IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO

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IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO
GEOLÓGICO E INGENIERÍA SÍSMICA Y DE DESASTRES
8 al 11 de mayo de 2012, Santiago de Cuba. Cuba.
UNIVERSIDAD DE ORIENTE. Facultad de Construcciones
THE INVOLVEMENT OF ARCHITECTURE IN SEISMIC DESIGN
Base Isolation in Italy before and after the 2009 Abruzzo earthquake
Magistral Conference
Prof. Ing. Alberto Parducci
Universidad de Oriente, Santiago de Cuba may 8, 2012
1
THE INVOLVEMENT OF ARCHITECTURE IN SEISMIC DESIGN
Base Isolation in Italy before and after the 2009 Abruzzo earthquake
Almost every progress in science has been paid for by a sacrifice, for almost
every new intellectual achievement previous positions and conceptions had to
be given up.
Werner Heisenberg, Nobel Prize for Physics, 1932.
FOREWORD
The underlying concepts of earthquake engineering have evolved substantially over recent
years. The objectives have become more ambitious and procedures for building design have
become more complex. Countries subject to seismic risk have updated their standards: some
to a considerable extent, others less so. In any case, greater attention is now focused directly
on the prevention of building collapse by controlling the post-elastic behaviour that
constructions bring into play to defend themselves when attacked by violent earthquakes.
Consequently, design requirements are no longer met by performing only traditional
resistance checks on assigned forces systems, because the checks must now refer to
requirements with predefined performance criteria.
There are now two explicitly expressed performance objectives. The first is an ethical act,
aimed at preventing those disastrous collapses which can injure or kill people. The second is
an economic objective, aimed at reducing building and repair costs in the event of damage,
i.e. aiming for optimal use of available resources. The two objectives are a clear reference to
new earthquake engineering concepts which can only be achieved with the compliance of all
those who design and manufacture buildings, especially the ordinary type, because these are
where most of those exposed to earthquake consequences are to be found. Thus it is the duty
of both the engineering and the architectural (now being offered new openings) sectors to
address these problems and suggest new design prototypes.
When such transformations occur, it is not unusual to find that previously neglected
problems actually become important. In a complex context like seismic design, modification
of reference standards easily begins to challenge consolidated practice. 1 The validity of these
practices, however, must be assessed in the light of new knowledge.
1
Thomas S. Kuhn, The Structure of Scientific Revolution, University of Chicago, 1970.
2
Today’s business climate fosters the separation of expertise. In general, the definition of a
building’s morphological aspects is performed by the “architect”, who works by solving
distribution and style issues, using the models learned during their training and which were in
vogue at that time. When the design of the building has been established and the outlines of
the structural system defined, the result is delivered to an “engineer” who may be able to
make only marginal adjustments.2 The engineer is endowed with mathematical skills and
filters everything through complex calculations, performed via automated processes,
undertaking the verifications required by design codes. The calculation methods used are
certainly reliable, but only when solving assigned numerical problems and providing that the
models used do not differ too much from those presumed by the methods applied.
It is probably the excessive confidence based on the (numerical)
reliability of the automatic systems available today that supports
the erroneous opinion that this simple “code-regularization” will
ensure that all constructions are all equally safe. 3 Everyone seems
to agree that this is sufficient, whatever configuration is under
examination, failing to consider that often the bare structure used
for the analysis will later acquire “non-structural” factors affecting
the validity of preceding calculations. For example, the addition of
masonry infill to frame structures may be beneficial if conditions
are static but will be detrimental in the event of an earthquake.
Recourse to this procedure is so widespread that it leaves no room
for other considerations worth dwelling on.
2
3
Christopher Arnold, Robert Reitherman, Building Configuration and Seismic Design, John Wiley & Sons,
1982. The authors are inspired by Henry Degenkolb and believe that if the initial configuration is mediocre,
the only remedy possible is to put a sticking plaster on it.
“There is no more common error than to assume that, because prolonged and accurate mathematical
calculations have been made, the application of the result to some fact of nature is absolutely certain.”
(aphorism attributed to Alfred N. Whitehead).
3
PART 1
ARCHITECTURE IN SEISMIC DESIGN
4
THE DESIGN IDEA
We intend to support the hypothesis that in a holistic conception of the design process, a
building can be rendered truly anti-seismic only if there is accountable architectural
involvement. Although underpinned by sophisticated numerical analysis, anti-seismic design
must still be regarded as an operation with extensive empirical content. Indeed, it requires a
professionalism that right from the start will visualize an appropriate and seemly design idea.
To support this argument, we should reflect on the topics illustrated below:
• the unpredictable intensity of an earthquake may exceed the resistance capacity of
materials used for a building especially if it is erected according to resistant notions of the
traditional type;
• design standards inevitably have a conventional content.
• for a long time professional training concentrated on the concept of resistance. Eurocodes
introduced the limit state concept, but the situation has changed only in part because often
 by habit or learning  resistance is still really perceived as the objective that will define
design strategy;
• anti-seismic design, in reality, must be based on the idea of performance from which to
obtain  and this is the fundamental point  a suitable design idea.
All this is confirmed by the fact that observation of
actual damage produced by earthquakes always shows
that the most disastrous situations can be attributed
primarily to configuration factors. Apart from “non
engineered” constructions, the biggest failures are
triggered by inadequate structural configurations or
morphological choices, rarely by poorly developed
calculations. This is what emerges from site inspections
and examination of reports drawn up by the most
accredited research centres.
OLD CONFIGURATION PARADIGMS
The architectural paradigms that guide design choices do not always correlate with antiseismic requirements and some are even detrimental. There is no lack of examples and the
most significant is the pilotis building plan inspired by the “Maison Domino” concept that Le
5
Corbusier proposed at the turn of the 20th century. It was the first of the five points of
“Nouvelle Architecture.”4 Le Corbusier, however, had no interest in anti-seismic
morphologies and neither did Seismic Engineering.
“Maison Domino” was a truly innovative concept because it associated architectural
composition with the features of a new material: reinforced concrete. The system is rational,
elegant and facilitates configuration aspects, so it became the most frequent design reference.
Nevertheless, as far as seismic episodes are concerned, if they exceed a certain intensity,
reinforced concrete becomes dangerous or, worse still, disastrous. Indeed, it is always
indicated as the primary cause of disasters. The “soft storey effect” it generates is not caused
by pillar resistance (this would be revealed by traditional checks) but by the poor dissipation
potential of the collapse mechanism that occurs when the elastic limits of materials are
exceeded. Result: the upper part of the building may shift significantly and the ensuing “pidelta” effect produces high stresses in the columns because of moments caused by upper
weight. The ruinous collapse of ground floors often occurs without serious damage, except
that caused by the actual fall. This vicious circle can also be caused by an architectural factor:
the stiffening effect of masonry infill on upper floors. Yet in earthquake-prone areas this was
and still is a classic building method.
During discussions these arguments are always widely addressed but it cannot be said that
they are given due attention by active designers nor is there adequate research into
composition. This lack of interest is rooted precisely in the difficulties encountered when
proposing new models in milieus where other practices and other concerns are already
entrenched and prevalent.
4
Le Corbusier, Vers une architecture, Edition Crès, 1923.
