covering at least
six inches from the structural elements. Composite
steel and concrete sections or steel plating of concrete
columns can provide higher levels of protection. Columns in unsecured areas should be designed to span
two or three stories without buckling in the event the
floor below and possibly above the detonation area
have failed, as previously discussed.
Interior Walls
Interior walls surrounding unsecured spaces are designed to contain the explosive effects within the
unsecured areas. Ideally, unsecured areas are located
adjacent to the building exterior so the explosive pressure may be vented outward as well.
Fully grouted CMU (concrete masonry unit) block
walls that are well reinforced vertically, horizontally
and adequately supported laterally are a common solution. Anchorage at the top and bottom of walls should
be capable of developing the full flexural capacity of
the wall. Lateral support at the top of the walls may
be achieved using steel angles anchored into the floor
system above. Care should be taken to terminate bars
at the top of the wall with hooks or heads, and to ensure the upper course of block is filled solid with grout.
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The base of the wall may be anchored by reinforcing
bar dowels.
Interior walls can also be effective in resisting progressive collapse if they are designed properly with
sufficient load-bearing capacity and are tied into the
floor systems below and above.
This design for hardened interior wall construction is
also recommended for primary egress routes to protect
against explosions, fire, and other hazards trapping occupants.
Checklist – Structural
• Incorporate measures to prevent progressive collapse.
• Design floor systems for uplift in unsecured areas
and in exterior bays that may pose a hazard to occupants.
• Limit column spacing.
• Avoid transfer girders.
• Use two-way floor and roof systems.
• Use fully grouted, heavily reinforced CMU block
walls that are properly anchored in order to separate unsecured areas from critical functions and occupied secured areas.
• Use dynamic nonlinear analysis methods for design of critical structural components.
Summary
Chapter three began with an introduction of progressive collapse and the methods which could cause it to
happen. Next, building structural systems were talked
about, including mass, shear capacity and capacity for
reverse loads. The importance of the structural layout of a building was discussed in detail, including the
roof, floors and interior and exterior columns.
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Chapter Four:
Building Envelope
and Mechanical and
Electrical Systems
Overview
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Exterior Wall/Cladding Design
Window Design
Other Openings
Checklist – Building Envelope
Emergency Egress Routes
Emergency Power System
Fuel Storage
Transformers
Ventilation Systems
Fire Control Center
Smoke and Fire Detection and Alarm System
Sprinkler/Standpipe System
Smoke-Control Systems
Checklist - Mechanical and Electrical Systems
Communication System
Summary
Learning Objectives
• Describe the importance of the exterior wall
• Identify the significance of the window design,
including glass design and mullion design
• Be able to discuss the design of doors, louvers
and other openings
• Understand specific recommendations for
mechanical and electrical systems
• Understand options for emergency power systems
• Identify the emergency elevator system
Exterior Wall/Cladding Design
The exterior walls provide the first line of defense
against the intrusion of air-blast pressure and hazardous debris into the building. They are subject to direct
reflected pressures from an explosive threat located
directly across from the wall, along the secured perimeter line. If the building is more than four stories
high, it may be advantageous to consider the reduction
in pressure with height due to the increased distance
and angle of incidence. The objective of design, at a
minimum, is to ensure these members fail in a ductile
mode (such as flexure) rather than a brittle mode (such
as shear). The walls also need to be able to resist loads
transmitted by the windows and doors. It is not uncommon, for instance, for bullet-resistant windows to have
a higher ultimate capacity than the walls to which they
are attached. Beyond ensuring a ductile failure mode,
the exterior wall may be designed to resist the actual
or reduced pressure levels of the defined threat. Note
that special reinforcing and anchors should be provided around blast resistant window and door frames.
Poured-in-place, reinforced concrete will provide the
highest level of protection, but solutions like pre-cast
concrete, CMU block, and metal stud systems may
also be used to achieve lower levels of protection. For
pre-cast panels, consider a minimum thickness of five
inches exclusive of reveals, with two-way, closely
spaced reinforcing bars to increase ductility and reduce the chance of flying concrete fragments. The
objective is to reduce the loads transmitted into the
connections, which need to be designed to resist the
ultimate flexural resistance of the panels. Also, connections into the structure should provide as straight a
line of load transmittal as is practical.
For CMU block walls, use eight-inch block walls, fully grouted with vertically centered heavy reinforcing
bars and horizontal reinforcement placed at each layer.
Connections into the structure should be designed to
resist the ultimate lateral capacity of the wall. For infill walls, avoid transferring loads into the columns if
they are primary load-carrying elements. The connection details may be very difficult to construct. It will
be difficult to have all the blocks fit over the bars near
the top, and it will be difficult to provide the required
lateral restraint at the top connection. A preferred system is to have a continuous exterior CMU wall that
laterally bears against the floor system. For increased
protection, consider using 12-inch blocks with two
layers of vertical reinforcement.
For a metal stud system, use metal studs back-to-back
and mechanically attached in order to minimize lateral torsional effects. To catch exterior cladding fragments, attach a wire mesh or steel sheet to the exterior
side of the metal stud system. The supports of the wall
should be designed to resist the ultimate lateral outof-plane bending capacity load of the system. Brick
veneers and other nonstructural elements attached to
the building exterior are to be avoided; if used, they
should have strengthened connections to limit flying
debris and to improve emergency egress by ensuring
exits remain passable.
Window Design
Windows, once the sole responsibility of the architect,
become a structural issue when explosive effects are
taken into consideration. To mitigate the effects of
explosions, windows should first be designed to resist conventional loads, then be checked for explosive
load effects and balanced design.
Balanced or capacity design philosophy means that
glass is designed to be no stronger than the weakest
part of the overall window system, failing at pressure
levels that do not exceed those of the frame, anchorage, and supporting wall system. If the glass is stronger than the supporting members, the window is likely
to fail with the whole panel entering into the building
as a single unit, possibly with the frame, anchorage,
and wall attached. This failure mode is considered
more hazardous than if the glass fragments were to enter the building, provided the fragments are designed
to minimize injuries. By using a damage-limiting approach, the damage sequence and extent of damage
can be controlled.
Windows are typically the most vulnerable portion of
any building. Though it may be impractical to design
all the windows to resist a large scale explosive attack,
it is desirable to limit the amount of hazardous glass
breakage in order to reduce injuries. Typical annealed
glass windows break at low pressure and impulse levels; shards created by broken windows are responsible
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for many of the injuries incurred during a large-scale
explosive attack.
Designing windows to provide protection against the
effects of explosions can be effective in reducing the
glass laceration injuries in areas that are not directly
across from the weapon. For a large-scale vehicular
weapon, this pressure range is expected on the sides
of surrounding buildings not facing the explosion, or
for smaller explosions in which pressures drop more
rapidly with distance. Generally, it is not known on
which side of the building the attack will occur, so all
sides need to be protected. Window protection should
be evaluated on a case-by-case basis by a qualified
protective design consultant to develop a solution that
meets established objectives. Several recommended
solutions for the design of the window systems to reduce injuries to building occupants are provided in
Figure 6-7.
Figure 6-7 Safe laminated-glass systems and failure modes
Several approaches can be taken to limit glass laceration injuries. One way is to reduce the number and size
of windows. If blast-resistant walls are used, then fewer and/or smaller windows will allow less air blast to
enter the building, thus reducing the interior damage
and injuries. Specific examples of how to incorporate
these ideas into the design of a new building include:
1. limiting the number of windows on the lower
floors where the pressures would be higher during
an external explosion
2. using an internal atrium design with windows facing inward, not outward
3. using clerestory windows, which are close to the
ceiling, above the heads of the occupants
4. angling the windows away from the curb to reduce the pressure levels
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Glass curtain-wall, butt-glazed, and Pilkington type
systems have been found to perform surprisingly well
in recent explosive tests with low explosive loads.
In particular, glass curtain wall systems have been
shown to accept larger deformations without the glass
breaking hazardously, compared to rigidly supported
punched window systems. Some design modifications
to the connections, details, and member sizes may be
required to optimize the performance.
Glass Design
Glass is often the weakest part of a building, breaking at low pressures compared to other components,
such as floors, walls, or columns. Past incidents have
shown glass breakage and associated injuries may extend many thousands of feet in large external explosions. High velocity glass fragments have been shown
to be a major contributor to injuries in such incidents.
For incidents within downtown city areas, falling glass
poses a major hazard to passersby and prolongs postincident rescue and clean-up efforts by leaving tons
of glass debris on the street. At this time, the issue of
exterior debris is largely ignored by existing criteria.
As part of the damage-limiting approach, glass failure
is not quantified in terms of whether breakage occurs
or not, but rather by the hazard it causes to the occupants. Two failure modes that reduce the hazard posed
by window glass are:
• Glass that breaks but is retained by the frame; and
• Glass fragments that exit the frame and fall within
three to ten feet of the window.
The glass performance conditions are defined based
on empirical data from explosive tests performed in
a cubical space with a 10-foot dimension (Table 6-1).
The performance condition ranges from 1, which corresponds to not breaking, to 5, which corresponds to
hazardous flying debris at a distance of 10 feet from
the window (see Figure 6-8). Generally, a performance condition 3 or 4 is considered acceptable for
buildings that are not at high risk of attack. At this
level, the window breaks and fragments fly into the
building but land harmlessly within 10 feet of the window, or impact a witness panel 10 feet away, no more
than 2 feet above the floor level. The design goal is to
achieve a performance level less than 4 for 90 percent
of the windows.
Table 6-1: Performance Conditions for Windows
Figuare 6-8 Plan view of test cubicle showing glass
performance conditions as a function of distance from
text window
The preferred solution for new construction is to use
laminated annealed (i.e., float) glass with structural
sealant around the inside perimeter. For insulated
units, only the inner pane needs to be laminated. The
lamination holds the shards of glass together in explosive events, reducing its ability to cause laceration
injuries. The structural sealant helps to hold the pane
in the frame for higher loads. Annealed glass is used
because it has a breaking strength that is about onehalf that of heat-strengthened glass and about onefourth as strong as tempered glass. Using annealed
glass becomes particularly important for buildings
with lightweight exterior walls that use, for instance,
metal studs, dry wall, and brick façade. Use the thinnest overall glass thickness that is acceptable based on
conventional load requirements. Also, it is important
to use an interlayer thickness that is 60 mil thick rather
than 30 mil thick, as is used in conventional applications. This layup has been shown to perform well in
low-pressure regions (i.e., under about 5 psi). If a 60
mil polyvinyl butaryl (PVB) layer is used, the tension
membrane forces into the framing members need to be
considered in design.
To make sure the components supporting the glass are
stronger than the glass itself, specify a window breakage strength that is high compared to what is used in
conventional design. The breakage strength in window
design may be specified as a function of the number of
windows expected to break at that load. For instance,
in conventional design, it is typical to use a breakage
pressure corresponding to eight breaks out of 1000.
When a lot of glass breakage is expected, such as for
an explosive incident, use a pressure corresponding to
750 breaks out of 1000 to increase confidence that the
frame does not fail, too. Glass breakage strength values may be obtained from window manufacturers.
Mullion Design
The frame members connecting adjoining windows
are referred to as mullions. These members may be
designed in two ways. Using a static approach, the
breaking strength of the window glass is applied to the
mullion; alternatively, a dynamic load can be applied
using the peak pressure and impulse values. The static
approach may lead to a design that is not practical,
because the mullion can become very deep and heavy,
driving up the weight and cost of the window system.
It also may not be consistent with the overall architectural objectives for the project.
As with frames, it is good engineering practice to limit
the number of interlocking parts used for the mullion.
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Frame and Anchorage Design
Window frames need to retain the glass so the entire
pane does not become a single large unit of flying
debris. Frames also need to be designed to resist the
breaking stress of the window glass.
To retain the glass in the frame, a minimum of a ¼-inch
bead of structural sealant (e.g., silicone) should be
used around the inner perimeter of the window. The
allowable tensile strength should be at least 20 psi.
Also, the window bite (i.e., the depth of window captured by the frame) needs to be at least ½ inch. The
structural sealant recommendations should be determined on a case-by-case basis. In some applications,
the structural sealant may govern the overall design of
the window system.
Frame and anchorage design is performed by applying
the breaking strength of the window to the frame and
the fasteners. In most conventionally designed buildings, the frames will be aluminum. In some applications, steel frames are used. Also, in lobby areas where
large panes of glass are used, a larger bite with more
structural sealant may be needed.
Inoperable windows are generally recommended for
air-blast mitigating designs. However, some operable
window designs are conceptually viable. For instance,
designs in which the window rotates about a horizontal
hinge at the head, or sill, and opens in the outward direction, may perform adequately. In these designs, the
window will slam shut in an explosion event. If this
type of design is used, the governing design parameter
may be the capacity of the hinges and/or hardware.
Wall Design
The supporting wall response should be checked using
approaches similar to those for frames and mullions. It
does not make sense, and is potentially highly hazardous, to have a wall system that is weaker than windows.
Remember, the maximum strength of any wall system
needs to be at least equal to the window strength. If the
walls are unable to accept the loads transmitted by the
mullions, the mullions may need to be anchored into
the structural slabs or spandrel beams. Anchoring into
columns is generally discouraged, because it increases
the tributary area of lateral load that is transferred into
the columns and may cause instability.
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The balanced-design approach is particularly challenging in the design of ballistic-resistant and forcedentry-resistant windows, which consist of one or more
inches of glass and polycarbonate. These windows
easily can become stronger than supporting walls. In
these cases, the windows may need to be designed for
the design threat air-blast pressure levels under the implicit assumption that balanced-design conditions will
not be met for larger loads.
Multi-hazard Considerations
Under normal operating conditions, windows perform
several functions:
• They permit light to enter the building.
• They save energy by reducing thermal transmission.
• They make the building quieter by reducing acoustic transmission.
Explosions are one of a number of abnormal loading
conditions that the building may undergo. Some of the
others are:
• fire
• earthquake
• hurricane
• gunfire, and
• forced entry
When developing a protection strategy for windows
to mitigate the effects of a particular explosion threat
scenario, it is important to consider how this protection may interfere with some of these other functions
or explosion threat scenarios. Some questions that
may be worthwhile to consider are:
• If an internal explosion occurs, will the upgraded
windows increase smoke inhalation injuries by preventing the smoke from venting through windows
that would normally break in an explosion event?
• If a fire occurs, will it be more difficult to break the
protected windows to vent the building and gain
access to the injured?
• Will a window upgrade that is intended to protect
the occupants worsen the hazard to passersby?
Other Openings
Doors, louvers, and other openings in the exterior envelope should be designed so the anchorage into the
supporting structure has a lateral capacity greater than
that of the element.
There are two general recommendations for doors:
• Doors should open outward so they bear against
the jamb during the positive-pressure phase of the
air-blast loading.
• Door jambs should be filled with concrete to improve their resistance.
For louvers that provide air to sensitive equipment,
some recommendations:
• Provide a baffle in front of the louver so the air
blast does not have direct “line-of-sight” access
through the louver.
• Provide a grid of steel bars properly anchored into
the structure behind the louver to catch any debris
generated by the louver or other flying fragments.
Checklist – Building Envelope
Cladding
• Use the thinnest panel thickness that is acceptable
for conventional loads.
• Design cladding supports and the supporting structure to resist the ultimate lateral resistance of the
panel.
• Design cladding connections to have as direct a
load transmission path into the main structure as
practical. A good transmission path minimizes
shear and torsional response.
• Avoid framing cladding into columns and other
primary vertical load-carrying members. Instead,
frame into floor diaphragms.
Windows
• Use the thinnest glass section acceptable for conventional loads.
• Design window systems so the frame anchorage
and the supporting wall are capable of resisting the
breaking pressure of the window glass.
• Use laminated annealed glass (for insulated panels,
only the interior pane needs to be laminated).
• Design window frames with a minimum of a
½-inch bite.
• Use a minimum of ¼-inch silicone sealant around
the inside glass perimeter, with a minimum tensile
strength of 20 psi.
Mechanical and Electrical Systems
In the event of an explosion directed at a high-occupancy building, the primary objective is to protect people
by preventing building collapse. The secondary goal
is to limit injuries due to flying building debris and the
direct effects of air blast entering the building (e.g.,
impact due to being thrown or lung collapse). Beyond
these life-safety concerns, the objective is to facilitate
building evacuation and rescue efforts through effective building design. This last objective is the focus
of this section. Issues related specifically to chemical,
biological, and radiological threats are discussed under a separate section with that heading.
The key concept for providing secure and effective
mechanical and electrical systems in buildings is the
same as for the other building systems: separation,
hardening, and redundancy. Keeping critical mechanical and electrical functions as far from high-threat
areas as possible (e.g., lobbies, loading docks, mail
rooms, garages, and retail spaces) increases their ability to survive an event. Separation is perhaps the most
cost-effective option. Additionally, physical hardening
or protection of these systems (including the conduits,
pipes, and ducts associated with life-safety systems)
provides increased likelihood they will be able to survive the direct effects of the event if they are close
enough to potentially be affected. Finally, by providing redundant emergency systems that are adequately
separated, there is a greater likelihood that emergency
systems will remain operational post-event to assist
rescuers in evacuating the building.
Architecturally, enhancements to mechanical and
electrical systems will require additional space to accommodate additional equipment. Fortunately, there
are many incremental improvements that can be made,
requiring only small changes to the design. Additional space can be provided for future enhancements as
funds, or the risk, justify implementation.
Structurally, the walls and floor systems adjacent to
the areas where critical equipment is located need
to be protected by means of hardening. Other areas
where hardening is recommended include; primary
egress routes, feeders for emergency power distribution, sprinkler system mains and risers, fire alarm system trunk wiring, and ducts used for smoke-control
systems.
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From an operational security standpoint, it is important to restrict and control access to air-intake louvers,
mechanical and electrical rooms, telecommunications
spaces and rooftops by means of such measures as;
visitor screening, limited elevator stops, closed-circuit
television (CCTV), detection, and card access-control
systems. Specific recommendations are given below for
1. Emergency egress routes
2. The emergency power system
3. Fuel storage
4. Transformers
5. Ventilation systems
6. The fire control center
7. Emergency elevators
8. The smoke and fire detection and alarm system
9. The sprinkler/standpipe system
10. Smoke control system
11. The communication system
louvers are not vulnerable to attack. A remote radiator
system could be used to reduce the louver size.
Emergency Egress Routes
Fuel storage
To facilitate evacuation, consider these measures.
• Provide positive pressurization of stairwells and
vestibules.
• Provide battery packs for lighting fixtures and exit
signs.
• Harden walls using reinforced CMU block properly anchored at supports.
• Use non-slip phosphorescent treads.
• Do not cluster egress routes in a single shaft. Separate them as far as possible.
• Use double doors for mass evacuation.
• Do not use glass along primary egress routes or
stairwells.
A non-explosive fuel source, such as diesel fuel, is acceptable for standby use for emergency generators and
diesel fire pumps. Fuel tanks should be located away
from building access points, in fire-rated, hardened
enclosures. Fuel piping within the building should be
located in hardened enclosures, and redundant piping
systems could be provided to enhance the reliability
of the fuel distribution system. Fuel filling stations
should be located away from public access points and
monitored by the CCTV system.
Emergency Power System
An emergency generator provides an alternate source
of power, should utility power become unavailable
to critical life-safety systems, such as alarm systems,
egress lighting fixtures, exit signs, emergency communications systems, smoke-control equipment, and
fire pumps.
Emergency generators typically require large louvers
to allow for ventilation of the generator while running.
Care should be taken to locate the generator so these
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Redundant emergency generator systems, remotely located from each other, enable the supply of emergency
power from either of two locations. Consider locating
emergency power-distribution feeders in hardened enclosures, or encased in concrete, and configured in redundant routing paths to enhance reliability. Emergency distribution panels and automatic transfer switches
should be located in rooms separate from the normal
power system (hardened rooms, where possible).
Emergency lighting fixtures and exit signs along the
egress path could be provided with integral battery
packs, which locate the power source directly at the
load, to provide lighting instantly in the event of a utility power outage.
Transformers
Main power transformer(s) should be located interior
to the building if possible, away from locations accessible to the public. For larger buildings, multiple
transformers, located remotely from each other, could
enhance reliability, should one transformer be damaged by an explosion.
Ventilation Systems
Air-intake locations should be located as high up in the
building as is practical to limit access by the general
public. Systems that serve public access areas such as
mail receiving rooms, loading docks, lobbies, freight
elevators/lobbies should be isolated and provided with
dedicated air handling systems capable of 100 percent
exhaust mode. Tie air intake locations and fan rooms
into the security surveillance and alarm system.
Building HVAC systems are typically controlled by a
building automation system, which allows for quick
response to shut down or selectively control air conditioning systems. This system is coordinated with the
smoke-control and fire-alarm systems.
See Chapter 5 on chemical, biological, and radiological protection for more information.
Fire Control Center
A Fire Control Center should be provided to monitor
alarms and life safety components, operate smokecontrol systems, communicate with occupants, and
control the fire-fighting/evacuation process. Consider
providing redundant Fire Control Centers remotely
located from each other to allow system operation
and control from alternate locations. The Fire Control
Center should be located near the point of firefighter
access to the building. If the control center is adjacent
to the lobby, separate it from the lobby by using a corridor or other buffer area. Provide hardened construction for the Fire Control Center.
Emergency Elevators
Elevators are not used as a means of egress from a
building in the event of a life-safety emergency event,
because conventional elevators are not suitably protected from the penetration of smoke into the elevator
shaft. An unwitting passenger could be endangered if
an elevator door opened onto a smoke-filled lobby or
other area. Firefighters may elect to manually use an
elevator for firefighting or rescue operation.
A dedicated elevator, within its own hardened, smokeproof enclosure, could enhance the firefighting and
rescue operation after a blast/fire event. The dedicated
elevator should be powered from the emergency generator, fed by conduit/wire that is protected in hardened enclosures. This shaft/lobby assembly should be
sealed and positively pressurized to prevent the penetration of smoke into the protected area.
Smoke and Fire Detection
and Alarm System
A combination of early-warning smoke detectors,
sprinkler-flow switches, manual pull stations, and audible and visual alarms provide quick response and
notification of an event. The activation of any device
will automatically start the sequence of operation of
smoke control, egress, and communication systems to
allow occupants to go quickly to a safe area. System
designs should include redundancy, such as looped infrastructure wiring and distributed intelligence, such
that the severing of the loop will not disable the system. Install a fire-alarm system consisting of distributed intelligent fire alarm panels, connected in a peerto-peer network, such that each panel can function
independently, process alarms and initiate sequences
within its respective zone.
Sprinkler/Standpipe System
Sprinklers will automatically suppress fire in the area
upon sensing heat. Sprinkler activation will activate
the fire alarm system. Standpipes have water available
locally in large quantities for use by professional firefighters. Multiple sprinkler and standpipe risers limit
the possibility of an event severing all water supplies
available to fight a fire.
Redundant water services would increase the reliability of the source for sprinkler protection and fire
suppression. Appropriate valving should be provided
where services are combined.
Redundant fire pumps could be provided in remote locations. These pumps could rely on different sources;
for example, one electric pump supplied from the utility and/or emergency generator and a second diesel
fuel source fire pump.
Diverse and separate routing of standpipe and sprinkler risers within hardened areas will enhance the system’s reliability (i.e., reinforced masonry walls at stair
shafts containing standpipes).
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Smoke-Control Systems
Appropriate smoke-control systems maintain smokefree paths of egress, for building occupants, by utilizing a series of fans, ductwork, and fire smoke dampers. Stair pressurization systems maintain a clear path
of egress for occupants to reach safe areas or to evacuate the building. Smoke control fans should be located
higher in a building, rather than at lower floors, to
limit exposure/access to external vents. Vestibules at
stairways with separate pressurization provide an additional layer of smoke control.
Communication System
• Place a transformer interior to building, if possible.
• Provide access to the fire control center from the
building exterior.
Summary
At this chapter’s beginning, we examined exterior
wall design as a first line of defense against attack.
Next, window design was discussed in detail; including glass design, mullion design, frame and anchorage design, and wall design. In the second half of the
chapter, we studied mechanical and electrical systems
and how their design should be set up to mitigate a
terrorist attack.
A voice communication system facilitates the orderly
control of occupants and evacuation of the danger area
or the entire building. The system is typically zoned
by floor, by stairwell, and by elevator bank for selective communication to building occupants. Emergency communication can be enhanced by providing:
• Extra emergency phones, separate from the telephone system, connected directly to a constantly
supervised central station;
• In-building repeater system for police, fire, and
EMS (Emergency Medical Services) radios; and
• Redundant or wireless fireman’s communications
in building.
Checklist – Mechanical
and Electrical Systems
• Place all emergency functions away from high-risk
areas, in protected locations with restricted access.
• Provide redundant and separated emergency functions.
• Harden the enclosures around emergency equipment, controls, and wiring and/or provide physical
buffer zones.
