An integrated fire safety plan to manage smoke movement during a high-rise fire.
Designing a smoke control system for use in modern buildings is a challenging task. Many interlacing and sometimes conflicting factors influence the movement of smoke throughout the building during a fire. Smoke movement is influenced by minute changes in pressure so that any smoke control plan must be able to anticipate the small changes in the structure's pressure caused by building design, environmental factors, and operation of air-handling equipment in the building. Seemingly small variations in the building design can have profound influence on the movement of toxic gases. Furthermore, tall buildings highlight several fire safety challenges, including long escape paths to the exterior and the large pressure changes that result from the variation in atmospheric pressure between the top and bottom floors of the building.
Smoke movement is largely influenced by the pressure distribution developed within the structure, with smoke moving from regions of high pressure to those with lower pressure. The fire floor typically establishes the maximum pressure within the building. On the other end of the pressure spectrum is the low pressure that naturally occurs in the upper elevations of the atmosphere near the top of the building. The combination of the low atmospheric pressure and the high fire pressure, in addition to the buoyancy forces resulting from the hot smoke, draws the smoke to the upper floors. The floor spaces and stairwells are relatively tight enclosures, but the elevator shafts are frequently open and vented at the top to prevent the piston-effect of the air in the shaft as the elevator car travels upward. Therefore, the elevator shafts are exposed to the low pressure in the atmosphere and they provide a low pressure, low resistance conduit to vent smoke to the exterior. Smoke will naturally find its way to this path of least resistance to the exterior. A prudent life safety scheme should therefore recognize these facts and not attempt to resist the natural movement of smoke to the upper floors via the elevator shafts. By capitalizing on a smoke movement plan that relies on the elevator shafts to vent smoke to the exterior, a smoke management plan will stand a greater chance of improving the smoke quality inside the building.
Using the elevator shafts to vent smoke during a fire is a reasonable plan because it does not reverse the long-standing fire safety culture that has warned people to avoid the use of elevators during a fire. It does not preclude the possibility of pressurizing a few elevator shafts, perhaps the service elevators, to keep them smoke-free. These shafts would then be under fire department control and available to transport fire personnel and equipment to upper floors or be used to transport building occupants who are unable or unwilling to use the stairwells to escape the fire. The selected elevator shaft would have to be modified by sealing all construction openings and installing dampers at the top of the shaft that would automatically close when a fire alarm is activated or when flow of water through the sprinkler system is detected. Dedicated pressurization fans could then be used to maintain a sufficient pressure gradient to keep smoke from invading the selected elevator shafts. It should be recognized, however, that elevator shaft pressurization is a demanding task due to the shaft's naturally low pressure relative to the other areas within the building and due to its normally leaky construction. The remaining unpressurized elevator shafts would then be assigned the task of venting smoke produced on the fire floors and conducting it to the exterior of the building.
Quantifying the pressure distribution within a high-rise building is essential in the development of a smoke movement plan that provides the greatest potential for safe egress from the building. Knowledge of the pressures in the structure is necessary so that equipment used to manage the smoke will have the greatest potential to provide smoke-free environments for the building's occupants. One method of determining the pressure distribution within the building is to apply computer modeling to predict the smoke movement during an anticipated fire scenario. Computer modeling is a particularly attractive way to design smoke control systems because multiple applications of the program using a wide range of input variables can reveal how the trends in building design and the application of pressurization equipment will influence the routes that smoke will take during afire. In that way, fire safety engineers can craft a smoke control system that provides the best opportunity for safe egress from the building. Computer modeling is often the only alternative to economically and extensively studying the influence of building design on smoke movement because full-scale testing of high-rise buildings is often not practical from either a facility or an expense standpoint.
A computer program that has been described in previous papers (Black 2009a, 2009b, 2009c, 2010, 2011) is used to study an integrated plan for moving smoke into desired paths so that occupants have the best opportunity of surviving a fire. The smoke movement plan initially considers the option of pressurizing the fire escape stairwells so that occupants have a smoke-free route to the exterior. Stairwell pressurization is then coupled with floor pressurization to maintain smoke-free air in the habitable floor spaces so that people who are unable to exit can remain in the building, relying on sprinkler systems and fire-fighting activity to extinguish the fire. The plan is then supplemented by exhausting the elevator shafts with the objective of encouraging the smoke produced on the fire floors to travel to the outside via the elevator shafts.
Pressurizing stairwells has been studied extensively and has long been recognized as a method of improving life safety during a fire (Tamura 1983, 1989, 1991, 1992; Tamura and MacDonald 1993; Klote 1980,1988; Shaw and Tamura 1976). The smoke control plan studied here expands on the concept of stairwell pressurization by supplementing it with floor pressurization to create an integrated and more far-reaching smoke control plan. The beneficial effects of stairwell pressurization coupled with floor pressurization complement each other. By pressurizing the floor spaces, the demand on the pressurization equipment in the stairwells is diminished and combined pressurization has a greater chance of maintaining clean air in the stairwells. An added benefit of floor pressurization is the reduction of the force necessary to open stairwell fire escape doors, which is always a concern for those attempting to enter the fire escapes. The proposed integrated pressurization plan can be further enhanced by installing exhaust fans in the elevator shafts to route the smoke into the elevator shafts and vent it to the exterior of the building.