6
CONVENTIONAL CONTENTS OF DESIGN PROCEDURES
We have already said that design procedures are extremely conventional in content and this
depends on a number of reasons, including the fact that to be applied in design practice a
number of factors of uncertainty must be sidestepped. We do not intend to confute these
standard design codes, because they are the result of extensive studies carried out in the most
accredited research centres: it would be tantamount to refuting shooting competitions where
different weapons (arrows, rifles, darts) are used, each with their own degree of (im)precision,
and not all equally suited to various targets (distant, close, in movement). The irreplaceable
value of the codes lies in their being a corpus and designers must apply their professional
expertise to use them properly each time.
Even without going into detail, it is enough to consider the situations listed below to
acknowledge this and accept the consequences. The analysis is based on the following
statements:
• the earthquake imposes a movement at the base of a construction and transmits a certain
amount of energy to it, which the construction has to manage with the dissipations
associated to its elastic and, if necessary, inelastic oscillations;5
• seismic motion is complicated and unpredictable: it is described by the response spectra
with which rules establish design input;
• spectral intensity and shapes are defined by averaging the recordings obtained in situations
considered to be geologically similar;
• they affect mainly, but not exclusively, horizontal motion components;
• design spectra refer to an elastic response in structures, but the intensity of seismic attacks
can largely overcome the material’s elastic limits.
• the earthquake exhibits characteristics that differ every time and even at the same site can
occur in a different way each time; it is sufficient that an event originate from new sources
for it to determine significant differences in surface motion.
The notes below offer a brief explanation about the uncertainties whose origins lie in these
assumptions.
Conclusion: It is not rational to entrust the delicate problem of seismic safety in buildings
only to automatic numerical calculations based on these assumptions and relinquishing the
need for an appropriate design idea.
--------------------------------------------------------------------------------------------------------------UNCERTAINTIES IN DESIGN PROCEDURES
The question marks that appear in the figures below indicate the main causes of
uncertainty found in a normal design process. They depend on the nature of its phenomena or
the schematization of calculation models. An overview is given in the notes below.
.• The standards define the design spectra for a conventional damping value, assumed as
viscous and equal to 5% of the critical value. In reality, however, the buildings can
dissipate energy through various mechanisms (plastic, friction, etc) and with very different
capacity.
5
This assumption does not correspond to the concept of stresses induced by systems of “equivalent forces”
that has long governed anti-seismic design.
7
• Design intensity is defined by the spectrum’s PGA  Peak Ground Acceleration. This is the
most significant but not the only parameter that governs destructive potential.
• Often soil-structure interaction is not considered, while recordings are obtained in free
soil.
• Spectral representations do not indicate the duration of the earthquake, although this
factor is important for assessing progressive damage preceding collapse.
• Spectral representations will not reveal possible presence of long acceleration pulses
which might shift the oscillatory phenomenon towards impulsive-type behaviour; the
difference may be of importance in studying overturning.
Other aspects are related to the modal analysis used for elastic calculation which, with
appropriate adjustments, constitutes the habitual foundation for design methods.
8
• Modal forms do not always correspond to actual construction deformation, partly because
of variability in mechanical properties of materials, and more so for the interaction of
elements considered non-structural.
• Information is lost in the frequency domain time and practical rules, like the quadratic
type, must be applied to combine modal responses.
• There is no way of predicting how axial forces produced by vertical components of seismic
motion in supporting elements (columns or walls)will combine with the axial forces
associated with rocking effects.
Other problems arise when numerical analysis models, of which several are listed below, are
defined.6
• “Beam” elements reproduce beams and columns, eliminating cross dimensions and
considering a continuous succession of transverse sections. This requires various
arrangements to assess the behaviour of many single points, like frame knots, crucial for
their horizontal resistance.
• The state of cracking is not taken into account and this is physiological in reinforced
concrete, fluctuating with variation in stress intensity.
• Materials, chiefly masonry, do not behave in a linear manner even in the field of small
deformations.
• The non-linear behaviour of plastic deformations, in particular plastic hinges, may only be
loosely reproduced (an aspect that is at the basis of Capacity Design principles).
• Etc, etc.
Some considerations also apply when designing structures outside of seismic zones, but all
have a particular relevance in specific seismic situations, especially when designing buildings
in high-risk areas where earthquakes are expected to be more violent.
--------------------------------------------------------------------------------------------------------------DEVELOPMENT AND LEGACY OF SEISMIC ENGINEERING
The mechanical sciences evolved in Europe during the Enlightenment period of the
eighteenth century, when it was realized that knowledge of the physical world is obtained by
direct observation of natural phenomena and without preconceived attitudes. Structural
Engineering developed a century later, finding fertile ground in the Theory of Elasticity and
meeting great success because it allowed the development of many important works that
characterized the late nineteenth century. Even today, the Theory of Elasticity is still a reliable
scientific basis with great potential, but (careful!) only within the framework of its
fundamentals: linear elastic behaviour and continuity of deformations. Seismic Engineering
developed later, when these concepts became consolidated and established the basic way of
addressing structural design. It was noted later, however, that Seismic Engineering poses
further problems that fall outside of its scope.
In the early twentieth century, several earthquakes hit intensely built-up cities (1906 in San
6
Structural engineering was defined tongue-in-cheek (Kelsey, Finite Element Method in Civil Engineering) as
“the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyze, so as to
withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect
the extent of our ignorance.”
9
Francisco, USA; 1908, Messina and Reggio Calabria, in Italy; 1923 in Tokyo, the Great
Kanto Earthquake, etc). Consequently standards began to develop, introducing numerical
computation in structural design.
The idea of isolating constructions at their base was already in circulation. After the
Messina earthquake, in Italy, several patents were awarded, but they were simple ideas that
were never applied and did not even attract the interest of academic circles.
Initially there were attempts to copy seismic action by means of horizontal static forces
calculated at about 10% of the weight of the construction. The Elasticity Theory reference led
to structural calculations in the linear field for verification of resistance by evaluation of
“allowable stress”, which was an approach of a static nature with checks replicating normal
working conditions. The implicit but unreliable assumption envisaged that the usual safety
margins would be sufficient to address the
most severe attacks. Moreover, information
was absent for addressing two topics whose
importance was recognized only later: the
dynamic aspect of the problem and the postelastic dissipative capacity of structures. The
application of these criteria lasted for some
time with regulatory updates that did not
change the basic approach. This has left a
substantial legacy in the minds of many
designers and to some extent also in learning.
The first step towards a dynamic approach was taken in the middle of the last century,
when the first accelerograms of real earthquakes were recorded. The “response spectrum” 7
technique was developed by processing these data. In quantitative terms, this tool brought
forth important information about the actual frequency distribution with which earthquakes
release their energy. Much of this energy reaches the surface in quite a high frequency range,
7
Hudson illustrated this calculation tool in 1956, at the first WCEE (World Conference on Earthquake
Engineering), in San Francisco.
10
over 1 hertz, which includes many
standard constructions. 8
The first recordings used to define
design spectra came from El Centro
(California, 1940) and indicated a peak
acceleration of 0.34g (NS component).
For a time it was thought that this value
was indicative of maximum seismic
intensity, but this was far from accurate.
A very important earthquake in the modern history of Seismic Engineering was that of
Imperial Valley (San Fernando, California, 1971). Numerous detection positions were
present, many in the epicentral area, where the peak acceleration registered was in excess of
1g (!). New information and analysis of very serious damage, this time caused by important
new reinforced concrete construction works (buildings and bridges), made a decisive impact
on anti-seismic design. It was clear how important it was to define new criteria in the
approach to and further investigation of the post-elastic behaviour in structures subjected to
repeated cycles of alternating large plastic deformations (plastic fatigue), before collapsing.
