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Smoke control in high-rise buildings
Canadian Building Digest
Division of Building Research, National Research Council Canada
CBD 134
Smoke Control in High-Rise
Buildings
Originally published Febuary 1971 J.H. McGuire, G.T. Tamura
Please note
This publication is a part of a discontinued series and is archived here as an historical reference. Readers should consult design and regulatory experts for guidance on the applicability of the information to current construction practice.
The mechanisms primarily responsible for the movement of smoke within buildings and examples of the extent of the hazards that can be created were presented in CBD 133. It has been shown that in many high buildings complete evacuation is not the solution to the problem of preserving life safety in the event of fire. Stair and elevator shafts may become contaminated within a few minutes of the attainment of a severe level of pollution on the fire floor. Complete evacuation, on the other hand, could take a much longer time (say 30 minutes), even if all elevators and stairs were used simultaneously.
The obvious alternative of permitting occupants to remain in the building is often not practical so far as upper storeys are concerned. Visibility and carbon monoxide considerations suggest that smoke concentrations more than 1 per cent of those that can prevail in a fire area are likely to be unacceptable for any more than the briefest of periods. Analysis of a hypothetical 20-storey building under winter conditions(1)indicates that smoke concentrations of more than
fifteen times this level could develop in the upper half of the building after some considerable time.
To a close approximation, the steady-state concentration (C) to be expected in the upper half of a simple tall building as a result of a fire at a low level in the building, and of stack action associated with building heating, is given by
C = 3 Co/N.n where
Co = the steady-state smoke concentration on the fire floor, N = the number of storeys in the building,
n = the number of similar compartments into which the fire floor is divided.
The expression relates to the situation where all windows are intact and the doors to all the compartments on the fire floor are closed. Adopting the 1 per cent criterion, the expression
suggests that, under the conditions specified, serious smoke conditions should not arise in the upper storeys of a high building provided n.N>300. Thus for an apartment building having ten apartments per floor, upper floors would remain tenable in the event of fire on a lower floor provided the building were higher than 30 storeys. Fracture of windows in the fire area (when this is at a low level in a building) will in general increase smoke densities in the upper parts of a building, as a result of stack action, by a substantial factor, e.g. 3. If the door to the compartment involved in fire is open, then the advantage of compartmentation, at least as regards its influence on smoke levels in remote parts of the building, is nullified. On the other hand, if the door and window of one or more adjacent compartments are opened, the resulting flow of diluent air, provided the door to the compartment involved in fire is closed, gives much lower maximum smoke densities in remote parts of the building.
Objectives of Smoke Control Measures
Discussion of the objectives in developing smoke control measures has hitherto been very limited, but all the authorities involved appear to be unanimous in suggesting that one or more stairwells, ultimately connecting to the exterior, should be maintained permanently tenable. As evacuation can take considerable time it also becomes necessary to provide smoke-free areas throughout the building sufficient to accommodate virtually all the occupants. These areas might, in fact, constitute the whole of the building with the exception of the fire floor, the one above it and perhaps the one below it. Alternatively, one might plan to maintain only limited areas smoke-free such as every fifth floor in a building. Requirements in addition to control of smoke levels can be envisaged for such areas.
Most fire-fighters hold the view that provision of one or more functional elevators for their use is an important objective. As to the extent to which such elevators should be maintained smoke-free, some doubt may exist. Some fire authorities comment that the fire-fighter himself will accept quite high smoke levels (using breathing apparatus), the only limitation being the effect of the smoke and products of combustion on the operation of the elevator. Others suggest that the smoke level in the chosen elevators should be minimal to permit their use for controlled evacuation (e.g. of regions near to the fire or of the topmost floors).