METHOD OF ANALYSIS
The task of designing an integrated smoke control plan for a high-rise structure utilized a program called COSMO (COtrol of SMOke). Details of the mathematical model behind the code have been provided in previous publications (Black 2009b, 2010). The program output compares favorably with CONTAM, which is a frequently used smoke control program developed by NIST (Black 2009b, 2009c; Klote and Milke 2002). COSMO simultaneously solves the conservation equations of mass, momentum, and energy for the distribution of properties throughout the building for a given fire scenario. The conservation equations are written in differential form so that the resulting model is a differential-network model as opposed to a discrete-network model that relies upon the solution of a matrix of algebraic equations. The output of the program is very extensive and includes gas pressures, temperatures, densities, mass flow rates, velocities, and other parameters that help assess the effect of building construction and operation of pressurization equipment on the design of a fire safety system.
The number and magnitude of variables that COSMO is able to evaluate is extensive. To narrow the focus of the smoke management design and to evaluate its potential success for moving the smoke into desired paths, the results that follow are limited to a set of parameters that describe a simple yet typical high-rise building. Therefore, the results displayed here are strictly applicable to the specified building under consideration and for the specified fire scenario. While the trends of the computer output are more significant than the precise quantitative values, the results provided by the program can be used to lead the way toward a reasonable smoke control design. Even though the program can be used to simulate fire conditions in a broad range of building designs, the results shown here should only be interpreted for the specific building under consideration. A fire safety system designed for other buildings with vastly different designs and geometries would require that the program be applied to the conditions of that specific class of buildings.
The specific fire scenario and particular building design considered here involves a 40-story building with a gross floor area of 2000 [m.sup.2] (21,500 [ft.sup.2]) and a floor height of 4 m (13 ft). The structure has four interior fire escapes and eight interior elevator shafts. All elevator doors remain closed, including those at the fire floor, and the program assumes the elevator cabs do not block the flow of smoke. Both the stairs and elevator shafts are constructed from cast concrete, and the exterior of the building consists of precast concrete panels. (1) The gap around the stairwell doors averages 1 mm (0.040 in.) and the gap around the elevator doors averages 3 mm (0.120 in.). A vent at the top of the elevator shafts is assumed to be open at either 5% or 20% of the total shaft cross-sectional area. The floors have an open construction and elevator lobbies are not considered in the formulation of the fire model. A fire with an average temperature of 700[degrees]C (1290[degrees]F) is limited to the first floor, and the pressure on that floor is 10 Pa (0.04 in. [H.sub.2]O) above the local atmospheric pressure. The surface temperature of both the elevator shafts and the stairwells is 40[degrees]C (103[degrees]F), and the air temperature on all floors is 22[degrees]C (71[degrees]F). The ambient air temperature is either 40[degrees]C (103[degrees]F), which simulates summer conditions, or -17[degrees]C (1[degrees]F) for winter conditions, and there is no wind incident on the external surfaces of the building.
SMOKE MOVEMENT PLAN
The curves in Figure 1 provide an illustration of the computer-generated pressure distribution throughout the high-rise building as a function of elevation. These curves were generated for summer conditions, a fire temperature on the first floor of 700[degrees]C (1290[degrees]F), no pressurization of any spaces within the building, and an elevator vent that is 20% of the total shaft area. The bulge in the floor pressure on the first floor represents the pressure rise of 10 Pa (0.04 in. [H.sub.2]O) resulting from the expanding gases on the fire floor. The intersections of the pressure curves identify the location of the neutral pressure planes (NPPs) for the various areas in the building. The elevator shaft NPP is on the 36th floor, the NPP for the stairwells is on the 21st floor, and the location of the NPP of the building is on the 34th floor. These NPP values suggest that smoke will enter the stairwells on the fire floor and be transported to the top of the building. The fire escapes will therefore be contaminated throughout the entire height of the building, and they will exhaust smoke to the floors above the 21st floor. The elevator shafts will likewise be contaminated for the entire height of the building, but they will contribute smoke to floors above the 36th floor.
If the same conditions apply, except the fire occurs during the winter (ambient temperature equal to -17[degrees]C [1[degrees]F]), the NPP in the stairwells rises to the 23rd floor, reflecting the lower pressures on the upper floors during cooler weather. The shift in the location of the stairwell NPP predicts a more difficult task of pressurizing the fire escapes when a fire develops during the winter. Pressurization fans with a greater capacity will be needed during cold weather compared to the size of the fans required during summer conditions. Repeated computer runs comparing the size requirements of pressurization fans have shown an approximate increase in fan capacity during the winter of about 30%-35% above the size required during summer operation when all other factors remain identical.
The pressure distributions inside the building suggest ways in which the pressures can be manipulated to improve the air quality in areas where occupants will likely reside during a fire. Obviously the stairwells are one area that should be maintained free of smoke during a fire. Also, due to extended travel paths that exist in high-rise fires, occupants may be unable or unwilling to descend to safety through the stairwells. In anticipation of this behavior, it would be prudent to provide smoke-free areas on floors by pressurizing habitable spaces so that individuals who remain in the building will have a safe haven to survive the fire. Obviously a fire will always produce smoke, and it will try to find the least resistive path to the outside. Since the elevator shafts provide that path, it would be wise to encourage the smoke to remain inside the elevator shafts by operating exhaust fans that would vent it to the outside.