Many experimental and theoretical studies were conducted after these events.9 Attention
focused on energy issues and the dissipative capacity of inelastic structural responses. The
Berkeley school was very active in this field, while many experiments were conducted,
mainly in Japan. The innovative concept came from the Christchurch school in New Zealand,
8
9
This information was only used for practical purposes many years later, to introduce and apply modern base
isolation methods.
Experimental research into these aspects was already under way, especially in Japan, but became more
intense in the years to follow.
11
where the principles of Capacity Design were defined in the early 1980s. The expression
describes the design criterion aimed at showing maximum energy dissipation, which then
became the main basis of seismic legislation.
It was at this time that Base Isolation applications began to spread as they had now gone
beyond the pioneering stage.10 Base Isolation was confirmed not only by the theoretical and
experimental evaluations of the effectiveness of the system, based on the dynamic decoupling
of the building from the ground, but by recordings made during the Loma Prieta (Los
Angeles, 1994) and Kobe (Japan, 1995) earthquakes, and later by many other Japanese
recordings.
Together with the previous systems, the use of various types of dissipative devices
(viscous, elasto-plastic, shape memory alloys, etc) has increased and they can also be used to
build Base Isolation systems. In the 1980s and 1990s, the decks of several Italian viaducts
were protected in this way. 11 In construction, similar systems can operate in parallel with the
main structure, but it must be flexible enough to withstand the necessary deformation.
We will not be discussing other special methods (tuned masses, active, semi-active and
hybrid systems): they are fascinating but very complex applications and can only be justified
for large-scale constructions of considerable importance, like the Landmark Tower in
Yokohama or the Applause Building in Osaka.
We may conclude this brief summary with Design Capacity and Base Isolation concepts,
thus coming to current notions of anti-seismic design.
PERFORMANCE REQUIREMENTS
Returning to what was said earlier, in high risk areas where demand may exceed the
material’s resistance capacity, the main design requirements are prevention of collapse and
minimization of damage. These are pursued mainly by controlling two quantities:
dissipation capacity and deformations12,13,14,15.
10
11
12
A. Parducci: Seismic isolation: why, where, when: design options for ordinary isolated structures,
International Post-Smirt Conference Seminar on Isolation, Energy Dissipation, Cheju (Korea), August 1999.
A significant example is the Coltano viaduct, comprising continuous 450-metre sections across multiple
spans, for a total length of almost 10 kilometres (FIP and ALGA devices by A. Parducci and E. Ciampi,
respectively).
E. Elsesser, New ideas for structural configurations, 8th U.S. NCEE, San Francisco (CA), April 2006.
12
We will simplify descriptions to make it easier to explain. Thus, stating that the structure’s
cyclic response can be modelled by bilinear elastoforce
plastic behaviour; that various viscous dissipations of
plastic step
limited effectiveness correspond to the elastic phase,
elastic
depending on velocity not on deformation; that
displacement
step
hysteretic plastic dissipations, more dissipative that
cyclic
residual
the aforementioned, correspond to the plastic phase,
dissipated
damage
energy
(especially if compliant with Capacity Design
principles), depending on deformation, not velocity.
The purpose of simplification is to highlight the hysteretic contribution associated with
damage to the building. Therefore, let:
Ei
be the energy fed into the overall construction;
Ek be the kinetic energy of the moving masses;
Ev be the energy associated with viscous elastic response;
Ei
be the potential energy of elastic deformations;
Ei
be the hysteretic deformation energy associated with irreversible plastic deformations
(damage).
During an earthquake the building accumulates
ENERGY
stored
structural
energy. If it is able to absorb the energy with
viscous dissipations
Ei
Ek + Ev + Ee + Eh
during the elastic
phase (this requires
kinetic
elastic
reversible
extensive structural
deformability),
the
viscous
hysteretic
dissipate
structure
oscillates
d
without
being
9
damaged. If this is not enough, it must bring into play the
inelastic deformations of the plastic phase and damage will
occur. If these mechanisms are sufficient, the structure does not
collapse and can be repaired, if this is cost effective. Disastrous
collapse, on the other hand, can be attributed to the inadequacy
of performance, since the latter cannot easily be guaranteed.
----------------------------------------------------------------------------------------------------------------AN OVERVIEW OF ITALIAN LEGISLATION
Italian codes have recently been updated to comply with Eurocode directives. The procedures
described below are applicable to design of new buildings. Existing bridges and constructions
are addressed separately. The requirements are expressed in terms of Limit States (LS) and
13
14
15
A. Parducci, Nuove concezioni per il progetto sismico - Una sfida per l'architettura e per l'ingegneria, “Eda,
esempi di architettura”, Edizioni Il Prato, special edition, June 2007 English translation available).
A. Parducci: Nuovi orizzonti per un'architettura antisismica, Atti del Seminario CNR, Roma, September 2007
(published in “Nuovi Sistemi e Tecnologie Antisismici”, 21° Secolo, Roma, February 2008)
M. Mezzi, Deformation vs stiffness  motion vs fixity  New vision in seismic conceptual design, The 14th
WCEE, October 2008, Beijing, China.
13
defined according to performance criteria. In fact, we will consider two kinds of LS:
• LLS = Life-saving Limit State, applied for rare events of high intensity expected to recur
every 5001,000 years.16 The structure may be damaged, even significantly, but must
retain part of its resistance in vertical and horizontal actions.
• DLS = Damage Limit State, applicable to more frequent events, with recurrence periods of
50100 years. The construction must be guaranteed immediately usable, even if there is
limited local damage to non-structural elements and plant.
Building design is addressed in two different sections. The first is more general, for
traditional buildings, with “fixed” foundations connected directly to the ground; the other is
for buildings using Base Isolation systems.
In the first case, Capacity Design principles are applied to ensure adequate dissipative
capacity giving significance to a linear calculation of reduced intensity compared to that of
the elastic spectrum. The LLS is met by adjusting calculation results according to practical
rules defined as follows:
• critical areas are taken into consideration, designed for high local ductility, placed in
strategic positions (plastic hinges near frame nodes);
• prevention of inelastic deformation occurring outside of critical areas (upscaling of noncritical areas);
• promotion of the formation of collapse mechanisms that effectively mobilize envisaged
critical areas, avoiding inadequate poorly dissipative mechanisms like soft storey (weak
beam-strong pillar rule).
These rules serve to ensure the availability of a global dissipative capacity that makes it
reasonable to reduce elastic spectrum values (e.g. 45 times more or less) significantly,
according to the configurations of the different structural systems. Morphological
architectural choices are thus involved in this design aspect (!).
DLS requirements, on the other hand, addresses deformations for which limitations are
assigned on the basis of structural type (relative storey drift).
When buildings are provided with Base Isolation the decoupling is required to bring about
a very low spectral input. Consequently, for Base Isolation to make sense, LLS must be
obtained ensuring the structure remains practically in the elastic field, without mobilizing
important plastic dissipation.
---------------------------------------------------------------------------------------------------------------RESISTANCE AND DEFORMATION CONCEPTS
The resistance paradox: a typical aspect of seismic design. The seismic input received by
a construction depends on the fields in which the two frequency distributions are located: that
of seismic movement, shown by the shape of the response spectrum, and that of the structures
own oscillations. Increasing resistance will lead to an increase in the size of structural
elements, which will bring an increase in rigidity. The oscillation period decreases and in
most cases an increase in demand ensues. Persevering with the path of resistance is like
challenging the earthquake in a battle that will surely be lost. Moreover, horizontal
16
Return periods are assigned according to different reference times conventionally attributed to various
building uses, applying a uniform hazard criterion.
