Stair pressurisation

Stair pressurisation is a smoke control method in which fans blow outside air into a stair enclosure, so that the air pressure in the stair is slightly higher than on the floors around it. Air then leaks out of the stair through the gaps around closed doors, and this outward flow stops smoke from leaking in. The main goal is a smoke-free escape route; a second goal is a smoke-free staging area for firefighters. It is a positive pressure system, and building codes in the United States require it in high-rise buildings.

The system has to work against the natural forces that move smoke through a building, above all the stack effect in tall shafts. A pressure difference that is too small lets smoke in, while one that is too large makes the doors too heavy to open. For closed doors, minimum design values of about 5 to 25 Pa are suggested, depending on the fire and on sprinklers. In the United States, the Life Safety Code limits the force needed to open an escape door to 133 N. Open doors are the weak point. When many people leave a building at once, many stair doors stand open and the pressure in the stair collapses, as studies of the World Trade Center towers showed.

Purpose and principle

Smoke is a major threat in building fires, and it often travels far from the fire itself. Stairs and lift shafts frequently fill with smoke, which blocks or slows evacuation. In the late 1960s the idea of pressurising stairs to keep smoke out began to attract attention. The term smoke control was later coined for systems that use pressure produced by mechanical fans to limit smoke movement. A glossary of the US Nuclear Regulatory Commission describes stair pressurisation as increasing the air pressure in stair wells, usually with fan systems, to provide a refuge area from fire and smoke. The book Introduction to Fire Safety Engineering likewise notes that a smoke management system can keep an area free of smoke by overpressure.

Smoke control rests on two principles. First, airflow by itself can control smoke if its average velocity is high enough. Second, a pressure difference across a barrier, such as a wall with a door, can stop smoke from crossing it. Pressurisation creates small airflows through the cracks around closed doors and through gaps in the construction, and these flows keep smoke out of the high-pressure side.

A fan supplies outside air to the stair. The higher pressure in the stair drives air out through the gaps around the closed doors, which keeps smoke on the fire floor out of the stair. Based on Klote (2002), pp. 4-278 and 4-286.

For a closed door the velocity in the gaps is hard to determine, so the pressure difference is the quantity used in design. For an open door the air velocity is the relevant quantity. A pressurised stair therefore has two tasks: keep a positive pressure across each closed stair door and, ideally, push enough air through the door on the fire floor when it is opened.

The World Trade Center investigation describes stair pressurisation as a positive pressurisation system that limits smoke spread into the stairways of buildings such as high-rise buildings. Its aim is to keep the primary escape routes of the building clear of smoke. Positive pressure systems also have drawbacks. Too much pressure, or unbalanced pressure, may push smoke from the fire zone into other parts of the building, and such systems do not remove smoke from the building. A pressurised stair is a special form of protected escape route: the walls and doors form the barrier, and the fans make that barrier work even when it leaks.

Forces that the system must overcome

The main forces that drive smoke through a building are the stack effect, the buoyancy of hot smoke, expansion, wind and the ventilation system. The stack effect is the vertical movement of a gas caused by a difference in density, as in a chimney. In cold weather the warm air inside a building rises through shafts such as stairs and lift shafts. Klote gives the example of a building 60 m tall with the neutral plane at mid-height. With −18 °C outside and 21 °C inside, the stack effect alone gives a pressure difference of up to 55 Pa between a shaft and the outside.

Hot smoke adds its own buoyancy. In a fire room at 800 °C, the pressure difference 1.52 m above the neutral plane is about 13 Pa. In a series of full-scale fire tests, the largest pressure difference measured across the wall of the burn room was 16 Pa, at the ceiling. Wind can produce much larger pressures on the outside of a building. A wind of 15.6 m/s with a pressure coefficient of 0.8 gives about 117 Pa. Its effect inside a tight building with closed windows is small, but it becomes important once a window in the fire room breaks.

In field tests of smoke control systems, pressure differences across closed doors often fluctuated by as much as 5 Pa, probably because of the wind. A stair pressurisation system must therefore produce a pressure difference large enough not to be overcome by these fluctuations, by the stack effect, by smoke buoyancy or by the wind. Weather matters too. A smoke control system has no thermal time lag, so it meets the full stack effect of the moment it starts. If the outside temperature is below the winter temperature used in design, stack effect problems may result.