[FIGURE 1 OMITTED]
The stairwells can be made smoke-free by pressurizing them and pushing the NPP to below the fire floor so that no smoke generated by the fire can enter the fire escapes which are the most likely exit paths for residents of the building. This scheme can be accomplished by increasing the stair pressure so that it exhausts clean air over the entire height of the building and no contaminated gases can enter the stairwells even on the fire floor. On the other hand, the floors will remain smoke-free if they are pressurized to such an extent that the elevator NPP is pushed above the top floor in the building. By doing so, no smoke that enters the elevator shafts can escape on the upper floors above the NPP. The integrated smoke movement plan could also be improved by providing exhaust fans in the elevator shafts to further decrease the pressure and reduce the resistance to the flow of smoke to the exterior.
The issue of pressurizing various areas in the building is complicated by the lack of general safety and code requirements needed to control the movement of smoke. The capacity of air-handling units used to pressurize stairwells so that they remain smoke-free is left unstated in the codes. In its place, NFPA codes (NFPA 2000, 2003) require a pressure difference of 12.5 Pa (0.05 in. [H.sub.2]O) across stairwell walls when the building is protected by sprinklers and they require 25 Pa (0.1 in. [H.sub.2]O) when the structure is not protected by sprinklers. The capacity of pressurization equipment is obviously dependent upon the building construction, fire intensity, and environmental conditions. However, codes do not address the issue of how these factors influence the capacity of pressurization equipment needed to provide the necessary pressure rise stated in the codes. One of the major goals of this paper is to provide guidance on selecting pressurization equipment for a comprehensive smoke control plan that is specifically applicable to high-rise structures.
Pressurization of the Stairwells
The objective of pressurizing stairwells is to provide sufficient fresh air to push the stairwell NPP below the fire floor. In doing so, the pressure inside the stairwells will be above the local floor pressure and uncontaminated air will exhaust from the stair shaft through any gaps to all floors in the building. With no stairwell pressurization, the stair NPP is on the 21st floor for the conditions used to generate the curves in Figure 1. Figure 2 illustrates how the location of the stair NPP is lowered as the volumetric flow rate of pressurized air is increased. As the volume of fresh air increases, the NPP drops while the outflow of air increases on the upper floors and the inflow on the lower floors decreases. When the flow rate of pressurized air in each stairwell reaches about 0.20-0.25 [m.sup.3]/s (425-530 cfm) on each floor, the entire height of the building experiences outflow from the stairs, except on the fire floor where the elevated pressure of the fire pushes smoke into the stairs. If the amount of fresh air is increased to above approximately 0.3 [m.sup.3]/s (635 cfm) on each floor, the stair pressure is finally sufficiently high to overcome the pressure on the fire floor. With this elevated level of pressurization, an outward flow of air exists on all floors, creating smoke-free conditions in the fire escapes throughout the entire building height.
The building design selected for illustration of the smoke control plan is fairly tight. For example, the average gap around the stairwell doors is 1 mm (0.040 in.) and the gap around the perimeter of the elevator doors averages only 3 mm (0.120 in.). These values were arbitrarily selected, but they are representative of modern building construction and they satisfy code requirements (NFPA 2007; ASME 2004). The tightness of the two shafts tends to diminish the demands on pressurization equipment, and the stated value of 0.3 [m.sup.3]/s (635 cfm) needed to create safe conditions in the fire escapes should be viewed as a minimum required capacity for the stairwell equipment. A safer value of approximately 0.6 [m.sup.3]/s (1270 cfm) on each floor would provide a safety factor of two and would account for the factors that are difficult to quantify, such as variations in construction or conditions difficult to anticipate such as fire escape doors that are propped open by people leaving the building. For the given volume of the stairwells, a volumetric flow rate of 0.6 [m.sup.3]/s (1270 cfm) on each floor translates to approximately 35 air changes per hour (ACH) in each of the four stairwells.
Pressurizing interior stairwells that have no surface exposure to the exterior of the building has a significant effect on the properties and flow of the gases in the stairwells, but it also affects the floor and elevator shaft pressures, although to a somewhat lesser degree. Some of the pressurized air in the stairwells naturally escapes through the unavoidable construction openings in the shaft and through gaps around the doors, so that once the stairwell pressure reaches an equilibrium value, nearly all the supplied stairwell air eventually finds its way into the floors. As a result, pressurizing the interior stairwells also increases the floor pressure and helps force fresh air into the elevator shafts that exist at the lowest level of pressure inside the building. Therefore, a stair pressurization scheme not only provides safe conditions in the fire escapes but also has an added benefit of helping prevent smoke in the elevator shafts from entering the upper floors. The beneficial effects of stair pressurization on the floor pressure distribution are magnified as the construction of the stairwells is less tight and as the stairwell doors are more loosely fitted, even though this type of open construction places a greater demand on the size of the stairwell pressurization equipment necessary to keep the stairs free of smoke.