14
accelerations transmitted to each floor will also increase.
Design codes do not address this aspect and do not lay down requirements for limiting
storey accelerations as such, even though it is equally important because it defines the
protection of non-structural parts, plant and content. For
some intended uses, such as hospitals and rooms containing
hazardous or valuable materials, it can become the main
feature.
Earthquake and wind. Both actions stress structures
horizontally but in addition to the different intensities, there
is also a fundamental difference in the way they manifest
themselves. Regardless of the aeroelastic phenomena that
bear no relevance to civil constructions, the airflow around a
building is actually unaffected by structural deformations and
therefore does not alter design input. Wind gust frequencies
are also much slower than those causing building oscillation.
Wind action and effects can therefore be interpreted using
equivalent static action, while seismic action depends on mass inertia forces and is linked to
motion velocity. Consequently, the more slowly the structures oscillate, the smaller seismic
action will be. In addition, hysteretic dissipative capacity mobilized during damage stages
depends directly on the extent of deformations.
Resistance and deformability. Ultimately, the concepts of resistance and deformability
must be properly balanced, giving due attention to the latter, because it can also be beneficial
in the elastic field. So the firmitas indicated by Vitruvius in his famous triad firmitas,
venustas, utilitas (strength, beauty, usefulness), when describing Imperial Rome in the 1st
century BC17 (always quoted in architecture courses), is not entirely true.
PAST EXPERIENCE
Apart from a few  some quite important  experiences, a culture that will achieve decisive
architectural design committed to seeking morphologies that address seismic issues is still
struggling to establish itself. We could say, simplifying again, that a number of causes may
affect the interest of Architecture as a discipline,18 hoping to stimulate at least some of the
younger architects.19 Several of the following causes also affect behaviour dictated by socalled common sense.
Vertical perception. Buildings are designed to be in the gravitational field: vertical and
17
18
19
Marcus Vitruvius Pollon, De architettura, Giulio Einaudi Editore, Torino 1997 (Latin and Italian texts).
Umberto Garimberti states that: “... even science can be psychoanalyzed and subjected to therapy for the
purpose of exposing our intellectual laziness that supports certain conceptual and operational choices, the
subconscious motivations that lead to certain concepts being taken for granted, the practical needs that go in
one direction rather than another, and the stubbornness of insisting on ideas that are proven but lack
prospects …” Paesaggi dell’anima, Oscar Mondadori, Milano, 1996.
Max Planck, who won the Nobel Prize for Physics in 1918, said that: “A new scientific truth does not
triumph by convincing its opponents and making them see the light, but rather its opponents eventually die,
and a new generation grows up that is familiar with.” Wege zur physikalischen Erkenntnis, article published
on 9 July 1932.
15
permanent. Often architectural design enhances this feeling. Resistant systems are designed in
a similar way, just as we all conceive space: “... all horizontal directions are equal and form a
plane of unlimited extension. The most elementary model of existential space is a horizontal
plane crossed by a vertical axis.”20 This vision contrasts with the perception that is necessary
for tackling seismic problems.
Static perception. Vitruvian firmitas, which has already been mentioned and which has
always influenced structural design. Collective wisdom perceives buildings as solid and
steady: this is usually what is stated in Faculties of Architecture.21
Compositional elements. Bricks are no longer used in building structures, having been
replaced by a frame that has become the basic construction element. The trilithon of ancient
history became the frame when steel and reinforced concrete building techniques began to be
used for achieving structural continuity with uprights and crossbeams through the knots. If we
examine the behaviour of a mesh
frame, however, it is easy to see that
such a popular element can be
adapted to seismic requirements (for
example, Capacity Design does this),
but intrinsically it does not have the
best attributes required for offsetting
seismic action. The horizontal inplain deformability of a spatial frame depends on bending (OK) and shear deformations of its
elements, especially the uprights, associated with storey drift. Maximum moment is found at
the knots (critical areas) where plastic hinges may form. The actual knots are critical, because
their failure makes the whole system unstable. Without Capacity Design22 precautions, a
frame’s dissipation capacity would be inadequate for a high-risk seismic zone?
Intensity of seismic impact. The macroseismic scales used by geophysicists are
20
21
22
Norberg and Schulz quoting Rudolf Arnheim in The Dynamics of Architectural Form, University of
California Press, Berkeley, 1977.
E. Torroja, Razón y ser de los tipos estructurales, Istituto técnico de la construcción y del cemento, 1960.
Even though we are at a high rational level, this argument is somehow reminiscent of C. Arnold’s sticking
plasters (see footnote 2).
16
logarithmic. The statement may seem trivial, but anyone
unaccustomed to using them (like media information
workers) may underestimate the great difference that
exists when magnitude grows by “only” two degrees, for
example. As a guideline, the ratio is 1:100 in terms of
displacement, and 1:1,000 in terms of energy (!).
Conventional value of numerical analyzes. The topic
was discussed above, pointing out the very conventional
aspect of numerical analysis which makes it impossible to base anti-seismic design on a
computational power that does not really exist.
EXAMPLES OF TRADITIONAL ANTI-SEISMIC ARCHITECTURE
We have said that in the past, anti-seismic design was based primarily on the concept of
resistance. When spectral representations showed that demand decreases with the increase of
the oscillation period, building height no longer appeared to be a limiting factor. Thus it may
be interesting to examine how the problem of lateral resistance was addressed in the most
demanding cases, that of skyscrapers, although for these buildings more attention is given to
the problem of hurricanes than that of earthquakes. We are still
referring to the spatial frame already mentioned, but with proper
precautions and additions. Over a certain height, to protect nonstructural parts and plant, lateral deformation must be limited and
there are various solutions.
Technical storey. The height of the frame is broken up into
blocks separated by technical storeys that act as non-flexible
slabs. Pier Luigi Nervi used this principle for the Montreal
skyscraper. The design idea has harmonized many aspects:
• structural, so each block works as an element interlocked
above and below to reduce column bending and transfer total
deflection to axial forces (traction-compression) in the corner
uprights;
• aesthetic, which interprets structural functioning by assigning a
strong architectural value to the corner uprights;
17
• technological, which uses rigid storey to distribute plant along the height.
At the time of the design (1963) dissipation of energy was not considered important.
Schemes of this kind still allowed for balancing design
parameters to achieve optimal performance.
Diagonal stiffening. The concept is similar to that of
triangular geometry used on racing bicycles, which
responds to all the athlete’s demands. Only axial loads
(traction-compression) are mobilized to obtain maximum
strength with minimum weight. This system has been used
to build a number of skyscrapers of which the most famous is the John Hancock Building, in
Chicago (Illinois, USA, 1969). This 100-storey building is 344 metres high and its exterior
tubular structure is slightly
tapered, with large exposed
diagonal
stiffened
elements.
Analysis of the design showed
that the diagonals play a
significant role in withstanding
vertical
loads.
The
latter
observation sparked the interest
of I. M. Pei, who designed the
amazing Bank of China building
in Hong Kong, surpassing
vertical-horizontal
hierarchical
values. The reticular structure
takes both loads to the peripheral
uprights and discharges them onto
the rocky terrain below.