• For egress routes, provide battery packs for exit
signs, use non-slip phosphorescent stair treads, and
double doors for mass evacuation.
• Avoid using glass along primary egress routes or
stairwells.
• Place emergency functions away from structurally
vulnerable areas such as transfer girders.
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Chapter Five:
Chemical, Biological,
And Radiological
Protection
Overview
•
•
•
•
•
•
•
•
•
•
•
Introduction
Air Intakes
Mechanical Areas
Return-Air Systems
Lobbies, Loading Docks, and Mail Sorting Areas
Zoning of HVAC Systems
Positive Pressurization
Airtightness
Filtration Systems
Detection Systems
Emergency Response Using Fire/HVAC Control
Center
• Evolving Technologies
• Checklist - Chemical, Biological & Radiological
Protective Measures
• Summary
Learning Objectives
• Discuss three types of airborne hazards
• Understand the importance of air intake systems
and return air systems and their locations
• Explain vulnerable internal areas where hazards
may be brought into a building
• Describe an HVAC system, how to zone an HVAC
system and the benefits of using the system
• Understand the importance of a detection system
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Introduction
This section discusses three types of air-borne hazards.
1. A large exterior release, originating some distance
away from the building (includes delivery by aircraft).
2. A small, localized exterior release at an air intake
or other opening in the exterior envelope of the
building.
3. A small interior release in a publicly accessible
area, a major egress route, or other vulnerable area
(e.g., lobby, mail room, delivery receiving).
Like explosive threats, chemical, biological and radiological (CBR) threats may be delivered to the building externally or internally. External, ground-based
threats may be released at a standoff distance from the
building or delivered directly through an air intake or
other opening. Interior threats may be delivered to accessible areas, such as the lobby, mailroom, or loading
dock, or they may be released into a secured area, such
as a primary egress route. This discussion is limited to
air-borne hazards.
There may not be an official or obvious warning prior to
a CBR event. While you should always follow any official warnings, the best defense is to be alert to signs of a
release occurring near you. The air may be contaminated
if you see a suspicious cloud or smoke near ground level,
hear an air blast, smell strange odors, see birds or other
small animals dying, or hear more than one person complaining of eye, throat or skin irritation or convulsing.
Chemicals will typically cause problems within seconds or minutes after exposure, but they can sometimes have delayed effects that do not appear for hours
or days. Symptoms may include blurred or dimmed
vision; eye, throat, or skin irritation; difficulty breathing; excess saliva; or nausea.
Biological and some radioactive contaminants typically will take days to weeks before symptoms appear,
so listen for official information regarding symptoms.
With radioactive “dirty” bombs, the initial risk is from
the explosion. Local responders may advise you to either shelter-in-place or evacuate. After the initial debris falls to the ground, leaving the area and washing
will minimize your risk from the radiation.
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Buildings provide a limited level of inherent protection against CBR threats. To some extent, the protection level is a function of how airtight the building is;
to a greater degree it is a function of the HVAC system’s design and operating parameters.
The objectives of protective building design, as they
relate to the CBR threat are: first, to make it difficult
for terrorists to successfully execute a CBR attack;
and second, to minimize the impact (e.g., life, health,
property damage, loss of commerce) of an attack if it
does occur.
In order to reduce the likelihood of an attack, use security and design features that limit terrorists’ ability
to approach the building and successfully release the
CBR contaminant. Some examples are:
• Use security stand-off, accessibility, and screening
procedures similar to those identified in the explosive threat mitigation sections of this document
• Recognize areas around HVAC equipment and
other mechanical systems to be vulnerable areas
requiring special security considerations.
• Locate outdoor air intakes high above ground level
and at inaccessible locations.
• Prevent unauthorized access to all mechanical areas and equipment.
• Avoid the use of ground-level mechanical rooms
accessible from outside the building. Where such
room placement is unavoidable, doors and air vents
leading to these rooms should be treated as vulnerable locations and appropriately secured.
• Treat operable, ground-level windows as a vulnerability and either void their use or provide appropriate
security precautions to minimize the vulnerability.
• Interior to the building, minimize public access to
HVAC return-air systems.
Further discussion of some of these prevention methods is provided on the following pages.
Air intakes
Air intakes may be made less accessible by placing
them as high as possible on the building exterior,
with louvers flush with the exterior (see figure 6-9).
All opportunities to reach air intakes through climbing should be eliminated. Ideally, there is a vertical
smooth surface from the ground level to the intake
louvers, without such features as high shrubbery, low
roofs, canopies, or sunshades, as these features can
enable climbing and concealment. To prevent opportunities for a weapon to be lobbed into the intake, the
intake louver, ideally, should be flush with the wall.
Otherwise, a surface sloped at least 45 degrees away
from the building and further protected through the
use of metal mesh (a.k.a. bird screen) should be used.
Finally, CCTV surveillance and enhanced security is
recommended at intakes.
In addition to providing protection against an air-borne
hazard delivered directly into the building, placing air
intakes high above the ground provides protection
against ground-based standoff threats, because concentration of the air-borne hazard diminishes somewhat with height. Because air-blast pressure decays
with height, elevated air intakes also provide modest protection against explosion threats. Furthermore,
many recognized sources of indoor air contaminants
(e.g., vehicle exhaust, standing water, lawn chemicals, trash, and rodents) tend to be located near ground
level. Thus, elevated air intakes are a recommended
general practice for healthy indoor air quality.
In the event a particular air intake does not service an
occupied area, it may not be necessary to elevate it
above ground level. However, if the unoccupied area
is within an otherwise occupied building, the intake
should either be elevated, or significant precautions
(tightly sealed construction between unoccupied/occupied areas, unoccupied area maintained at negative
pressure relative to occupied area) should be put in
place to ensure contaminants are unable to penetrate
into the occupied area of the building.
Figure 6-9 Schematic
showing recommended
location for elevated airintakes
on
exterior
of building
Mechanical Areas
Another simple measure is to tightly restrict access
to building mechanical areas. These areas provide access to equipment and systems (e.g., HVAC, elevator,
building exhaust, and communication and control) that
could be used or manipulated to assist in a CBR attack.
Additional protection may be provided by including
these areas in those monitored by electronic security
and by eliminating elevator stops at levels that house
this equipment. For rooftop mechanical equipment,
ways of restricting (or at least monitoring) access to the
roof that do not violate fire codes should be pursued.
Return-Air Systems
Similar to the outdoor-air intake, HVAC return-air
systems inside the building can be vulnerable to CBR
attack. Buildings requiring public access have an increased vulnerability to such an attack. Design approaches that reduce this vulnerability include the use
of ducted HVAC returns within public access areas
and the careful placement of return-air louvers in secure locations not easily accessed by public occupants.
The second objective is to design with the goal of minimizing the impact of an attack. For many buildings,
especially those requiring public access, preventing
a determined terrorist from initiating a CBR release
will be a significant challenge. Compared to buildings
in which campus security and internal access can be
strictly controlled, public-access buildings may require a greater emphasis on mitigation. However, even
private access facilities can fall victim to an internal
CBR release, whether through a security lapse, or perhaps, a delivered product (mail, package, equipment,
or food). Examples of design methods to minimize the
impact of a CBR attack are listed below.
• Public access routes to the building should be designed to channel pedestrians through points of noticeable security presence.
• The structural and HVAC design should isolate
the most vulnerable public areas (entrance lobbies,
mail rooms, load/delivery docks) both physically
and in terms of potential contaminant migration.
• The HVAC and auxiliary air systems should carefully use positive and negative pressure relationships to influence contaminant migration routes.
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Let’s look closer at some of these prevention methods.
Lobbies, Loading Docks,
and Mail Sorting Areas
Vulnerable internal areas where airborne hazards
may be brought into the building should be strategically located. These include lobbies, loading docks,
and mail sorting areas. Where possible, place these
functions outside the footprint of the main building.
When incorporated into the main building, these areas should be physically separated from other areas by floor-to-roof walls. Additionally, these areas
should be maintained under negative pressure, relative to the rest of the building, but at positive-toneutral pressure relative to the outdoors. To assist in
maintaining the desired pressure relationship, necessary openings (doors, windows, etc.) between secure and vulnerable areas should be equipped with
sealing windows, doors, and wall openings due to
ductwork, utilities, and other penetrations should be
sealed. Where entries into vulnerable areas are frequent, the use of airlocks or vestibules may be necessary to maintain the desired pressure differentials.
Ductwork that travels through vulnerable areas
should be sealed. Ideally, these areas should have
separate air-handling units to isolate the hazard. Alternatively, the conditioned air supply to these areas
may come from a central unit, as long as exhaust/
return air from these areas is not allowed to mix into
other portions of the building. In addition, emergency exhaust fans activated upon internal CBR release
within the vulnerable area will help to purge the
hazard from the building and minimize its migration into other areas. Care must be taken to be sure
the discharge point for the exhaust system is not colocated with expected egress routes. Consideration
should also be given to filtering this exhaust with
High Efficiency Particulate Air (HEPA) filtration.
For entrance lobbies that contain a security screening location, it is recommended that an airlock or
vestibule be provided between the secured and unsecured areas.
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Zoning of HVAC Systems
Large buildings usually have multiple HVAC (heating,
ventilation, air conditioning) zones, each zone with its
own air-handling unit and duct system. In practice,
these zones are not completely separated if they are
on the same floor. Air circulates among zones through
plenum returns, hallways, atria, and doorways that are
normally left open. Depending upon the HVAC design and operation, airflow between zones on different floors can also occur through the intentional use
of shared air return/supply systems and through air
migrations via stairs and elevator shafts.
Isolating the separate HVAC zones minimizes the potential spread of an airborne hazard within a building,
reducing the number of people potentially exposed if
there is an internal release. Zone separation also provides limited benefit against an external release, as it
increases internal resistance to air movement produced
by wind forces and chimney effect, thus reducing the
rate of infiltration. In essence, isolating zones divides
the building into separate environments, limiting the
effects of a single release to an isolated portion of the
building. Isolation of zones requires full-height walls
between each zone and the adjacent zones and hallway
doors.
Another recommendation is to isolate the return system (i.e., no shared returns). Strategically locate return
air grilles in easily observable locations, preferably in
areas with reduced public access.
Both centralized and decentralized shutdown capabilities are advantageous. To quickly shut down all HVAC
systems at once, in the event of an external threat, a
single-switch control is recommended for all air exchange fans (including bathrooms, kitchens, and other
exhaust sources). In the event of a localized internal
release, redundant decentralized shutdown capability is also recommended. Controls should be placed
in a location easily accessed by the facility manager,
security, or emergency response personnel. Duplicative and separated control systems will add an increased degree of protection. Further protection may
be achieved by placing low-leakage automatic dampers on air intakes and exhaust fans that do not already
have back-draft dampers.
Positive Pressurization
Filtration Systems
Traditional good engineering practice for HVAC design strives to achieve a slight overpressure of 5-12
Pa (.02-inch-.05-inch w.g.) within the building environment, relative to the outdoors. This design practice
is intended to reduce uncontrolled infiltration into the
building. When combined with effective filtration, this
practice will also provide enhanced protection against
external releases of CBR aerosols.
To offer effective protection, filtration systems should
be specific to the particular contaminant’s physical
state and size. Chemical vapor/gas filtration (a.k.a. air
cleaning) is currently a very expensive task (high initial and recurring costs) with a limited number of design professionals experienced in its implementation.
Specific expertise should be sought if chemical filtration is desired. Possible application of the air cleaning
approach to collective protection zones (with emergency activation) can assist in significantly reducing
the cost, though protection is limited to the reduced
size of the zone.
Using off-the-shelf technology (e.g., HEPA), manually triggered augmentation systems can be put into
place to over-pressure critical zones to intentionally
impact routes of contaminant migration, and/or to
provide safe havens for sheltering-in-place. For egress
routes, positive-pressurization is also recommended
(unless, of course, the CBR source is placed within
the egress route). Design parameters for such systems
will depend upon many factors specific to the building
and critical zone in question. Care must be taken that
efforts to obtain a desired pressure relationship within
one zone will not put occupants in another zone at increased risk. Lastly, the supply air used to pressurize
the critical space must be appropriately filtered (see
filtration discussion below) or originate from a noncontaminated source in order to be beneficial.
Airtightness
To limit the infiltration of contaminants from outside the
building into the building envelope, building construction should be made as airtight as possible. Tight construction practices (weatherization techniques, tightly
sealing windows, doors, wall construction, continuous
vapor barriers, sealing interface between wall and window/door frames) will also help to maintain the desired
pressure relationships between HVAC zones. To ensure
the construction of the building has been performed
correctly, building commissioning is recommended
throughout the construction process, and prior to taking ownership to observe construction practices and to
identify potential airflow trouble spots (cracks, seams,
joints, and pores in the building envelope and along the
lines separating unsecured from secured space), before
they are covered with finish materials.
Most “traditional” HVAC filtration systems focus on
aerosol type contaminants. The CBR threats in this category include radioactive “dirty bombs”, bio-aerosols,
and some chemical threats. Riot-control agents and
low-volatility nerve agents, for example, are generally
distributed in aerosol form. However, a vapor component of these chemical agents could pass through a
filtration system. HEPA filtration is currently considered adequate by most professionals, to achieve sufficient protection from CBR particulates and aerosols.
However, HEPA filtration systems generally have a
higher acquisition cost than traditional HVAC filters
and they cause larger pressure drops within the HVAC
system, resulting in increased energy requirements to
maintain the same design airflow rate. Due to recent
improvements in filter media development, significant
improvements in aerosol filtration can be achieved at
relatively minimal increases in initial and operating
costs. Also important is that incremental increases in
filtration efficiency will generally provide incremental
increases in protection from the aerosol contaminant.
In 1999, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) released Standard 52.2-1999. This standard provides
a system for rating filters that quantifies filtration efficiency in different particle size ranges to provide a
composite efficiency value, named the Minimum Efficiency Reporting Value (MERV). MERV ratings range
between 1 and 20, with a higher MERV indicating a
more efficient filter. Using the MERV rating table, a
desired filter efficiency may be selected according to
the size of the contaminant under consideration. For example, a filter with a MERV of 13 or more will provide
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a 90% or greater reduction of most CBR aerosols (generally considered to be at least 1-3 um in size or larger)
within the filtered air stream with much lower acquisition and maintenance costs than HEPA filtration.
Efficiency of filtration systems is not the only concern. Air can become filtered only if it actually passes
through the filter. Thus, filter-rack design, gasketing,
and good quality filter sources should all play a role in
minimizing bypass around the filter. The use of returnair filtration systems and the strategic location of supply
and return systems should also be carefully employed
to maximize effective ventilation and filtration rates.
Detection Systems
Beyond the measures discussed above, there is the
option of using detection systems as part of the protective design package. In general, affordable, timely,
and practical detection systems specific to all CBR
agents are not yet available. However, for aerosol
contaminants, nonspecific detection equipment can be
employed to activate response actions, should a sudden spike in aerosol concentration of a specific size
range be detected. If the spike were detected in an outdoor intake, for example, this could trigger possible
response options, such as damper closure, system
shutdown, bypass to alternate air intake, or rerouting
the air through a special bank of filters. Such protective actions could occur until an investigation was performed by trained personnel (i.e., check with adjacent
alarms, and review security tape covering outdoor air
intake). Unless foul play was discovered, the entire
process could be completed within 10 minutes or less
,and without alarming occupants. The initial cost of
such a system is relatively modest (depending upon
the number of detectors and response options incorporated into the design), but the maintenance requirements are relatively high. Similar monitoring systems
could be employed to trigger appropriate responses in
high-threat areas, such as mailrooms, shipping/receiving areas, or entrance lobbies. The approach could also
be expanded to incorporate some of the newer chemical detection technologies, though the low threshold
requirements may generate a substantial number of
false positives. As technology progresses, detector
availability and specificity should continue to expand
into the general marketplace.
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It is recognized at this time, detection systems are not
appropriate for many buildings. Consider using higherefficiency filtration systems initially, and design HVAC
systems so detection systems can be easily integrated
into the HVAC control package at a later date.
Emergency Response Using Fire/HVAC
Control Center
Certain operations that are managed at the Fire Control Center can play a protective role in the response to
a CBR incident. Examples of such operations and how
they could be used are given below.
• Purge fans. These can be used to purge an interior CBR release or to reduce indoor contaminant
concentrations, following building exposure to an
external CBR source. (Note: In practice, some jurisdictions may recommend purging for chemical
and radiological contaminants but not for biological contaminants, which may be communicable
and/or medically treatable.)
• Communication Systems. Building communication systems that allow specific instructions to be addressed to occupants in specific zones of the building can play a significant role in directing occupant
response to either an internal or external release.
• Pressurization Fans. These provide two functions. First, the ability to override and deactivate
specific positive-pressure zones may be beneficial
in the event a known CBR source is placed into
such an area. Second, areas designated for positive
pressurization (generally for smoke protection)
may also become beneficial havens for protection
from internal and external CBR releases, if they are
supplied by appropriately filtered air.
• HVAC Controls. The ability to simultaneously
and individually manipulate operation of all HVAC
and exhaust equipment from a single location may
be very useful during a CBR event. Individuals empowered to operate such controls must be trained
in their use. The provision of simple floor-by-floor
schematics showing equipment locations and the
locations of supply and return louvers will aid the
utility of this control option.
• Elevator Controls. Depending upon their design
and operation, the ability to recall elevators to the
ground floor may assist in reducing contaminant
migration during a CBR event.
Evolving Technologies
Many of the challenges relating to CBR terrorism
prevention will be facilitated with the introduction of
new technologies developed to address this emerging threat. As vendors and products come to market,
it is important that the designer evaluate performance
claims with a close level of scrutiny. Vendors should
be willing to guarantee performance specs in writing,
provide proof of testing and show certified results by
an independent, reputable lab. The testing conditions
(e.g., flow rate, residence time, incoming concentrations) should be consistent with what would be experienced within the owner’s building. For CBR developments, proof of federal government testing and
acceptance may be available.
Checklist – Chemical, Biological &
Radiological Protective Measures
• Place air intakes servicing occupied areas as high
as practicably possible (minimum 12 feet above
ground). GSA may require locating at fourth floor
or above when applicable.
• Restrict access to critical equipment.
• Isolate separate HVAC zones and return air systems.
• Isolate HVAC supply and return systems in unsecured areas.
• Physically isolate unsecured areas from secured
areas.
• Use positive pressurization of primary egress
routes, safe havens, and/or other critical areas.
• Commission building throughout construction and
prior to taking ownership.
• Provide redundant, easily accessible shutdown capabilities.
• For higher levels of protection, consider using contaminant-specific filtration and detection systems.
• Incorporate fast-acting, low-leaking dampers.
• Filter both return air and outdoor air for publicly
accessible buildings.
• Select filter efficiencies based upon contaminant
size. Use reputable filter media installed into tightfitting, gasketed and secure filter racks.
• Provide separate HVAC, with isolated returns capable of 100% exhaust.
• Operate these areas at negative pressure relative to
secure portion of the building.
• Use airtight construction, vestibules, and air locks
if there is high traffic flow.
• Consider installation of an emergency exhaust fan
to be activated upon suspected internal CBR release.
• Lock, secure, access-log, and control mechanical
rooms.
• In public access areas, use air diffusers and return
air grilles that are secure or under security observation.
• Zone the building communication system so it is
capable of delivering explicit instructions, and has
back-up power.
• Create safe zones using enhanced filtration, tight
construction, emergency power, dedicated communication systems, and appropriate supplies (food,
water, first aid, and personal-protective equipment).
Summary
In this chapter we were introduced to three types of
airborne hazards. Chemical, biological and radiological warfare were discussed in detail. Next, air intakes
and HVAC systems were examined. This included
how to prevent CBR threats and also how to detect
them. Evolving technology was the final topic, as we
addressed challenges and new developments in CBR
threats.
For higher threat areas (mail room, receiving, reception/screening lobby):
• Preferably, locate these areas outside the main
building footprint.
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Chapter Six:
Occupancy Types and
Cost Considerations
Overview
•
•
•
•
•
•
•
•
Introduction
Multi-Family Residential Occupancy
Commercial Retail Space Occupancy
Light Industrial Buildings
Initial Costs
Life-Cycle Costs
Setting Priorities
Summary
Learning Objectives
• Recognize multi-family residential buildings
and the types of occupants
• Distinguish the unique features of commercial
retail space occupancy and the challenges
associated with them
• Understand the two types of initial construction
costs
• Describe the setting of priorities if the initial cost
of construction is too high
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Introduction
The previous chapters focused on the protective design of office buildings, because this occupancy is
one that has been of most concern within the public
and private sectors to date. The concepts discussed,
however, are largely applicable to any type of civilian
building serving large numbers of people on a daily
basis. This chapter considers the unique challenges
associated with three other high-occupancy building
types: multi-family residential buildings, commercial
retail buildings, and light-industrial buildings. (Protection of schools and hospitals is addressed in other
FEMA reports and is not explicitly addressed here.)
The uniqueness of these other occupancy types from
the perspective of protective design is a function of
many factors, including hours of peak usage, dominant population, size of building, and construction
type. For dual-use facilities, such as those that incorporate retail and commercial office uses, two important recommendations for the HVAC are:
• to provide separate HVAC zones; and
• to strictly adhere to isolation principles (that is, to
treat any public area as equivalent to an entrance
lobby in a single-use building).
Multi-Family Residential Occupancy
Multi-family residential buildings are unique because
they tend to house more elderly, handicapped, and
children than office buildings, which tend to have
more able-bodied occupants within the working age
(18-65). Office buildings, of course, can have a certain
percentage of less-able-bodied populations, depending
on the tenancy (e.g., medical offices, social services,
or child care centers), and such populations need to be
accounted for in the design of these buildings as well.
In any case, the occupancy will have a major effect on
evacuation and rescue efforts.
For multi-family residential buildings, it becomes
even more imperative that primary egress routes, including hallways leading to stairwells, remain as clear
of debris and smoke as possible during the evacuation
period. This criterion demands a higher level of protection than has been discussed for office buildings.
Here are some recommendations for providing enDesigning Buildings to Mitigate Terrorist Attacks
hanced protection to facilitate evacuation and rescue
of distressed populations:
• Place hallways in a protected location, away from
the building exterior.
• Do not use glass in the hallways used for primary
egress.
• Egress routes should lead to exits that are as far
as possible from high-risk areas, such as the lobby,
mail room, and delivery entrance.
• Create pressurized safe havens in elevator vestibules and stairwells, using tightly constructed, airtight enclosures placed in a protected core area of
the building.
• Emergency exits should be easily accessible by
emergency vehicles, and spacious enough to accommodate rescue workers entering the building,
as well as injured persons exiting the building.
• The side(s) of the building with emergency exits
should be free of any canopies, overhanging balconies, or other ornamentation that may fall and
block the exits.
• Emergency power should provide sufficient lighting and/or phosphorescence to lead persons safely
out of the building.
• Avoid using false ceilings in hallways. These can
become falling debris that interferes with evacuation.
• Attach light fixtures to the floor system above to
avoid hazardous debris in the exit path and to provide emergency lighting.
Multi-family residential construction is more likely
than office building construction to incorporate flat
plate/slab or pre-fabricated components, and therefore
tends to be more structurally vulnerable. To improve
performance, robust connection detailing becomes
paramount to ensure the connections are not weaker
than the members to which they are attached. Also,
balconies are more common in multifamily residential
buildings. These present a debris hazard, due to their
inherent instability and connection weakness.
Commercial Retail Space Occupancy
Commercial retail space such as malls, movie theaters, hotels, nightclubs, casinos, and other spaces that
house large public populations, gathering for shopping or entertainment, have unique features that inPage 161
crease their vulnerability, compared with that of office
buildings. Often, these spaces are low-rise buildings
that have large interior spaces with high, laterally
unsupported walls, long-span roofs, and interior columns spaced relatively far apart. They are generally
constructed using lightweight construction and may
be prefabricated. This type of construction has little,
if any, redundancy, which significantly increases the
structural vulnerability.
struction. Typically, they are located in industrial or
commercial complexes and may have significant setbacks from public streets. They are serviced by surface
parking lots or parking structures outside the building.
Security may vary widely, depending on the use of the
building. For a building used for laboratories or manufacturing, there may already be significant security
measures at the perimeter and inside the building. For
office buildings, security may be light to negligible.
The primary goal for this type of construction is to prevent progressive collapse of the building in response
to a large-scale attack. Where possible, floor-to-floor
height and bay spacing should be reduced, and lateral
bracing of the columns and roof joists should be provided. Connections should be designed to be at least
as strong as the members. Secondary structural framing systems further enhance protection. To limit laceration injuries, lamination of glass is recommended.
Consider structural partition walls or shelving units
placed within the space that will stop the roof system
from falling directly on the occupants, in the event of
collapse. If this approach is used, take care that the
partitions have sufficient lateral support so that they
do not topple over.
