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Smoke Control for High Rise Buildings
By: Giovanni Cosma, Jensen Hughes S.r.l, Italy. Luciano Nigro, Jensen Hughes S.r.l, Italy
Staircases usually cover a key role within a building as they create a connection among several floors interlinking different spaces and occupants which can be used on a daily basis. In the event of an emergency, such as the outbreak of a fire, the staircase acquires a more important role since there’s the need for the occupants to leave the building safely.
It is therefore important to design new buildings in such way that stairwell protection becomes a primary strategy with the intent to provide acceptable and tenable conditions long enough for occupants to evacuate in the event of an emergency. At the same time, staircase protection becomes fundamental, along with other measures, to allow firefighters to safely enter the building and establish a line of a successful firefighting operation.
Designers, architects, and engineers should therefore understand the fundamental role of the staircase in the event of an emergency.
In residential buildings, hotels, or hospital, the escape times might be prolongated (sleeping occupants or with low levels of physical and/or mental abilities) and thus it is essential that escape routes must be kept available and smoke-free for an extended period of time . A combination of passive and active means, such as fire walls, fire doors, and smoke control systems can be the key elements to protect the vertical escape routes.
For many decades, fire incident reports showed us that fires in flats rarely spread beyond the flat on fire , . However, it has been proved that smoke may enter into corridors when the residents leave the flat on fire, or firefighters enter the flat to extinguish the fire. That’s one of the reasons why “stay put” strategies were developed. When a “stay put” strategy is adopted, the risk of being affected by smoke reduces, as entering unnecessarily in a smoky corridor may result in being overcome by the toxicants and combustion products of the fire. Staying put also means firefighters can tackle the fire safely and quickly without being delayed by many residents evacuating down the stairways. However, for certain buildings is not always possible to implement a defend-in-place strategy. Hence, maintaining vertical escape free of smoke enables occupants to leave safely, also on a later stage of the fire, whilst enabling firefighters to reach upper levels in case of total evacuation. As written within the UK national high-rise firefighting guidance  “Incident Commanders should understand when a partial or full evacuation strategy might become necessary in a residential building where a “Stay Put” policy is normally in place” meaning that in multi-storey residential building, as circumstances and incidents may change in a dynamic way, there could be a chance where evacuees and firefighters are using vertical escapes at the same time. It is therefore vital to guarantee tenable conditions for prolongated periods within the vertical escape routes.
To assist means of escape and firefighter access in high-rise buildings it is common practice to provide some form of smoke control to the stairwells or adjacent spaces, which could take the form of mechanical schemes or provisions of natural ventilation . When natural ventilation is used for tall buildings, special attention should be given to the design of the system, as the smoke-driven flow may be affected and altered by loss of buoyancy, piston effect, stack effect, or wind pressures. Natural smoke venting is usually not recommended in fire-fighting shafts serving floors above 30 m above ground level .
Several international fire safety standards provide some common guidance on different types of smoke control systems that can be implemented to protect vertical escape paths within high rise buildings. These solutions may include:
Stair pressurization – Most building codes require high-rise buildings to be provided with fire-rated enclosures for final exit stairs provided with pressurization. Mechanical stair pressurization is the most commonly used approach to meet this requirement for high-rise buildings. For example, the IBC code, which is primarily adopted in the United States of America, specify for tall buildings, which needs to be equipped throughout with an automatic sprinkler system, a mechanical means of smoke control applied by utilizing outside air to pressurize the stair and create a pressure difference between the stair and the occupied space. Similarly, the National Construction Code (NCC) of Australia, which incorporates all minimum necessary requirements for the design and construction of buildings, requires, for fire-isolated stairs serving any storey 25 meters above ground, an automatic stair pressurization system. The primary effect of the pressurization system is to create a minimum air velocity through openings (i.e. doors) to minimize the spread of smoke into the stairs doors. A Similar approach is followed within the UK and Germany [6,], .
Egress stairs in high-rise or tall buildings are typically pressurized to prevent smoke leakage through construction cracks and doors opened to provide access. Stair pressurization systems typically require pressures in the range of 25-50 Pa, depending on the applicable local national code.
Zoned smoke control system – This type of system divides the building into separate smoke control zones and creates pressure differentials to minimize the smoke spread. Mechanical ventilation is normally provided in the affected areas to exhaust smoke and a positive pressurization system is normally provided for the other contiguous zones. In a high-rise building, zones may consist of entire floors, but sometimes floors are subdivided into multiple zones. An example is the pressure "sandwich" effect in which the fire floor is exhausted, and the floors above and below are pressurized.
Smoke Clearance– Post-fire smoke removal systems are intended to facilitate smoke removal after fire-fighting intervention. This can be achieved via operable windows/panels, or via mechanical ventilation capable of providing a prescribed number of air changes per hour. It must be stressed that this type of system is not meant to maintain smoke-free areas.
Atrium smoke control systems – Modern buildings may contain one or more atria or large-volume spaces connecting multiple floors which will require means of smoke venting. This would typically consist of a system designed to establish a stable smoke layer to enable safe escape of the occupants or to dilute the smoke in order to maintain tenable conditions. Mechanical or natural smoke exhaust systems will extract smoke from the top of the atrium with low-level, low velocity make-up air at the bottom of the atrium for a specified time period. Some international building codes allow a combination of all the above smoke ventilation systems (de-pressurization, clearance or temperature control systems) to be installed within such big volumes.
There is agreement that properly implemented stairwell protection systems can provide tenable conditions for evacuating occupants in tall buildings. A variety of different methods are currently in use worldwide ranging from corridors depressurization systems to depressurization of entire smoke zones. As these systems are now being fine-tuned and improved over the years, in the same manner as sprinkler systems or building energy systems have, there is very little agreement and different opinions as to which method is to be used for specific building types. It is therefore down to the fire safety engineer to analyse and draw the conclusions, in coordination with all the stakeholders and AHJs, how to design a system aimed to limit smoke movement to the fire origin zone and guarantee a safe escape of the occupants.
The increasing building heights combined with reduced number of vertical escape routes, are nowadays creating new challenges for fire safety engineers. A smoke control system, if properly designed, reduces the migration of smoke allowing occupants to safely exit the building, even on a later stage of the fire. In the event of a fire, the number of stairwells within a building, if unprotected, might become an irrelevant number, as smoke-logged escape routes will become not usable, causing delays on the required occupants escape time and increasing the efforts that the attending fire brigade needs to put in place.
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