The modern pagoda. The 1972 Transamerica Pyramid, which soars against the San
Francisco skyline, deserves special attention because its movements during the Loma Prieta
earthquake of 1989 (M = 7.2) were recorded. The building vaunts significant lateral
deformability with an oscillation period of 3 seconds. Oscillations of the amplitude of ± 20
18
cm were measured on the 49th floor (five times greater than at the base), lasting for almost a
minute without causing damage. Dissipations of a “viscose” nature were slowly able to absorb
the energy transmitted by the earthquake, maintaining a sufficiently elastic deformable
structure. The building has an internal metal structure, the façades are prefabricated, and the
base is formed by large interwoven structural elements. The latter leave open spaces free on
several floors without creating a “pilotis effect.” It is possible that some stiffening was
produced by the pre-cast façade elements and that their relative limited sliding contributed to
dissipation effects.
The building has a unique shape and some feel that in general the tapered silhouette, which
is very marked here, is a morphological construction requisite. Moreover, similarity with the
configuration of Japanese pagodas is evident and these are buildings that over the centuries
have withstood strong earthquakes. The figure compares the San Francisco Pyramid with the
Horinji pagoda in Nara (Japan), which is 1˙300 years old. The seismic behaviour of pagodas
was studied by Japanese scholars, who highlighted various effects that should be explored in
depth. At the moment, however, no possible loans to current construction needs have been
tested.23
Critical configurations. The stiffening of frames achieved with shear walls, box-shaped
structures or fascia of high beams can produce undesirable consequences if the resulting
configurations create shear beams in the other elements with which they interfere. This will
foster shear failure, which lacks dissipation, in the shear beams. The situation is particularly
critical in the case of reinforced concrete captive columns exposed to strong shattering.
23
K. Fujita et Al., Earthquake response of ancient five-story pagoda structure of Horyu-Ji temple in Japan, 13th
WCEE, Vancouver, Canada, 2004.
19
PART 2
THE BASE ISOLATION
20
BASE ISOLATION
Base Isolation (BI) is a seismic protection system that reduces significantly in the energy
transmitted to a building by the earthquake. The result is obtained by arranging the
construction on supports having conspicuous horizontal flexibility, thus modifying the
oscillation period. The principle is based on the fact that earthquakes carry energy in a fairly
narrow range of frequencies. The highest
response accelerations, amplified with respect
to the ground, are spread over periods of less
than one second. Most low and mid-rise
buildings fall in the same range of elastic
response periods and are therefore exposed to
increased risk. If the period passes this critical
area, demand drops rapidly and becomes very
small when it comes to periods of at least 2
seconds. Reduction in terms of acceleration,
however, corresponds to an increased demand in terms of displacement.
The situation offers two considerations. Firstly, understanding why earthquakes often
distribute their effects in a non-uniform (“leopard-spot”) fashion. Secondly, suggesting a way
to avoid earthquake effects by cunning rather than by force. 24
On standards. The figure
shows a typical case of an elastic
response spectrum expressed in a
“capacitive” form (Sa response
accelerations shown vs Sd
displacements)
that
Italian
standards applied to the city of
L’Aquila (high-risk area). The
spectrum refers to the LLS (Lifesaving Limit State) for a public
building subject to large crowds (rare event, with a 10% probability and reference period of
VR = 712 years). It may be a school located on flat land with medium quality soil (soil C).
Fixed-base design. Italian standards harmonized with Eurocode 8. Designs for the ULS
(Ultimate Limit State), accepting damage estimates even as severe but without collapse. For a
calculation in the elastic range to be significant, the input is evaluated by dividing spectral
accelerations Sa by structure factor q. The greater the reduction in q, the greater the potential
inelastic dissipation capacity of the building, and the more extensive and flexible the plastic
deformation the structure can withstand. Applying the principles of Capacity Design,
standards define q by virtue of:
• the configuration of a resisting system;
24
“Why fight an earthquake? Why not join it and beat it with astuteness?” wrote Frank Lloyd Wright in his
memoirs, remembering that for the Tokyo Imperial Hotel he had created elements to “float” on sludge.
“Rigidity was not the right answer, but flexibility and resilience were.” The building survived the 1923 Great
Kanto Earthquake in Tokyo (M≈8) unscathed.
21
• assigned classes of ductility.
For reinforced concrete frame structures of regular shape, the q factor varies from 3.0 (very
dissipative configuration with low ductility) to 5.85 (dissipative configuration with high
ductility).
Base-isolated building design. The design again uses elastic spectrum Sa, but the q factor
is very small, about 1.5, because the spectral structure reduction is required to be isolated in
the elastic range, at most with slight damage. No special requirements are therefore needed
for ductility.
Isolator displacements are calculated without any reduction of the elastic spectrum. In the
case of L’Aquila, ± 35 cm would be needed, plus some marginal guarantee requirements.
Elastomeric isolators. The most
popular isolating devices for these
applications are HDRB  high damping
rubber bearings, which are multilayer
rubber-steel devices or LRB, lead rubber
bearings. They are made using thin layers
of silicone rubber (610 mm) alternating
with vulcanized metal sheets (23 mm).
These devices are very deformable for
horizontal shear (distortions up to 200%,
upper standard limit). When confined, the
crushing of the rubber is reduced so that
the devices deform little in the vertical direction and can be used as supports. The figure
shows the response curve trend obtained with “quasi static” (slow alternating deformation
cycles) force-displacement tests of different amplitudes. The areas shown in the diagrams
measure the energy dissipated in each cycle. Secant stiffness k depends on deformation
amplitude: large (k1) for small amplitudes; small (k2) for large amplitudes. This behaviour is
useful because large displacements are needed for the system to be effective against violent
earthquakes, while more stiffness is needed to respond to normal operating actions, like that
of wind, without significant displacements.
Isolating devices do not have a large dissipation capacity. In LRB devices it is increased
by a lead cylinder fitted inside.
In any case, as will be seen, BI
reduces seismic input by means
of dynamic decoupling rather
than by dissipation. The
equivalent viscous damping can
be evaluated through an equienergy criterion.25 With HDRB
isolating devices damping is in
25
Within specific limits the relationship =(1/4)×(WD/WEL) is usde, linking viscose damping  to the energy
that the device dissipates in each cycle. The quantities WD and WEL=½kd2 correspond to the areas indicated
in the figure.
22
the order of 10% of the critical value ( ≈ 0.10); for LRB devices it can reach 2530%, a
value that may be useful for reducing displacements.
Practical considerations. For multi-storey buildings, BI
is best applied when the building has a basement (garage
or cellar). This avoids having to construct an extra floor
that is otherwise unnecessary. The isolators can be placed
on the containment walls and inner columns to insulate the
upper building, leaving the basement set into the ground.
The oscillation period can be increased by fitting sliders
with a low friction coefficient in parallel with the isolating
devices.
----------------------------------------------------------------------------------------------------------------THEORETICAL ANALYSIS OVERVIEW
The figure sums up an isolated structure
q
with a system that has one degree of freedom,
m
building
deformable as a shear-type frame, defined by
mass m and stiffness k. Building deformation
k, c
isolators
is (q - q0). The inclusion of an isolation
q0
m0
k,c
0
0
system and addition of mass m0 bring a
system with two degrees of freedom. Seismic
input is defined by a history of soil accelerations ag(t) compatible with the design spectrum.
The structure is defined by the following dimensions:
• non-isolated structure: m = building mass,
k = building stiffness,
c = building dissipation coefficient.
• isolation system:
m0 = isolated foundation mass,
k0 = elastic stiffness of the isolation system,
c0 = isolation system damping,
• displacements:
q0 = displacements by isolator deformation,
q = building displacement (mass m compared to the soil).