Airflow through door gaps

The amount of air a fan must supply follows from the leakage of the stair. Air flows from the stair through every gap, and the flow through each gap grows with the square root of the pressure difference across it. Klote takes a flow exponent of 0.5 as reasonable for all flow paths except extremely narrow cracks. With a flow coefficient of 0.65 and an air density of 1.2 kg/m³, the volume flow through a gap is:

$$\dot{V}=K_{f}A\sqrt{\mathrm{\Delta }P}$$

Symbol	Description	Unit
$\dot{V}$	volume flow of air through the gap	m³/s
$K_f$	coefficient, 0.839 for a flow coefficient of 0.65 and an air density of 1.2 kg/m³	m/(s·Pa0.5)
$A$	flow area of the gap (leakage area)	m²
$\mathrm{\Delta }P$	pressure difference across the gap	Pa

The flow coefficient depends on the shape of the gap and is generally between 0.6 and 0.7. A closed door with a crack area of 0.01 m² leaks about 0.013 m³/s at 2.5 Pa and 0.073 m³/s at 75 Pa. Leakage areas depend mainly on workmanship, for example on how well a door is fitted. For the walls of an average stairwell, Klote gives a leakage area of 0.11 × 10⁻³ times the wall area, excluding the gaps around doors. For an open stair door, a flow area of half the door area is recommended, because the flow pattern through a real doorway is complex. In New Zealand, C/VM2 (second edition, 2025) likewise treats an open egress door as half its width for smoke flow calculations.

Air leakage through the gap of a closed door against the pressure difference, for gap areas of 0.01 m² and 0.024 m². Based on Klote (2002), pp. 4-280 and 4-288.

A worked example uses the stair of Klote's design example: 20 floors, each with a leakage area of 0.030 m² between stair and floor, made up of the door gap and the stair wall. The pressure difference is taken as 20 Pa, the bottom value of that example. One floor then leaks 0.839 × 0.030 × √20 = 0.11 m³/s, and 20 floors leak about 2.3 m³/s.

This simple estimate assumes the same pressure difference on every floor. Klote's full analysis includes the stack effect for an outside temperature of −10 °C and a stair at 21 °C. The pressure difference then rises from 20 Pa at the bottom of the stair to 106 Pa at the top, and the fan must supply 3.9 m³/s. Hand calculations of this kind assume no vertical leakage between floors. For vertical leakage and open stair doors, computer analysis is recommended.

Pressure difference between a pressurised 20-storey stair and the building, with −10 °C outside and 21 °C in the stair. Based on Klote (2002), p. 4-288.

The result is sensitive to leakage. If the leakage area per floor in the same example were 0.050 m² instead of 0.030 m², the required supply would be 6.5 m³/s. In practice these areas are difficult to estimate and even more difficult to control. A fan with an adjustable drive is one way to match the supply to the real leakage of the finished building. Network computer programs such as CONTAM model each stair as a vertical series of spaces, one per floor, and can include wind, stack effect and open doors.

Design criteria

Klote names five groups of design parameters for smoke control: leakage areas, weather data, pressure differences, airflow and the number of open doors. The minimum pressure difference follows from the criterion that no smoke should leak in during the evacuation. To withstand an intense fire next to a door, a minimum of 20 to 25 Pa is suggested, slightly above the 16 Pa measured in fire tests. For hot smoke from a remote fire, 15 to 20 Pa is suggested. In a sprinklered building the smoke is cooled, and 5 to 10 Pa is suggested to allow for pressure fluctuations.

The maximum pressure difference is set by the force needed to open a door. A person's physical condition largely determines what force is reasonable, so the limit is hard to define. The US Life Safety Code (NFPA 101), as cited by Klote, limits the force to open any door in a means of escape to 133 N. The total opening force is the force of the door closer plus the force needed to overcome the pressure difference. The closer alone usually needs more than 13 N and sometimes as much as 90 N. For a door 2.13 m high and 0.91 m wide, a pressure difference of 75 Pa with a closer force of 53 N gives a total of 133 N.

Where stair doors are fire doors with self-closers, C/VM2 (New Zealand, second edition, 2025) assumes them to be closed unless occupants are using them. During egress it treats each door as open for three seconds per person, or for the whole queuing time when people queue. In Australia, the 2020 data sheets for the Fire Safety Verification Method note that pressurisation can push opening forces above the permitted limits and can stop fire and smoke doors from closing automatically.

Open doors need an air velocity criterion. Klote estimates about 4 m/s to stop smoke from an intense fire of 2.4 MW and 1.5 m/s for a fire of 125 kW. Where sprinklers cool the smoke, 0.25 to 1.25 m/s may be used.

In Australia, the deemed-to-satisfy provisions of the National Construction Code reference AS 1668.1. According to the 2020 data sheets, this standard requires an airflow of at least 1 m/s through each open doorway to the fire floor. That applies when all doors to that floor are open together with the main discharge doors of all fire-isolated exits. For a door of 0.91 m by 2.13 m, 1 m/s means 1.94 m³/s through a single doorway. That is about half the supply in Klote's whole closed-door example. Because such flows are large, pressure relief is needed, so that excessive pressures do not make doors hard to open or stop them from closing.

How many doors will be open at the same time depends on the use of the building. In a densely occupied building it is very likely that all doors will be opened during evacuation. With a staged evacuation plan, a refuge area concept or sparse occupation, only a few doors may be open at once. Designing for all doors open makes sure the system always works, but it probably adds to the cost.