[FIGURE 2 OMITTED]
Pressurizing stairwells with a volume of fresh air sufficient to move the stairwell NPP below the fire floor assures compliance with the NFPA minimum requirements of 25 Pa (0.1 in. [H.sub.2]O) pressure difference across the fire escape doors (NFPA 2000, 2003). With the stairwell pressure raised to such an extent that it is above the elevated fire pressure at ground level, the pressure difference across the doors continues to increase with elevation until it reaches a maximum value at the top floor of the building and all stairwell doors have pressure differences sufficient to satisfy NFPA requirements. Code compliance can be seen by the flow rate of air out of the stairwells (positive values) on all floors, as indicated by the far right curve in Figure 2.
Pressurization of the Floors
When the elevator shafts transport smoke upward, some of that smoke will be exhausted on upper floors and present a danger to occupants who might be hesitant to leave the building due to the long escape route to the exterior. If air-handling equipment could be used to increase the pressure on all floors (or perhaps, more economically, only on upper floors), then the NPP in the elevator shafts could be pushed to above the top floor in the high-rise building, and no smoke would be able to contaminate upper floors. Since floors are more leaky than stairwells due to their greater surface area and their large border with the exterior surface of the building, it should be anticipated that the capacity of pressurization equipment used for floor spaces will be substantially larger than the equipment dedicated for use in the stairwells.
The companion to Figure 2 for floor pressurization is Figure 3. For no floor pressurization, the elevator's NPP is on the 36th floor. As the flow rate of the floor AHUs is increased, the floor pressure increases and the location of the NPP is raised. As the amount of air supplied to the floors increases the flow into the elevator shafts increases slightly below the NPP, and it decreases out of the elevator shafts above the NPP. Once the volumetric flow rate exceeds about 0.6 [m.sup.3]/s (1270 cfm) on each floor, the NPP is pushed above the top floor and the flow of smoke inside the elevator shafts is forced to stay inside the shafts and exit through the elevator vents at the top of the building.
As expected, the capacity of the AHUs used for floor pressurization necessary to keep smoke inside the elevator shafts is approximately twice the size of the stair pressurization equipment required to maintain smoke-free fire escapes. If a safety factor of two is applied to the size of the floor pressurization units, a reasonable capacity of these units of approximately 1.2 [m.sup.3]/s (2540 cfm) on each floor or 0.57 ACH would provide a measure of spare capacity to account for the factors that are not considered by the computer model.
When a smoke movement plan involves only pressurization of the floor spaces and the stairs are left unpressurized, pressure differences across the elevator doors will meet code specifications. The smoke will be forced to remain in the elevator shafts, and the floors will remain smoke-free. This trend is indicated in Figure 3, which shows that the flow of gases is always into the elevator shafts (negative values) as long as the floor pressurization rate exceeds 0.6 [m.sup.3]/s (1270 cfm) on each floor. However, if at the same time the stairwells are left unpressurized, smoke can enter the stairwell at the fire floor and it will be forced to remain there due to the elevated floor pressure. In this case, the pressure differences across the fire escape doors will not satisfy code requirements (pressures greater inside the stairwells than on the floors), which would suggest that a fire safety plan that relies solely on floor pressurization should be coupled with fire escape pressurization to improve air quality throughout the entire building.
Combination of Stair and Floor Pressurization
Using a combination of floor and stairwell pressurization as part of a fire safety plan has several advantages. First of all, it provides a safe escape route for those who choose to use the stairwells to exit the structure, and secondly it provides smoke-free areas within the building so that those who are forced or choose to remain in the building have an improved opportunity of surviving the event. In fact, stair and floor pressurization equipment complement each other because pressurizing the floors not only keeps smoke contained inside the elevator shaft, but also reduces the smoke leakage around the stairwell doors and enhances the performance of the stair pressurization system. Furthermore, pressurizing the floors reduces the force necessary to open fire escape doors that swing in the direction of egress, which is one of the major concerns when adapting a stairwell pressurization system as the only facet of a fire safety plan.
[FIGURE 3 OMITTED]
The interaction of the floor and stair pressurization and their effect on the NPPs in the elevators and stairs are illustrated in Figures 4 and 5. The locations of the NPPs in the elevator shafts are plotted in Figure 4 as a function of the volumetric flow rate applied to the floors and stairs when the size of the elevator vents is 20%. As the amount of fresh air supplied to the stairwells increases, there is a dramatic increase in the height of the NPP in the elevator shafts. Absent of any floor pressurization, a stairwell pressurization rate in excess of about 0.35 [m.sup.3]/s (740 cfm) per floor is sufficient to prevent smoke from leaving the elevator shafts on upper floors as long as the elevator vent is large. If floor pressurization is used without any assistance from stairwell pressurization, a volumetric flow rate in excess of 0.7 [m.sup.3]/s (1480 cfm) per floor is required to elevate the NPP of the elevators to the top of the building. Figure 4 also shows how a combination of floor and stair pressurization can reduce the size of both pressurization equipment while still keeping combustion products inside the elevator shafts. By adding stair pressurization, the size of the floor pressurization equipment can be significantly reduced because all air introduced into the interior stairs eventually ends up on the floors, which augments the air added by the floor pressurization equipment.