Control of Materials Involved in Fire
An elementary approach to resolving the smoke problem in buildings is to limit the amount of materials with a propensity for generating smoke and toxic gases that can be involved in fire. By applying available information on smoke generation to the air-flow conditions in the 20-storey model building discussed in CBD 133, the rate of destruction of material necessary to maintain pollution in the shafts and upper floors that are already polluted to the critical level of tolerance can be predicted. The steady rates of burning required are very low, for example as little as 0.2 lb/min of some materials, although the critical levels would only be approached in about 3 hr. The time to foul the shafts would, however, be reduced to a few minutes by the initial destruction of 100 times this amount.
It is unlikely that the objective of limiting the amount of material that can be involved in fire can be achieved by absolute restriction of the amount of combustible material within a building. The suggestion that it might be achieved by the use of a sprinkler system appears to be much more feasible and is in fact encouraged by some fire authorities as being the most reliable and effective approach to the problem.
Dilution of Smoke
It is logical to consider next, as an approach to the problem, the possibility of diluting smoke to acceptable levels as it issues from a region involved in fire and flows to an adjacent region. The flow rates required for adequate dilution can be deduced from a knowledge of the rate of pollutant flow and of the criterion of tenability. using the 1 per cent value, dilution by a factor of 100 is called for. For the hypothetical 20-storey building(1)about 2,800 cfm of pollutant was
shown to be flowing into the shafts. The diluent air requirement to maintain tenable conditions could thus be as high as 280,000 cfm for this particular example. Wintertime conditions were
considered, but similar pollutant flow rates could be maintained for periods of about 20 minutes as a result of the expansion mechanism in the fire area. For this period a diluent flow rate of several hundred thousand cfm could be called for regardless of exterior temperature if dilution were the only smoke control measure involved.
Consideration of further alternative smoke control techniques will often lead to the conclusion for Canadian winter conditions, that the dilution approach is unlikely to be the most economic. In conjunction with venting of the fire area, it could become more competitive with other methods under summer conditions.
Measures involved in some techniques yet to be discussed, with the primary objective of establishing certain pressure differentials, will involve air flows which will often serve to dilute smoke that has penetrated certain areas prior to the initiation of the smoke control measures. If the diluent air mixes with the atmosphere in the enclosure the concentration of smoke decreases to about a third of its previous value for every volume of air entering. In general, the region will thus become tenable after two or three volumes of air have entered it.
Creation of Favourable Pressure Differentials
An approach that will often be found appropriate, particularly where Canadian winter conditions are involved, is the creation of favourable pressure differentials across various partitions within a building, so that smoke will not flow out of the compartment already polluted. This can be achieved either by natural venting or by mechanical pressurization of shafts. In devising possible methods, the assumption can be made that the primary problem is smoke transfer via the vertical shafts as a result, firstly. of expansion and secondly, of stack action associated with building heating.
Natural Venting of Shafts
The use of a smoke shaft in the fire area will effectively check smoke spread caused by both thermal expansion and building heating stack action. A smoke shaft consists of a shaft with an opening to the fire floor and a larger opening to the outside, at the top. To describe its effect it is convenient to refer to pressure characteristic diagrams as used in CBD 104and CBD 107. Figure 1 illustrates the pressure characteristics of a typical building in winter; Figure 2 shows the influence on the fire region that would be created by a smoke shaft. The large opening at the top of the smoke shaft maintains pressures that are lower than those of adjacent floor spaces at all levels in the shaft. At the fire floor it also induces a relatively low pressure in the fire region itself. It thus follows that flows through all leakage areas between the fire region and adjacent regions will be towards the fire. The smoke shaft in turn will accept the whole discharge from the fire area and the rest of the building will remain uncontaminated.
Figure 2. Pressure differences produced by a smoke shaft.
Under summer conditions, stack action might not immediately be initiated in a smoke shaft. Rather than wait for the shaft to be warmed by gas flow resulting from expansion, it might be considered desirable to install a fan within it.
Unfortunately, appreciable window breakage in the fire region nullifies the effect of a smoke shaft so far as building heating stack action is concerned. The fire floor pressure characteristic moves over to coincide with the outside air characteristic (at that level) and smoke will then flow from the fire region to all adjacent regions.