The main focus of this section is on light industrial
buildings that house office space. These are the buildings with potentially high populations and, therefore,
life safety is a primary concern. For warehouses and
manufacturing plants, the primary objective is more
likely to be protection of the contents and processes.
For laboratories, the primary objectives are to prevent
release or deflagration of hazardous materials, and to
protect processes.
In these large spaces, it is virtually impossible to isolate HVAC to protect against CBR-type threats. In this
case, negative zone pressurization or smoke-evacuation methods become critically important. Also, mechanical areas should be protected with restricted access and a hardened shell (walls, ceiling and floor).
It is also recommended to have centralized redundant
control stations easily accessible by appropriate personnel. Consideration should be given to providing
additional, clearly marked, easily located egress routes
to facilitate mass evacuation. If there are business offices serving these buildings with a sizable workforce,
consider relocating these and other mixed-use functions to a separate, offsite location.
Light Industrial Buildings
Light industrial buildings are used throughout the
United States for offices, light manufacturing, laboratories, warehouses, and other commercial purposes.
Typically, these buildings are low-rise buildings, three
to five stories high, often using tilt-up concrete conPage 162
Office parks inherently have an open character with
medium-to-large setbacks from the street and public
parking. In this environment, the most effective way
to protect the building from moving vehicle threats
is to use landscaping methods between public streets
and parking to prevent the intrusion of vehicles. Devices such as ponds, fountains, berms, and ditches can
be very effective in reducing the accessibility of the
building exterior to high-speed vehicles.
Parking should be placed as far as practical from the
building. Driveways leading directly to the building
entrance should have a meandering path from the public streets that does not permit high velocities to be
achieved. Separation between the driveway and building may be achieved through a number of devices such
as a pond with a bridge leading to the entrance, a knee
wall with foliage in front, or other landscape features.
The design of parking structures servicing these buildings should fulfill two main objectives in order to prevent explosions in the parking structure from seriously
damaging the main office building. The first is to control the lines of sight between the parking structure
and the building to limit air-blast effects on the building. One solution is to use a solid wall with a berm and
landscape on the side of the parking structure facing
the building. Second, design the parking structure to
withstand the design level explosion without strucDesigning Buildings to Mitigate Terrorist Attacks
tural failure, in order to reduce the potential for debris
from a parking structure failure to damage the office
building. This second objective can be achieved while
still allowing the parking structure to sustain significant levels of damage.
For the tilt-up walls, use continuous vertical reinforcement with staggered splices, preferably on both
sides of the wall to resist large lateral loads. It may
be advantageous to consider designs that permit the
wall to bear against floor diaphragms to resist loads.
Connections between the walls and structural frame
should be able to accept large rebound forces in order
to prevent the wall from being pulled off the exterior.
Care should be taken to prevent the wall from bearing directly against exterior columns, so as to limit
the opportunity for progressive collapse. Using laminated glass on the exterior reduces the potential for
laceration injuries. For the roof, a concrete slab, with
or without decking, is preferred over a solution using
metal decking only.
If additional land is not available to move the secured
perimeter farther from the building, the required floor
area of the building can be distributed among additional floors. As the number of floors is increased, the
footprint decreases, providing an increased stand-off
distance. By balancing the increasing cost of the structure (due to the added floors) and the corresponding
decrease in protection cost (due to added stand-off),
it is possible to find the optimal number of floors to
minimize the cost of protection.
These methods for establishing an optimum stand-off
distance are generally used for the maximum credible explosive charge. If the cost of protection for this
charge weight is not within the budgetary constraints,
the design charge weight must be modified. A study
can be conducted to determine the largest explosive
yield and corresponding level of protection that can
be incorporated into the building, given the available
budget.
Initial Costs
The initial construction cost of protection has two
components: fixed and variable. Fixed costs include
such items as security hardware and space requirements. These costs do not depend on the level of an
attack; that is, it costs the same to keep a truck away
from a building whether the truck contains 500 or 5000
lbs. of TNT. Blast protection, on the other hand, is a
variable cost. It depends on the threat level, which is a
function of the explosive charge weight and the standoff distance. Building designers have no control over
the amount of explosives used, but are able to define
a stand-off distance by providing a secured perimeter.
The optimal stand-off distance is determined by defining the total cost of protection as the sum of the
cost of protection (construction cost) and the cost of
stand-off (land cost). These two costs are considered
as a function of the stand-off for a given explosive
charge weight. The cost of protection is assumed to be
proportional to the peak pressure at the building envelope, and the cost of land is a function of the square of
the stand-off distance. The optimal stand-off is the one
that minimizes the sum of these costs.
Figure 8-1 Plots showing relationship between cost of
upgrading various building components, standoff distance,
and risk
Though it is difficult to assign costs to various upgrade
measures because they vary based on the site specific
design, some generalizations can be made (see Figure
8-1). Here is a list of enhancements arranged in order
from least expensive to most expensive:
• Hardening of unsecured areas
• Measures to prevent progressive collapse
• Exterior window and wall enhancements
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Life-Cycle Costs
Life-cycle costs need to be considered as well. For
example, if it is decided that two guarded entrances
will be provided, one for the visitors and one for the
employees, they may cost more during the life of the
building than a single well designed entrance serving
everyone. Also, maintenance costs may need to be
considered. For instance, the initial costs for a CBR
detection system may be modest, but the maintenance
costs are high. Finally, if the rentable square footage
is reduced as a result of incorporating robustness into
the building, this may have a large impact on the lifecycle costs.
Setting Priorities
If the costs associated with mitigating man-made hazards are too high, there are three approaches:
1. reduce the design threat
2. increase the building setback
3. accept the risk
In some cases the owner may decide to prioritize enhancements, based on their effectiveness in saving
lives and reducing injuries. For instance, measures
against progressive collapse are perhaps the most effective actions that can be implemented to save lives,
and should be considered above any other upgrades.
Laminated glass is perhaps the single most effective
measure to reduce extensive non-fatal injuries. If the
cost is still considered too great, and the risk is high
because of the location or the high-profile nature of the
building, the best option may be to consider building
an unobtrusive facility in a lower-risk area instead. In
some cases – financial institutions with trading floors,
for instance – business interruption costs are so high
they outweigh all other concerns. In such a case, the
most cost-effective solution may be to provide a redundant facility.
Early consideration of man-made hazards will significantly reduce the overall cost of protection and
increase the inherent protection level provided to the
building. If protection measures are considered as
an afterthought, or not considered until the design is
nearly complete, the cost is likely to be greater because more areas will need to be structurally hardened
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due to poor planning. An awareness of the threat of
manmade hazards from the beginning of a project also
helps the team to decide early what the priorities are
for the facility. For instance, if extensive teak paneling of interior areas visible from the exterior is desired by the architect for the architectural expression
of the building, but the cost exceeds that of protective
measures, a decision needs to be made regarding the
priorities of the project. Including protective measures
as part of the discussion regarding trade-offs early in
the design process often helps to clarify such issues.
Ultimately, the willingness to pay the additional cost
for protection against man-made hazards is a function
of the “probability of regrets” in the event a sizable
incident occurs. In some situations, the small probability of an incident may not be compelling enough
to institute these design enhancements. Using this type
of logic, it is easy to see why it is unlikely they will
be instituted in any other than the highest-risk buildings, unless there is a mandated building code or insurance that requires these types of enhancements.
This scenario is likely to lead to a selection process
in which buildings stratify into two groups: those that
incorporate no measures at all or only the most minimal provisions and those that incorporate high levels
of protection. It also leads to the conclusion that it may
not be appropriate to consider any other than the most
minimal measures for most buildings.
Summary
In this chapter we first examined multifamily residential occupancy buildings and their added risks. The
major concern in this section was placing egress routes
in the most accessible places. Next, commercial retail
space and light industrial buildings were discussed,
with the primary concern focusing on the safety of
the people. Then, initial costs of construction were
brought up, and we discussed how to save money by
taking measures during construction to mitigate attacks, rather than after construction is complete.
Student Assessment
Final Exam
1. Which of the following is the most likely
source of a terrorist threat?
a. Airborne chemicals
b. Bombings
c. Biological warfare
d. Radiological warfare
6. What is typically the most vulnerable portion
of any building?
a. Windows
b. Exterior walls
c. Interior walls
d. Lobby
2. A/an _________ is an extremely rapid release
of energy in the form of light, heat, sound, and
a shock wave.
a. Explosion
b. Force
c. Bomb
d. Implosion
7. Ductwork that travels through vulnerable
areas should be:
a. Easily accessed
b. Open
c. Sealed
d. Completely closed
3. What are bollards?
a. Sidewalks
b. Planters secured into the pavement
c. A type of fountain
d. Concrete filled steel pipes
8. What is the primtary concern of a light
industrial building?
a. Life safety
b. Progressive collapse
c. Airborne hazards
d. Bombings
4. Active systems include all of the following
except:
a. Rotating wedge systems
b. Planters
c. Crash beams
d. Crash gates
9. ______ costs include such items as security
hardware and space requirements.
a. Fixed
b. Variable
c. Initial
d. True
5. Which method is the preferred approach to
preventing progressive collapse?
a. Specific local resistance
b. Alternate load path
c. Indirect
d. Direct
10. What are the frame members connecting
adjoining windows called?
a. Ledges
b. Window panes
c. Window seals
d. Mullions
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Municipal Wastewater
Treatment Systems
Course Description
This course will focus on wastewater treatment, including pretreatment and disposal of residuals. The course
objective is to give the professional engineers an overview of wastewater treatment, pollutants which may be
involved, and onsite and cluster symptoms. Information for this course was taken from the Environmental Protection Agency (EPA).
Chapters
• Chapter One: Treatment Of Wastewater
• Chapter Two: Use and Disposal of Wastewater
Learning Objectives
• Understand the Clean Water Act requirements
• Discuss centralized collection of wastewater
• Recognize primary and secondary treatments
• Identify advanced methods of wastewater treatment
• Discuss disposal of wastewater biosolids
• Understand onsite and cluster systems
• Comprehend asset management, including operation and maintenance
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Chapter One:
Treatment of
Wastewater
Overview
• Introduction
• Clean Water Act Requirements for Wastewater
Treatment
• The Need for Wastewater Treatment
• Effects of Wastewater on Water Quality
• Challenges Faced by Professionals
• Collecting and Treating Wastewater
»» Centralized Collection
• Pollutants
• Wastewater Treatment
• Summary
Learning Objectives
•
•
•
•
•
•
Understand the Clean Water Act requirements
Identify the need for wastewater treatment
Recognize the effects of wastewater on water quality
Discuss centralized collection of wastewater
Identify pollutants such as nutrients and pathogens
Recognize primary and secondary treatments
Introduction
Clean water and treatment of wastewater are essential
to life as we know it. Engineers need to be aware of
water treatment systems and the terminology that goes
along with those systems. Clean Water Act Require-
ments for Wastewater Treatment
The 1972 Amendments to the Federal Water Pollution Control Act (Public Law 92- 500, known as the
Clean Water Act (CWA), established the foundation
for wastewater discharge control in this country. The
CWA’s primary objective is to restore and maintain the
chemical, physical, and biological integrity of the Nation’s waters.
The CWA established a control program for ensuring
communities have clean water by regulating the release of contaminants into the country’s waterways.
Permits that limit the amount of pollutants discharged
are required of all municipal and industrial wastewater dischargers under the National Pollutant Discharge
Elimination System (NPDES) permit program. In addition, a construction grants program was set up to
assist publicly owned wastewater treatment works to
build improvements required to meet these new limits.
The 1987 Amendments to the CWA established State
Revolving Funds (SRF) to replace grants as the current principal federal funding source for the construction of wastewater treatment and collection systems.
Over 75 percent of the nation’s population is served
by centralized wastewater collection and treatment
systems. The remaining population uses septic or
other onsite systems. Approximately 16,000 municipal wastewater treatment facilities are in operation
nationwide. The CWA requires municipal wastewater
treatment plant discharges meet a minimum of secondary treatment. Over 30 percent of the wastewater
treatment facilities today produce cleaner discharges
by providing even greater levels of treatment than secondary.
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from growing development activities, continues to
be a source of significant pollution, as runoff washes
off the land.
Water pollution issues now dominate public concerns
about national water quality and maintaining healthy
ecosystems. Although a large investment in water
pollution control has helped reduce the problem,
many miles of streams are still impacted by a variety
of pollutants. This, in turn, affects people’s ability to
use the water for beneficial purposes. Past approaches to controlling water pollution must be modified to
accommodate current and emerging issues.
Effects of Wastewater on Water
Quality
The Need for Wastewater Treatment
Wastewater treatment is needed so we can use our rivers and streams for fishing, swimming and drinking
water. For the first half of the 20th century, pollution
in the Nation’s urban waterways resulted in frequent
occurrences of low dissolved oxygen, fish kills, algal
blooms, and bacterial contamination. Early efforts
at water pollution control prevented human waste
from reaching water supplies or reduced floating debris that obstructed shipping. Pollution problems and
their control were primarily local, not national, concerns. Since then, population and industrial growth
have increased demands on our natural resources,
altering the situation dramatically. Progress in abating pollution has barely kept ahead of population
growth, changes in industrial processes, technological developments, and changes in land use, business
innovations, and many other factors.
Increases in both the quantity and variety of goods
produced can greatly alter the amount and complexity of industrial wastes and challenge traditional
treatment technology. The application of commercial
fertilizers and pesticides, combined with sediment
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The basic function of the wastewater treatment plant
is to speed up the natural processes by which water
purifies itself. In earlier years, the natural treatment
process in streams and lakes was adequate to perform
basic wastewater treatment. As our population and
industry grew to their present size, increased levels
of treatment prior to discharging domestic wastewater became necessary.
Challenges Faced by Professionals
• Many of the wastewater treatment and collection
facilities are now old and worn, and require repair, improvement, or replacement to maintain
their useful life
• The character and quantity of contaminants presenting problems today are far more complex than
those that presented challenges in the past
• Population growth is taxing many existing wastewater treatment systems and creating a need for
new plants
• Farm runoff and increasing urbanization provide
additional sources of pollution, not controlled by
wastewater treatment
• One third of new development is served by decentralized systems (e.g., septic systems), as population has migrated farther from metropolitan areas
Collecting and Treating Wastewater
The most common form of pollution control in the
United States consists of a system of sewers and
wastewater treatment plants. The sewers collect municipal wastewater from homes, businesses, and industries and deliver it to facilities for treatment before
it is discharged to water bodies or land, or reused.
Centralized Collection
During the early days of our nation’s history, people
living in both the cities and the countryside used cesspools and privies to dispose of domestic wastewater.
Cities began to install wastewater collection systems
in the late nineteenth century because of an increasing
awareness of waterborne disease and the popularity of
indoor plumbing and flush toilets. The use of sewage
collection systems brought dramatic improvements to
public health, further encouraging the growth of metropolitan areas. In the year 2000, approximately 208
million people in the U.S. were served by centralized
collection systems.
in cities. Later, lines were added to carry domestic
wastewater away from homes and businesses. Early
sanitarians thought these combined systems provided
adequate health protection. We now know the overflows designed to release excess flow during rains also
release pathogens and other pollutants.
Sanitary Sewer Systems
Sanitary sewer collection systems serve over half
the people in the United States today. EPA estimates
that there are approximately 500,000 miles of publicly owned sanitary sewers, with a similar expanse of
privately-owned sewer systems. Sanitary sewers were
designed and built to carry wastewater from domestic, industrial and commercial sources, but not to carry
storm water. Nonetheless, some storm water enters
sanitary sewers through cracks, particularly in older
lines, and through roof and basement drains. Due to
the much smaller volumes of wastewater that pass
through sanitary sewer lines, compared to combined
sewers, sanitary sewer systems use smaller pipes and
lower the cost of collecting wastewater.
Simplified Urban Water Cycle
Combined Sewer Systems
Many of the earliest sewer systems were combined
sewers, designed to collect both sanitary wastewater
and storm water runoff in a single system. These combined sewer systems were designed to provide storm
drainage from streets and roofs to prevent flooding
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Pollutants
Oxygen-Demanding Substances
Dissolved oxygen is a key element in water quality
that is necessary to support aquatic life. A demand is
placed on the natural supply of dissolved oxygen by
many pollutants in wastewater. This is called biochemical oxygen demand, or BOD, and is used to measure
how well a sewage treatment plant is working. If the
effluent, the treated wastewater produced by a treatment plant, has a high content of organic pollutants or
ammonia, it will demand more oxygen from the water
and leave the water with less oxygen to support fish
and other aquatic life. Organic matter and ammonia
are “oxygen-demanding” substances.
Oxygen demanding substances are contributed by domestic sewage, as well as agricultural and industrial
wastes, of both plant and animal origin, such as those
from food processing, paper mills, tanning, and other
manufacturing processes. These substances are usually destroyed or converted to other compounds by bacteria if there is sufficient oxygen present in the water,
but the dissolved oxygen needed to sustain fish life is
depleted in this breakdown process.
Pathogens
Disinfection of wastewater and chlorination of drinking water supplies have reduced the occurrence of waterborne diseases such as typhoid fever, cholera, and
dysentery. These remain problems in underdeveloped
countries, though they have been virtually eliminated
in the U.S.
Infectious micro-organisms, or pathogens, may be
carried into surface and groundwater by sewage from
cities and institutions, by certain kinds of industrial
wastes, such as tanning and meat packing plants, and
by the contamination of storm runoff with animal
wastes from pets, livestock and wild animals, such
as geese or deer. Humans come in contact with these
pathogens either by drinking contaminated water or
through contact activities such as swimming or fishing. Modern disinfection techniques have greatly reduced the danger of waterborne disease.
Nutrients
Carbon, nitrogen, and phosphorus are essential to living organisms and are the chief nutrients present in
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natural water. Large amounts of these nutrients are
also present in sewage, certain industrial wastes, and
drainage from fertilized land. Conventional secondary biological treatment processes do not remove the
phosphorus and nitrogen to any substantial extent -- in
fact, they may convert the organic forms of these substances into mineral form, making them more usable
by plant life.
When an excess of these nutrients over-stimulates the
growth of water plants, the result causes unsightly
conditions, interferes with drinking water treatment
processes, and causes unpleasant and disagreeable
tastes and odors in drinking water. The release of large
amounts of nutrients, primarily phosphorus but occasionally nitrogen, causes nutrient enrichment which
results in excessive growth of algae. Uncontrolled
algae growth blocks out sunlight and chokes aquatic
plants and animals by depleting dissolved oxygen in
the water at night. The release of nutrients in quantities that exceed the affected water body’s ability to
assimilate to them results in a condition called eutrophication or cultural enrichment.
Inorganic and Synthetic Organic Chemicals
A vast array of chemicals is included in this category. Examples include detergents, household cleaning
aids, heavy metals, pharmaceuticals, synthetic ororganic pesticides and herbicides, industrial chemicals, and the wastes from their manufacture. Many
of these substances are toxic to fish and aquatic life;
many are also harmful to humans. Some are known
to be highly poisonous at very low concentrations.
Others can cause taste and odor problems, and many
are not effectively removed by conventional wastewater treatment.
Thermal
Heat reduces the capacity of water to retain oxygen.
In some areas, water used for cooling is discharged to
streams at elevated temperatures from power plants
and industries. Even discharges from wastewater
treatment plants and storm water retention ponds affected by summer heat can be released at temperatures above that of the receiving water, and elevate
the stream temperature. Unchecked discharges of
waste heat can seriously alter the ecology of a lake, a
stream, or estuary.
Wastewater Treatment
In 1892, only 27 American cities provided wastewater
treatment. Today, more than 16,000 publicly-owned
wastewater treatment plants operate in the United
States and its territories. The construction of wastewater treatment facilities blossomed in the 1920s and
again after the passage of the CWA in 1972 with the
availability of grant funding and new requirements
calling for minimum levels of treatment. Adequate
treatment of wastewater, along with the ability to provide a sufficient supply of clean water, has become a
major concern for many communities.
Primary Treatment
The initial stage in the treatment of domestic wastewater is known as primary treatment. Coarse solids
are removed from the wastewater in the primary stage
of treatment. In some treatment plants, primary and
secondary stages may be combined into one basic operation. At many wastewater treatment facilities, influent passes through preliminary treatment units before
primary and secondary treatment begins.
Preliminary Treatment
As wastewater enters a treatment facility, it typically
flows through a step called preliminary treatment. A
screen removes large floating objects, such as rags,
cans, bottles and sticks that may clog pumps, small
pipes, and downstream processes. The screens vary
from coarse to fine. Some are constructed with parallel
steel or iron bars with openings of about half an inch,
while others may be made from mesh screens with
much smaller openings. Screens are generally placed
in a chamber or channel and inclined towards the flow
of the wastewater. The inclined screen allows debris
to be caught on the upstream surface of the screen,
and allows access for manual or mechanical cleaning.
Some plants use devices known as Comminutor or
Barminutor, which combine the functions of a screen
and a grinder. These devices catch and then cut or
shred the heavy solid and floating material. In the process, the pulverized matter remains in the wastewater
flow to be removed later in a primary settling tank.
Primary Sedimentation
With the screening completed and the grit removed,
wastewater still contains dissolved organic and inorganic constituents along with suspended solids. The
suspended solids consist of minute particles of matter
that can be removed from the wastewater with further
treatment, such as sedimentation or gravity settling,
chemical coagulation, or filtration. Pollutants that are
dissolved or are very fine and remain suspended in
the wastewater are not removed effectively by gravity
settling. When the wastewater enters a sedimentation
tank, it slows down and the suspended solids gradually sink to the bottom. This mass of solids is called
primary sludge. Various methods have been devised to
remove primary sludge from the tanks. Newer plants
have some type of mechanical equipment to remove
the settled solids from sedimentation tanks. Some
plants remove solids continuously, while others do so
at intervals.
After the wastewater has been screened, it may flow
into a grit chamber where sand, grit, cinders, and small
stones settle to the bottom. Removing the grit and
gravel that washes off streets or land during storms
is very important, especially in cities with combined
sewer systems. Large amounts of grit and sand entering a treatment plant can cause serious operating
problems, such as excessive wear of pumps and other
equipment, clogging of aeration devices, or taking
up capacity in tanks that is needed for treatment. In
some plants, another finer screen is placed after the
grit chamber to remove any additional material that
might damage equipment or interfere with subsequent
processes. The grit and screenings removed by these
processes must be periodically collected and trucked
to a landfill for disposal, or they may be incinerated.
Basic Wastewater Treatment
Processes
Physical
Physical processes were some of the earliest methods
to remove solids from wastewater, usually by passing
wastewater through screens to remove debris and solids. In addition, solids that are heavier than water will
settle out from wastewater by gravity. Particles with
entrapped air float to the top of water and can also be
removed. These physical processes are employed in
many modern wastewater treatment facilities today.
Biological
In nature, bacteria and other small organisms in water
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consume organic matter in sewage, turning it into new
bacterial cells, carbon dioxide, and other by-products.
The bacteria normally present in water must have oxygen to do their part in breaking down the sewage. In
the 1920s, scientists observed these natural processes could be contained and accelerated in systems to
remove organic material from wastewater. With the
addition of oxygen to wastewater, masses of microorganisms grew and rapidly metabolized organic pollutants. Any excess microbiological growth could be
removed from the wastewater by physical processes.
Chemical
Chemicals can be used to create changes in pollutants that increase the removal of these new forms by
physical processes. Simple chemicals, such as alum,
lime, or iron salts, can be added to wastewater to cause
certain pollutants, such as phosphorus, to create floc
(large, heavier masses), which can be removed faster
through physical processes. Over the past 30 years, the
chemical industry has developed synthetic inert chemicals known as polymers to further improve the physical separation step in wastewater treatment. Polymers
are often used at the later stages of treatment to improve the settling of excess microbiological growth or
biosolids.
Secondary Treatment
After the wastewater has been through Primary Treatment processes, it flows into the next stage of treatment, called secondary. Secondary treatment processes can remove up to 90 percent of the organic matter
in wastewater by using biological treatment processes.
The two most common conventional methods used to
achieve secondary treatment are attached growth processes and suspended growth processes.
Attached Growth Processes
In attached growth (or fixed film) processes, the microbial growth occurs on the surface of stone or plastic media. Wastewater passes over the media, along
with air to provide oxygen. Attached growth process
units include trickling filters, biotower, and rotating
biological contactors. Attached growth processes are
effective at removing biodegradable organic material
from the wastewater.
A trickling filter is simply a bed of media (typically
rocks or plastic) through which the wastewater passPage 172
es. The media ranges from three to six feet deep and
allows large numbers of microorganisms to attach
and grow. Older treatment facilities typically used
stones, rocks, or slag as the media bed material. New
facilities may use beds made of plastic balls, interlocking sheets of corrugated plastic, or other types
of synthetic media. This type of bed material often
provides more surface area and a better environment
for promoting and controlling biological treatment
than rock.
