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Construction Canada, 34, 1, pp. 45-50, 1992-01
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Smoke management in atria
Tamura, G. T.
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https://nrc-publications.canada.ca/eng/view/object/?id=d375a365-02a0-441c-946e-fbeb81789ea2 https://publications-cnrc.canada.ca/fra/voir/objet/?id=d375a365-02a0-441c-946e-fbeb81789ea2Smoke Management in Atria
by G. T. Tamura
G. T. Tamura is a senior research officer in the National Fire Laboratory of the Institute for Research in Construction.
Originally published in "Construction Canada" 34(1), 1992, p. 45-50
The concept of an atrium in a large building is a relatively new architectural feature introduced in 1963 by John Portman, architect/developer of the 23 storey Hyatt Regency Hotel in Atlanta. Since then, atrium buildings, which provide large interior spaces protected from inclement weather, have gained considerable popularity. Occupants are in touch with activities outside their office space and share the interior atrium features such as sculptures, decorations, and landscaping.
Problem
Atria, which interconnect floor spaces in a building, are at odds with the principle of floor to floor fire compartmentation whose codes require to confine fire spread and smoke movement. Because of the increased fire hazard thus created, all U.S. and Canadian building codes mandate sprinkler systems in atria and adjacent floor spaces. Sprinklers are extremely effective in suppressing fires in floor spaces, but because of delayed response they are not so effective in controlling fires in atria exceeding 20 m in height. Also, water droplets from sprinklers make smoke less buoyant and drive it down to the atrium floor level or to lower levels of the adjacent floors. Thus, the entire atrium and adjacent floor spaces can, in a short time, be contaminated with smoke, endangering the lives of occupants, particularly those
located in upper floors.
Figure 1. Smoke plume of an
atrium fire
Technical
Considerations
Above a fire in an atrium, a column of hot smokey gases rises toward the ceiling (Figure 1). Along with it, surrounding cool air is entrained into the rising stream and mixes with it. The rate of air entrainment and the quantity of smoke produced depend on the fire size, temperature of the smoke plume, and the height of the column of hot gases between the floor and the bottom of the layer of hot gases that has collected underneath the ceiling. With time, the smoke layer will gradually spread downward toward the floor to fill the atrium. Another scenario is that smoke may stratify below the ceiling if the plume loses buoyancy caused by its cooling with the entrainment of surrounding air.
The rate of smoke production, the time to fill the atrium with a steady fire, the mass of fuel required to fill an atrium with a steady fire, and the time to fill the atrium with a growing fire are given in Tables 1, 2, 3, and 4, respectively, by Milke, 19881. The values in the tables were obtained using a 2 zone fire model. Table 1, which gives the volumetric smoke production, shows that smoke production is strongly affected by the clear height. Doubling the clear height results in a three times increase in smoke production and tripling the clear height results in a five times increase in smoke production. Tables 2 and 4, for a steady and growing fire
respectively, indicate that 80% of the volume of an atrium can be filled with smoke in a matter of minutes. Table 3 shows that burning of only a few pounds of combustibles can fill an atrium with smoke.
Hotel Atrium Fire
In April 1973, a fire started in a second floor nightclub in the 10 storey Hyatt Regency O'Hare Hotel (Sharry, 1973)2. Guest rooms were arranged along the walls surrounding the atrium with corridors open to the atrium and balconies on the exterior walls. Circular towers containing guest rooms and a stairshaft were located at the corners of the atrium. Elevators were located at the centre of the atrium with walkways from the corridors to the elevators.
A fire in the nightclub, which was not sprinklered, was discovered at 4:30 a.m. when smoke was issuing from its entrance into the atrium. The firefighters found the atrium charged with smoke, with visibility down to 3 m in most areas of the atrium to the point of obscuring exit signs. The atrium smoke exhaust system had failed to operate. There were no fatalities and only one guest required medical attention, but the fire graphically illustrated the potential for rapid smoke buildup in an atrium unless measures are taken to prevent that.
Smoke Management
The objectives of a smoke management system are to accomplish life safety and minimize property losses. For buildings with an atrium, the special concern is smoke movement from the atrium space to communicating floor spaces, stairs, and elevators where occupants are
present. The specific objectives of a smoke management system are: providing safe means of tenable egress for occupants,
providing adequate visibility for firefighting and occupant rescue, and assisting in smoke removal.
