Floor Pressurization As A Means Of Controlling Smoke During A High-Rise Fire
Three out of four deaths resulting from structural fires are from smoke inhalation. Consequently, being able to predict and control the paths that smoke will travel throughout a building is of vital importance. The goal of maintaining smoke-free areas is particularly important in high-rise buildings due to the world-wide trend in construction of super-tall buildings. With extended escape routes and the added challenges of extinguishing a fire in a tall building, a well-planned smoke management scheme is essential to a successful pre-fire life safety plan.
COSMO (COtrol of SMOke in buildings), a smoke control program, is used to predict where smoke will travel during a fire in a high-rise building. The results are then applied to the design of a smoke-control scheme that can manage the smoke and keep it away from occupants throughout the fire.
The smoke control concept is based on pressurizing floor spaces with AHUs that are capable of providing smoke-free spaces so that people can stay inside a high-rise building and let sprinklers and firefighting activities extinguish the fire.
Program results show that the success of such a smoke control plan depends on the tightness of the building construction and the capacity of the air pressurization equipment. Volume flow rates required from the AHUs are modest and can be generated by existing A/C equipment if the building construction is relatively tight and if the elevator shaft design is modified to reduce flow resistance to the outside of the building. Applying a well-designed and well-conceived floor pressurization plan can greatly improve the chances of surviving a fire in a high-rise building.
Smoke that is generated in a fire will follow predictable paths throughout a building. At the source of the fire, the local pressure inside the fire compartment will increase and the generated smoke will be forced under pressure to move away from the area where burning takes place. At the same time, smoke will rise due to the fact that the heated gases are lighter than the cooler surrounding air.
As a result, buoyant smoke will try to move upward in the building as it seeks the path of least resistance to the top of the structure. Smoke is also pulled to upper regions in the building by decreasing atmospheric pressure encountered by the smoke as it rises. While the decrease in atmospheric pressure may be negligible in short buildings, the change in outdoor pressure is quite large in tall buildings, and it plays a sizeable role in drawing the smoke into upper floors. The smoke tends to favor large unobstructed openings that extend all the way to the top of the building. Therefore, two of the most logical paths for it are stairwells and elevator shafts.
As long as stairwell doors remain closed, the elevator shaft presents a more desirable route to the top of the building, in part because it is open to the exterior via a pressure relief vent at the top of the shaft. Elevator shafts also become the preferred path for smoke, because they have a large cross-section and no internal impediments to restrict the flow of gases.
On the other hand, in well-designed buildings, the stairwells are airtight, and they have no vent at the top of the shaft. They are designed to prevent smoke from entering at all floors so that occupants can use them as a means of safe egress to the outside. For these reasons, elevator shafts often carry the bulk of smoke during a fire as it travels upward. It is only logical, then, to warn occupants to avoid using the elevators and use only the stairwells in the event of a fire. Warnings to avoid using an elevator have been widely distributed, and most people realize that they should not use an elevator during a fire. Avoiding the use of elevators as a means of escaping a fire has become part of our fire-behavior culture. Given the present fire-escape philosophy, it seems logical to maintain stairwells free of smoke and encourage the smoke to follow the path that it prefers, which is through the elevator shafts.
An additional method of further improving life safety and one that is particularly well-suited when fires occur in a high-rise structure is to pressurize the floors so that occupants have a safe environment to survive a fire that exists on lower floors.
Evolving PhilosopyIn super-tall buildings that often exceed 100 floors, fire-escape plans that were conceived for much shorter buildings need to be carefully examined. When occupants are expected to escape a high-rise fire down only the stairwells and not use elevators, there may be safer ways to protect the lives of the residents in the highrise.
One way would be to pressurize the floor spaces throughout the building to prevent smoke from entering the occupied areas. This concept is similar to pressurizing fire-escape stairwells so that they remain smoke free and provide a route of escape from the building. If a floor-pressurization plan can be successfully achieved, then people can stay in place and wait for sprinklers and firefighters to extinguish the fire, thereby avoiding the long descent to the ground through stairwells that can often be filled with smoke.
COSMO has been written so that different fire safety schemes can be evaluated with the goal of determining which is the most feasible for improving fire safety during a high-rise fire.
The program is based on fundamental principles of physics, and it considers all the interactions that influence the movement and control of smoke as it travels throughout a building. The program can be used to identify those factors that strongly affect the smoke movement, and just as importantly, it isolates those issues that have little or no influence on smoke travel. By parametrically considering all factors, the program can be used to design a life-safety plan for a highrise. These factors include building construction, weather conditions, fire characteristics, and the capacity of the AHUs that are used to pressurize the building to provide safe smoke-free areas inside the building.