Bacteria, algae, fungi, and other microorganisms grow
and multiply, forming a microbial growth or slime layer (biomass) on the media. In the treatment process,
the bacteria use oxygen from the air and consume most
of the organic matter in the wastewater as food. As the
wastewater passes down through the media, oxygendemanding substances are consumed by the biomass,
and the water leaving the media is much cleaner. However, portions of the biomass also slough off the media
and must settle out in a secondary treatment tank.
Suspended Growth Processes
Similar to the microbial processes in attached growth
systems, suspended growth processes are designed to
remove biodegradable organic material and organic
nitrogen-containing material by converting ammonia nitrogen to nitrate, unless additional treatment is
provided. In suspended growth processes, the microbial growth is suspended in an aerated water mixture
where the air is pumped in or the water is agitated sufficiently to allow oxygen transfer. Suspended growth
process units include variations of activated sludge,
oxidation ditches, and sequencing batch reactors.
The suspended growth process speeds up the work of
aerobic bacteria and other microorganisms that break
down the organic matter in the sewage by providing
a rich aerobic environment, where the microorganisms suspended in the wastewater can work more efficiently. In the aeration tank, wastewater is vigorously
mixed with air and microorganisms acclimated to the
wastewater in a suspension for several hours. This allows the bacteria and other microorganisms to break
down the organic matter in the wastewater. The microorganisms grow in number, and the excess biomass is
removed by settling before the effluent is discharged
or treated further. Now, activated with millions of additional aerobic bacteria, some of the biomass can be
used again by returning it to an aeration tank for mixing with incoming wastewater.
The activated sludge process, like most other techniques, has both advantages and limitations. The units
necessary for this treatment are relatively small, requiring less space than attached growth processes. In
addition, when properly operated and maintained, the
process is generally free of flies and odors. However,
most activated sludge processes are more costly to
operate than attached growth processes, due to higher
energy use to run the aeration system. The effectiveness of the activated sludge process can be impacted
by elevated levels of toxic compounds in wastewater,
unless complex industrial chemicals are effectively
controlled through an industrial pretreatment program.
An adequate supply of oxygen is necessary for the
activated sludge process to be effective. The oxygen
is generally supplied by mixing air with the sewage
and biologically active solids in the aeration tanks by
one or more of several different methods. Mechanical
aeration can be accomplished by drawing the sewage
up from the bottom of the tank and spraying it over
the surface, thus allowing the sewage to absorb large
amounts of oxygen from the atmosphere. Pressurized
air can be forced out through small openings in pipes
suspended in the wastewater. A combination of mechanical aeration and forced aeration can also be used.
Also, relatively pure oxygen, produced by several different manufacturing processes, can be added to provide oxygen to the aeration tanks.
From the aeration tank, the treated wastewater flows
to a sedimentation tank (secondary clarifier), where
the excess biomass is removed. Some of the biomass
is recycled to the head end of the aeration tank, while
the remainder is “wasted” from the system. The waste
biomass and settled solids are treated before reuse as
biosolids or disposal.
Lagoons
A wastewater lagoon, or treatment pond, is a scientifically constructed pond, three to five feet deep, that allows sunlight, algae, bacteria, and oxygen to interact.
Biological and physical treatment processes occur in
the lagoon to improve water quality. The quality of
water leaving the lagoon, when constructed and operated properly, is considered equivalent to the effluMunicipal Wastewater Treatment Systems
ent from a conventional secondary treatment system.
However, winters in cold climates have a significant
impact on the effectiveness of lagoons, and winter
storage is usually required. Lagoons have several advantages when used correctly. They can be used for
secondary treatment or as a supplement to other processes. While treatment ponds require substantial land
area and are predominantly used by smaller communities, they account for more than one-fourth of the municipal wastewater treatment facilities in this country.
Lagoons remove biodegradable organic material and
some of the nitrogen from wastewater.
Land Treatment
Land treatment is the controlled application of wastewater to the soil where physical, chemical, and biological processes treat the wastewater as it passes across
or through the soil. The principal types of land treatment are slow rate, overland flow, and rapid infiltration. In the arid western states, pretreated municipal
wastewater has been used for many years to irrigate
crops. In more recent years, land treatment has spread
to all sections of the country. Land treatment of many
types of industrial wastewater is also common.
Whatever method is used, land treatment can be a feasible economic alternative where the land area needed
is readily available, particularly when compared to
costly advanced treatment plants. Extensive research
has been conducted at land treatment sites to determine treatment performance and study the numerous
treatment processes involved, as well as potential impacts on the environment; e.g. groundwater, surface
water, and any crop that may be grown.
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Slow Rate Infiltration
In the case of slow rate infiltration, the wastewater is applied to the land and moves through the soil where the
natural filtering action of the soil along with microbial
activity and plant uptake removes most contaminants.
Part of the water evaporates or is used by plants. The remainder is either collected via drains or wells for surface
discharge or allowed to percolate into the groundwater.
Slow rate infiltration is the most commonly used land
treatment technique. The wastewater, which is sometimes disinfected before application, depending on
the end use of the crop and the irrigation method, can
be applied to the land by spraying, flooding, or ridge
and furrow irrigation. The method selected depends
on cost considerations, terrain, and the type of crop.
Much of the water and most of the nutrients are used
by the plants, while other pollutants are transferred to
the soil by adsorption, where many are mineralized or
broken down over time by microbial action.
ment processes. It is also effective in cold or wet
weather and has been successfully used in Florida,
northeastern and arid southwestern states. Large
amounts of wastewater are applied to permeable
soils in a limited land area and allowed to infiltrate
and percolate downward through the soil into the water table below. If the water is to be reused, it can
be recovered by wells. The cost-effectiveness of this
process depends on the soil’s ability to percolate a
large volume of water quickly and efficiently, so suitable soil drainage is important.
Overland Flow
This method has been used successfully by the food
processing industry for many years to remove solids,
bacteria and nutrients from wastewater. The wastewater is allowed to flow down a gently-sloped surface
that is planted with vegetation to control runoff and
erosion. Heavy clay soils are well suited to the overland flow process. As the water flows down the slope,
the soil and its microorganisms form a gelatinous
slime layer, similar, in many ways, to a trickling filter
that effectively removes solids, pathogens, and nutrients. Water not absorbed or evaporated is recovered at
the bottom of the slope for discharge or reuse.
Constructed Wetlands
Wetlands are areas where the water saturates the
ground long enough to support and maintain wetland vegetation, such as reeds, bulrush, and cattails. A
“constructed wetlands” treatment system is designed
to treat wastewater by passing it through the wetland.
Natural, physical, chemical, and biological wetland
processes have been recreated and enhanced in constructed wetlands, designed specifically to treat wastewater from industries, small communities, storm runoff from urban and agricultural areas, and acid mine
drainage. Significant water quality improvements, including nutrient reduction, can be achieved.
Disinfection
Rapid Infiltration
The rapid infiltration process is most frequently used
to polish and recover wastewater effluents for reuse
after pretreatment by secondary and advanced treatPage 174
Untreated domestic wastewater contains microorganisms or pathogens that produce human diseases.
Processes used to kill or deactivate these harmful organisms are called disinfection. Chlorine is the most
widely used disinfectant, but ozone and ultraviolet radiation are also frequently used for wastewater effluent disinfection.
Chlorine
Chlorine kills microorganisms by destroying cellular
material. This chemical can be applied to wastewater
as a gas, a liquid, or a solid form similar to swimming
pool disinfection chemicals. However, any free (uncombined) chlorine remaining in the water, even at
low concentrations, is highly toxic to beneficial aquatic life. Therefore, removal of even trace amounts of
free chlorine by dechlorination is often needed to protect fish and aquatic life. Due to emergency response
and potential safety concerns, chlorine gas is used less
frequently now than in the past.
Ozone
Ozone is produced from oxygen exposed to a high
voltage current. Ozone is very effective at destroying
viruses and bacteria and decomposes back to oxygen
rapidly without leaving harmful by products. Ozone is
not very economical due to high energy costs.
Summary
In this chapter, we were introduced to the Clean Water
Act and its objective of restoring the chemical, physical and biological integrity of the nation’s waters. We
discussed the importance of wastewater treatment for
rivers and streams for fishing, swimming, and drinking water. We also learned the effects of wastewater
on water quality and the challenges professional engineers face when it comes to wastewater treatment.
Engineers use centralized collection systems which
include both combined sewer systems and sanitary
sewer systems. We learned about various pollutants
that affect our water. Finally, we discussed the treatment of wastewater, including primary, basic, and secondary treatment.
Ultraviolet Radiation
Ultra violet (UV) disinfection occurs when electromagnetic energy in the form of light in the UV spectrum produced by mercury arc lamps penetrates the
cell wall of exposed microorganisms. The UV radiation retards the ability of the microorganisms to survive by damaging their genetic material. UV disinfection is a physical treatment process that leaves no
chemical traces. Organisms can sometimes repair and
reverse the destructive effects of UV when applied at
low doses.
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Chapter Two:
Use and Disposal
of Wastewater
Overview
• Pretreatment
• Advanced Methods of Wastewater Treatment
• The Use or Disposal of Wastewater Residuals
and Biosolids
• Decentralized (Onsite and Cluster) Systems
• Asset Management
• Summary
Learning Objectives
• Gain basic knowledge of advanced methods
of wastewater treatment
• Discuss the disposal of wastewater biosolids
• Understand onsite and cluster systems
• Comprehend asset management, including
operation and maintenance
Pretreatment
The National Pretreatment Program, a cooperative effort of Federal, state, POTWs and their industrial dischargers, requires industries to control the amount of
pollutants discharged into municipal sewer systems.
Pretreatment protects wastewater treatment facilities
and their workers from pollutants that may create hazards or interfere with the operation and performance
of the POTW, including contamination of sewage
sludge, and reduces the likelihood that untreated pollutants are introduced into the receiving waters.
Under the Federal Pretreatment Program, municipal
wastewater plants receiving significant industrial discharges must develop local pretreatment programs to
control industrial discharges into their sewer system.
These programs must be approved by either EPA or
a state acting as the Pretreatment Approval Authority. More than 1,500 municipal treatment plants have
developed and received approval for a Pretreatment
Program.
Advanced Methods of
Wastewater Treatment
As both our country and the demand for clean water
have grown, it has become more important to produce
cleaner wastewater effluents. Yet some contaminants
are more difficult to remove than others. The demand
for cleaner discharges has been met through better
and more complete methods of removing pollutants
at wastewater treatment plants (WWTPs), in addition to pretreatment and pollution prevention, which
helps limit types of wastes discharged to the sanitary
sewer system. Currently, nearly all WWTPs provide a
minimum of secondary treatment. In some receiving
waters, the discharge of secondary treatment effluent
would still degrade water quality and inhibit aquatic
life. Further treatment is needed. Treatment beyond
the secondary level is called advanced treatment.
Advanced treatment technologies can be extensions of
conventional secondary biological treatment to further
stabilize oxygen-demanding substances in the wastewater, or to remove nitrogen and phosphorus. Advanced treatment may also involve physical-chemical
separation techniques such as adsorption, flocculation/
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precipitation, membranes for advanced filtration, ion
exchange, and reverse osmosis. In various combinations, these processes can achieve any degree of pollution control desired. As wastewater is purified to
higher and higher degrees by such advanced treatment
processes, the treated effluents can be reused for urban, landscape, and agricultural irrigation, industrial
cooling and processing, recreational uses and water
recharge, and even indirect augmentation of drinking
water supplies.
Nitrogen Control
Nitrogen, in one form or another, is present in municipal wastewater and usually not removed by secondary
treatment. If discharged into lakes, streams or estuary
waters, nitrogen in the form of ammonia can exert a
direct demand on oxygen or stimulate the excessive
growth of algae. Ammonia in wastewater effluent can
be toxic to aquatic life in certain instances.
By providing additional biological treatment beyond
the secondary stage, nitrifying bacteria present in
wastewater treatment can biologically convert ammonia to the non-toxic nitrate through a process known
as nitrification. The nitrification process is normally
sufficient to remove the toxicity associated with ammonia in the effluent. Since nitrate is also a nutrient,
excess amounts can contribute to the uncontrolled
growth of algae. In situations where nitrogen must be
completely removed from effluent, an additional biological process can be added to the system to convert
the nitrate to nitrogen gas.
The conversion of nitrate to nitrogen gas is accomplished by bacteria in a process known as Denitrification. Effluent with nitrogen in the form of nitrate is
placed into a tank devoid of oxygen, where carboncontaining chemicals, such as methanol, are added,
or a small stream of raw wastewater is mixed in with
the nitrified effluent. In this oxygen free environment,
bacteria use the oxygen attached to the nitrogen in the
nitrate form, releasing nitrogen gas. Because nitrogen
comprises almost 80 percent of the air in the earth’s atmosphere, the release of nitrogen into the atmosphere
does not cause any environmental harm.
Biological Phosphorus Control
Like nitrogen, phosphorus is also a necessary nutrient
for the growth of algae. Phosphorus reduction is often
necessary in order to prevent excessive algal growth
before discharging effluent into lakes, reservoirs
and estuaries. Phosphorus removal can be achieved
through chemical addition and a coagulation sedimentation process. Some biological treatment processes,
called biological nutrient removal (BNR), also can
achieve nutrient reduction, removing both nitrogen
and phosphorus. Most of the BNR processes involve
modifications of suspended growth treatment systems,
so the bacteria in these systems also convert nitrate
nitrogen to inert nitrogen gas and trap phosphorus in
the solids that are removed from the effluent.
Coagulation-Sedimentation
A process known as chemical coagulation-sedimentation is used to increase the removal of solids from
effluent after primary and secondary treatment. Solids
heavier than water settle out of wastewater by gravity. With the addition of specific chemicals, numerous
solids can become heavier than water and will settle.
Alum, lime, or iron salts are chemicals added to the
wastewater to remove phosphorus. With these chemicals, the smaller particles “floc” or clump together into
large masses. The larger masses of particles will settle
faster when the effluent reaches the next step: the sedimentation tank. This process can reduce the concentration of phosphate by more than 95 percent.
Although used for years in the treatment of industrial
wastes and in water treatment, coagulation-sedimentation is considered an advanced process because it
is not routinely applied to the treatment of municipal
wastewater. In some cases, the process is used as a
necessary pretreatment step for other advanced techniques. This process produces a chemical sludge, and
the cost of disposing of this material can be significant.
Carbon Adsorption
Carbon adsorption technology can remove from
wastewater organic materials that resist removal by biological treatment. These resistant, trace organic substances can contribute to taste and odor problems in
water, taint fish flesh, and cause foaming and fish kills.
Carbon adsorption consists of passing the wastewater
effluent through a bed or canister of activated carbon
granules or powder which removes more than 98 percent of trace organic substances. These substances adhere to the carbon surface and are removed from the
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water. To help reduce the cost of the procedure, the
carbon granules can be cleaned by heating and used
again.
The Use or Disposal of Wastewater
Residuals and Biosolids
When pollutants are removed from water, there is always something left over. It may be rags and sticks
caught on the screens at the beginning of primary
treatment. It may be the solids that settle to the bottom of sedimentation tanks. Whatever it is, there are
always residuals that must be reused, burned, buried,
or disposed of in a manner that does not harm the environment.
The utilization and disposal of the residual process
solids is addressed by the CWA, Resource Conservation and Recovery Act (RCRA), and other federal
laws. These Federal laws reinforce the need to employ environmentally sound residuals management
techniques and to beneficially use biosolids whenever
possible. Biosolids are processed wastewater solids
(“sewage sludge”) that meet rigorous standards allowing safe reuse for beneficial purposes. Currently,
more than half of the biosolids produced by municipal
wastewater treatment systems is applied to land as a
soil conditioner or fertilizer, and the remaining solids
are incinerated or landfilled. Ocean dumping of these
solids is no longer allowed.
Prior to utilization or disposal, biosolids are stabilized
to control odors and reduce the number of diseasecausing organisms. Sewage solids, or sludge, when
separated from the wastewater, still contain around 98
percent water. They are usually thickened and may be
de-watered to reduce the volume to be transported for
final processing, disposal, or beneficial use. De-watering processes include drying beds, belt filter presses,
plate and frame presses, and centrifuges.
To improve de-watering effectiveness, the solids can
be pretreated with chemicals, such as lime, ferric chloride, or polymers, to produce larger particles which
are easier to remove. Digestion is a form of stabilization where the volatile material in the wastewater
solids can decompose naturally and the potential for
odor production is reduced. Digestion without air in
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an enclosed tank, (anaerobic solids digestion) has the
added benefit of producing methane gas, which can be
recovered and used as a source of energy. Stabilization
of solids may also be accomplished by composting,
heat treatments, drying, or the addition of lime or other alkaline materials. After stabilization, the biosolids
can be safely spread on land.
Land Application
In many areas, biosolids are marketed to farmers as
fertilizer. Federal regulation defines minimum requirements for such land application practices, including contaminant limits, field management practices,
treatment requirements, monitoring, recordkeeping,
and reporting requirements. Properly treated and applied biosolids are a good source of organic matter for
improving soil structure. They supply nitrogen, phosphorus, and micronutrients that are required by plants.
Biosolids also have been used successfully for many
years as a soil conditioner and fertilizer, and for restoring and revegetating areas with poor soils due to construction activities, strip mining, or other practices.
Under this biosolids management approach, treated
solids in semi-liquid or de-watered form are transported to the soil treatment areas. The slurry, or dewatered biosolids, containing nutrients and stabilized
organic matter, is spread over the land to give nature
a hand in returning grass, trees, and flowers to barren
land. Restoration of the countryside also helps control the flow of acid drainage from mines that endangers aquatic life and contaminates the water with acid,
salts, and excessive quantities of metals.
Incineration
Incineration consists of burning the dried solids to reduce the organic residuals to an ash that can be disposed or reused. Incinerators often include heat recovery features. Undigested sludge solids have significant
fuel value as a result of their high organic content.
However, to take advantage of the fuel potential of the
biosolids, the water content must be greatly reduced by
de-watering or drying. For this reason, pressure filtration dewatering equipment is used to obtain biosolids
which are sufficiently dry to burn without continual
reliance on auxiliary fuels. In some cities, biosolids
are mixed with refuse or refuse-derived fuel prior to
burning. Generally, waste heat is recovered to provide
the greatest amount of energy efficiency.
Beneficial Use Products from Biosolids
Heat dried biosolids pellets have been produced and
used extensively as a fertilizer product for lawn care,
turf production, citrus groves, and vegetable production for many years. Composting of biosolids is
also a well-established approach to solids management that has been adopted by a number of communities. The composted peat-like product has shown
particular promise for use in the production of soil
additives for revegetation of topsoil depleted areas,
and as a potting soil amendment. Effective pretreatment of industrial wastes prevents excessive levels
of unwanted constituents, such as heavy metals (i.e.
cadmium, mercury, and lead) and persistent organic compounds, from contaminating the residuals of
wastewater treatment and limiting the potential for
beneficial use.
Effective stabilization of wastewater residuals and
their conversion to biosolids products can be costly.
Some cities have produced fertilizers from biosolids
which are sold to help pay part of the cost of treating
wastewater. Some municipalities use composted, heat
dried, or lime stabilized biosolids products on parks
and other public areas.
Decentralized (Onsite and Cluster)
Systems
A decentralized wastewater system treats sewage from
homes and businesses not connected to a centralized
wastewater treatment plant. Decentralized treatment
systems include onsite systems and cluster systems.
An onsite system is a wastewater system relying on
natural processes, although sometimes containing mechanical components, to collect, treat, disperse or reclaim wastewater from a single dwelling or building.
A septic tank combined with a soil adsorption field is
an example of an onsite system. A wastewater collection and treatment system, under some form of common ownership that collects wastewater from two or
more dwellings or buildings and conveys it to a treatment and dispersal system, located on a suitable site
near the dwellings or buildings, is a cluster system.
Decentralized systems include:
• media filters
• constructed wetland systems
• aerobic treatment units
• soil dispersal systems
Soil dispersal systems include pressure systems such
as low pressure pipe and drip dispersal systems. These
systems treat and disperse relatively small volumes of
wastewater and are generally found in rural and suburban areas. While septic tanks and soil adsorption
systems have significant limitations, decentralized
systems can effectively protect water quality and public health from groundwater and surface water contamination, if managed properly (i.e. properly sited,
sized, designed, installed, operated, and maintained).
Nitrate concentrations in groundwater that exceed the
drinking water standards can cause health problems.
Treatment
Onsite wastewater systems contain three components:
a treatment unit, which treats water prior to dispersal into the environment; a soil dispersal component,
which assures that treated water is released into the
environment at a rate which can be assimilated; and a
management system, which assures proper long-term
operation of the complete system. Disinfection of the
treated effluent may be provided prior to dispersal.
A typical onsite system consists of a septic tank followed by an effluent distribution system. Alternative
treatment systems include aerobic treatment and sand
filtration systems.
Conventional Septic Tanks
A septic tank is a tank buried in the ground used to
treat sewage without the presence of oxygen (anaerobic). The sewage flows from the plumbing in a home
or small business establishment into the first of two
chambers, where solids settle out. The liquid then
flows into the second chamber. Anaerobic bacteria in
the sewage break down the organic matter, allowing
cleaner water to flow out of the second chamber. The
liquid typically discharges through a subsurface distribution system. Periodically, the solid matter in the
bottom of the tank, referred to as septage, must be removed and disposed of properly.
Aerobic Treatment Units
Aerobic treatment units are also used to provide onsite
wastewater treatment. They are similar to septic tanks,
except that air is introduced and mixed with the wastewater inside the tank. Aerobic (requiring oxygen) bacPage 179
teria consume the organic matter in the sewage. As
with the typical septic system, the effluent discharge
from an aerobic system is typically released through a
sub-surface distribution system, or may be disinfected
and discharged directly to surface water. Aerobic treatment units also require removal and proper disposal of
solids that accumulate in the tank.
Media Filters
Media filters are used to provide further treatment of
septic tank effluent, and provide high levels of nitrification. They can be designed to pass the effluent once
or multiple times through the media bed. Media, such
as sand, acts as a filter. The media is placed two to
three feet deep above a liner of impermeable material
such as plastic or concrete. Septic tank effluent is applied to the filter surface in intermittent doses and is
further treated as it slowly trickles through the media.
In most media filters, wastewater is collected in an under drain, then either pumped back to the filter bed or
to other types of treatment.
Dispersal Approaches
is commonly used. A mound system is a distribution
system constructed above the original ground level
by using granular material, such as sand and gravel
to receive the septic tank effluent before it flows to
the native soil below. The effluent flows to a dosing
tank that is equipped with a pump. Here, the effluent
is stored until there is sufficient liquid. Once the liquid
is pumped out, it moves evenly throughout the mound
before reaching less permeable soil or ground water.
The granular material acts as a treatment medium and
improves the removal of aerobic treatment unit pollutants in ways that may not be provided by substandard
native soils.
Drip Dispersal System
Where soils are very thin or have reduced permeability, drip dispersal systems can be utilized. The typical
drip system operates like drip irrigation at a moderately high pressure. The components of a drip system
include: filters to remove solids, a network of drip
tubes to disperse liquid into soil, tanks to hold liquid,
and controllers to regulate the flow to the drip system.
Traditional onsite systems include treatment units followed by a drain field or absorption field. Wastewater
from the treatment unit is dispersed through a suitable
soil layer, where it receives additional treatment by the
soil microorganisms and filtering properties of the soil.
If the soil is unsuitable for the installation of a soil absorption field, alternative methods can be used to further
treat or distribute the treated effluent. The most common
alternative dispersal systems include low pressure pipe,
mounds, drip disposal, and evapotranspiration beds.
Evapotranspiration Beds
Absorption Field
Ensuring performance of decentralized wastewater
treatment systems is an issue of national concern, because these systems are a permanent component of
our nation’s wastewater infrastructure. Twenty-five
percent of households nationwide and one-third of the
new homes being constructed are served by onsite systems. Many of the existing systems do not perform adequately, due to lack of management. Therefore, EPA
promotes the sustained management of decentralized
wastewater systems to enhance their performance and
reliability.
When soil conditions permit, the most common method of dispersing septic tank or aerobic system effluent is an absorption field consisting of a series of
perforated parallel pipes laid in trenches on gravel
or crushed stone, or as a direct discharge to the soil
through trenches. Typically, effluent flows into the absorption field from a distribution box, which maintains
an even flow of effluent to the absorption field. From
there, the effluent drains through the stone and into the
soil, which provides further treatment.
Mound System
When the soil is not conducive to percolation or
when the groundwater level is high, a mound system
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An evapotranspiration (ET) bed is an onsite dispersal
system where pretreated wastewater evaporates from
the soil surface or is transpired by plants into the atmosphere. Usually, ET beds are used in arid climates
and there is no discharge, either to surface or ground
water. Vegetation is planted on the surface of the sand
bed to improve the transpiration process, and landscaping enhances the aesthetics of the bed.