[FIGURE 4 OMITTED]
Figure 5 is a companion to Figure 4 that shows the location of the stair NPP when both the floors and stairwells are pressurized and the elevator vent size is 20%. This figure illustrates how the stairwell NPP drops as the stair pressurization rate is increased. AHUs with a capacity of about 0.25 [m.sup.3]/s (530 cfm) per floor are necessary to lower the NPP to below the fire floor and prevent smoke from invading the entire height of the fire escapes. Changing the rate of fresh air supplied to the floors has very little effect on the location of the NPPs of the stairs because the air supplied to the floors can more easily vent through the large surface area of the building rather than enter the relatively tight, small surface area of the exposed stairwells. The curves in Figure 5 clearly show that the only effective way to rid the stairs of smoke is to apply pressurized air directly to them. No reasonable amount of clean air added to the floors will be able to greatly influence the quality of air in the stairwells.
The conclusions that evolve from Figures 4 and 5 give an overly optimistic picture of the pressurization scheme of improving fire safety in a high-rise building. It appears that stair pressurization alone will be capable of clearing both the stairs and floors of smoke during a fire. In part, this conclusion is due to the relatively tight construction of the elevator shafts, stairwells, and exterior surface of the building that were selected for the example because these choices tend to improve the effectiveness of any pressurization plan. In addition, the choice of a very large elevator vent encourages smoke to stay in the elevator shafts and not be forced into the adjacent floors. (2) The size of the elevator venting system has been shown to be one of the most influential factors that governs smoke movement during a fire (Black 2009b). A slightly different picture of an effective smoke management system emerges when the size of the elevator vent is decreased to only 5% of the total shaft area. By reducing the size of the elevator vent to a value close to code requirements, the elevator shafts become more of a choke point for all the smoke produced by the fire and small elevator vents resist efforts to maintain clean air on all floors.
Figures 6 and 7 show the locations of the NPPs in the specified high-rise building for an elevator vent size of 5%. These figures suggest a much more difficult task of providing smoke-free floors when the elevator vent is close to the code-required size. The greatest influence of changing the size of the elevator vents occurs in the selection of the size of floor pressurization equipment. As the locations of the elevator NPPs in Figure 6 indicate, without any stairwell pressurization, AHUs with a capacity approaching 3 [m.sup.3]/s (6350 cfm) on each floor are necessary to maintain clean air on the floors. Adding stairwell pressurization helps the matter somewhat, but the effect is marginal. Even with a significant contribution from the stair pressurization system, the floor AHUs must be fairly large to combat the high pressure in the elevator shafts when the elevator vent is restrictive. If the floors are not pressurized, no reasonable amount of stairwell pressurization will rid the floors of smoke during the fire.
[FIGURE 6 OMITTED]
The locations of the NPPs in the stairwells are shown in Figure 7 for various levels of building pressurization. Changes in the size of the elevator vent have a minor influence on the ability to keep smoke out of the stairwells. Without any floor pressurization, stairwell pressurization rates of about 0.3 [m.sup.3]/s (635 cfm) are necessary to move the NPP to below the fire floor. This value is practically the same as the one that is required for the larger vent size. By adding floor pressurization, the stair NPP is lowered, but the change in elevation is small. This trend reinforces the fact that only a small portion of air from the floors is able to enter the stairs and influence the fire escape pressure distributions.
If stairwell and floor pressurization equipment are used in tandem, and the fan capacities are sufficient to move the NPPs to their desired locations, then the pressure gradients in the building can satisfy NFPA pressure difference standards. In these cases, the established pressure gradients within the building will always be from the high-pressure stairwells to the intermediate pressure floor spaces and finally to the low-pressure elevator shafts. The stairwells and floor spaces will remain smoke-free, and the smoke produced by the fire will be forced to remain inside the elevator shafts and vent to the exterior at the top of the building.
Comparison of Figures 4 and 5 with Figures 6 and 7 illustrates how changes in one aspect of the building design, in this case the size of the elevator vents, can affect the chances of success of a smoke management plan. A complete evaluation of all aspects of the building design, environmental conditions and specifics of the fire must be done before a rational fire safety design can evolve. That is, the details of a smoke management system are highly individualistic, and they involve an iterative process that includes the consideration of many factors that are known to influence the movement of smoke during a fire.
[FIGURE 7 OMITTED]
The computer results suggest that the capacity of equipment to maintain smoke-free conditions in the stairs can be achieved by moderately sized dedicated fans mounted in the fire escapes that draw in 100% fresh air in order to raise the pressure in the fire escapes. The pressurization equipment necessary to maintain uncontaminated air on the floors can be achieved with existing air-handling equipment used for normal comfort control. The necessary air changes on the floors are within the range of equipment required for normal comfort control and fresh air requirements in commercial buildings. Air-handler controls could be designed to fully close return air dampers in the event of a fire and to energize existing fans to circulate 100% outdoor air. With floor pressurization requirements in the neighborhood of the computer-predicted 1-2 ACH, existing comfort control equipment could provide sufficient pressurization to keep smoke from invading the floor spaces in a high-rise structure.
Venting of Elevator Shafts
The general smoke management scheme proposed thus far involves pressurizing spaces people are likely to occupy during a fire and using the elevator shafts as a conduit to vent the smoke to the exterior of the building. Since the elevator shafts are naturally the lowest pressure region inside the high-rise structure, they create a favorable pressure gradient and thereby provide the least-resistant path for the smoke to take to the exterior. The performance of the smoke movement plan can be further enhanced by installing exhaust fans at the top of the elevator shafts that can be activated when the fire alarm system detects the presence of a fire in the building. If exhaust fans are installed in the elevator shafts, they would further reduce the pressure in the elevator shafts and enhance the use of the elevator shafts as chimneys to vent the smoke to the exterior. The smoke movement software was used to evaluate the effectiveness of elevator venting and to determine if elevator exhaust is capable of augmenting floor and stairwell pressurization while further improving life safety issues during a fire.