Measures to achieve these objectives are discussed in the following section.
Means to Control Fire
One approach is to limit the quantity of smoke production by limiting the combustibles in an atrium. The National Building Code of Canada (NBC)3limits combustibles to 16 g/m3of
interconnected floor space if the ceiling height is over 8 m. Comparing this with Table 3 indicates that controlling combustibles to this limit may still result in a smoke filled atrium. Therefore, further measures, usually fire suppression, are required for life safety. For high atria, one approach for fire suppression is to install sprinklers under canopies on the atrium floor; another is to install fixed fire nozzles on one of the lower floors for extinguishing fires on the atrium floor. The latter approach is employed in a high rise building in Osaka, Japan, where two platforms are placed on the second floor diagonally opposite each other across the atrium.
Means to Exhaust Smoke
The height of a smoke layer can be controlled by either natural or mechanical venting at the top of an atrium space. Natural venting relies on the buoyancy force caused by the elevated temperatures of the layer of hot gases. Mechanical venting is more positive with regard to the venting rate and less affected by wind than natural venting.
A buildup of the smoke layer underneath a ceiling is difficult to prevent with mechanical venting because of the large exhaust rate required. Mechanical venting can help slow the rate of
descent of the smoke layer, and limit the descent to a given height above the floor of an atrium. A specified depth of smoke layer can be achieved by exhausting at a rate equal to the rate of smoke supplied to the smoke layer. Table 1 can be used to select the required exhaust capacity for a fan for a specified design steady fire and clear height. Where occupants are likely to be exposed to smoke in floor spaces communicating with the atrium, smoke temperatures, concentrations of CO, CO2 and other combustion products, and smoke obscuration need to be calculated. Also, the time for smoke to reach upper levels and the time needed to evacuate those areas should be compared to ascertain that there is sufficient time for safe egress of occupants. Technical data to carry out such calculations are given in NFPA 92B, Smoke Management for Malls, Atria and Large Areas.4
Currently, many building codes specify the required rate of smoke exhaust in terms of air changes per hour based on the interconnected spaces. The NBC specifies four air changes per hour for smoke exhaust systems that are intended to be operated by firefighters for smoke removal and not specifically for smoke control.
Mechanical venting of an atrium for smoke control other than controlling the depth of smoke layer will be discussed later.
Means to Separate Adjacent Spaces
Fire separations are often required to prevent smoke from encroaching public escape routes. If floor spaces are sprinklered, codes often permit unrated fire separations to be used for public corridors, except in institutional and residential occupancies. Even unrated fire separations, however, must remain in place until sprinklers are activated to prevent the spread of fire and smoke. Tests by Kim and Lougheed5indicated that early activation of sprinklers is essential in preventing failure due to water spray on hot glazing. Small scale tests with water spray
indicated that maximum glass temperatures to withstand water spray were 80 to 90°C for plain glass, 150 to 165°C for heat strengthened glass, and 200°C for tempered glass. To maintain the integrity of fire separations, dedicated fast response sprinklers are required on the floor side in the event of a fire on one of the floors and also on the atrium side if the smoke layer temperatures in the atrium are expected to exceed critical glass temperatures. These designed glazing systems, in fact, provide a fire rating as long as the water spray continues.
Except for institutional and residential buildings, the NBC also permits a fully open atrium, provided that there are protected vestibules attached to the stairshafts and that the stairs have an area of treads and landings sufficient to take all occupants of a building. If not, then a protected floor space with an area sufficiently large to accommodate the remaining occupants is required. Protected floor spaces require fire separations with a fire resistance rating equal to that of the floor assembly and openings in such separations need to be protected by vestibules to prevent smoke entry.
Means to Provide Negative Pressures
As indicated previously, mechanical exhaust as specified in the NBC is not specifically intended for smoke control but for exhausting smoke from an atrium as required by firefighters. Also, mechanical exhaust, as intended in NFPA 92B, is for controlling the depth of smoke layer and smoke filling time. By mechanically exhausting the atrium, however, pressures in the atrium can be made lower than those outdoors with flow of outdoor air through leakage openings in the exterior walls into floor spaces and through leakage openings in the vertical fire separations into the atrium. This will help inhibit smoke from a fire in an atrium from entering adjacent floor spaces with fire separations. Also, negative pressures created by exhausting the atrium can help prevent smoke from entering stairshafts with protected vestibules in floor spaces without fire separations. There are several hotels in the United States with corridors fully open to the atrium. Although these corridors may be filled with smoke during a fire, negative pressures in the atrium can inhibit smoke from migrating into guest rooms.