Management of Decentralized Systems
EPA strongly encourages communities to establish
management programs for the maintenance of onsite
systems in addition to improving local requirements
for onsite system siting and design. Communities
benefit from effective onsite system management
programs by enjoying improved protection of public
health and local surface water and groundwater resources, preserving rural areas, protecting property
owners’ investments through increased system service
life, and avoiding the need to finance costly central
wastewater collection and treatment systems.
Asset Management
America’s public water based infrastructure – its water supply, wastewater, and storm water facilities and
collection/distribution systems – is integral to our economic, environmental and cultural vitality. Much of this
country’s public wastewater system infrastructure has
crossed the quarter-century mark, dating back to the
CWA construction grant funding of the 1970s. Many of
our collection systems date from the end of World War
II and the population boom of the post-war era.
The oldest portions of the collection system pipe network exceed 100 years of service. Significant parts of
this infrastructure are severely stressed from overuse
and the persistent under-funding of repair, rehabilitation, and replacement. In an increasing number of
communities, existing systems are deteriorating, yet
the demand for new infrastructure to accommodate
growth presses on unabated. A revitalized approach to
managing capital wastewater assets for cost effective
performance is emerging in this country. This asset
management approach focuses on the cost effective
sustained performance of the wastewater collection
and treatment system assets over their useful life.
Maintenance
Wastewater collection and treatment systems must
provide reliable service and avoid equipment breakdowns. Most equipment breakdowns can be prevented
if system operators inspect the equipment, including
sewer lines and manholes, regularly. Preventive maintenance uses data obtained through the inspections in a
systematic way to direct maintenance activities before
equipment failures occur. A good program will reduce
breakdowns, extend equipment life, be cost-effective,
and help the system operators better perform their jobs.
For more information: www.epa.gov
Summary
In this chapter, we examined the pretreatment of
wastewater, as well as advanced methods of wastewater treatment, which typically include nitrogen
control, biological phosphorous control, and carbon
absorption. We also discussed the use and disposal of
wastewater residuals and biosolids. This section included land application, incineration, and the creation
of beneficial use products from biosolids. In addition,
we looked at decentralized systems including media
filters, soil dispersal systems, and aerobic treatment
units. We concluded by touching briefly on asset management, which specifies that systems must be operated as designed to protect water quality and human
health.
Operation
Wastewater collection and treatment systems must
be operated as designed in order to adequately protect water quality and human health. Most systems
are in operation every day of the year, rain or shine.
Licensed and trained operators are responsible for the
day-to-day performance of the wastewater system.
Their responsibilities include budget and business administration, public relations, analytical testing, and
mechanical engineering, as well as overseeing the collection system and wastewater treatment processes.
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Student Assessment
(last page of book) or complete your assessment online at www.McKissock.com/Enginering.
Final Exam
1. How much of the nation’s population is
supplied by wastewater collection?
a. 50%
b. 65%
c. 75%
d. 25%
2. Which of the following statements is true
about flocculation?
a. Sewage may increase in size due to chemical
action
b. Floc is resistant to penetration by fluids
c. It refers to compounds which do not contain
carbon
d. It refers to the oxidation of ammonium to
nitrate
c. Lagoons
d. Filters
6. 6. Which of the following statements is true
about nitrogen?
a. It is usually removed by secondary treatment
b. It may stimulate the growth of algae
c. It comprises up to 90% of the earth’s air
d. When released, it may cause environmental
harm
7. When soils are thin, _____ may be utilized.
a. Mound systems
b. Drip dispersal systems
c. Evapotranspiration beds
d. Absorption fields
3. The measure of the ability of a material to
transmit fluids is called:
a. Clarification
b. Permeability
c. Denitrification
d. Evapotranspiration
8. In America, __________ of all households are
served by onsite systems.
a. 50%
b. 75%
c. 25%
d. 40%
4. The term “aerobic” can be defined as:
a. Suspended growth process for removing
organic matter from sewage
b. A life or process that occurs in the presence of
oxygen
c. Removal of solids from wastewater by gravity
settling
d. Killing of microbes
9. Biosolids are used by farmers for:
a. Fertilizers and conditioners
b. Revegetating areas with poor soils
c. Strip mining
d. All of the above
5. Biodegradable materials and nitrogen are
removed from wastewater by using:
a. Estuaries
b. Detoxification
10. A/an ___________ is buried in the ground and
is used to treat anaerobic waste.
a. Aerobic treatment unit
b. Conventional septic tank
c. Media filter
d. Mound system
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The History and
Future of Domes
Course Description
This course reviews the essential principles in designing a dome. It uses the Romans’ Pantheon Dome to introduce the structural elements of domes: meridians, parallels and materials such as concrete. It outlines how
modern domes are engineered, and what loads must be considered in the design, emphasizing how to calculate
the loads on a dome from wind using the ASCE Standard, and how to size I-beams for a modern dome. It also
shares the stories behind the building of Istanbul’s Hagia Sophia – a cathedral built 14 centuries ago that can
still astonish a modern-day engineer – as well as one of the most revolutionary domes in the history of building:
The Duomo (Dome) of the Basilica di Santa Maria del Fiore in Florence. The double-masonry dome and unique
structure allowed its designer to span a greater distance than the Pantheon.
Chapters
• Chapter One: History of Dome Engineering
• Chapter Two: Calculation of Wind Load
• Chapter Three: Sizing an I-Beam Used in a Dome
Learning Objectives
• Explain the basic principles of dome engineering
• Identify the structural elements of domes
• Describe the considerations for determining dome load, such as wind and snow
• Use the ASCE Code to calculate the load from wind on a dome
• Understand the technical aspect of Brunelleschi’s Dome in Florence
• Explain how to size I-beams used in a modern triangulated dome
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Chapter One:
History of Dome
Engineering
Overview
• Introduction
• The Pantheon
»» Brief History
»» Structural Elements of a Dome
• Why a Dome “Works”
• The Pantheon Dome
• Concrete
»» How the Romans Poured Concrete
to Make the Pantheon
• Modern Domes
• Snow Loading
• Summary
Learning Objectives
•
•
•
•
Explain the basic principles of dome engineering
Identify the structural elements of domes
Explain how a dome structure works
Describe considerations for determining dome
load, such as wind and snow
Introduction
In this course, the huge impact engineers have had in
creating spaces where the human spirit can soar will be
examined, where it can celebrate its relationship to the
mysteries of the universe, experience the adrenaline of
a sporting event, or even just feel majesty. This structure is something that is taken for granted: the dome!
Think about it for a moment: Domes cap the world’s
greatest churches and crown sporting arenas around
the globe, places where twenty or forty or even eighty
thousand people commune to rejoice, to carouse, to
consecrate, to jubilate... to engage in all the activities
that make us human.
In this course you will review how to estimate the loads
from wind on a dome, and how to size the I-beams for
a modern dome made of triangular aluminum plate.
These calculations are necessary to construct a safe
building. That role for engineers – great protector of
the human population – is often underrated by the public because it is done so well. Just imagine the havoc if
sporting arenas or cathedrals routinely collapsed.
Take this journey back through time and also around
the world to examine three marvelous achievements
of engineers: The Romans’ great Pantheon; the sixth
century’s magnificent Hagia Sophia; and Florence’s
amazing masterpiece, Bruncelleschi’s Duomo, the
Dome of the Cathedral of Florence. The architect Mario Salvadori astutely said, “Perhaps the dome is the
nearest materialization of heaven, the only man-made
representation of the sky, and this is why a dome seems
to protect us like the sky of a clear night, embracing
us and our smallness and solitude.” Learn about the
mysteries of how these magnificent engineers built
structures that have remained standing for centuries.
The Pantheon
The Romans built the Pantheon as a temple to all the
gods of Ancient Rome, and then rebuilt it in 126 AD.
Its exact use is still debated, and it is unknown who designed it, but there is no debate it is capped with the
most majestic and oldest large-scale dome in Rome. It
has been in continuous use throughout its history; in
fact, one of the reasons it is so well preserved is that it
became a Roman Catholic Church in the 7th Century.
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Michelangelo found it so beautiful he called it “angelic” and declared it “not of human design.” In the
Middle Ages, most people believed the Pantheon’s
dome stayed put because of sinister forces of demons
as it did not have the dense row of arches as did conventional cathedrals. Yet, of course, it is human-built,
a unique structure created by engineers.
The Pantheon’s dome provides us with a beautiful way
to study how building materials must be used to create
an elegant, but safe, structure. Here are some details to
see why it stays standing.
startles a first time viewer. The height to the oculus
and the diameter of the interior circle are the same:
142 feet. Since the “Great Eye” at the dome’s apex
is the source of all light to the interior, it draws the
viewer’s eye toward it like a moth toward a flame. The
interior is seamless: no straight lines, axial vistas, flat
walls, or angles to give an observer a solid focal point.
The dome simply dominates, yet, many of the features
serve more than an aesthetic function. The oculus,
for example, also serves as a cooling and ventilation
method. During storms, a drainage system below the
floor handles the rain that falls through the oculus.
Brief History
The current building dates from about 126 AD, during
the reign of the Emperor Hadrian. You can actually
see “date-stamps” on its bricks. Hadrian was a cosmopolitan emperor who traveled widely in the East,
where Greek culture strongly influenced him. It is telling that he used a Greek name for the structure, (Pantheon comes from the Greek pan for “all” and theon
for “gods”) rather than a Latin name which would be
more typical of Rome. With its hemispherical dome
and orderly division of interior walls into different
levels, the Pantheon represents an architectural embodiment of the Greek idea of cosmos.
Structural Elements of a Dome
The building is circular with a portico of three ranks
of huge granite Corinthian columns (eight in the first
rank and two groups of four behind) under a pediment opening into the rotunda, which is capped with
a coffered, concrete dome, itself topped by a central
opening – the oculus or Great Eye – to the sky. The
columns of the portico work as “foils” to break up the
huge expanse of the rotunda. Thus, when the doors are
opened, the vast, round, seemingly unsupported space
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arches have what are called “flying buttresses.” These
exist because the arch thrust outward in addition to
downward. By contrast, a dome is a monolithic structure where all “arches” are tied together, thus eliminating the need for buttresses. The linked meridians
create a structure that works like the hoops that hold
the staves in a barrel.
When the load of the dome presses downward, it tends
to expand at the bottom and flatten at the top.
Why a Dome “Works”
Think of the dome as a perfect half-sphere, the thickness of which is very small, relative to its span. It isn’t
exactly, but it is close enough. This dome must carry
its own weight and the weight of any load on its exterior surface; for example, wind, rain or snow. The
dome is made up of meridians, curved vertical lines
that remind one of an arch. The meridians meet at the
top and have a common keystone.
Source: Why Buildings Stand Up: The Strength of Architecture by Mario Salvadori (Norton 1980)
This allows the dome to be made thinner than one might
expect because, first, the compression makes the dome
much stiffer; and, second, it keeps the bottom from
opening up. In this drawing the degree is exaggerate
to which this deformation takes place: A 100 foot reinforced concrete dome two or three inches thick will
deflect by less than one-tenth of an inch.
The loads on these meridians accumulate from top to
bottom; the members become more and more compressed as they approach the dome’s support at its base.
If you are familiar with cathedrals, you will recall their
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Before looking at the Pantheon in detail, a crucial difference between an arch and a dome should be noted.
An arch supports its load by compression in its elements, but also by bending if necessary. A dome does
not bend because it is monolithic; this means a dome
can carry any kind of load without changing shape and
without developing bending stresses – until, of course,
it fails catastrophically. This means domes, even
though thin, are stiffer and stronger than any structure
created by humankind and, as seen with the Pantheon,
among the most beautiful.
The Pantheon Dome
The Pantheon holds the record for the largest unreinforced concrete dome. The 5,000 ton weight of the
concrete dome is concentrated on a 30-foot-diameter
ring of voussoirs (the wedge-shaped pieces that form
the vault) which forms the oculus, while the downward
thrust of the dome is carried by eight barrel vaults in
the 21-foot-thick drum wall; these are divided into
eight piers. The thickness of the dome varies from 21
feet at the base of the dome to 4 feet around the oculus.
The height to the oculus and the diameter of the interior circle are both 142 ft, so the whole interior would
fit exactly within a cube.
Concrete
The Romans build the Pantheon after they discovered
how to make pozzolana concrete. This type of concrete – also known as pozzolanic ash – is a fine, sandy
volcanic ash, originally discovered and mined in Italy
around the volcano Vesuvius. The origin of the word
“concrete” gives away its ancientness. It is made by
combining the Latin prefix con, meaning “together,”
and crescere, meaning “to grow.” The name comes
about because when the ingredients making up concrete – water, gravel, sand, and a bit of cement – are
mixed, they turn into a hard, rigid solid. The Romans
discovered concrete by accident. A builder who was
making some mortar happened to be working near
Mount Vesuvius. He tossed in some volcanic ash and
noticed that when his mixture dried it made a very
hard substance. From this serendipitous beginning,
the Romans fine-tuned the recipe for concrete. This
was in contrast to simple lime mortar, which is made
by mixing water with quicklime and sand; it sets when
the water has been evaporated into the atmosphere or
absorbed into surrounding masonry. Roman concrete
sets by combining chemically with water; the cement
doesn’t need to dry out like lime mortar. In fact, pozzolana concrete is called “hydraulic” because it can
even set when immersed in water.
A simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4) forms a calcium silicate hydrate (CaH2SiO4 2 H2O). This reaction is:
Ca(OH)2 + H4SiO4 → Ca2+ + + H2SiO42-+ 2 H2O →
→ CaH 2 SiO · 2 H2O
In addition to this silicate, other substances can be
added to increase the strength of the concrete. For example, the Romans mixed horsehair to reduce shrinkage during hardening. They also added blood, which
made the concrete frost-resistant. Today plastics are
used instead of horsehair, and industrial chemicals
instead of blood, but the same principles apply. With
these innovations, Roman concrete reached a high level of quality unmatched until the 20th century.
Today’s structural concrete improves on Roman concrete for two main reasons:
1. Its mix consistency is more fluid and homogeneous, partly due to carefully prepared aggregate
instead of the nearly random rubble used by the
Romans. This means it can be poured into forms,
thus requiring less hand layering; and
2. Reinforcing steel gives modern concrete great
strength in tension.
Even though concrete is now high-tech – with smart
concrete that can conduct electrical signals, for example – it is a testament to the ingenuity of engineers that,
after 2,000 years, the concrete dome of the Pantheon
is still standing. This concrete was both easier to work
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with than lime mortar, and far superior in its compressive strength. Although there are no tensile test results
available for the Pantheon’s concrete, other Roman ruins of the same era give us a good guess. Roman ruins
in Libya show a compressive strength of 2.8 ksi (20
MPa) and a tensile strength of 213 psi (1.47 MPa).
is an experience not readily forgotten.” But the oculus
is more than this; it functions as a compression ring,
or sort of a keystone, of all the “arches” in the dome.
How the Romans Poured Concrete to Make
the Pantheon
Like the Romans, there are several domed structures
built to celebrate our culture. Houston’s once mighty
Astrodome showcased Muhammad Ali fights and Elvis Presley concerts. It played a part in our nation’s
politics when it housed Hubert Humphrey’s 1968
presidential campaign rally and George H.W. Bush’s
1992 nominating convention, and it presented spectacles such as Evel Knievel’s motorcycle jumps and
Billie Jean King’s defeat of Bobby Riggs in the 1973
tennis match billed, as a “Battle of the Sexes.”
The heroes in the pouring of the mighty concrete
dome of the Pantheon are carpenters. Master carpenters skillfully built the wooden molds that shaped the
concrete.
Carpenters built a complicated system of supporting
timbers, propped on top of the lower walls, sturdy
enough to hold the weight of the concrete. The inside
of the dome was lined with coffers – sunken rectangular sections – so intricate, they had to be built by
master carpenters. More than decorative, these coffers
helped reduce the weight of the dome. They remove
only five percent of the weight, but on a 5,000-ton
dome, that’s a significant 250 tons. All work on the
dome was dangerous, but perhaps the most dangerous
was the job of those men who stood on the scaffolding
and poured concrete layer by layer, as the dome grew
higher and higher.
The dome is made of tapering courses, or steps, that
are thickest at the base and thinnest at the oculus. The
Romans used the heaviest aggregate, mostly basalt, at
the bottom and lighter materials such as pumice at the
top. They embedded empty clay jugs into the dome’s
upper courses to further lighten the structure and make
the concrete cure. In fact, the Roman engineers overdid it and created too massive a base on the dome;
later, they had to scrape out great chambers to reduce
the dome’s weight.
At the very top, they left an opening, the oculus, to
avoid pouring concrete on a horizontal scaffold set
high at the crown. They rimmed this opening with
hard-burnt bricks, cemented by mortar, and then circled it with a bronze cornice. The area encircled by the
oculus is about four percent of the surface of the floor
below. One visitor to the Pantheon notes, “There is no
railing around either the oculus or the flashing. To lie
upon the flashing and look down through the oculus
to the paving forty-five meters (about 150 feet) below
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Modern Domes
Unlike the Romans, there are other building options
for creating domed structures. Perhaps the most amazing of modern times is the Skydome – now renamed
the Rogers Centre – in Toronto.
Opened in 1989, the Skydome houses the Toronto Blue
Jays. While it is mainly a sporting arena, it also hosts
conventions, trade fairs, concerts, fun fairs, and – my
favorite – monster truck shows. The Skydome is the
first stadium to have a fully retractable motorized roof.
Snow Loading
This roof is an amazing engineering achievement. Although it covers eight acres and weighs 6,500 tons, it
can be opened in twenty minutes, even in forty-mileper-hour winds. But perhaps, more amazing than even
these statistics is its greatest achievement: It has done
this safely thousands of times since its debut in 1989.
In the early morning hours of January 1978, the huge
domed roof of the C.W. Post Center at Long Island
University collapsed. “It looks like a giant cracked
eggshell”, said a police officer as he viewed the wreckage. Indeed, the 300-foot-diameter roof had shattered
into thousands of fragments. Unlike the monolithic
concrete dome of the Pantheon, this dome was reticulated: A space frame of hundreds of triangular trusses,
made of galvanized steel, held together by tension
suppression rings, with a compression ring at the top
like the Pantheon’s oculus, and a tension ring at the
base of the dome.
Today, domes are not only for spectacular purpose,
but also for such common uses as roofs of libraries,
mosques, and synagogues, as well as water, sewage,
and chemical tanks. Their safe use comes, of course,
from the incredible body of knowledge about building
materials developed over many millennia.
For example, in designing a modern dome, the engineer must consider loads from wind. In tall buildings,
wind pressures require a structure that resists the wind
load separate from the one that resists the gravity load.
In some tall buildings, up to ten percent of the structural weight, and thus ten percent of the cost, goes to
wind bracing. The first question to be answered about
a tall building’s structure is, “What would be the strongest wind ever at the building site?” For large projects
such as the Toronto Skydome, an engineer would use
a “100-year wind”, which is defined as a wind speed
that has one chance in one hundred years of occurring.
Engineers can use the ASCE Standard 7-05 to estimate the design pressures for wind loading of a dome.
Engineering standards are one of the greatest triumphs
of human ingenuity, saving countless lives. To design
a dome safely, an engineer needs to find the maximum
pressure exerted by the wind; that is, the wind in the
worst possible conditions. We will learn how to calculate loads in the next chapter. It isn’t always wind
that can overload a dome; unlike the Romans, in North
America we need to worry about snow.
A post-mortem analysis on the building concluded it
had been designed using a simplified theory that didn’t
apply to reticulated domes. The principal error was in
assuming that the dome would have uniform dead and
live loads, or the weight of the dome itself and the load
from wind or snow, respectively. Instead, the wind
blew snow only on one side of the dome, thus stressing the structure unevenly. In fact, this uneven loading was only one-fourth of the load required by code
if the load were applied evenly. ASCE Standard 7-05
now in effect, requires engineers to estimate loads for
wind and snow that are both balanced and unbalanced.
Since the collapse occurred during the winter break at
the university, the campus was deserted, no one was
even near the dome. Nevertheless, this highlights the
importance of the ASCE Standards and their role in
preserving human life.
The Roman engineers, without the benefit of computing abilities and rigorous theory of structure, simply
overbuilt the great dome of the Pantheon. So it is no
surprise that the span of the Pantheon was unsurpassed
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for 1,300 years, until the octagonal dome of Santa Maria del Fiore (The Cathedral of Florence) exceeded it
by three feet in maximum span. In the final chapter of
this course, the amazing engineering achievement will
be examined in detail.
Summary
We began this chapter by discussing the Pantheon
Dome, which is the oldest large-scale dome in Rome,
and looked at the structural elements of domes, which
include meridians and parallels. We learned the loads
on meridians accumulate from top to bottom. In addition to meridians and parallels, concrete was used to
hold everything together. Finally, we received a preview of how modern domes are engineered and what
loads must be considered in the design.
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Chapter Two:
Calculation of Wind Load
Overview
•
•
•
•
•
•
•
Hagia Sophia
Calculating the Loading from Wind on a Dome
General Method
Building Description
Velocity Pressure
Dome Roof Pressure
Summary
Hagia Sophia
Hagia Sophia – from the Greek for “Holy Wisdom”
– built 14 centuries ago, can still astonish a modern
day engineer. No engineer today would fail to marvel
at this magnificent structure, which was built in less
than six years, and, unlike the Gothic Cathedrals of the
Middle Ages, did not result from a careful step-by-step
evolution, but instead appeared as if from nowhere.
Nothing comparable had been built for 200 years.
Learning Objectives
• Summarize how the dome of the Great Cathedral
Hagia Sophia is supported
• Use the ASCE Code to calculate the load from
wind on a dome
• Identify how wind affects three components
of a building
Justinian I, the fourth Christian Emperor of Rome,
egged on by his wife, a former theater performer and
courtesan, forcibly put down a tax revolt by the people
of Constantinople. This “Nika Revolt” (citizens cried
“Nika!” (“Conquer!”) as they took over the city) lasted for a week in A.D. 532. Nearly half the city was
burnt and 30,000 people were killed. The protesting
populace had burned down the church Hagia Sophia.
Justinian, outraged at this act, decided to rebuild the
church on a scale never seen before.
The Emperor amassed material from all over the Roman Empire. As the English historian Edward Gibbon
noted in his Decline and Fall of the Roman Empire:
“The memory of past calamities inspired Justinian with a wise resolution, that no wood, except
for the doors, should be admitted into the new edifice; and the choice of the materials was applied
to the strength, the lightness, or the splendor of
the respective parts.”
Page 191
Justinian brought in Hellenistic columns from the
temple of Artemis; beautiful red-purple porphyry from
Egypt; green marble from Thessaly in Greece, black
stone from the Bosporus region; and yellow stone
from Syria.
Although Gibbon was not a structural engineer, his
careful eyes zeroed in on the main structural elements.
Work on the structure commenced on February 23,
532. Ten thousand workers were split into two rival
groups: One worked on the southern half the building,
the other on the northern. The competition was designed to spur them to make tremendously fast progress. As Gibbon notes, Justinian took an avid interest
in the construction:
“The emperor himself, clad in a linen tunic, surveyed each day their rapid progress, and encouraged their diligence by his familiarity, his zeal,
and his rewards.”
The completed cathedral, Hagia Sophia, was consecrated by the Emperor in an amazing five years, eleven
months, and ten days from the first foundation. Gibbon
poked fun at the final structure, but also praised precisely the aspect which still stuns a modern engineer:
“The dome, illuminated by four-and-twenty windows, is formed with so small a curve, that the
depth is equal only to one sixth of its diameter;
the measure of that diameter is one hundred and
fifteen feet, and the lofty center, where a crescent
has supplanted the cross, rises to the perpendicular height of one hundred and eighty feet above
the pavement. The circle which encompasses the dome lightly reposes on four strong arches, and
their weight is firmly supported by four massy
piles, whose strength is assisted, on the northern
and southern sides, by four columns of Egyptian
granite.”
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At no time in history had a structure so completely
broken with the past. Its composition of space, surface, and light were like no other structure before it.
The building’s interior does not impart a feeling of
structural mass, but instead one of an enfolding surface. This effect has a specific purpose, because any
building with a social or societal function conveys a
message. Pope Nicholas V outlined why the Roman
Catholic Church built magnificent structures:
“To sustain the faith of the unlettered masses
there must be something that appeals to the eye
or it will wither away. Bu if authority were visibly
displayed in magnificent building, imperishable
monuments and everlasting witnesses seemingly
built by God himself, the belief implanted by the
doctrine would be confirmed and strengthened.”
Hagia Sophia still stunned Byzantine Emperor John
VI Kantakouzenos 800 years after its construction: “In
size the greatest of all Churches under the sun, surpassing in beauty and magnitude all others wheresoever they may be.” Indeed, until the Dome built by
Brunelleschi, described in the next chapter, no dome
even came close to the dome of Hagia Sophia.