The capacity of the exhaust fans installed in the elevator shafts was quantified by the pressure drop across the fan. The elevations of the NPPs in the elevator shafts are shown in Figure 8 for three different cases. When the vent area is small, the flow of air through the elevator vent is very restricted. If the floors are not pressurized, a relatively high-capacity fan with a pressure difference in excess of 500 Pa (2 in. [H.sub.2]O) is needed to raise the NPPs in the elevators to the top floor of the structure. If elevator exhaust is used in conjunction with 2.0 [m.sup.3]/s (4240 cfm) of pressurized air on each floor, then the pressure drop required from the exhaust fans needed to keep smoke in the elevator shafts drops to about 300 Pa (1.2 in. [H.sub.2]O). If the elevator vent size is increased to 20%, the exhaust capacity of the elevator fans drops to only 50 Pa (0.2 in. [H.sub.2]O) for safe conditions, even if no floor or stair pressurization is provided. Additional computer output has shown that evacuating the elevator shafts has practically no influence on the location of the stairwell's NPP and it is incapable of improving the air quality in the fire escapes.
Summaries of the predicted AHU capacities for the two vent sizes required to keep smoke away from the floors and stairwells appear in Table 1 for the smoke movement plans that have been proposed. The values listed in the table apply to the specific fire scenario and building design described herein, and they may not be appropriate for buildings with different designs and construction styles. The values do not consider a safety factor, and therefore they should be viewed as minimum safe values.
The values in Table 1 reinforce the conclusions that pressurizing stairwells without floor pressurization can maintain smoke-free fire escapes with rather modest-capacity equipment, but the floors will be contaminated with smoke that is routed to upper floors via the elevator shafts. The size of the elevator vents has practically no influence on the capacity of the stairwell pressurization equipment needed to create smoke-free fire escapes. Pressurizing the floors alone can free them of smoke, but the size of the equipment is greater than that of the stairwell equipment. The size of the elevator vents plays a significant role in determining the volumetric capacity of the floor AHUs. Small vents place a stiff penalty on the AHU capacity. If the stairwells and floors are simultaneously pressurized, the capacity of the equipment, particularly on the floors, can be reduced and still achieve smoke-free areas in the high-rise structure. Combining all three aspects of the integrated smoke management plan--pressurizing the stairs and floors coupled with evacuation of the elevator shafts--further minimizes the size of the air-handling equipment without jeopardizing fire safety issues. The values in the table emphasize the fact that the elevator vent size has a controlling influence on smoke movement, and the size of the elevator vent can spell either success or failure of a well-conceived fire safety plan.
[FIGURE 8 OMITTED]
FORCES REQUIRED TO OPEN STAIRWELL DOORS
One concern when using stair pressurization alone to keep smoke from invading the fire escapes is the possibility of excessive force being necessary to open stairwell doors during a fire. Pressurization of a stairwell will increase the effort required to open the door that swings in the direction of occupant egress. If the amount of air introduced into the stairwell is excessive, then the resulting pressure rise can become so large that people will have difficulty opening the fire escape door.
When stairwell pressurization is used without the benefits that come with floor pressurization, there is no counterbalancing pressure on the opposite side of the door to assist in opening the door. However, if floor pressurization is used in conjunction with stair pressurization, not only is air quality throughout the building improved but also the force necessary to open the door is reduced. Therefore, pressurizing both areas will provide an added benefit to a fire safety plan.
Table 2 shows the forces necessary to open fire escape doors during a fire when different pressurization schemes are utilized and the elevator vent area is 5%. Values in the table are determined for a door on the 40th floor, which is the location of the greatest adverse pressure gradient across the door. Any opening force that exceeds 133 N (30 lbs) violates life safety codes (NFPA 2000, 2003). When the stairwells are pressurized to a level that will prevent smoke from entering at the fire floor, the forces necessary to open the doors at the highest levels of the building can exceed code limits, as seen by the magnitude of forces shown in Table 2. If pressurization levels are lowered to reduce the opening forces, the stairwell pressure may not be sufficient to maintain a smoke-free escape route. However, if stairwell pressurization is supplemented with floor pressurization, the opening forces are reduced and moderate pressurization rates on the floors can successfully reduce forces so that they satisfy code requirements.
Trends in the forces necessary to open fire escape doors that are not addressed in Table 2 should also be mentioned. Opening forces for winter conditions are greater than those shown in Table 2 due to the overall lower pressures imposed on the building during winter conditions. Forces required to open stairwell doors are also greater when the size of the elevator vent is increased because the larger area open to the atmosphere creates lower overall pressures in the elevator shaft and floors. Stair shafts are more immune to changes in atmospheric conditions due to the fact that they are more isolated from the environment than, for example, the elevator shafts that are exposed to atmospheric pressure via the elevator vent.