23
For an indicative analysis evaluating the system’s elastic response using modal analysis
methods considering the following parameters:
• frequency corresponding to oscillation period TBF of the fixed-base structure:
m
k
• reference frequency  0 and corresponding oscillation period T0 for the entire isolated
system, considering non-deformable, mobile above isolators:
2 
k
m
TBF  2
 02 
k0
m0  m
T0  2
m0  m
k0
Two dimensionless parameters are assumed:  = mass ratio;  = period ratio:
 
   0
  
m
 
mo  m
2
T
  BF
 T0




2
It is recognized that  is less than 1, but of the same order of magnitude, because the mass m
of the entire building is predominant with respect to m0.
Supposing that the parameter  is small ( <1), namely that the reference period T0 is
substantially greater than the TBF of the structure with a fixed base (e.g.: T0≈3TBF, so ≈0.1).
With these assumptions we can write the system of the two equations that describe, in the
linear field, the system motion with two degrees of freedom:
m0 q  ( k0  k ) q0  k q  m a( t )
m
 k q0
 k q  m a( t )
Separating variables, proceeding with modal analysis methods and considering small
quantities as negligible infinitesimals, we calculate the characteristics of the two modal
shapes of the isolated system [sample values correspond to = 0.1].
First method
period
T(1) ≈ T0
modal form
u0 = 1
u ≈ 1+
2
m ( 1 ) = (m0 + m)(1 -  ) [> 0.99 (m0 + m] (!)
participating mass
Second method
period
modal form
T(2) ≈ TBF
u0 = 1
u
participating mass
= 
m(2)
1

(1 )
= (m0 + m)(1 - ) 2 [<.01] (!)
----------------------------------------------------------------------------------------------------------------Compactness requirement. The modal analysis carried out in the previous note leads to an
important result, valid when the ratio = (TBS/T0)2 from fixed-base period to isolated period
is small. This happens when the protected construction is stiff. The first way reproduces
almost completely the entire system response in which most of the construction mass
participates (m+m0). The isolated period in effect corresponds only to isolator deformation
while the building oscillates almost without deformation. The system is hardly sensitive to the
second mode, the one that deforms the structure, because the participating mass is almost null.
24
The result indicates the following
conditions for optimal BI functioning:
• the
isolated
oscillation
period
(immediately assessable TIS≈T0) must
be large because the construction is
certainly located in the low acceleration
response zone (TIS>2, but may well be
more);
• it is better if the structure located on the
isolators is rigid, barely deformable
(T0>3TBF), for dynamic decoupling to be completely effective.
The second condition re-evaluates the use of many traditional building techniques, such as
those of masonry, with a favourable contribution to current bio-architecture problems.
BASE ISOLATION PERFORMANCE
BI has many advantages. The first is to reduce the stresses transmitted by violent seismic
attacks. BI allows an almost elastic response to be obtained without significant damage and
offers the advantages described below.
BI eliminates the resistance paradox because its separates tasks: deformation is entrusted
completely to isolator devices, specially designed and experimented, while the more resistant
upper structure must be rigid so storey dirft is very limited.
Moreover, BI reduces plane accelerations and the damage inside buildings. The images26
below show the effects of the 1994 Northridge earthquake inside Olive View Hospital, which
had been rebuilt after 1971 as a fixed-base structure, with very strong and very rigid, stiffened
walls. Accelerations of 0.8g recorded at the base, and 1.53g at the top, did not damage
structures. Damage to plant and non-structural parts was significant and caused the hospital
huge economic problems.
26
The illustration and information on this topic were provided by Prof. Edoardo Cosenza, of the University of
Naples.
25
BASE ISOLATION AND ARCHITECTURAL DESIGN
Passive seismic protection contrasts with the
firmitas concept. Even Vitruvius would have
defined requirements with the terms motus,
scissio, deformatio (movement, separation,
deformation). The two models in the figure
correspond to a simple BI application and to a
variant known as a “bell-building” whose
fluctuations are controlled by dissipation
devices.27,28 These are not the only models consistent with BI, because developing patterns
like these opens new avenues for experimenting architectural forms that expand freedom of
configuration. More complex configurations, like that shown in the figure, can also become
anti-seismic.
Union
House
in
Auckland (New Zealand,
1980) is a perfect example
of an architectural layout
that spotlights exactly how
BI works. The foundations
are in “mobile” piles that
cross a quite erratic terrain
and rest on a stiff soil. The piles are bound to the structure by hinges and are housed in a
sleeve of a larger diameter that allows their heads to move. A system of elasto-plastic metal
dissipating devices connected to
a fixed platform at the base of the
building contrasts horizontal
displacements. The elevated
structure is stiffened by diagonal
braces that emphasize the
design’s anti-seismic conception
by stiffening the structure and
passing vertical and horizontal
loads to the isolators. The plastic
threshold of the isolators is
calibrated so that it does not
transmit horizontal forces in
excess
of
the
structure’s
resistance capacity.
27
28
A. Parducci, Seismic isolation and architectural configuration, Conceptual Conference on the Conceptual
Approach to Structural Design, Singapore, 2001.
M. Mezzi, A. Parducci, Aseismic suspended building based on energy dissipation, 10th ECEE, Wien, 1994.
26
27
PART 3
SOME APPLICATIONS OF BASE ISOLATION IN ITALY
BEFORE L’AQUILA EARTHQUAKE 2009
28
THE NEW EMERGENCY MANAGEMENT CENTRE FOR CENTRAL ITALY
The initiative was funded by Umbria Regional Government. The building complex is the
result of design research performed through material achievements with a dual purpose. The
first was to circulate BI techniques in a country where they were beginning to spread,
although with some difficulty; secondly, to test the importance of architecture in today’s
seismic problems. The intention was to show that in a holistic conception of the design
process, BI does not limit architectural choices and actually opens doors for the definition of
compositional forms able to mobilize
significant anti-seismic synergies.
The plan shows buildings designed
with differentiated BI solutions, taking
into account their use and functions. In
the following descriptions there is greater
focus on the two achievements
considered most significant, which were
designed by the author of this report in
collaboration with the architect Guido
Tommesani, the engineer Alfredo
Marimpietri and professor Marco Mezzi.
(A) OPERATIONS ROOMS BUILDING.
This is the nerve centre of the complex, shaped like a false cupola, 32 metres in diameter.
The structure rests on 10 HDRB elastomeric isolators with 1000 mm diameter, arranged along
the base perimeter. A double-vault system returns the loads above, exposing a slab floor
composed of ribs that intersect in a complex series of solids and voids. From the first level, by
the supports, 10 reinforced concrete semi-arches rise to meet at the keystone, where a central
29
unit comprising two concentric tubes of pre-stressed reinforced concrete is suspended and
contains the vertical courses. The central tube is extended below the ground surface to contain
the excess lift travel. The floors connect the central unit with the perimeter arches. The
building has a compact structure: inside there are ample spaces without columns and the
ground floor is accessible even though it does not have a pilotis configuration.
Design factors that optimize BI in this building
• High oscillation period (TIS=2.6). A value far from the seismic frequencies that
characterize the site. This ensures a sharp drop in demand which becomes almost
independent of seismic intensity in this field .
The building was designed for PGA=0.49g, indicated by regulations for ULS. It
corresponds to a return period of 950 years. Performance has been extensively verified.
The construction can oscillate slowly, almost undeformed, with displacements of ±40 cm.
• High oscillation period ratio (TIS/TBF > 3). The protected building has more compactness
than resistance. Deformations are controlled by the isolators. Although it is large and
devoid of columns, it has considerably more compactness than a frame structure. It
oscillates horizontally in the first manner, with a 99% participation of the total mass.