The key engineering problem in building Hagia Sophia was how to support the dome. Four main arches
make up the sides of a square.
receive the weight of the dome, concentrating it at
the four corners where it can be received by the piers
beneath. This was revolutionary. Prior to the pendentive’s development, builders used corbeling – a piece
of stone jutting out of the wall - to carry weight, or columns inside the dome’s rotunda. Without pendentives,
Hagia Sophia would not have its majestic interior that
provides the awesomeness of an immense open space,
unobstructed by columns.
The dome of Hagia Sophia sits only on the top
of the arches that make up the four walls of the
Church. This alone would not support the dome
sufficiently; it would crack under its own weight.
Source: http://videos.howstuffworks.com/hsw/13391discover-magazine-engineering-secrets-video.htm
Each arch is a half-circle whose rise equals half its
span. Hagia Sophia’s great round dome must sit on
this square; in fact, it only touches at the very top of
the rounded arches, essentially at a point on the top of
each rounded arch. This would not support the dome
well enough; it would crack under its own weight. Its
designers used a most ingenious and original method
to overcome this flaw. Between the arches, its builders
placed pendentives.
To “fill in” the spaces between the arches, Hagia
Sophia’s designers used pendentives, triangular
segments of a sphere. Source: http://videos.howstuffworks.com/hsw/13391-discover-magazine-engineering-secrets-video.htm
Pendentives are triangular segments of a sphere, which
taper to points at the bottom and spread at the top.
They fill in between the arches to create a continuous
circular structure for the dome. In other words, they
Calculating the Loading
from Wind on a Dome
Let’s imagine for a moment we are building a replica
of Hagia Sophia in the United States. A key design
consideration would be the load caused by wind and
snow on the structure. Each dome has, of course, a
dead load from its own material, but we need to consider the extra load created by wind and snow. In this
example, I will focus on how to use the ASCE Standard 7-05 Minimum Design Loads for Buildings and
Other Structures. While this was the most current
standard when I performed the calculations below, be
sure to check the ASCE Website to find the most upto-date standards.
To estimate the wind load so a structure can be safely
designed, we need to assess the impact of wind on
three aspects of the structure:
1. The first is the impact on the main wind forceresisting system (MWFRS);
2. The second is that on the components and cladding (C&C); and
3. The third is the loads directly on the walls.
The first is, of course, the overall structure receiving
wind loading from more than one face. The second receives wind loads directly and generally transfers the
load to other components or to the main system. Generally the C&C design pressures will be higher than the
MWFRS, because localized pressures act over small
areas; also, the MWFRS receives pressure from several
surfaces. So, with spatial averaging and correlation, the
pressures are likely to be smaller than for C&C. In general, an engineer calculates all these loads and then uses
them to design the various components. For instance,
in the next chapter you will see an example of how to
size the I-beams used to support a triangulated dome.
Page 193
General Method
To calculate MWFRS and C&C, we need to know
some details of the building beyond its dimensions: its
location and use are key in applying the ASCE Standard. For small structures there are simple ways to estimate the wind load, but typically a domed structure
is large enough that we need to use the analytical procedure described in the Standard.
In brief, the procedure is:
1. Calculate the velocity pressure caused by the
wind; then
2. Use the appropriate gust effect factors and force
coefficients to estimate the loads felt by the MWFRS and then the C&C.
Use: The Standard defines four categories that range
from low hazard to human life to essential facilities
such as hospitals and fire stations. This sets the value
of I – the “importance factor” – as described below.
Since this building will be used as a church with a
capacity of more than 300 people, it is a Category III
building as per Table 1-1 of the Standard.
Wind Speed: You can use the maps in the Standard to
determine the basic wind speed, defined as 3-second gust
speed at 33 feet above the group for a particular exposure category. Associated is a typical occurrence of once
every 50 years. From the map shown below, we can estimate V=120 miles per hour for Marion County, Florida.
In this example, calculate only the wind load on the
MWFRS holding up Sophia Hagia; the standard can
be used to calculate C&C and the load on the walls in
a similar way.
Building Description
In order to apply the standard, it is necessary to know
its use and location, in addition to building dimensions. So, let’s say you’re building a replica of Hagia
Sophia in Marion County, Florida, but using modern
techniques of steel construction.
Location: Marion County, Florida
Topography: Homogeneous
Terrain: Open
Dimensions:
• 102 feet diameter for the dome
• 156 feet height to eave (the spring line of the dome)
• 186 feet distance from the ground to the top of the
dome
Framing: Steel frame dome roof, metal decking roofing
Exposure: The Standard lists three ground exposure categories, which depend on surface roughness;
roughness can dramatically affect wind. Our Sophia
Hagia replica will be in an area with scattered obstructions having heights less than 30 feet; typically flat
open country and grasslands. This would be, then, exposure category C.
Page 194
Source of map: American Society of Civil Engineers.
2006. Standard 7-05. In Standard Minimum Design Loads
for Buildings and Other Structures. Reston, VA. ASCE.
Velocity Pressure
The wind loading calculation begins with an estimate
of the velocity pressure felt by the structure. Again, we
estimate this and use the appropriate gust coefficients
to apply this to the MWFRS, C&C, and the load on the
walls. In this example, we start with:
The velocity pressure, , is given by equation 6-15 in
the Standard:
•
is a coefficient based on the exposure category
•
corrects for topography; in this case 1.0, since
it is homogeneous
•
is a wind directionality factor of 0.95 as per
Table 6-4 of the Standard
• V = 120 mph
• I = 1.15 for Category III as in Table 6-1 of the Standard
With the data above, we can calculate the velocity
pressure. This is the key calculation for all the necessary wind loads for design: MWFRS roof and wall and
the C&C. The engineer simply uses different pressure
coefficients to calculate the forces and loads on the
walls and roof, and on the cladding and components.
Here it is calculating only the roof pressures, so we
need this value at the top of the dome.
G is the gust effect factor, which is always 0.85 for
rigid buildings as noted in 6.5.8.1 of the Standard.
From Figure 6-5 of the Standard, shown below, you
learn that for enclosed buildings
= +/- 0.18.
From Table 6-3 of the Standard, the value of
for
calculating MWFRS is for a height of 180 feet (the top
of the dome) and exposure category C, we find
=
1.43 – the difference between 180 and 200 feet is so
small interpolation isn’t needed. This is the final value
we need in order to calculate the velocity pressure:
Section of Table 6-3 Velocity Pressure Exposure Coefficients
Height
above
ground
(ft)
Exposure (Note 1)
B
C
D
Case 1
Case 2
Cases 1
&2
Cases 1
&2
180
1.17
1.17
1.43
1.58
200
1.20
1.20
1.46
1.61
Notes for Table 6-3:
Case 1: a: All components & cladding; b: Main wind force
resisting system in low-rise building designed using Figure
6-10 of the Standard.
Case 2: a. All main wind force resisting systems in buildings except those in low-rise building designed using Figure 6-10 of the Standard; b. All main wind force resisting
systems in other structures.
Domed Roof Pressure
Source: American Society of Civil Engineers. 2006. Standard 7-05. In Standard Minimum Design Loads for Buildings and Other Structures. Reston, VA. ASCE.
For an enclosed building,
pressure at the dome, and so
the pressures are
is always the velocity
= 57.59 psi. Thus
To use this equation, you must estimate the values of
the pressure coefficients. These vary with height,
and according to the Standard you need to calculate
them in two ways using figure 02-103:
Next up, use this velocity pressure to calculate the
domed roof pressures, or the loads caused by wind.
This is the net pressure on any surface which arises
from a difference of the external and internal pressures. To calculate this, we use equation 6-17 from the
Standard:
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Figure #02-103 [width of cylinder is 102 ft, height of cylinder is 156 feet, base of cylinder to dome top-point B is 186
feet – should be drawn to scale]
point B. For a Distance of 20 feet from the spring line,
this yields:
Cp = slope*distance from edge of dome + intercept
intercept = -1.52 [We start from the edge of the dome
where the distance from along the spring line is zero]
Cp (@ 20 feet along spring line) = -0.0075*20 - 1.52 =
= -1.37
Case A: Use a linear interpolation to calculate values
of
from Point A to Point B
Step 2: Estimate Pressures
Case B: Use the pressure coefficient at A for the entire
= 25 defront area of the dome up to an angle of
grees (as measured from the spring line to the top of
the dome), and then interpolate the values for the rest
of the dome as in case A.
To calculate the coefficients, we use Figure 6.7 and
find the values at points A, B, and C using the ratio of
the height at spring line (hD) to the dome diameter (D)
and the ratio of the dome height above the spring line
(f) to the diameter of the dome (D).
For our replica of Hagia Sophia, these values are:
Reading Figure 6.7 of the Standard, we find
In Table 02-101 below, I’ve tabulated the values for
Case A for the windward side. A typical line is calculated as follows:
Step 1: Estimate Cp by interpolation
The Standard recommends using linear interpolation
from the windward point A to the top of the Dome
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Table 02-101:
Case A: Pressures along Dome Case from Windward Point A to Top of Dome Point B
Distance
Cp
Pressure- Pressure- External
along
(pressure
minus
plus
pressure
spring
coefficient)
(psi)
(psi)
(psi)
line (ft)
0
-1.52
-84.8
-64.0
-74.4
5
-1.48
-83.0
-62.2
-72.6
10
-1.45
-81.1
-60.4
-70.8
15
-1.41
-79.3
-58.6
-68.9
20
-1.37
-77.5
-56.7
-67.1
25
-1.33
-75.7
-54.9
-65.3
30
-1.30
-73.8
-53.1
-63.5
35
-1.26
-72
-51.3
-61.6
40
-1.22
-70.2
-49.5
-59.8
45
-1.18
-68.4
-47.6
-58.0
50
-1.15
-66.5
-45.8
-56.2
Table 02-102:
Case B: Pressures along Dome Case on Leeward
Side from Top of Dome Point B to Point C
Distance
Cp
valong
(pressure
minus
plus
pressure
spring
coefficient)
(psi)
(psi)
(psi)
line (ft)
51
-1.14
-66.2
-45.4
-55.8
60
-1.03
-60.6
-39.9
-50.3
70
-0.9
-54.5
-33.8
-44.1
80
-0.78
-48.4
-27.6
-38.0
90
-0.65
-42.2
-21.5
-31.8
100
-0.53
-36.1
-15.3
-25.7
Case C: Using Edge Coefficients Up to
o
= 25
The Standard requires we calculate the pressures on
the Dome in a second way: Determine the point of the
o
dome at which = 25 ; use the values at the leading
edge (Point A) until that point. From there, the calculation follows that of the Case A. For the replica of
o
Hagia Sophia, = 25 when we have traveled 14.38
feet along the spring line – from the outer edge to the
center.
So, use the pressures calculated from Case A at the
point A until 14.38, from there we would interpolate
between the value at 14.38 and the top of the Dome,
Point B.
In Table 02-103 below, you’ll see the tabulated the
values for Case B for the windward side. A typical line
after = 25o is calculated as follows:
Step 1: Estimate Cp by interpolation
The Standard recommends using linear interpolation
from the windward point A to the top of the Dome,
point B. For a Distance of 20 feet from the spring line,
this yields:
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Summary
Table 02-103:
In this chapter, we focused on how to calculate wind
loads on a dome. We were introduced to one of the
most magnificent domes in the world: Hagia Sophia,
which still astonishes modern-day engineers. We detailed the calculations necessary to estimate loading
from wind. To safely assess the impact of wind the
engineer needs to consider its effect on three aspects
of the structure: a) the main force-resisting system, b)
the components and cladding, and c) the loads directly
on the walls. To illustrate the calculation, we looked at
how to build a full-sized steel replica of Hagia Sophia
in Marion County, Florida.
Case D: Pressures along Dome Case from Windward Point A to Top of Dome Point B
Distance
Cp
along
(pressure
minus
plus
pressure
spring
coefficient)
(psi)
(psi)
(psi)
line (ft)
0
-1.52
-84.8
-64.0
-74.4
14.38
-1.52
-84.8
-64.0
-74.4
20
-1.46
-81.9
-61.2
-71.6
25
-1.41
-79.4
-58.6
-69.0
30
-1.36
-76.8
-56.1
-66.5
35
-1.25
-74.3
-53.6
-63.9
40
-1.25
-71.8
-51.0
-61.4
45
-1.2
-69.2
-48.5
-58.9
50
-1.15
-66.7
-45.9
-56.3
The pressures shown on the dome in the figure below
for cases A and B.
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Chapter Three:
Sizing an I-Beam
Used in a Dome
Overview
Brunelleschi’s Dome
The dome of the Basilica di Santa Maria del Fiore
dominates the skyline of Florence, Italy. Finished in
1436, its “peaked” dome – a shape called “a quinto
acuto” – distinguishes it immediately from the Pantheon, Hagia Sophia, and other ancient domes.
• Brunelleschi’s Dome
• Sizing I-Beams in a Modern Dome
• Summary
Learning Objectives
• Understand the technical aspect of Brunelleschi’s
Dome in Florence
• Explain how to size I-beams used in modern
triangulated domes
• Analyze the charts of the Aluminum Association
Standard to select an I-beam
A contemporary observer remarked: “It was incredible
in our own time that it could be done; it was not conceived or known by the ancients.” Indeed, this was a
step beyond the Pantheon: Instead of that dome’s thick
walls, the Duomo, amazingly, uses only two comparatively thin domes nested together. It was the first to
exceed the Pantheon’s diameter (142 vs. 145). From
its completion in the 1st century A.D. no dome came
close to the Pantheon. The mighty Hagia Sophia had
a diameter only two-thirds that of the Pantheon. The
Duomo is still the largest brick dome ever built.
In 1417, the town leaders of Florence decided to commission a dome for their unfinished cathedral. Its first
stone had been laid in 1296 and its construction continued for 170 years, interrupted by the death of its architect and the Black Plague of 1348. Work continued
fitfully until 1418, when only the dome remained incomplete. The semi-finished cathedral had an octagonal drum on which the dome was to be built.
This drum stood 177 feet high and presented the tower
leaders with a difficult problem: With such a tall drum,
how could enough resources – timber and other materials – be amassed, both logistically and financially, to
build scaffolding to support a dome as massive as Santa Maria del Fiore deserved? Even though the Opera
Page 199
del Duomo (the committee in charge of building the
cathedral) owned several forests, it would still require
a huge expenditure: At the time, timber was nearly
as difficult to harvest and transport as marble. Today,
power saws and sophisticated trucking make it look
simple compared to mining stone. But in 1417, even if
they could muster the resources to build the centering,
removing it would be another huge task. Typically,
this type of demolition was one of the most hazardous
parts of construction in the Middle Ages. One member of the Opera proposed (perhaps in jest) they fill
the cathedral with coin all the way to the top of the
drum, so the workers would have a “platform” to work
on. The virtue of this, in the eyes of its proposer, was
there would be no expense to having them removed:
Once done, they could simply open the doors and let
the greedy townspeople carry the coins away!
The leaders held a contest in 1417 to find the right
person and the right design for the dome:
“Whoever desired to make any model or design
for the vaulting of the main Dome of the Cathedral under construction by the Opera del Duomo
– for armature, scaffold or other thing, or any
lifting device pertaining to the construction and
perfection for said cupola or vault – shall do so
before the end of the month of September. If the
model be used he shall be entitled to a payment of
200 gold Florins.”
This gave interested parties six weeks to build models
or draw a design. The winner was from a well-known
local goldsmith.
Filippo Brunelleschi, known as “Pippo”, submitted a
model that called for a unique double-dome construction – two domes nested together – and required, he
claimed, no need for “centering” the scaffolding traditionally used to hold the dome as it was built. The
committee awarded the prize and a contract to Pippo, but because they were unsure of his abilities as
a builder, they required him to partner with his main
rival, a fellow goldsmith named Lorenzo Ghiberti.
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The committee did not realize Brunelleschi was a mechanical genius. His father had intended him to become, like his father, a civil servant, but as a young
boy he showed no interest for this. Instead he displayed an uncanny talent for solving mechanical problems. Although disappointed his son would not follow
in his footsteps, Brunelleschi’s father set him up as an
apprentice to a goldsmith, where Young Pippo thrived
on learning to mount gems with precision, engrave on
silver, and also embossing. This led him to study the
engineering of motion: how to use wheels, weights,
and gear to move things. He is said to have used his
mechanical knowledge to build the world’s first alarm
clock.
This, though, was not enough to convince the Opera he
could build a huge stone dome alone. When the time
came to start building, Brunelleschi feigned illness,
letting his rival head up the effort. Soon it became apparent that Ghiberti was not up the task and Brunelleschi, alone, directed the building of the dome for the
next sixteen years. (In fairness to Ghiberti, Brunelleschi had omitted key elements from his model so no
one but he could build it.)
Brunelleschi was a most exacting builder. He chose
the clay and specified the exact dimensions of the
bricks. He gave exact instructions about which quarries could provide stone, wanting only the highest
quality. He managed the project with an iron hand, going so far as to construct kitchens inside the cathedral,
so his workers would not waste time looking for food.
He refused to pay dues to the stonemasons and woodworkers guilds, and even trained workers to replace
them when they struck.
The most important aspect of his design was its double-masonry dome. It had a thick inner octagonal shell
connected by meridianal arched ribs connected to a
thinner outer shell. In fact, one can still walk between
these inner and outer domes. The main structural element was six horizontal rings of sandstone reinforced
on their outer surface with iron chain. These chains
prevented the dome from bursting. This is in contrast
to the Pantheon, where there are extremely thick concrete walls to keep the dome from collapsing. In the
Duomo, the stone and iron hoops allowed Brunelleschi
to make two thin domes instead. Probably Brunelleschi chose this method, in part, because he had likely
studied the Pantheon during an extended stay there in
his youth and understood the problem with cracking;
and partly because the secrets of Roman concrete has
long ago been lost. So, how did Brunelleschi build his
dome without using centering? (Recall that, for the
Pantheon, building the wooden scaffolding used for
center was one of the most exacting and dangerous
jobs in that dome’s construction.)
He used two clever methods, one of which is reflected
in the herringbone pattern for the bricks. Between the
ribs, he constructed horizontal “arches”, creating nine
concentric circles from the bottom of the dome to the
top. Thus, his Duomo works, structurally, just like a
circular dome, which is stable at all times during its
construction because the uppermost complete ring
acts as a keystone for its meridianal arches and keeps
them for falling inward.
To accomplish this, Brunelleschi needed to use one
more trick. During construction, the uppermost ring
can act as a keystone, only when it is complete, because only then can it resist compression. How did
Brunelleschi solve this problem? He connected the
uppermost incomplete ring to the completed ring below it. For the bricks and masonry to function like the
concrete in a dome, the bricks could not be laid on the
horizontal, but instead with an inward inclination so
that every brick increases the height of the dome. This
yields the herringbone spirals characteristic of the famous Duomo.
Perhaps most amazing of all, is the great engineering
genius, Brunelleschi had only an intuitive feel for the
stresses in the dome. He would not have thought in
terms of “compression” and “tension” as we do today, but would have understood the dome’s structure
and stability as manifestations of crushing and pulling
apart or tearing. He also would have known, roughly, in what direction these catastrophic events would
happen. Today, of course, we have very sophisticated
ways to estimate the structural properties of a roof. In
the remainder of this chapter, we’ll look at how we can
use engineering standards to size the components of a
modern-day dome roof.
Sizing I-Beams in a Modern Dome
The most common type of dome roof is the triangulated
dome, popularized by Buckminster Fuller. Thousands
of examples exist. Perhaps the most famous is Spaceship Earth at EPCOT Center in Orlando, Florida. This
iconic structure dominates the skyline at Disney World.
Page 201
In addition to being the main focal point of EPCOT, the
18-story geodesic sphere takes guests on a “time machine tour” using a conveyor belt: The 13-minute ride
celebrates the achievements of engineers in advancing human communication, by taking the viewer from
prehistoric times to the 21st century. Ray Bradbury, the
great science fiction writer, helped to write the story told
inside the structure, although its exterior was inspired by
the work of Buckminster Fuller, shown below.
Over forty thousand workers took 26 months to build
Spaceship Earth. The “sphere” sits on a massive foundation formed by deep trusses and six legs, none of which
are directly under the sphere, in order to give the illusion
of the ball floating in air. Quadrupod structures support
the smaller beams, on which the skin is built. Interestingly, a small service car at the top of Spaceship Earth
carries an engineer down the sides to make any necessary repairs. Geometrically, Spaceship Earth is a pentakis dodecahedron; that is, it consists of 60 isosceles triangle faces divided into 16 smaller equilateral triangles,
which are then divided further. As in most such roofs,
each individual triangle is flat, but the lattice approximates a sphere when connected on a spherical surface.
In analyzing such a roof, we can treat it like a dome.
There are many calculations one could make in designing such a dome, but here I was to use the Aluminum
Association’s Specification for Aluminum Structures
to illustrate how to size the members in a triangulated
dome. The type of dome we’ll look at is a single-layer
triangular lattice. To make this calculation concrete,
here’s the dome we’ll analyze:
• Diameter, D, at base of dome is 150 feet
• Height of dome, f, above spring line is 30 feet
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As above, although each member is flat, they tile the
structure in such a way the resulting structure can be
treated as a dome. The main roof structure is a series of I-beams, which compose the triangular lattice.
Clamped to these beams are panels made from light
gauge aluminum. Batten bars hold the panels in place
by clamping the edges to the top flanges of the I-beams.
In addition to I-beams, panels, and batten bars, there
must be a way to hold the “I-beam triangle” together.
Typically this is done with a dished gusset plate that
sandwiches the joint. The question asked is: What size
standard I-beam should be used to build this dome?
While one could use extremely sophisticated computer programs to calculate all the properties of the
dome, the goal will be to make a “quick and dirty”
calculation that sizes the I-beams. To do this, we
make use of research that shows single-layer lattice
domes to be similar to thin solid shells. On the latter,
there has been a lot of analysis. This means we can
use a thin, solid shell to model the lattice dome; bearing in mind, though, the validity of this model relies
on the assumption the joints in the lattice frame are
rigid enough for the frame to act like a continuum –
not an unreasonable assumption for the method typically used to connect the I-beam triangles. Also, the
method assumes no significant imperfections from
its fabrication, nor errors in tolerance when the structure is erected.
An equation developed by engineers Douglas Wright
and Kenneth Buchert relates the structural properties
of a dome:
• Where pcr = critical pressure applied uniformly
and normally to the shell
• A = cross section area of the individual members
in the lattice
• E = modulus of elasticity
• r = radius of gyration
• L = typical length of members
• R = spherical radius of dome
sphere that forms the basis of the dome is a function
of the height of the dome above the spring line and the
dome’s radius (length of the spring line). Using a little
solid geometry one can show:
To estimate the proper size of the members (I-beams)
in the dome’s lattice, we’ll use this equation to find
the typical value of the product Ar, and then use the
Aluminum Association’s Standard I-Beam Tables to
pick out the correct size I-beam. Thus, we’ll rearrange
our equation to be:
For the dome under consideration:
Let’s consider the value of each term.
Safety Factor SF: Note we’ve added in a safety factor against buckling, SF. We’ll use a value of 1.65;
this is 65% greater than where we estimate buckling
would occur. Typically this value would be listed in
local codes, or even some kind of office standard.
Critical Pressure pcr: In order to do this calculation,
we need to determine the loads on the dome based on
local conditions. In the previous chapter we learned,
for example, how to calculate the loading from wind.
For this calculation, let’s assume that the dead load is
about 10 psf and the snow/wind load is 70 psf. This
means the total load is 80 psf. We’ll use this as the
critical pressure in our equation.
Typical member size L: The value of L, the typical
length of an I-beam, is determined by the materials
available. For an aluminum roof this limit would be
the width of the available coil material. For this dome
we’ll use 3003-H16 alloy, for which the greatest width
manufactured is 108 inches. Since the roof will be
covered with equilateral triangles, the side lengths will
be about 125 inches, using 108 inches as the altitude
of the triangle. Some members might be a little longer
and others a little shorter, but L = 130 inches will give
a good enough estimate.
Elastic Modulus, E: We can easily look up the elastic
modulus for this type of aluminum alloy. Its value is
10,100,000 psi.
Using these values in our main equation, we find a
value for Ar of:
Recall we are using the product of the area (A) and
the major axis of gyration (r). We can use the Aluminum Association Standard I-beams to pick out a beam.
Table 03-101 from the Aluminum Association shows
many properties of I-beams. Of greatest interest to us
is the Area and the radius of gyration along the X-X
axis. If we extract these two columns, we can make a
table (Table 03-102) that contains the product of the
two. Scanning down this list shows that I-beam “I 4 x
2.79” has an Ar value of 4.00; I-beam “I 5 x 3.70” has
a value of Ar equal to 6.65. The value we calculated is
between these two, so we’ll choose the larger I-beam.