COMPARISON OF COMPUTER AND TEST RESULTS
The computer-predicted capacities of equipment necessary to keep smoke out of the fire escape stair shafts and away from the occupied floor spaces can be substantiated by several stairwell pressurization tests conducted on small burn towers and simulated fire tests carried out in large high-rise buildings. Unfortunately there are only a few tests (Tamura and McGuire 1973) involving floor pressurization, so an accurate comparison of the computer estimates of floor pressurization equipment cannot be stated.
Table 3 shows the capacity of air-handling equipment employed to pressurize stairwells in an effort to maintain smoke-free conditions. Two of the tests that are listed in the table were carried out in a 10-story fire tower for two different fire temperatures, and the third test involved a 16-story apartment building. The three tests utilized pressurization fans with capacities ranging from 0.29 to 0.43 [m.sup.3]/s (615 to 910 cfm) per floor. These values fall within the range suggested in Table 1 which are those predicted by the smoke management program.
Table 4 lists the fan capacities used in five stairwell pressurization tests that were performed in full-scale buildings, but under non-fire conditions. The buildings ranged from 11 to 42 stories in height. The capacity of the air handlers used in these five tests covered a broad range from 0.11 to 0.92 [m.sup.3]/s (233 to 1950 cfm) per floor. The lowest-capacity fans were judged to be ineffective because they were unable to prevent smoke from invading the stairs, and the higher-capacity fans were employed in an intentional attempt to overpressurize the stairs while venting excess air via dampers. The computer-predicted fan capacities for stairwell pressurization fall within the mid-range of the values used in these full-scale buildings tests.
A computer program is used to predict smoke movement within a simulated high-rise building during a fire event. An integrated fire safety plan involving stairwell and floor pressurization plus venting of the elevator shafts is discussed. The plan provides for smoke-free fire escapes and floor spaces so that those leaving the building via the fire escapes and those selecting to remain in the building have safe areas that are uncontaminated by smoke. Exhaust fans mounted in the elevator shafts enhance the plan because they force the smoke produced by the fire to remain inside the elevator shafts and away from people in the building. The use of elevator shafts to route smoke to the exterior continues the long-standing fire protection culture of prohibiting the use of elevators in the event of a fire, and it also recognizes the normal pressure distribution in a building that favors the flow of hot gases via the elevator shafts. The addition of floor pressurization to the scheme of pressurizing fire escapes has an added safety feature of reducing the force required to open fire escape doorways. Interaction between the pressurized air introduced inside the stairwells helps to minimize the size of the pressurization equipment used on the floors. Further reduction in the capacity of the pressurization equipment can be realized by incorporating elevator venting. Predicted capacities of the air-handling equipment in both the stairs and floors fall within the range of equipment used in several stairwell pressurization tests carried out in fire towers and full-size buildings. The predicted capacities of pressurization equipment necessary to create smoke-free conditions in the fire escapes and on the floors are within the capabilities of existing comfort control fans and moderately sized dedicated air handlers that could be installed in the stairwells.
ASME. 2004. ASME 17.1 Safety Code for Elevators and Escalators. New York: American Society of Mechanical Engineers.
Black, W.Z. 2009a. Pressurization of floors to improve life safety during a high-rise fire. ASHRAE Transactions 115(2):278-89.
Black, W.Z. 2009b. Smoke movement in elevator shafts during a high-rise structural fire. Fire Safety Journal 44(2):168-82.
Black, W.Z. 2009c. Use of air handling equipment to manage smoke movement during a high-rise fire. ASHRAE Transactions 115(1):165-81.
Black, W.Z. 2010. COSMO--Software for designing smoke control systems in high-rise buildings. Fire Safety Journal 45(6-8):337-48.
Black, W.Z. 2011. Computer modeling of stairwell pressurization to control smoke movement during a high-rise fire. ASHRAE Transactions 117(1):786-800.
Chow, W.K., and L.W. Lam. 1993. Evaluation of a staircase pressurization system. ASHRAE Transactions 99(2):194-99.
ICC. 2006. International Building Code, Section 3004.3. Falls Church, VA: International Codes Council.
Kerber, S., and D. Madrzykowski. 2007. NIST internal report 7468, Evaluating positive pressure ventilation in large structures: High-rise fire experiments. Gaithers burg, MD: National Institute of Standards and Technology.
Klote, J.H. 1980. Stairwell pressurization. ASHRAE Transactions 86(1):604-22.
Klote, J.H. 1988. An overview of smoke control technology. ASHRAE Transactions 94(1):1211-21.
Klote, J.H. and J.A. Milke. 2002. Principles of Smoke Management. Atlanta: ASHRAE.
NFPA. 2000. NFPA 92A: Recommended practice for smoke control systems. Quincy, MA: National Fire Protection Association, Inc.
NFPA. 2003. NFPA 101: Life Safety Code. Quincy, MA: National Fire Protection Association.
NFPA. 2007. NFPA 80: Standards for fire doors and other opening protectives. Quincy, MA: National Fire Protection Agency.
NFPA. 2006. NFPA 5000: Building Construction and Safety Code, Section 188.8.131.52. Quincy, MA: National Fire Protection Agency.
Shaw, C.Y., and G.T. Tamura. 1976. Design of a stairshaft pressurization system for tall buildings. ASHRAE Journal 18(2):29-33.
Tamura, G.T. 1983. Review of the DBR/NRC studies on control of smoke from a fire in high buildings. ASHRAE Transactions 89(1B):341-61.