• Rocking-effect form. The low centre of mass reduces rocking effects (typical of a tapered
shape). This reduces compression variations on the isolators during seismic oscillations.
• Uniform form. The regular, substantially symmetrical shape meets uniformity
requirements.
• Centrifuging of stiffness. The peripheral layout of the isolators ensures minimal disruption
due to torsion effects.
30
Factors for enhancing strength capacity of reinforced concrete
• Compression of semi-arches. Much of the weight is supported by the core which
discharges the load onto the semi-arches, stressing mainly by compression.
• Centring of compression. In the presence of seismic actions, arch compression centring is
assumed by horizontal connections with the core. The dimensions are required more for
architectural reasons (sunshade elements) than for strength requirements. The calculated
C/D (capacity/demand) factors are almost 2.
• Critical points. The configuration has no critical areas where significant stress
concentrations may arise.
31
Awards
• AICAP Award 2010-11. Project Award winner AICAP (Italian Association for prestressed and reinforced concrete).
• The significance of the project has been recognized internationally and is found on the
Earthquake Architecture homepage of the CUREE (Consortium of Universities for
Research in Earthquake Engineering) website.
Noi abbiamo pensato, abbiamo discusso, abbiamo progettato, abbiamo calcolato;
loro hanno sudato ed hanno sofferto i disagi del lavoro di cantiere.
Sono loro, gli operai, che con le loro mani hanno costruito l'opera.
32
(B) CULTURAL HERITAGE WAREHOUSE.
The project is an example of seismic protection for industrial buildings which cause major
accident risks29,30 in the event of an earthquake and in relation to their processes or contents.
The building has an octagonal plan of 2000 m2 and is about 9 metres high. The perimetral
walls are suspended. The main structure supporting the roof is formed by four steel beams
arranged in a cross, resting on 12 round reinforced concrete columns by means of an isolation
system, of which 8 are peripheral columns at the corners of the octagon, and 4 are central
pillars. The four roof beams are cantilevered to hold a perimeter beam from which the
peripheral closing walls are suspended. The insulation was achieved with LRB devices
arranged on the top of the perimeter columns and by sliding bearings arranged on the inner
columns. The floor is separated from the insulated part and is integral with the terrain. The
isolation system will therefore protect all of the elevated structure: roofs and closing walls.
The decision to suspend the walls not only attained protection of all the building masses,
but also made it possible to avoid the need for foundations for the peripheral walls that would
have had to sustain the seismic actions transmitted by the walls themselves, which are of
considerable height.
The building was designed with the same input as its neighbour and has a very similar
period of oscillation, so performance is also similar.
29
30
A. Parducci, F. Brancaleoni, Terremoto del 6 maggio 1976 nel Friuli. Considerazioni sul comportamento
degli edifici industriali, Industria Italiana del Cemento, Roma (Italia) 1976.
F. Menegotto, La prefabbricazione strutturale: aspetti teorici, Giornate AICAP, Stresa (Italia) 1987.
33
(C) OTHER BUILDINGS IN THE COMPLEX.
These are of no particular interest for our specific topic, because their architecture had
already been defined before deciding to switch to isolation. Necessary adaptations were then
required, for instance in the long single block which the Fire Department uses in part for
accommodation and in part for garages. It was necessary to provide for two different isolator
positions and to maintain roof continuity the two parts were separated with an internal sliding
connection.
THE NEW JOVINE SCHOOL.
In 2002, an earthquake destroyed a school in San Giuliano di Puglia, killing 27 children.
The architectural design of the new school consisted of two adjacent buildings of irregular
shape. Alignment with new codes would have required an estimated 42% increase of the
resistance envisaged, with fallout on
the architecture. The BI option,
commissioned by the Civil Protection
Service, solved the problem and
ensured even better safety than is
offered by fixed-base design.31
This intervention is interesting for
the application of an idea already
mentioned in connection with a
number of Japanese constructions of a
larger size. Instead of isolating the
buildings directly, an entire base
platform is isolated. The same idea,
albeit with a different scope, was
successfully taken up by the C.A.S.E.
Project described below.
31
Design adaptation predefined by P. Clemente and G. Buffarini (ENEA), with M. Dolce and A. Parducci as
external consultants.
34
MASONRY BUILDINGS.
ATER Building in Corciano. Masonry buildings today are viewed with great interest by
bio-architecture. In the past, traditional conceptions tended to consider masonry unsuitable for
seismic zones, mainly due to the poor ductility of resistant elements. BI re-evaluates masonry
as an interesting seismic system. A consistent, well-organized configuration that guarantees
proper “box” behaviour, is a simple design option that alone ensures excellent performance
for a base-isolated building, especially for the “compactness” required to achieve dynamic
decoupling on which BI constructs its effectiveness. In 2007, for experimental purposes,
ATER of Perugia built the masonry residential block (seen in the image below) in Corciano, a
high-mid seismic hazard zone (PGA = 0.25 g).
SIRICA 2010 Award. The BI combines in a synergistic way with the energy-saving
demands of bio-architecture. This was highlighted by the Bio-Sisma project, winner of the
2010 Sirica Award (an architectural competition dedicated to this topic), 32 in which the author
32
M. Carli (project manager), Residenze BioSisma - Perugia, CNAPPC (National Council of Architects),
“Premio Sirica 2010. Sicurezza nell'abitare”, Di Baio Editore, Milano.
35
of these notes participated. Here a single reinforced concrete foundation platform resting on
insulators was again included, acting as a thermal flywheel. Weights at low levels are not a
problem when BI is applied. The air used for ventilation of the building circulates in the
space below, where the insulators are found.
36
37
PART 4
THE ITALIAN EXPERIENCE ON BASE ISOLATION
AFTER L’AQUILA EARTHQUAKE 2009
THE C.A.S.E. PROJECT
38
L’AQUILA EARTHQUAKE 2009
6 April 2009 Earthquake. On 6 April 2009 a violent earthquake (IX-X on the MKS scale)
centred L’Aquila, the capital of the Abruzzo Region, a historic art city of 75˙000 inhabitants.
Not only the age of the buildings, but also the quality of more recent reinforced concrete
constructions, nonetheless designed with outdated regulations and sometimes without
adequate technical applications, rendered the situation
catastrophic. The result was 308 dead, 1500 injured,
20˙000 buildings collapsed or declared uninhabitable,
more than 25˙000 people left homeless.
THE PROJECT C.A.S.E.
The Civil Protection Service intervened promptly
and besides dealing with immediate relief operations,
also planned and implemented the “C.A.S.E.
Project”33 to cover the first phase of reconstruction. In
just over six months, it was able to complete
construction of 185 new buildings applying a new BI
concept. From 29 September 2009 to 19 February 2010, a total of 4450 new, fully-furnished
lodgings were handed over to approximately 15˙000 homeless.
One of the most significant aspects of this operation was the way in which BI was used.
The following description is focused on use
of BI and the central role it played in
completing the project. Descriptions of the
whole
operation,
complete
with
photographic documentation, are available
online in Italian and English, on the Civil
Protection Service website.
33
Acronym for “Complessi Antisismici Sostenibili Ecocompatibili” [sustainable and eco-compatible antiseismic complexes]. The scheme was completed but aroused criticism and debate in the political sphere. Here
we describe it in relation to the Base Isolation function used in the project.
39
The project layout. The “isolated
deck” project was partly inspired by the
idea tested in San Giuliano di Puglia
where the entire foundations were
isolated instead of the buildings. Here a
variant was to use the space below,
primarily with a different scope.