Spherical Radius R: A dome is, of course, a section
of a sphere; although the spring line doesn’t necessary contain a radius. Typically, the spring line of a
dome is a chord, rather than a radius. The radius of the
Page 203
Table 03-101: Aluminum Association Standard I-beams (excerpt)
Designation
Width
Flange
Web
Fillet Area
Depth
(Dept x
bf
Thickness Thickness Radius
A
d (in.)
width)
(in.)
tf (in.)
tw (in.)
R (in.) (in.2)
Axis X-X
Ix
(in4)
Sx
(in4)
Warping Torsion
Constant Constant
Cw (in6)
J (in4)
Axis Y-Y
rx
(in)
Iy
(in4)
Sy
(in4)
ry (in)
I 4 x 2.31
4
3.00
0.23
0.15
0.25
1.96
5.62 2.81 1.69 1.04 0.691 0.727
3.68
0.033
I 4 x 2.79
4
3.00
0.29
0.17
0.25
2.38
6.71 3.36 1.68 1.31 0.872 0.742
4.5
0.061
I 5x 3.70
5
3.50
0.32
0.19
0.30
3.15
13.9 5.58 2.11 2.29
1.31
0.853
12.5
0.098
I 6 x 4.03
6
4.00
0.29
0.19
0.30
3.43
22.0 7.33 2.53 3.10
1.55
0.951
25.3
0.089
Table 03-102: Ar for selected I-beams
Designation
(Depth x width)
Area A (in.2)
rx (in)
Ar
I 4 x 2.31
1.96
1.69
3.31
I 4 x 2.79
2.38
1.68
4.00
I 5x 3.70
3.15
2.11
6.65
I 6 x 4.03
3.43
2.53
8.68
Summary
We opened this chapter by describing one of the most revolutionary domes in the history of building: The Duomo of the Basilica di Santa Maria del Fiore. We revealed how to
size the I-beams in a modern dome. The most common type of dome is the triangulated
dome popularized by Buckminister Fuller. Using a method developed by two engineers
in the 1960s, which assumes that the joints in the dome lattice frame are rigid enough
for the frame to act like a continuum, we showed how to estimate the area and radius of
gyration of a beam for particular loading conditions. The product of these quantities can
then be used to select proper sized I-beam using tables from the Aluminum Association
Standards.
Page 204
Student Assessment
Final Exam
1. A dome is, in a way, a set of arches linked
monolithically together. What are these
“arches” called in a dome?
a. Parallels
b. Meridians
c. Medians
d. Voussoirs
2. One of the main differences between how an
arch and a dome supports a load is:
a. The arch not only compresses, but bends; a
dome does not bend, but only compresses
throughout
b. An arch does not bend, but compresses
throughout; a dome not only compresses,
but bends slightly
c. A dome expands as a load is placed on it,
while an arch contracts
d. There are no differences in how a dome
and an arch support a load
3. The key ingredient in Roman concrete is:
a. Potassium
b. Nickel
c. Sand
d. Volcanic ash
4. The Rogers Centre in Toronto, Canada, has
this feature that surpasses the Pantheon:
a. Doors
b. A motorized roof
c. Better concrete
d. All of the above
5. The key engineering issue in building
Hagia Sophia was:
a. Find the proper type of stone to use
b. Mixing the concrete
c. Supporting a round dome on a square base
d. Choosing the site
6. A pendentive is:
a. A five-sided regular polygon
b. A type of concrete
c. The shape of Hagia Sophia’s dome
d. A triangular segment of a sphere
7. The Standard lists “exposure” as a category
which influences windspeed. Exposure depends
primarily on:
a. The location of the building
b. The type of material used in the building
c. The use of the building
d. The roughness of the ground
8. The spring line of a dome is typically the ___ of
the sphere associated with the dome’s dimensions.
a. Radius
b. Chord
c. Diameter
d. Circumference
9. Bruncelleschi could make his dome thinner
than the Pantheon because:
a. He used iron chain to reinforce the dome and
keep it from bursting
b. He had better quality concrete
c. He had less span to cover than in the Pantheon
d. He had more workers
10. In sizing an I-beam for a reticulated dome,
the structure may be treated as:
a. A space frame
b. A thin-shelled structure
c. A solid block
d. A membrane
Page 205
Course Description
This course examines the reasons for, sources for and application of ethics in the engineering profession. There is
a need to understand why ethics for engineers is important, and who the audience or evaluators are. A summary
of engineering application, ethics history and sources to consider ethics are discussed. Several examples of ethics
results are presented based on cases from the State of Florida, but generally applicable to all states. Finally some
discussion on licensure, obtaining licensure and the need for licensure is interwoven into the course.
Chapters
• Chapter One: Engineers and Society
• Chapter Two: Where do ethics come from?
• Chapter Three: Ethics and Licensure
Learning Objectives
Upon completion of the course, the participant will be able to:
• Understand the role of engineers in society and where ethical dilemmas may arise
• Identify ethical people
• Identify ethical professions
• Understand how and why we differentiate the two
• Learn where ethics come from
• Identify ethic issues when they arise in their career
• Understand how to evaluate ethic situations that confront them
• Know what role licensure play in ethics
• Recognize and describe wetlands and their regulation
• Know what the requirements are for licensure
• Relate recent efforts to elevate the ethics discussion
• Understand what the consequences are of unethical behavior
Page 206
Chapter 1:
Engineers and Society
Overview
•
•
•
•
•
Introduction
What Engineers do
Ethics
Ethical Professions
Summary
Learning Objectives
• Understand the role of engineers in society and
where ethical dilemmas may arise
• Identify ethical people
• Identify ethical professions
• Understand how and why we differentiate the two
Introduction
There is a need to understand why ethics for engineers
is important, and who the audience or evaluators are.
A summary of engineering application, ethics history
and sources to consider ethics are discussed. Several
examples of ethics results are presented based on cases from the State of Florida, but generally applicable
to all states. Finally some discussion on licensure, obtaining licensure and the need for licensure are interwoven into the course.
Most people do not really know what engineers do.
Many people, if asked what engineers do, will respond,
“They drive trains.” Or maybe they will respond,
“Isn’t Dilbert ® one?” While we all love Dilbert and
Wally, this is not what we should expect for society’s
understanding of engineers. Society needs engineers.
So, in this chapter, let’s talk about what engineers do
and see if we can identify where there are ethical issues that arise in the everyday efforts of engineers. We
will explore historical contexts with an eye to identifying how we figure out the ethical issues, with the
hope that our effort will be fruitful and we find a clear
outline for ethical behavior by engineers.
Figure 1 – train “engineer”
What Engineers Do
Society will always be undertaking the construction
of capital projects to erect buildings or install the
supporting infrastructure needed for economic and
housing opportunities, such as: water mains, sewer
lines, storm water drainage, transportation, parks,
power lines, developing mechanical equipment to
Page 207
operate these systems, and developing transportation system and control systems. These are the purview of the engineer. For efficient operation, these
newly constructed facilities must be developed in
accordance with the latest technical and professional standards to protect the health, safety and welfare
of the customers, served both now and in the future.
Such daunting tasks require engineers to keep pace
with the latest technological advancements. It is the
engineer’s responsibility to identify these standards
and technologies, and ensure their designs meet or
exceed all requirements. Some improvements may
be constructed by developers, while others will be
undertaken by governments. Many of these projects are big, sometimes involving investments of
multi-millions or even billions of dollars. The risks
and rewards may be significant and, in many cases,
government investment in infrastructure will be required. In most cases, those government projects
will be designed and constructed by the private sector, using money borrowed by governments. The
expectation is these facilities will last many years
and provide substantial improvements to society.
Most entities encounter the need to construct or
acquire capital facilities through an ongoing basis.
With proper planning and anticipation of the needs,
new capital projects can be constructed or acquired
with a minimum impact to the operation of the built
environment. The impact of these improvements
and inventions on the built environment has become
a piece of the purview of engineers. Planning is required to anticipate needs, as the current processes
used to deliver these facilities take time and effort.
While time, quality, and cost are most frequently
the determining issues, they are not the only concerns of owners. Control over the final product is
also important. Clients typically believe that in controlling the project, they can achieve schedule, cost
and quality goals. Additionally, the owner’s personnel often have specific ideas pertaining to the project details. Retaining control means more involvement by the owner in both design and construction
efforts.
Because owners generally want the best, qualified
people to provide the engineering required, trust is
of great importance to selection (Bloetscher, 1999
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and 1999a). It is often clear that not all engineers
are equally qualified, despite the consultant’s perceptions. For instance, if an owner wants to build a
membrane water treatment plant, a consultant who
has already designed a similarly-sized membrane
treatment plant would be preferred. Yet, despite
this obvious fact, many engineers will apply for
the work and some will actually lobby elected officials to obtain that work, despite the fact they have
no prior experience (Bloetscher, 1999 and 1999a).
This violates the honesty and forthrightness criteria, and is probably the first potential ethical violation as well. It is far better for consultants who do
not have that type of experience to team up with
consultants who do; thereby, their lobbying efforts
are maintained within more appropriate parameters
(Bloetscher, 1999 and 1999a). Lobbying is a second
ethical issue.
In addition to being honest, staff looks for the right
skills to do the job. This is not only the known capabilities of the firm, but also the specific talents of
the personnel who are actually selected by consultants to perform the work. Performing in the area of
your expertise is a third ethical issue. If a consultant
submits a proposal that includes work done by people who will never work on the proposed project, in
an attempt to show that the firm has the expertise to
do a job, this is misleading and dishonest; another
ethical issue. Ultimately, if the owner is not comfortable, or does not trust the engineer’s personnel,
the relationship will be difficult.
Why do we immediately identify certain actions as
having ethical questions in the engineering field?
Forget about the fact that engineers are responsible for the delivery of water used in the morning
shower, the sewer lines for the drain, the roads and
bridges to get to work, the cars, trains and airplanes
that we use to get to work, the storm water system
that drains the roadways, the structure of the buildings that people work in, the mechanical systems in
the buildings we use to accomplish work, and many
more unmentioned; people do not understand the
depth of engineering, they just have an expectation
that things will work. So what are ethics? Let’s see
if we can clear the fog.
acts this way? These people share many of the same
beliefs and conform to an accepted set of “rules” and
acceptable behavior. Engineers are among the groups
with common values, but alas, so are religions, political parties, terrorists, fascists, Nazis, and many others
not generally associated with ethical behavior.
Source: Florida Board of Professional Engineer Newsletter
The third definition is of a person with a set of values
that are universally accepted. What do we make of this
definition? Do we accept it? Do we accept a person
who acts this way? Find one example of a universally
accepted ethical value. Just one! That doesn’t work
very well either, so the philosophical answers are not
very helpful.
Ethics
Ethical Professions
The concept of licensure stems from a responsibility
to the public and the expectation of the public that engineers will act to protect their interests. Licensure is
an ethical responsibility. Ethics is an issue that comes
up in the engineering business on an ongoing basis,
but what are ethics? To begin to answer this question,
we must turn to philosophy for the study of ethics. A
cursory review indicates three potential definitions of
ethical people (Popkin and Stroll, 1993):
• One who establishes a set of values and lives by
them.
• One who lives by any set of values which are shared
by a group of people.
• One who lives by a set of values that are universally accepted.
Another approach is to look at professions. What are
the professions that most people perceive as unethical? Professions that are perceived to be unethical by
the general public, on a routine basis include:
• Salesmen
• Lawyers
• Politicians
• Financial brokers and bankers
• Realtors
Figure 2 – the Fog
Let’s take a look at each one of these. The first definition is a person who has a set of values and who lives
by them. What do we make of this definition? Do we
accept it? Do we accept a person who acts this way?
In reality, few people buy into this first definition as an
ethical person. The values a person holds can vary and
this definition may include individuals with a highly
personalized set of ideals (e.g. Robin Hood) or individuals with frequently unaccepted behaviors (e.g. serial killers).
The second definition is of a person who lives by any
set of values which are shared, lived and determined
by a group of people. What do we make of this definition? Do we accept it? Do we accept a person who
Sometimes, illegal enterprises are included, but illegality is not necessarily an ethical issue. Organized
crime typically has ethical values and core principles
that are shared and lived by. That does not make these
syndicates acceptable to society. Ethics and legality
are different principles, for example: Prostitution is
considered by many to be a common “unethical activity,” however, if you get what you pay for, what is
unethical? The commonality with the professions perceived to be unethical is those who are in said professions all want your money.
Now compare professions that are generally perceived
as being ethical:
• Engineers (at least we hope)
• Scientists
• Medical personnel
• Teachers
• Public Safety
• Health Care Providers
• Social Workers
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Generally speaking, the perception is these people
work to serve the public and protect the public interest. In addition, most have to be licensed and are regulated. If things go wrong with these people and their
projects, they can be brought before judicial boards
and reprimanded. The public has expectations of such
professions and presume competency with accountability.
Another common trait amongst these professions is
most of their decisions are based on judgment. Doctors, like engineers, have imperfect information, but
they make a diagnosis based on their “best guess,”
given the facts. However, the public expects that they
will come up with the correct assessment every time.
Many situations do not have definitive answers, and
furthermore, things are always changing.
Summary
Engineers have a major role in society and everything
we do involves some form of engineering process.
The fact that engineering is all around us may lead to
a lack of understanding of the importance of engineers
to society.
It is easier to identify unethical behavior than define
it. It may not be possible to find a set of universal ethics, but social or public expectations go a long way toward defining what is and is not ethical. It is the public
expectations of competence that differentiates ethical
from unethical professions, but there is no clear answer.
Though there is a public expectation of competency
and application of judgment, we are still left with only
a foggy idea of a perception of ethics and no real answers. Perhaps a historical review would help?
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Chapter 2:
Where do ethics
come from?
Overview
•
•
•
•
•
Introduction
Civilization and Ethics
The Philosophers Weigh In
Creeds, Codes and Canons
Summary
Introduction
In this chapter we will take a look back in time to learn
when the concept of ethics started, realizing that we
can identify professions that seem ethical, but perhaps
not what makes them so. A historical review can be illuminating, so we will start 10,000 years ago to look at
how and why professions have developed. In addition,
we will study what professional societies say about
engineers and their ethical behavior. The professional
societies led directly to state regulatory bodies as far
back as 100 years ago.
Civilization and Ethics
Learning Objectives:
• Understand a brief history of the development
of ethics for the engineering industry
• Understand how ethics developed and where
ethics come from
• Identify ethic issues when they arise in their
career
• Understand what the consequences are of
unethical behavior
Around 10,000 B.C.E., human civilization consisted
of disparate bands, or tribes, of hunter/gatherers that
subsisted on wild animals and plants. The person
who brought home the food was respected, as were
elders and leaders. Shelter providers, and those loyal
to the group, also ranked highly. Disrespected members were food hoarders, selfish individuals, thieves,
etc. Offenders were, in many cases, banished from the
group, condemned to survive alone in the wild (the old
Druidic solution). At that time, few people, if any, survived alone in the wilderness, therefore making this
banishment a quasi-death sentence. The tribe, in order
to be successful, learned how to control its members,
how to pull together and how to ensure success and
survival of the collective. Only those tribes that figured this out were successful.
Three thousand years later, agriculture began and
changed the way people lived and proliferated. Agriculture meant better nutrition, which meant healthier,
longer lives, and in turn created a need for more agriculture to sustain the ever growing population of tribes.
With more harvests came the opportunity of a surplus,
and efficiency could lead to excess products to trade
with other tribes. Since agriculture is stationary, there
needed to be a place to trade goods. Since people settled in one place for agriculture, the tribes established
villages and commercial systems for trading purposes.
With surplus, people became free to specialize within
the community. Some specialized in growing produce,
while others specialized in raising animals, making
tools, healing, etc. There was an expectation that traders would deal fairly with the farmers and merchants.
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The villages would need roads, buildings, and storage
areas to be constructed.
Protecting the harvest was paramount and members
of the villages had an expectation of carpenters and
stone masons to do the job correctly. The penalty for
not dealing fairly was exile from the community.
Successful groups of villages became city-states that
traded on a larger scale and eventually began to create
laws for regulating trade amongst people unknown to
each other. Laws were later recorded and trade centers
with specialties were created. The best of the products
were in high demand but were dependent upon others
to provide packaging supplies for shipping. This dependency created high expectations of the packager’s
knowledge. As the demand outstripped a given merchant’s ability to supply, more packagers were needed,
and the expectation of quality did remain. Products
that failed could be catastrophic for the village, so
training and apprenticeships became an important role
of the village. Guilds were created to ensure the workers provided what the public expected of them. Ethics, thus, sprang up from the public expectation that
people would do their job correctly and efficiently.
Example
Let’s assume that your society developed a high quality beer. The beer became a highly demanded good in
other communities. People of other communities were
willing to trade for it. Your society invested heavily in
developing beer, became highly successful, and soon,
your society was very wealthy. Then, an issue arose
where by the barrel maker was no longer able to keep
up with the supply of beer, so a new vendor started
making barrels. However, the new vendor’s barrel
leaked, a violation of outlaws against alcohol abuse
and spillage. What to do?
One obvious solution would be to penalize the new
vendor in some manner, but a smart leader would realize that such a solution hardly resolves the larger problem, needing more barrels to sell more beer. A better
solution is to have the new vendor’s enterprise apprentice under the current barrel-maker. In that manner,
the knowledge is passed down, the new vendor makes
better barrels and the society becomes wealthier by
selling more beer.
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In fact, this is what happened with carpentry, stone
masonry, and a host of other professions – apprenticeships helped younger people hone their skills so
they would be the future craftsman. The 4 year waiting period to take the professional engineer’s exam is
an apprenticeship. The PE exam is focused on practice problems – things expected to be performed in the
profession as opposed to academic exercises that are
the focus of the fundamentals exam.
The Philosophers Weigh In
Most ancient philosophers lived in challenging times.
As the Greeks developed mathematical laws, philosophers tried to discover behavioral laws to explain why
life went awry. They were trying to figure out if their
difficulties were caused by society’s failure to follow
some form of behavioral laws. The ancient philosophers thought education and leading a proper life were
important to avoiding difficulty. As a result, Philosophy is the study of consequences for implementation
of a series of behavioral principles. Ethics are those
behavioral principles.
Over the last 2500 years, there have been numerous
philosophers pursuing an explanation of this elusive
concept. One philosopher’s work that directly affects the engineering profession is Emmanuel Kant
(1724-1804). Kant tried to define morality and good.
He stated that people have a duty to act in a certain
manner and that every action could be evaluated by
looking at a universal code of behavior. The question
raised was “If everybody did _______, would society
function?”(Mantell, 1964) If the answer was no, the
behavior was deemed unethical. If the answer was yes,
the practice was considered ethical. Kant contributed
the concept of “duty” and the need to evaluate whether
an action was ethical or not based on the impact to
society. This relates directly to the engineer’s responsibility to protect the health, safety and welfare of the
public, and therefore has direct applicability to the engineering profession.
Utilitarianism appeared in the 18th century and outlined that an action was ethical and right if it produces
the greatest number of happy people. This scenario is
how most democratic governments function. If there
was a close split, the court system was employed to
make the determination. Engineers use this to do alternative analyses. We pick the preferred options using an objective set of selection criteria established a
priori. However, note that for many democratic governments, there is a court system that permits the unhappy to challenge a law or policy that disadvantages
them.
Creeds, Codes and Canons
As the engineering profession developed formally in
the late 19th century, societies of engineers were created. Relevant to civil engineering is the American Society of Civil Engineers (ASCE). ASCE established a
set of codes, creeds, and canons to guide engineers to
the proper actions to take, given certain circumstances. This was a time before licensing boards were created and self-regulation was the rule.
As the engineering profession developed its rules, it
began to self-regulate and create disciplinary actions
for failure to adhere to its rules. There is a theme that
underlies all of the rules established by the ASCE: the
top priority under ALL CIRCUMSTANCES is the
health, safety and welfare of the public. Legal issues are second, and third is the engineering profession itself. Engineers must be perceived to be ethical
because damage to that perception damages all engineers and the profession. The client is fourth on the
priority list, and the engineer is last, on the hierarchy,
unfortunately.
There are a series of other provisions to observe. Engineers must perform work within their area of specialty, and not in any area where they may lack experience or competence. Engineers must not accept a
project if they do not have the skills to design it. In
other words, they must recognize their competencies
as well as their personal limitations. This is directly
related to the protection of the public’s health, safety
and welfare. Engineers are required to issue objective
and truthful comments and avoid conflicts of interest. Engineers must build their personal reputation on
merit, uphold the dignity of the profession and continue professional development throughout their careers.
Most states require a specific number of hours in professional development for each license renewal cycle.
Continuing education is how engineers keep up with
new technology, as well as changes to relevant building and design codes.
No one is allowed to review the plans of a professional engineer unless they are also a professional engineer. Engineers may be paid by someone to review
another engineer’s plans, but the other engineer must
be notified in writing. One reason for doing this is all
information may not on the plans and specifications,
so more information may be needed to provide appropriate judgment or review. Engineers need to do their
“due diligence” when reviewing other’s work. Due
diligence is a legal concept that involves ensuring that
all information required to make a decision is properly
considered. Performing due diligence will be necessary in order to write an accurate report for whomever
is paying for the review.
Engineers are paid by their clients. The client’s fee is
the only source of income. Engineers cannot accept
money from suppliers, competing clients, manufacturers, salesmen, or others, as this compromises the
ability to enforce the requirements in the plans and
specifications. Accepting compensation from someone other than the client on a project is a conflict of
interest, which engineers must avoid. ASCE goes on
to note that engineers should protect the client’s interests, as long as doing so does not conflict with protecting the public.
If things go wrong, engineers are required to freely and
openly admit errors, and then offer a solution. If engineers have no involvement in an issue, they should not
comment (a legal concept called “standing”).
The National Society of Professional Engineers
(NSPE) also has a code of ethics. It covers all engineering disciplines. Familiar highlights are:
• Give utmost performance.
• Participate in honest enterprises.
• Live in accordance with the highest standard of
professional conduct. This is nebulous at best, but
can be used in lawsuits. This is why there is NEVER just one engineer on a project.
• Service before profit.
• Honor and standing above personal advantage.
• Health and welfare above all else.
• Further, notify the client when judgment is overruled. This is a potential conflict point.
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•
•
•
•
•
Sign and seal only in areas of competency.
Report violations in the code of conduct.
Avoid conflicts of interest.
Do not accept outside compensation.
Review plans only when requested to do so and advise the standing engineer of the review and why.
In reviewing the history, licensure comes from codes,
which came from guilds, which came from the collective expectations of the public. The engineer has a
civic duty, as Kant notes, to protect the public. However, the public does not need to know exactly what
an engineer does; they need only have the confidence
the engineer is competent to do it correctly. Licensure
is that notice to the public that the engineer is indeed
competent.
Summary
In the early years of civilization, expectations of the
tribe were important. Do your job, do it correctly, and
you get to remain in the tribe. As agriculture grew, the
number of professions increased but the expectations
remained the same. Ancient philosophers attempted to
apply “rules” to behavior, but were only partially successful. They did, however, leave a legacy of concepts
that were useful in defining those public expectations.
The use of expectations continued into the professional societies in the nineteenth century as the engineer
professions became more defined. The result led to licensure, the discussion in the next chapter.
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Chapter 3:
Ethics and Licensure
Overwiew
•
•
•
•
•
•
Introduction
Licensure
Working as an Engineer
Documentation and Technical Reports
Failing to follow the Licensing Ethics and Rules
Summary
Learning Objectives
• Identify ethical issues when they arise in
their career
• Understand how to evaluate ethical situations
that confront them
• Know what role licensure play in ethics
• Know what the requirements are for licensure
• Relate recent efforts to elevate the ethics
discussion
• Understand what the consequences are
of unethical behavior
Licensure
If you are currently pursuing a degree in civil engineering, licensure is something that must be obtained to be
successful in the long term. As an engineer, there will
be many potential career opportunities in the job market, and many opportunities to grow and develop. Engineers are in high demand because there are many types
of engineering jobs available to choose from, such as:
regulators (state, federal, local governments), equipment manufacturing representatives, contractors, public
works and utility managers (both public and private),
engineering consultants, and academicians. Students
should ask themselves if they want to be involved with
project management, design, manufacturing and sales,
construction, instruction, and/or regulation. There are
exciting, interesting, and challenging job opportunities
in each field. It is encouraging to note that graduating
engineers rarely have much difficulty finding a job related to civil engineering, especially if their communication skills are excellent, and good communications
skills cannot be overstated. Careers depend on them.