The action of a smoke shaft suggests a useful technique whereby shafts in general can be prevented from transferring smoke from lower to higher floors as a result of building heating stack action. If a shaft is heavily vented at the top then again its characteristic will be moved to the left and shaft pressures are decreased relative to the floor space pressures. If it is, in fact, to the left of the floor space characteristic at all levels, then smoke flowing into the shaft at low levels will not return to any other floor spaces as the vent at the top will constitute the only outlet from the shaft. Two features of this technique must be noted, firstly that it involves fouling of the shaft and secondly that for high buildings its application is sharply limited by a requirement for excessive top vent areas.(2)
If a shaft is opened to the atmosphere at the bottom, i.e. bottom vented, conditions exactly the converse of those given by top-venting are initially created. The shaft characteristic is moved to the right, as shown by the dotted characteristic in Figure 3, fresh air flows in at the low level opening and it then flows to the floor spaces through the leakage areas at all levels. Such conditions would be ideal particularly as the shaft itself remains smoke free, but unfortunately they cannot be expected to prevail indefinitely. The shaft cools and its pressure characteristic tends to change, as indicated by the dashed line, approaching that of the outside air.
Figure 3. Pressure characteristics of a stair shaft open to exterior at the bottom
Mechanical Pressurization
Where shafts are to be maintained smoke free, their characteristics must be moved to the right (i.e. an over-all pressure increase created at all levels) and this can be achieved by injecting (warm) air into the shafts. The effect is similar to that of bottom venting of a shaft. In some buildings the ratio of the shaft/floor space leakage and the exterior skin leakage is sufficiently high that an attempt to raise the shaft pressures appreciably raises the floor space pressures. In such cases it may be more convenient to pressurize the building generally, venting the fire area. The ramifications associated with this and other methods involving one or more of the techniques mentioned so far are discussed in detail in Reference (3).
Other Approaches
A complete list of possible techniques of control of smoke movement in high buildings is beyond the scope of this Digest. Some reference should be made, however, to techniques that depend upon the layout of the building spaces.
Computer analysis of smoke movement has indicated that vertical shafts constitute the principal path whereby smoke is dispersed throughout a building. An obvious approach to combating this mechanism is to divorce the vertical shafts, physically, from the main floor areas. Few buildings however would prove acceptable, in the depths of a Canadian winter, if access to stairs, elevators, garbage chutes etc., could be only gained by passing through an area freely exposed to atmospheric conditions. The concept becomes more practical if the separation is achieved by lobbies which normally contain tempered air but can be heavily vented to the exterior in the event of a fire.
Another approach in the same category is to divide a building vertically in two, in the hope that, in the event of a fire, only one side of it will become contaminated. Where the two parts of a building are actually separated spatially and communication is solely by vestibules or bridges that can be heavily vented or pressurized in the event of fire, the approach can readily achieve its objective. Where the separation is by a partition, asymmetries in the pressure patterns in the two parts of the building can make the leakage through the partition very significant as regards smoke transfer. This impediment can usually be overcome, however, by a judicious but unusual application of venting (often of the side of the building not involved in fire)(3).
Studies of smoke control techniques suitable for high-rise buildings have only recently been undertaken consequent to an appreciation of the magnitude of the smoke problems that can be posed by fire in high buildings. It is therefore possible that, as these studies progress, and as
the principles are applied in the design of buildings, developments further to those mentioned in this Digest will permit other solutions to the problems.
References
1. Tamura, G. T. Computer Analysis of Smoke Movement in Tall Buildings, ASHRAE Trans., Vol. 75, Part II, p. 81-93, 1969.
2. Tamura, G. T., and A. G. Wilson. Natural Venting to Control Smoke Movement in Buildings via Vertical Shafts. Presented at ASHRAE Annual Meeting, Kansas City, 28 June - 1 July 1970.
3. Explanatory Paper on Control of Smoke Movement in High Buildings. National Research Council of Canada, Assoc. Com. Nat. Bldg. Code, Ottawa June 1970. NRC 11413.