Tamura, G.T. 1989. Stair pressurization for smoke control: Design considerations. ASHRAE Transactions 95(2):184-92.
Tamura, G.T. 1990. Field Tests Of Stair Pressurization Systems With Overpressure Relief ASHRAE Transactions 96(1):951-58.
Tamura, G.T. 1991. Stair pressurization systems for smoke control. ASHRAE Journal 33(7):14-18.
Tamura, G.T. 1992. Assessment of stair pressurization systems for smoke control. ASHRAE Transactions 98(1):66-72.
Tamura, G.T., and R.A. MacDonald. 1993. Comparative performances of mechanical smoke exhaust systems, zoned smoke control and pressurized building method of smoke control. ASHRAE Transactions 99(1):488-95.
Tamura, G.T., and J.H. McGuire. 1973. The Pressurized Building Method of Controlling Smoke in High-Rise Buildings, NRCC 13365. Ottawa, ON: National Research Council Canada.
Tamura, G.T., and C.Y. Shaw. 1976. Air leakage data for the design of elevator and stair shaft pressurization systems. ASHRAE Transactions 82(2):179-90.
Tamura, G.T., and C.Y. Shaw. 1978. Experimental studies on mechanical venting for smoke control in tall office buildings. ASHRAE Transactions 84(1):54-71.
Wang, Y., and F. Gao. 2004. Test of stairwell pressurization systems for smoke control in a high-rise building. ASHRAE Transactions 110(1):185-92.
W. Z. Black, PhD, PE
(1) Leakage areas for cast concrete shaft walls and for pre-cast concrete panels used for the exterior walls of the building were taken from Tamura and Shaw (1976) and Tamura and Shaw (1978).
W.Z. Black is Regents Professor Emeritus in the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta.
Table 1. Approximate Capacity of Pressurization Equipment for Different Smoke Control Schemes Smoke Control Vent Area = 20% Scheme VFR in VFR on Stairwells, Floors, ** * [m.sup.3]/ [m.sup.3]/ s (cfm) s (cfm) Stairwell 0.30 (635) 0 pressurization only stairs smoke-free Floor 0 0.6 (1270) pressurization only floors smoke-free Stairwell 0.20 (425) 0.2 (425) and floor pressurization both stairs and floors smoke-free Smoke Control Vent Area = 5% Scheme VFR in VFR on Stairwells, Floors, ** * [m.sup.3]/ [m.sup.3]/ s (cfm) s (cfm) Stairwell 0.30 (635) 0 pressurization only stairs smoke-free Floor 0 3 (6350) pressurization only floors smoke-free Stairwell 0.20 (425) 2.5 (5300) and floor pressurization both stairs and floors smoke-free Note: Summer conditions 40[degrees]C (103[degrees]F), fire temperature 700[degrees]C (2190[degrees]F), no safety factors applied to fan capacities, and no elevator shaft exhaust are considered. * Volumetric flow rate (VFR) of air to each stairwell on each floor required to move stairwell NPP to below fire floor. ** Volumetric flow rate of air to each floor required to move elevator shaft NPP to above top floor. Table 2. Forces Required to Open Fire Escape Doors for Different Smoke Control Schemes Stairwell Floor Pressurization, Pressurization, [m.sup.3]/s (cfm) per floor [m.sup.3]/s per floor (cfm) 0 1.0 (2119) 2.0 (4240) Force to Open Fire Escape Doors, * N (lbs) 0 28 (6.3) 16 (3.6) 6.1 (1.4) 0.1 (212) 58 (13) 44 (9.9) 32 (7.2) 0.2 (424) 109 (25) 92 (21) 81 (18) 0.3 (636) 190 (43) 172 (39) 170 (38) Note: Summer conditions 40[degrees]C (103[degrees]F), fire temperature 700[degrees]C (1290[degrees]F), elevator vent area 5% of total shaft area. * Minimum force to open fire escape door on 40th floor. Value excludes force to overcome closure mechanism. Table 3. Capacities of Stairwell Pressurization Equipment Used in Fire Tests Source Building Type Stair Comments Pressurization, [m.sup.3]/s (cfm) per floor Tamura 10-story NRCC 0.35 (740) Fire temperature (1989) Fire Tower 300[degrees]C (572[degrees]F) Tamura 10-story NRCC 0.43 (912) Fire temperature (1989) Fire Tower 580[degrees]C (1075[degrees]F) Kerber and 16-story 0.29 (615) Two fans Madrzykowski Building in installed in (2007) Chicago fire escape Table 4. Capacities of Stairwell Pressurization Equipment Used in Field Tests (Non-Fire Conditions) Source Building Stair Pressurization, Comments Type [m.sup.3]/s (cfm) per floor Chow and 11-story 0.11 (233) Single fan on Lam (1993) commercial each floor-system building did not perform satisfactorily Wang and 32-story 0.30-0.42 (636-890) Single roof- Gao (2004) high-rise mounted fan to building pressurize stairs via vertical shaft Tamura 22-story 0.30 (636) Overpressurization (1990) apartment test building Tamura 39-story 0.47 (996) Overpressurization (1990) office test building Tamura 42-story 0.92 (1950) Overpressurization (1990) office test building
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|Date:||Jul 1, 2013|
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