Program ResultsThe volume flow rate predicted by the COSMO program to keep the smoke from entering all of the floor spaces is rather modest (1 to 3 ach in a typical high-rise building) and is within the capability of AHUs that are used for normal comfort control in commercial buildings.
However, the successful application of floor-pressurization methods is naturally a strong function of the type of building and its construction. A building that is relatively tight and has well-sealed ductwork, pipe chases, and well-designed fire dampers at all fire-rated walls and floor penetrations is a good candidate for a floor-pressurization fire-safety plan. Pressurizing floors that have leaky walls and many construction openings with unsealed ducts and pipes chases will not be a reasonable option to keep smoke from invading the floors, particularly those near the upper portion of the high-rise.
The program results have also shown that the size of the elevator vent at the top of the elevator shaft is one of the most important factors affecting the distribution of smoke throughout a high-rise structure. If the vent is designed to meet the minimum code size limitation, smoke will back up inside the elevator shaft, and large quantities of smoke will escape to the upper floors above the neutral pressure plane. By simply increasing the elevator vent area, the neutral pressure plane can be raised significantly, and more areas in the building will be kept smoke-free.
Increasing the elevator vent size need not conflict with energy conservation. The elevator vent can be designed to remain relatively small in non-fire conditions so that it effectively relieves the pressure pulse causes by moving elevator cars. In the event of a fire, a motorized damper could open and increase the vent area to the exterior so that it will offer less restriction to the smoke carried by the elevator shafts.
COSMO results have shown that the tightness of the elevator shaft is somewhat less important compared to the influence of the building construction. Smaller gaps around the elevator doors and more airtight shaft construction tend to make a floor-pressurization plan more successful and tend to raise the neutral-pressure plane in the building, although the effect on the neutral-pressure plane is rather small. While the tightness of the elevator doors and the construction of the elevator shaft will influence the smoke movement into the shaft, the type of building construction is more influential on the smoke travel than the construction of the elevator shaft.
If elevator doors open at the fire floor or burn open as a result of the fire, smoke can enter the shaft and travel toward the upper floors. The increase in smoke volume carried by the shaft places a greater danger to occupants on the upper floors in the high-rise. With an increase in smoke in the shaft, the volume-flow rate of floor pressurization equipment must increase in order to maintain smoke-free conditions on the upper floors.
When elevator doors open at the fire floor, the program has shown the capacity of the air-handling equipment must be increased by a factor of two to three to overcome the added volume of smoke in the shaft. Even if elevator doors at the fire floor open for any reason, the capacity of the floor pressurization equipment is still reasonable, and the smoke can still be forced to remain in the shaft and out of the upper floors.
Looking UpThe program has also pointed in the direction of further improvements in a fire safety plan that will reduce the number of AHUs that are used to pressurize the floors and also reduce the capacity of those units so that smaller fans could be used. A selective or intelligent floor-pressurization plan has been identified that calls for pumping air into only the upper floors while leaving lower floors unpressurized. By selectively pressurizing only the upper floors while leaving the lower floors unpressurized, the number of fans can be cut in half, and the capacity of the equipment can be downsized by about 10% while maintaining smoke-free conditions on all floors throughout the building.
The disadvantage of this type of smoke-control plan is the added complexity of a logical control system that must sense the location of the fire by monitoring the flow of water through the sprinkler system or by receiving input from fire department personnel. The control system must then decide which pressurization equipment would be used and which units would be left unenergized. Implementation of an intelligent floor pressurization scheme could lead to smoke-free conditions on upper floors so that occupants in the upper portion of the building would be able to retreat to upper floors and avoid having to descend many flights of stairs and possibly have to come into close contact with a fire and smoky conditions on lower floors.
Quantifying the effect of fire conditions, building construction, and weather conditions on the movement of smoke is a very complex science. Smoke is driven by extremely small pressure differences and very large temperature differences. Any tool designed to quantify the effects of all of the important factors that drive smoke throughout a building is only as valid as the assumptions that go into the model. A well-formulated computer model is particularly suited for such a complex task, because it can examine the trends caused by numerous changes that may be anticipated during a fire.
By closely examining the trends predicted by the program results, a clearer picture of how and where smoke will travel in the event of a fire will begin to emerge. Then by fitting together all of the trends predicted by the program, a fire safety plan will emerge which will have a reasonable probability of success when a fire breaks out in the building. The true value of a smoke control software program is its ability to assess trends in smoke movement, and to devise a fire safety plan that will be successful for a specific building design and a given fire scenario. ES