Students need to decide where their interests lie. Will
the career path lead to the private sector or the public
sector? If students want to have extensive hands-on
training, working for a large private firm, or perhaps
a government utility, transportation department, public
works entity, or a regulatory agency may have more
appeal. Most large entities offer some degree of mentoring and oversight to allow entry-level employees to
grow into a professional engineer, under the guidance
of a more experienced engineering supervisor. Keep in
mind, engineers need a license to practice engineering
(or to even call themselves engineers in some states), so
before starting a business, it is important to gain experience. Without a PE license, starting a business will be
extremely difficult. The downside to smaller companies
is there is less opportunity to get mentoring, but the excitement of working on many different types of projects
will keep engineers from getting bored with repetitive
work. Since engineers generally have many opportunities to advance and increase their salary, especially after
obtaining their professional engineering license, students should not worry too much about starting salaries
when first joining the workforce. Keep in mind, when
getting that first job; the starting salary should not be the
only criterion. It is vastly more important to enjoy the
job and the tasks being performed, so the job does not
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feel like work. It should be more than that – you want
to be challenged, do different types of projects, learn
about different settings, work with different people, etc.
- in other words continue the learning process.
Each state engineering licensing board has its own laws
regarding engineering licensure, but in general, the candidate must: 1) earn a degree from an ABET-accredited engineering program, 2) pass the Fundamentals
of Engineering (FE) exam, 3) gain 4 years of progressively responsible engineering work experience under
the supervision of a PE, and 4) pass the Principles and
Practice of Engineering Exam (PE) in the appropriate
discipline.
As state boards were being organized in the early 20th
century, there was a need to coordinate the effort nationally. The Accreditation Board for Engineering
and Technology (ABET) was established in 1932 as a
means to create a structured program for engineering
guidance, training, education, and recognition. ABET
accreditation is an assurance that a college or university program meets the minimum quality standards,
established by the profession for its students, and enables prospective employers to recruit graduates that
are well-prepared. Engineers need to graduate from an
ABET-accredited school to be eligible for a PE license
so universities zealously protect their accreditation.
Most of the coursework in an accredited undergraduate
engineering degree is dictated by ABET.
The FE is the first step in obtaining the PE license.
The exam is provided nationwide by the National
Council of Examiners for Engineering and Surveying
(NCEES) in Clemson, SC. The FE evaluates content
knowledge related to the subjects that ABET requires
for accredited undergraduate engineering degree programs. Historically, the FE was an 8-hour exam. From
1980 to 2010, the exam migrated from individual, hand
calculated problems, to an exam that contained 180
multiple-choice questions and was split evenly into a
4-hour morning session (120 questions) and a 4-hour
afternoon session (60 questions). During the morning
session, all examinees would take the same general
exam, common to all engineering disciplines. Essentially, this covers information from the general education requirements in the freshman and sophomore
years, and also includes subjects such as mathematics,
chemistry, computers, engineering mechanics, ethics,
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calculus and engineering economics. During the afternoon session, examinees can elect to take a disciplinespecific (chemical, civil, electrical, environmental,
industrial, mechanical, etc.) exam or a more general
exam labeled “Other Disciplines.” If the degree is in
one of the six major disciplines, it is recommended
to take the discipline-specific exam. Examinees must
participate in both sessions on the same day.
The FE is closed book, but an FE Supplied-Reference
Handbook is provided on exam day. The reference
handbook is available for download or purchase at
www.NCEES.org, along with practice exams, study
materials, and other references. In the spring of 2014,
NCEES will transition the FE to computer based testing and change the format of the exam. Some of the
reasons for converting the exams to a computer-based
format are: to allow greater scheduling flexibility for
candidates, more uniformity in testing conditions, and
enhanced security for exam content. In the past, the FE
and PE exams were offered only twice per year—in
April and October. Under the new CBT platform, examinees will be able to register and sit for exams yearround. It will remain closed book with an electronic
reference handbook provided. Examinees will have 6
hours to complete the exam, which contains 110 multiple-choice questions. The 6-hour time also includes a
tutorial, a break and a brief survey at the conclusion.
Candidates must bring their own calculators to the
exam; however, only models of calculators as specified by NCEES can be used. The NCEES calculator
policy was recently revised due to concern for the
security of examination content. Available calculator
technology has been used for exam subversion, and
calculators that can store and communicate text are
considered a security risk. It is highly recommended to check the NCEES calculator policy for compliance, and to become familiar with the allowable
calculators, so it will not be an issue on exam day.
Only NCEES-supplied marking and erasing instruments can be used for the exam. Mechanical pencils
are provided. Each pencil has an eraser and is preloaded with three pieces of 0.7-mm HB lead. This
is the only writing instrument that is allowed. Candidates may not bring their own writing instruments
or erasers. If necessary, proctors will issue additional
pencils during the exam. In addition, these items are
not allowed:
• Devices or materials that might compromise the security or the integrity of the exam or the examination
process are not permitted.
• Calculators not specified by NCEES.
• Devices having a QWERTY keypad arrangement
similar to a typewriter or keyboard
• Palmtop, laptop, handheld, and desktop computers,
calculators, databanks, data collectors, and organizers.
• Communication devices such as pagers and cellular
phones.
• Non-NCEES supplied writing utensils.
The diagnostic report summarizes the scores as follows:
• A low percentage (0-50%) in a content area signifies that substantial study of that content area is
recommended prior to retaking the exam.
• A marginal percentage (50-75%) in a content area
indicates that your understanding may be improved
by further study, thus improving your chances of
passing the examination.
• A high percentage (75-100%) indicates strong
grasp of the content area; however, further review
of this area may also improve your chances of passing the examination.
As with cell phones and other electronic devices, any
inappropriate calculators will be confiscated by the
proctor and the test invalidated.
For either the FE or the PE exams, those who fail are
allowed to retake the test up to 3 times. After the third
attempt without success, candidates will be required to
enroll and pass 12 credit hours of college classes of senior level and above to be eligible to retake the exam.
After passing the FE exam, the next step is to gain
acceptable work experience under the supervision of a
licensed professional engineer as a mentor.
A passing score on an NCEES exam is the number of
correct answers or points required to indicate a knowledge level necessary to meet the minimum performance
standard for a discipline, which is determined by an appointed committee of licensed subject-matter experts.
Beginning with the October 2005 administration, candidates received results of “Pass” or “Fail” only, with
no numerical score. There is no “curve,” but NCEES
scores each exam based on its own merits, with no regard for a predetermined percentage of examinees that
should pass or fail. All exams are scored the same way.
If a person fails the exam, a diagnostic report is generated, which lists the percentages of correctly answered
questions in each knowledge area of the exam. This is
the best guide for determining strengths and weaknesses with regard to specific subject areas.
After gaining the appropriate experience, candidates
are now eligible to take the PE exam. The exam is currently a traditional pen and paper 8-hour open book
exam, but is designed to test the engineering experience gained over the 4 years since the FE exam. As
a result, the PE exam tests the candidate’s ability to
practice competently in a particular engineering discipline. The PE exam is typically the last step in the process of becoming a licensed PE. This exam is extensive, and the list continues to grow as the engineering
field expands. The goal of every engineering student
should be to obtain a PE license.
While students who fail the test receive a report, the
university also receives one as a means to help improve its curriculum. (Figure 4). In this example, the
university needs to improve its curriculum with respect to structural analysis, computers, surveying and
hydraulics, areas where the institution’s students performed below the national average.
Figure 3 Diagnostic Report for student failing FE exam
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Figure 4 Example of a university diagnostic report
Let’s talk about getting work; this is the first place an
engineer can get into trouble.
some will actually lobby elected officials to obtain
that work, despite the fact they have no prior experience. This violates the honesty and forthrightness
criteria, but also violates the consistent ethical issue
of performing work in the areas where you have expertise. Lobbying to get work could involve damaging the reputation of other engineers to gain leverage.
This clearly violates the universal ethic to protect the
profession. Since engineering relies on trust, damaging the public trust by disparaging another engineer
is a clear violation of this ethical principle. If a consultant submits a proposal that includes work done by
people who will never work on the proposed project,
in an attempt to show the firm has the expertise to do
a job, this is misleading and dishonest, and it violates
the ethic of standing on your own. A poor relationship
in working with other engineers reflects negatively on
the firm and its employees. Ultimately, if the owner is
not comfortable, or does not trust the engineer’s personnel, the relationship will be difficult.
In Chapter 1 we noted owners generally want the most
qualified people to provide the engineering required.
We noted some engineers will apply for the work and
How do you solve the expertise issue? Engineers do apprenticing, which means they work with someone who
has this experience to learn from. Firms can do this as
Working as an Engineer
Protection of the public health, safety and welfare is
a public trust issue that rivals the expectations of doctors, and for comparable reasons. Holding a Professional Engineering (PE) license demonstrates to the
public this professional has obtained the required education, experience, and knowledge necessary to make
engineering judgments, and the public can rely upon
that person to protect their health, safety and welfare.
Having a PE license allows professionals to perform
engineering consulting, own their own businesses, and
bid for public funding. Now you have your license and
you renew every year (or two or three, depending on
the state), don’t forget about the rules for your states.
How do you conduct your business and what do you
need to look out for?
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well. Engineers do not need to be big, national or international consulting firms. Big, national firms may be
able to access the experts to provide information, but
their focus cannot be with one particular agency, especially smaller agencies. As a result, any local engineers
who have access or have prior collaborative efforts involving small, local providers, matched with a larger
consulting firm, may be a more palatable engineering
team and resolve many of the potential ethics issues. To
do all the design work, a team is required.
The best teams are made up of individuals who go out
of their way to make each other look good, as well as
provide an appropriate amount of decorum. An effective team has many open lines of communication and
meets frequently to discuss progress, plan for future
deliverables, evaluate concepts, make decisions, rehearse presentations, and edit written submittals. Effective team meetings can be very helpful in keeping
the project on schedule. However, if team meetings
are ineffective, they can be a source of dissention and
erosion of team cohesiveness. A good way to keep
team meetings successful is to evaluate how productive the time was spent.
• Did it start and end on time?
• Did everyone participate?
• Were important issues discussed and decisions
made?
• Did you reach a consensus solution?
• Did you all engage and add to the discussion?
• Did you allow for new ideas to come forward?
• Did you explore these and include them?
• Was there any negative issues brought up?
• Were you able to resolve the situation?
• Was the meeting efficient and effective?
• Did you make decisions?
• Did someone record them?
• Did you stay on the agenda?
No one said it would be easy, and often, just as in real
world situations, conflicts, difficulties and communication issues arise within teams. Decorum is part of
the ethics of engineers. We can disagree, but decorum
is required.
Teams that discuss the issues, share opinions, resolve
disagreements, and gain an understanding of all differing points of view offered are teams that function well
and perform effectively. Conditions need to be estabProfessional Engineering Ethics
lished where every team member has an opportunity
to speak and be heard. Some team members may be
uncomfortable expressing their opinions, questioning
others, or defending points of disagreement in group
settings. They will likely distance themselves for fear
of confrontation or humiliation. If this happens, one
or more important points of view will be silent or lost.
Ground rules are important teamwork, such as:
1. Speaking. Only one person speaks at a time. Speak
so everyone can hear. Make sure that everyone can
hear clearly. Never allow the loudest voice in the
group to seize control of the proceedings.
2. Listening. Give the speaker full attention. Stay receptive to what others have to say and open-minded
to new ideas. Listen without making assumptions
or judgments. Acceptance of others and having empathy for them is an important prerequisite. Do not
have side conversations while someone is speaking. Be aware and be perceptive. Take a moment
to understand the argument and comprehend the
meaning of the words before reacting.
3. Using Time Wisely. For all team meetings, rehearsals, presentations, and appointments, make a
commitment to be on time, start on time, and end
on time, as mutually agreed upon. Showing up on
time is not enough, so be prepared to get the work
done in the time allotted.
4. Focus. Remember to remain focused on the task
at hand by focusing on the specific problem that
needs to be solved, not the people on the team, or
other tangents which merely waste valuable time.
Stay on target, addressing what needs to be accomplished “in the now,” at this team meeting.
Use an agenda that states the goal of each meeting at the outset. If the discussion goes off course,
bring the team back to the goal of the agenda item.
5. Be Open to the Outcome. Keep an open mind
about what the outcome of the meeting might be.
It could be very different and more refreshing than
anticipated.
6. Honor Personal Commitments and Trust in the
Team Decisions. If the team has made a decision,
move forward. There is no sense in continuing to
argue a point that has been decided. If something
is not working out, be flexible enough to admit
mistakes and move on from there.
7. Always Make the Best Effort Possible. It is unethical to do the opposite. A team that aims at mediocrity will inevitably miss that goal. Setting the
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standard too low is a recipe for failure. Set the bar
high, and aim to be the best. Show competitive spirit and strive to reach the top. Always play to win!
tion of a statute, rule, ordinance, etc. Willful neglect of
a code could be construed as misconduct. The following is a Florida example.
Failing to Do Your Job
From a legal perspective there are three basic areas of
failure associated with engineering errors. These are:
• Negligence
• Incompetence
• Misconduct
Negligence is defined as the failure to exercise due care
in the performance of the work, or something which
an ordinarily prudent person would foresee as a risk
of harm to others if not corrected. Negligence can constitute grounds for disciplinary action by the Board of
Professional Engineers, but not criminal prosecution in
most states. Call your lawyer and insurance company to
understand your state board’s negligence laws. The following is a Florida case dealing with negligence.
Incompetence is defined as a lack of ability to perform
a function, a lack of qualification to perform a function, or a lack of physical or mental ability to perform.
Like negligence, incompetence can constitute grounds
for disciplinary action by the Board of Professional
Engineers, but not criminal prosecution. Again, call
your lawyer and insurance company to be clear on incompetence laws.
Misconduct is another matter. Misconduct is defined
as a transgression of some established rule of action
where no discretion is left. An example is any violaPage 220
So let’s take a look at some ethical issues that have
arose involving engineers.
Case Study 1 – Licensure in Multiple states
The first case study concerns James, who has a PE license in Florida and Idaho. He is having his license
acted upon by the licensing agency in the State of Idaho. They fined him $1,000, payable within 30 days,
and will suspend his license until the fine is paid, if
beyond 30 days. Does James have any issues with
his Florida License? The answer is likely, yes. In this
case, James was fined $1,000 for having action taken
on his license in another state. If you have action in
one state, you may be subject to the same in additional
states in which you are licensed.
Case Study 2 – Practicing Without a License
The second case study concerns Ernest, who has an
engineering degree and has worked for many years in
construction. He applies for and receives a job with
an agency where the job requirement is to have a PE
license. Ernest says he does, and it takes a while for
anyone to check if this is true. What could happen?
Case Study 5 – Sealing Documents that are
not Final
The fifth case study concerns Mary. Mary is a PE who
signed, dated and sealed plans without indicating they
were preliminary. What could happen? This case created
a lot of discussion and has altered the rules in Florida.
The seal means the plans meet all codes and rules and indicates the plan is final. Permit copies are not final. Mary
was fined, and issued the following decision:
Case Study 3 – Design Defect
The third case study concerns Joe, who designed a storm
water system and had to deviate from the permit conditions to make it work. He did not submit the revisions
to the permit agency and construction continued. Does
Joe have any issues with his Florida License? What could
happen? You need to engage permit agencies in major
changes, even during construction. In this case Joe did
not, and the Board fined him $2,000, plus administrative
costs of $3,129 as a result of negligence/defective design.
He also received a 1 year suspension and 2 years probation of his license. Permits are legal requirements.
Case Study 4 – Failing to Seal the Documents
The fourth case study concerns Bob. Bob is a civil
engineer by training and primarily does subdivision
work. He filed several documents to be recorded in
the Public Records. The Clerk of Courts notified the
Board that Bill had not properly signed, sealed and
dated the documents. Does Bob have any issues with
his Florida License? Yes. He did not properly seal the
documents so they fined Bob $1,000, payable within
30 days, as well as threatened to suspend his license
until the fine is paid, if beyond 30 days. Again this is a
rule requirement, so it is misconduct, not negligence.
Page 221
Case Study 6 – Practicing outside your area
This case is self explanatory:
Documentation and Reports
Case Study 7 – Practicing outside your area
This case is also self-explanatory:
Documentation is a key priority for engineers – both to
protect themselves and to demonstrate ethics. Writing
technical reports requires the same skill and attention
to detail as has been discussed throughout this chapter.
Many organizations will also have a set of strict guidelines that govern the format of these kinds of reports.
The final technical report for a project will typically
have an outline such as the following, of significant
importance.
The cover page should contain all the essential elements, including the title of the project, the contributors’ names, the date of submittal, etc. A table of contents provides the structural outline of the report, so
finding the exact location of key items is easier for the
reader. A list of figures and tables provides the same
ease for the graphics included in the report. If the captions were made appropriately, these are simply lists
of captions with the page number locations.
The introduction to the report describes the project
goals, the location of the project, and the objectives.
It must start strong with scope and objectives clearly
presented. This is where you note your assumptions
and documentation relied on in the design. The introduction should fully express the primary purpose and
scope in its context at the beginning of the report. The
introduction should also presents a clear statement
that demonstrates how the report will track the fundamental, secondary, and implied problems, questions,
Page 222
and issues described within. The body of the report
contains all the supporting information to address the
major items, subdivided in sections that are all related
to the goals of the project. These typically include: existing conditions, discussion of alternatives, selection
of alternatives, recommendations, and issues that need
owner input, among others. The text should convey a
professional level of knowledge of the subject matter,
with no important content left out, and no incorrect
material presented. The report’s focus must be clear
to the reader, such that paragraphs, logically and coherently, build upon each other through the complete
and fluent use of transitions towards a logical conclusion supported by the data presented or referenced.
The writing should exhibit substantial depth and complexity of thought supported by well-developed ideas,
analysis, or evidence that tie back to the original purpose and goals of the project. Facts should be presented in a logical sequence, and sections must transition
effectively between topics and different authors. Some
other characteristics of a well-written body are as follows:
• Seamlessly incorporates and explains the accuracy
and relevance of data/evidence/quotations/paraphrase/visuals
• Offers evidence from a variety of sources, including counterarguments, contrary evidence, and
quantitative analysis
• Presents data in graphical, tabular, or sketch format, following all rules for graphical format, including proper units and labels
• Spelling and grammar are checked for accuracy
• Sentences consistently communicate thoughts
clearly, while relatively free of sentence level patterns of error using a technically sound sentence
structure that is varied, convincing, nuanced, eloquent with appropriate tone
• Evidence of editing
Most importantly, the supporting material must build
towards an effective conclusion to finish strong with
a reasonable summary and/or recommendations presented, as justified from the body of the report using
regency techniques. A complete reference section is
included that cites and formats literature sources accurately and consistently.
Each report should be designed to be a stand-alone
technical memorandum or supporting materials deProfessional Engineering Ethics
signed to be a complete summary of the design for a
particular aspect of the project. It is expected that the
reader will be able to take this information and replicate the work. All calculations, codes and drawings
must be included. A written description of methods,
assumptions and application of technical data is required.
Failure to write TMs and Basis of
Design Reports
A couple of examples will illustrate why TMs are so
important for engineering projects. These two examples are water treatment plant expansion projects, but
the issues were related to electrical and structural components. One helped the engineer develop because the
basis of design report was complete, informative and
well crafted. The other report was of much less help.
In both cases, there were things that could have been
done to rectify the situation; however, the public interest was not served in either case.
Case Study 8 - Membrane Treatment Plant
The first case study concerns the design and construction of a large membrane water treatment plant for a
utility. The consultant chosen for the project had designed prior plans for similar construction, including
one in a neighboring municipality. The project cost
was expected to be over $20 million and had a short
construction timeframe. The construction consulting
fees were approximately $1.2 million. The services
to be provided by the consultant included design of
the facilities, contract administration, daily inspection,
services during bidding, negotiation of change orders,
and review of the schedule. The utility’s staff, while
limited, had experience with construction projects and
was providing periodic inspection and project coordination on less complex components of the project. As
a result, the consultant was receiving some direction
from the utility staff for the project on a routine, consistent basis, including reviews of the drawings and
discussions with the operational staff.
The project was constructed in two phases:
The first phase was nanofiltration, with the reverse osmosis phase to follow 3-5 years later. The design was
to incorporate all piping and electrical work to satisfy
the expansion. The contractor who was awarded the
first phase was litigation-oriented. As a result, the conPage 223
tract construction time became unachievable and significant animosity developed due to the delays. The
local government ignored the recommendations of its
project management staff to terminate the contractor,
and decided more personnel should get involved. After much effort, the project came in over a year late,
and litigation with the contractor ensued. The animosity created large enough issues the design consultant
was not selected to complete the second phase, despite
their familiarity with the project.
References
A new consultant was hired to perform the second
phase and design the upgrade. They based their design
on a prior project they had constructed elsewhere. It
was revealed later that little design work had actually
occurred, since many of the parameters used in this
project were exactly the same as the previous project,
particularly with respect to the electrical system design. The original design called for the pressures on
the reverse osmosis pumps to be 405 psi; however,
the electrical system was not capable of handling this
pressure. A claim was made by the utility against the
initial design consultant in the amount of $1.5 million,
which exceeded the initial payment to the consultant
for the design.
Mantell, M.L. 1964. Ethics and Professionalism in
Engineering, Collier-MacMillan Ltd, London.
Bloetscher, Frederick, 1999, Looking for Quality on
an Engineering Consultant, American City and County, December, 1999, p. 36.
Bloetscher, Frederick, 1999a. What You Should Expect from Your Consulting Professionals (and How
to Evaluate Them to Get It), Water Engineering and
Management, October, 1999, pp. 24-27.
Popkin, R. H. and Stroll, A. 1993, Philosophy Made
Simple, Broadway Books, New York, NY.
Summary
Ethics is an ongoing issue in this career. We highlighted 8 case studies, thanks to the state of Florida,
that indicate potential negligence, incompetence and
misconduct issues. Ethical violations can be any of
the three, but are primarily in the form of misconduct.
Potential risks are; loss of licensure, suspension fines,
etc. Every state is a little different. With case studies
and history, the issue should be clearer. Consult your
professional societies and state rules for more guidance on your ethical issue.
Page 224
Student Assessment
(last page of book) or complete your assessment online at www.McKissock.com/Engineering
Final Exam
1. What is a common answer to the question,
“What do engineers do?”
a. Drive trains
b. Get into politics
c. Go to the Moon
d. What Sheldon does in the Big Bang Theory
6. The concept of protecting the profession
suggest that engineers should never
a. Market
b. Notify the client when judgment is overruled
c. Report violations in the code of conduct
d. Lobby elected officials for work
2. Professions that are often deemed to be ethical
include:
a. Doctors
b. Lawyers
c. Politicians
d. The mafia
7. Engineers should not
a. Notify the client when judgment is overruled
b. Report violations in the code of conduct
c. Accept outside compensation
d. Service before profit
3. The commonality for engineers, social
workers, teachers, public safety personnel and
doctors is:
a. They are all regulated by state agencies
b. They all are government employees
c. They all have an obligation to protect
the public first
d. They are all highly ethical
4. In the ancient world people were expected to:
a. Be successful in the agriculture field
b. Do their share of the work
c. Be successful at hunting and gathering
d. Be strong warriors
5. The difference between what we perceive as
ethical and unethical professions is:
a. Money
b. Influence
c. Products
d. Happiness
8. Performing work without a license is called:
a. Negligence
b. Misconduct
c. Incompetence
d. Stupidity
9. Forgetting to include a spec for the generator
in your bid is called:
a. Negligence
b. Misconduct
c. Incompetence
d. Stupidity
10. The inability to perform work is called:
a. Negligence
b. Misconduct
c. Incompetence
d. Stupidity
Page 225
Page 226
Continuing Professional Competency Log
Pdhs earned
Duration
Type of
activity
Location
Date
Title or
specific
subject
TOTAL PDH’S EARNED: YEAR EARNED:
Sponsoring
organization
name
LICENSE NUMBER: LICENSEE’S NAME:
Briefly explain how this activity will maintain,
improve or expand the skills and knowledge
relevant to your field of practice
STATE:
CONTINUING PROFESSIONAL COMPETENCY LOG
Notes:
Notes Page
Page 227
Notes:
Page 228
Notes Page
Notes:
Notes Page
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Energy: Its Effect on
our World and Lives
– page 23
Amusing Ourselves
Safely
– page 47
1. a b c d
1. a b c d
– page 70
1. a b c d
2. a b c d
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10. a b c d
Heavy Loads
– page 120
2. a b c d
Designing
Buildings to
Mitigate Terrorist
Attacks
– page 165
3. a b c d
1. a b c d
4. a b c d
2. a b c d
5. a b c d
3. a b c d
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Page 232
Practical Repair
Materials for
Roadway Pavements
Municipal
Wastewater
Treatment Systems
– page 182
1. a b c d
2. a b c d
3. a b c d
4. a b c d
5. a b c d
6. a b c d
7. a b c d
8. a b c d
9. a b c d
10. a b c d
Building Safe
Structures in Flood
Zones – page 87
1. a b c d
The History and
Future of Domes
– page 205
Professional
Engineering Ethics
– page 225
1. a b c d
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Student Assessment Answer Sheet

For Professional Engineers

Transcription

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