A single prototype, consisting of two
parts, was applied to construction of all
the buildings.
The lower part comprises:
• two reinforced concrete plates, 21x57
metres and 50 cm high;
• supporting columns, usually metal, arranged on a 6x6-metre mesh;
• isolation devices of the friction pendulum type, placed above the columns.
The upper part included a three-storey building prototype to be implemented later, over the
upper isolated deck, with a settlement capacity of 70 persons minimum.
Plates and uprights. The design and construction of the two parts, although meant to be a
single unit, was conceived in two independent stages. In fact, a process had to be defined that
would resolve site needs and schedules as quickly as possible.
The lower structures consisted of a
repetitive prototype that was easy to make and
to be used for all buildings. The working
plans were therefore designed to assess
isolation system characteristics using
performance criteria, within the parameters
established for the building above. This
allowed work to start immediately.
Conversely, a preliminary draft was drawn
up for the upper part, based on tenders for
design and construction of the buildings,
which were to fall within parameters
compatible with the isolation system
designed. In this way it was possible to assign
projects and work to numerous qualified
contractors via tenders decided while the lower parts were under construction.
The strategy was compatible with the different characteristics of the construction
companies to be involved quickly and in large numbers. The prototype defined by the
preliminary draft was not shown as an example of an architectural solution, but as a detailed
reference to be interpreted according to performance logic, leaving maximum freedom for
configuration, choice of materials and construction systems.
The size of the concrete plates and the positions of the uprights were established using a
preliminary calculation that took into account distribution factors.
40
A load of approximately 1 MN was calculated for each column. Then a more detailed
sizing was performed, compatible with the idea of different types of building construction
(steel, wood, reinforced concrete, prefabrication) that contractors might present. Similarly, the
building oscillation period was estimated by basic formulas (T≈CS×H0.75, where H is the
height of the prototype building and Cs0.0500.085).
The three figures illustrate the flexural stress distribution on the top plate produced by local
lifting needed for any replacement or installation of a device (in some cases, to save time, the
devices were installed after construction of the plate).
The isolation devices. The choice fell to FPS  Friction Pendulum System34  devices,
which function allowing relative movement by sliding of an articulated element along the
surfaces of steel spherical caps. Over 7300 devices were installed. The contract allowed for
use of other types of isolators, like rubber-steel. The choice of devices was left to the
contractors and was influenced mainly by the tight turnaround for complying with work
schedules.
The figure shows one of the F-d (force vs displacement) diagrams obtained from laboratory
tests performed on unified radius isolators of R=4 m. The final design of the entire isolated
system was thus performed by assuming horizontal response F force corresponding to the
expression F=(mg)+(mg/R)d, where mg represents the weight of the building; =3% the
friction coefficient; d set displacement. Secant stiffness Keff=14.6 kN/mm corresponding to a
displacement of up to 0.20 m was evaluated as indicated in the chart. Result: isolated period
34
D.M. Fenz, M.C. Constantinou, Behaviour of The Double Concave Friction Pendulum Bearing, Earthquake
Engng Struct. Dyn. June 2006.
41
T=3.29 seconds; damping =20.1%. Seismic tests in the non-linear field were performed
using spectrum-compatible accelerograms with standard spectra derived from records of
known real events (L'Aquila 2009; Imperial Valley 1979; Loma Prieta 1989; Northridge
1994; Kobe 1995; Taiwan 1999).
Housing construction. The figures illustrate some of the building stages. An interesting
solution, adopted in some cases for plant, which was encased in a metal frame suspended
below the isolated top plate. The solution avoids the need for flexible connections.
42
43
44
Environmental sustainability and energy
consumption. A significant aspect, inherent to how
these issues were addressed, should be explored but
is not part of the scope of this presentation, which
focuses on BI.
Conclusion. The entire operation was certainly
costly: a total of 705˙000˙000 €, including
furnishings, utilities, installation of gardens and
initial maintenance, broken down as follows:
• anti-seismic foundations
160
• buildings
424
• furnishings and plant
56
• lifts
10
• urban planning and gardens
55
On the other hand, the cost of maintaining 15˙000 people for a longer duration would be
equally high: an estimated extra 1˙500˙000 € per day for those without a home.
In February 2010, 10 month, after the earthquake, almost 4˙500 fully-furnished dwellings
were completed and delivered. The Emergency Management Service had finished its
commitment and handed over to local authorities. But that is a very different story.
----------------------------------------------------------------------------------------------------------------BEFORE CALCULATIONS WERE USED, DESIGN IDEAS WERE REQUIRED
In conclusion, we take a step back: Lisbon earthquake,
1755, a major historical event that was the first occasion
when attempts were made to define a seismic technique with
some engineering content. The idea developed of a gajola
house, also known as a gajola pombalina from the name of
the Marquis of Pombal, who wisely and skilfully managed
reconstruction. The house was based on a structural system
comprising a wooden frame of cross-resistant elements, filled
with inert material. The filling was of mud brick or even
compacted earth. No calculations were performed, but the system was interesting because it
had three advantages First, it was resistant: the wood structure with cross elements was
45
carefully prepared to resist horizontal actions. Second, it was uniform: the structure was
compact and well distributed throughout its height. Third, the capacity to dissipate energy
due to deformation of the inert material. In the most severe seismic attacks this material,
preferably confined to the wooden structure, would play a useful role in dissipation. Perhaps
knowledge was not ripe for considering this important synergistic contribution , but in reality
this did not reduce its efficacy.
It is interesting to note that this building system is still used in Portugal, where it was
invented, to perform structural interventions in existing buildings. A similar construction
technique was also used in Italy during the Bourbon period, after the Calabria Ulteriore
(Southern Italy) earthquake in
1873. There are a few but
insignificant relics. The concept
behind this system is nonetheless
proved by the work of the
architect Ferraresi, published in
works of the time.35 In some
sketches, a wooden double frame
enabled space to be filled by
compacting material inside.
It is interesting to note that in several Latin American countries there are relics (not to be
forgotten) of buildings of historical interest made of wooden frames filled with “tierra
pisada.”
-----------------------------------------------------------------------------------------------------------------
35
Giovanni Vivenzio: Istoria e teoria de’ tremuoti in generale ed in particolare di quelli della Calabria, e di
Messina del 1783, Stamperia Regale, Napoli, 1783.
46
INDEX
FOREWORD
PART 1 -
ARCHITECTURE IN SEISMIC DESIGN
The design idea
Old configuration paradigms
Conventional content of design procedures
Uncertainties in design procedures
Development and legacy of seismic engineering
Performance requirements
An overview of Italian legislation
Resistance and deformability concepts
Past experience
Examples of traditional anti-seismic architecture
PART 2 -
THE BASE ISOLATION
Base isolation
Theoretical analysis overview
Base isolation performance
Base isolation and architectural design
PART 3 -
SOME APPLICATION OF BASE ISOLATION IN ITALY
BEFORE L'AQUILA EARTHQUAKE 2009
The new Emergency Management Centre for central Italy
(A) Operations room building
(B) Cultural heritage warehouse
(C) Other buildings in the complex
The new Jovine school
Masonry buildings
PART 4 -
THE ITALIAN EXPERIENCE ON BASE ISOLATION
AFTER L'AQUILA EARTHQUAKE 2009 - THE C.A.S.E. PROJECT
The L'Aquila earthquake
The project C.A.S.E.
Before calculations were used design ideas were required
47

IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO

Transcription

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