THE DESIGN OF MORE SECURE PRESSURIZATION SYSTEMS IN

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THE DESIGN OF MORE SECURE PRESSURIZATION SYSTEMS IN
STAIR CASES OF HIGHRISE BUILDINGS
Michael Haas, Peter van de Leur and Alexander Winthorst
Introduction
In high rise buildings it is essential that exit staircases remain free of smoke during evacuation. Around the world,
pressurizing systems are often used to keep exit stairs in tall buildings smoke free. The design of pressurization systems
are not easy, because of the stack effect: horizontal air flow in staircases, due to temperature difference between
outside air during cold or warm weather conditions and conditioned air inside the building. This effect has the
potential of overcoming the overpressure in the staircase and allowing smoke entry from the fire floor into the
staircase, or unacceptably high opening forces to enter the stair, possibly preventing a safe evacuation.
DGMR Consulting Engineers was involved in the design and testing of a pressurization system of the staircases of a
highrise office building in The Hague. Limitations imposed due to late introduction in the building process lead to a
design that may offer a solution for the difficulties in the design of pressurized stairs.
One person bedrooms
One person bedroom
Pressurization system
A pressurization system should function in two different settings to prevent smoke to enter a staircase. In the situation
where all stair doors are closed, the system maintains a pressure difference between the staircase and the fire floor.
During the evacuation of a building or during fire fighting proceedings, the door between the fire floor and the
staircase inevitably will be opened. In that case the system should provide an air flow from the staircase to the fire
floor, preventing smoke ingress in the staircase.
A pressurizing system consists of a fan and ductwork for air supply and an air or
smoke release device on every floor of the building. This air exhaust system is
formed by a dedicated shaft or of simple opening in the façade of the building. The
pressurizing system is controlled by a fire detection system and/or a sprinkler alarm
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system.
Pressurization systems are vulnerable to adverse effects of the stack effect and
smoke ingress during evacuation. Often a design minimum pressure differential is
maintained, using the lowest measured pressure differential from the sensors. In cold
weather conditions, modulating the fan based on the lowest value will lead to too
high pressure differences high in the staircase, due to the stack effect.
Another general problem is the response time of the system. When exit doors open
and close, a modulating system is constantly ramping up and ramping down the fan.
This could lead to unacceptable high opening forces to open exit doors.
of the doors. Practical tests on the spot with pressure and leakage measurements were held to assure the shaft doors
were air tight enough. These tests were also held to check that the doors and penetration seals would withstand the high
negative pressure in the shaft.
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Control system
When either the smoke detection system or the sprinkler system is activated on the fire
floor, the shaft door on that floor is released. The fan of the air release shaft is turned
Titel (36pt)
on and starts to accelerate, which leads to increasing pressure difference between the
fire floor and the stairs. At the same time the air supply fan is started which results in a
Naam (20pt)
positive pressure in the staircase. Both the air inlet shaft of the exit stairs and the air
release shaft are supplied with a variable frequency drive (VFD). The VFD of the inlet
shaft adjusts the air inlet to the stairs and therefore the pressure. The positive pressure
in the staircase is set on the pressure difference between the staircase and a reference
line. The staircases are divided into three sections, separated by doors, the reference
line was connected to a floor outside the section. E.g. the lower section ranges from
the ground floor to the 11th floor, the reference line is connected to the 12th floor.
Design challenges
When DGMR got involved in the project, construction work had started, and the
building design did not contain any form of air release for the pressurization system.
Half way through the building process exhaust ducts were added to the building.
Room for these shafts was limited.
ASF
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Analysis with building airflow model CONTAM developed by NIST revealed that the
system would not function without extremely powerful fans in the air release shafts.
The CONTAM model also pointed out that very high negative pressures would occur
in the air release shaft and therefore also on the fire floor. This could easily lead to
unacceptably high door opening forces. The challenge here was to design a system
that would never lead to blocked access doors to the staircases.
The VFD of the air release shaft adjusts the air flow of the air release shaft. Pressure
differential gauges on every floor measure the pressure difference between
occupancies and the staircase on the same level. The fan of the air release shaft
accelerates until, on any floor, the pressure difference exceeds a limiting value of 70
Pa. If this limiting value is reached the fan of the air release shaft is shifted down.
Another design challenge was imposed by the building having openable windows.
This meant the pressurization system should be able to operate in the situation
where all windows of the building were closed and the building is air-tight, as well
as in the situation where one or more windows were opened. The figures underneath
shows the pressure difference gradient between the staircase and the floors in case
of closed windows. Left: closed windows, right with door opened.
reference line
air
release
fan
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Reference
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air supply fan
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air release fan
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shunt
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air supply
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Finally wind will result in ever changing conditions in and outside the building.
Section 3
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fire compartment
Pressure differential gauges
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air supply
Stair case
Stair case
neutral
pressure levels
pressure difference across stair
door
1
neutral
pressure level
warm
Poutside
and
Pstairs
Pinside
2
cold
height
warm
cold
cold
Poutside
and
Pstairs
Pinside
Absolute pressure
Absolute pressure
all windows closed
all windows open
Solutions
In one of the two towers the only solution was to dedicate a small storage room on every floor to the pressurization
system. As mentioned before, because of structural reasons, the holes in the floors could not be larger than 0.5 m2 in
total in both towers. As stated, an analysis with building airflow model CONTAM showed that an extremely high
pressure drop would occur in the 140 meters high shafts, caused by the exhaust air accelerating and decelerating on all
floors. To prevent the necessity of even higher pressures a steel ducted shaft was installed to smoothly guide the air up
the shaft. High standards were imposed regarding the air tightness
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When the stair door on the fire floor is opened, an air flow occurs from the staircase to
the fire floor. The pressure in the staircase drops, both the fan of the air supply shaft
and the fan of the air release shaft accelerate. When the stair door on the fire floor is
closed, a peak pressure will develop that is picked up by the sensors. Both the air
supply fan and the air release fan are ramped down within seconds. This causes a short
period in which a lot of force is needed to open the staircase door. Because both the airSection 2
inlet and the air release are measured and modulated separately, both access to the
stairs and pressure difference is maintained. The principle is shown in the picture
below. The VFD of the air release shaft. Pressure differential gauges on every floor
measure the pressure difference between occupancies and the staircase on the same
level. The fan of the air release shaft accelerates until, on any floor, the pressure
difference exceeds a limiting value of 70 Pa. If this limiting value is reached the fan of
the air release shaft is shifted down. differential
Conclusions
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Scheiding 12 122
Pressurization systems are difficult to design because of the impact of stack effect. For
the new office building of the Ministries of Justice and Interior Affairs the Hague the
challanges to be solved were the addition of air release shafts in a building that was
under construction, the extremely high pressures that were expected in the air release
shafts and the ever changing pressure differences as a result of wind and of open or
closed windows.
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Section 1
A stair pressurization system was designed that will keep the pressure difference across
the stair door below dangerous levels even under these adverse circumstances. The
system set up guarantees an air flow across the door opening on the fire floor, also
when doors on other floors are opened during evacuation.
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Acknowledgement
A team, consisting of building services consultants (Deerns Installation Engineers), contractor (JuBi T), system supplier
(HC-RT) and DGMR Consulting Engineers, worked together to design and build a robust pressurizing system for the stairs
in a building already under construction. The Government Buildings Agency (Rijksgebouwendienst) and the project
supervisor (Brink Groep) were very cooperative and supportive in the process.
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2013
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