System types

Stair pressurisation systems are divided into single and multiple injection systems. A single injection system supplies air at one point, most often at the top of the stair. If smoke reaches the fan intake, the system can blow smoke into the stair it is meant to protect. Automatic shutdown should therefore be considered for that event. In tall stairs, single injection can fail when a few doors near the injection point are open, because all the supply air escapes through them. This failure is especially likely with injection at the bottom while the ground floor door is open.

Multiple injection systems supply air at several points over the height of the stair, with the fan at ground level or on the roof. The supply duct can run in a separate shaft or inside the stair itself. In that case the duct must not obstruct the evacuation. An alternative is to divide a tall stair into compartments of a few floors, each with its own injection point. The benefit is lost when the doors between compartments are open. Compartmentation is therefore unsuitable for densely occupied buildings where everyone leaves by the stair at once, but it can work with a staged evacuation.

Three ways to supply air to a tall stair: single injection at the top, multiple injection through a duct, and a stair divided into compartments. Based on Klote (2002), pp. 4-286 and 4-287.

Stair pressurisation is often combined with other measures. Lift shafts can be pressurised in the same way, and the same analysis applies with the lift shaft in place of the stair. A protected lift shaft matters for the fire service, because firefighters use lifts with special keys for rescue and to bring equipment to the fire floor. This is the role of a firefighting lift. A building with only stair pressurisation can still suffer smoke spread through floors and other shafts. Zone smoke control, in which the fire floor is exhausted and other floors are supplied with air, is meant to limit that spread.

Reliability and testing

A smoke control system must be tested after installation, because many problems have been found in practice. Klote mentions fans running backwards, fans without electrical power and controls that did not work. The first test is functional and checks each component. The second checks the performance of the system as a whole, usually by measuring pressure differences across doors and, where needed, air velocities through open doors. Chemical smoke from smoke candles is not recommended for performance tests, because it lacks the buoyancy of real hot smoke. Pressure difference testing is therefore the most practical performance test.

The International Fire Engineering Guidelines (2005) state that for smoke control, greater complexity means lower reliability. The Australian data sheets of 2020 summarise studies of stair pressurisation in service. Audits of Australian buildings found that the fans failed to start on alarm in 10 % of office buildings and 8 % of retail premises. A detailed study by Fazio treated door forces above 110 N or air velocities below 1 m/s as failure. On that basis the effectiveness for airflow was between 30 % and 63 % and that for door forces only 8 % to 10 %. The data sheets note that smoke may still be held back at lower velocities, and that some people can open doors with forces above 110 N.

Fazio attributed these low values to systems that were never commissioned, were not maintained, or had faults that were not repaired. The data sheets adopt a typical reliability of 50 % for Australian fire-isolated exit pressurisation systems, with a low value of 30 % and a high value of 60 %. For comparison, they give 70 % for lift shaft pressurisation. They add that most of the data predate the current design and servicing standards, so newer systems may perform better.

Limits in tall buildings and mass evacuation

The World Trade Center towers in New York illustrate the limits of the method. Between 1976 and 1979 the Port Authority tested a prototype system in one stair of WTC 1, the north tower. The criteria of New York City Local Law 5 were 12.5 Pa across closed stair doors with three doors open, 25 Pa with all doors closed, and a maximum door opening force of 156 N. With three doors open the pressure differences were generally low, and with all doors closed the opening forces approached the limit. Sprinklered existing buildings were later exempted, and the towers were never fitted with stair pressurisation.

After the bombing of 1993, many stair doors were held open by people leaving the building. This increased the airflow up the stairs caused by the stack effect, and much smoke spread through the stairs. The stairs met the code requirements for exit capacity, but they could not take all occupants quickly, so the doors stayed open.

A later computer study found stair pressurisation feasible with the doors closed. It also found that relief venting was needed at the top of each stair section to keep door forces acceptable under stack effect. With several doors open a slightly positive pressure could be kept, but not with all doors open, as in a mass evacuation. A review committee concluded that stair pressurisation would not work in a mass evacuation with more than three stair doors open at once. It also found that the New York code lacked rules for periodic testing and maintenance.

For the events of 11 September 2001, the investigation concluded that stair pressurisation would have been ineffective. The stairs ran through 110 storeys and changed position through transfer corridors, and the full evacuation of the undamaged floors would have lowered the stair pressure below the required value. In the impact zone the stairs were damaged and smoke entered them directly. Stair pressurisation is designed to keep smoke out of a stair, and its supply air would have been ineffective in diluting such large amounts of smoke. At the time, the main US building codes required stair pressurisation in new high-rise buildings. Full-scale tests in the 1970s had shown such systems to be effective against unsprinklered test fires.

See also

• Evacuation lift
• Positive pressure ventilation
• Network model

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• Ferreira, M. J., & Strege, S. M. (2005). Smoke management systems (NIST NCSTAR 1-4D). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Klote, J. H. (2002). Smoke control. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-274–4-291). National Fire Protection Association; Society of Fire Protection Engineers.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.