Engineered Systems - January 2009 - (Page 76) With advanced strategies and software, total evacuation need not be the last word(s) in life safety strategy. The size and role of elevator shafts, selective pressurization, and the ability to evaluate multiple reactions quickly may combine to form a safer and swifter reaction to the threat of flames and smoke. BY W. Z. BLACK, P.E. T hree 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. 76 En gi neer ed Sy stem s January 2009
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