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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

Pressurization system

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

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

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.

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.

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.

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.

Finally wind will result in ever changing conditions in and outside the building.

Solutions

1

2

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

Absolute pressure

cold warm

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co

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Stair case

neutral

pressure levels

Poutside

and

Pstairs

all windows open

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Absolute pressure

warm

Pinside

Poutside

and

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neutral

pressure level

pressure difference across stair

door

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Stair case

all windows closed

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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

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

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.

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.

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 air

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

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.

Control system

Conclusions

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.

2013

/

Naam (20pt)

Titel (36pt)

air supply

air supply

air

release

fan

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reference line

/

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.

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.

Acknowledgement ABG

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Scheiding 12 122

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Reference

ASF air supply fan

ARF air release fan

s shunt

RC-## fire compartment

Pressure differential gauges P