The amount of negative pressure would depend on the exhaust rate, the airtightnesses of the exterior walls and the fire separations, and the size of the opening on the ground floor for make up air supply. The negative pressures required would be those needed to overcome the adverse pressure differences across fire separations caused by the buoyancy force of the smoke layer and those caused by outside conditions of wind and temperature, which are discussed by Hansell and Morgan.6,7 This aspect of atrium exhaust still needs to be explored by field measurements and full scale experiments.
Closing Remarks
Because of the increased potential for occupant harm from fire in atrium buildings, it is necessary to properly assess the fire and smoke hazards and to develop effective smoke management systems. To date, there have been few fire experiences to allow designers to fully assess the hazard and to evaluate smoke control systems in atrium buildings. To manage smoke from a fire in an atrium, most codes use a rule of thumb approach and simply specify an exhaust rate in terms of air changes per hour. The recently adopted NFPA 92B4recommends an engineering approach based on equations for fire plumes to determine the required exhaust rate. Tests in buildings, preferably with hot smoke, are required to fully evaluate both approaches.
Table 1. Volumetric rate of smoke production
for axisymmetric plumes m3/s (1000 scfm )
Clear
Height Total Heat Release Rate (kW)
m (ft) 1000 2000 4000 8000 16000 3.0 4 6 9 15 25 (10) (9) (13) (20) (32) (54) 7.6 16 22 29 40 56 (25) (35) (46) (61) (84) (119) 15.2 50 64 82 107 141 (50) (106) (136) (174) (226) (299) 22.8 98 124 158 202 260 (75) (207) (262) (334) (427) (552) 30.5 157 199 252 320 410 (100) (333) (421) (534) (679) (869) 45.7 308 389 491 622 790 (150) (652) (824) (1040) (1318) (1673) 61.0 497 626 791 999 1265 (200) (1053) (1328) (1676) (2117) (2680) 76.2 720 907 1145 1446 1827 (250) (1526) (1923) (2426) (3063) (3872) 91.4 975 1229 1550 1956 2470 (300) (2067) (2605) (3285) (4145) (5234)
– steady fire, minutes per 93 m2(1000 ft2) of
cross sectional area
Clear
Height Total Heat Release Rate (kW)
m (ft) 1000 2000 4000 8000 16000 7.6 (25) 1.04 0.82 0.68 0.52 0.41 15.2 (50) 0.65 0.52 0.43 0.33 0.26 22.8 (75) 0.50 0.40 0.33 0.25 0.20 30.5 (100) 0.41 0.33 0.27 0.21 0.16 45.7 (150) 0.31 0.25 0.21 0.16 0.12 61.0 (200) 0.26 0.21 0.17 0.13 0.10 76.2 (250) 0.22 0.18 0.15 0.11 0.09 91.4 (300) 0.20 0.16 0.13 0.10 0.08 Reference: Milke, 19981
Table 3. Mass of fuel consumed to fill 80% of
the atrium volume with smoke, kg per 100 m2
of cross sectional area (lb per 1000 ft2) Clear
Height Total Heat Release Rate (kW)
m (ft) 1000 2000 4000 8000 16000 7.6 5.9 9.3 15.3 23.5 37.3 (25) (12.0) (19.0) (31.4) (48.0) (76.2) 15.2 3.7 5.9 9.7 14.7 23.5 (50) (7.5) (12.0) (19.8) (30.2) (48.0) 22.8 2.8 4.4 7.4 11.3 17.9 (75) (5.7) (9.1) (15.1) (23.1) (36.6) 30.5 2.3 3.7 6.1 9.3 14.8 (100) (4.8) (7.5) (12.5) (19.0) (30.2) 45.7 1.8 2.8 4.6 7.1 11.3 (150) (3.6) (5.8) (9.5) (14.5) (23.1) 61.0 1.5 2.3 3.8 5.9 9.3 (200) (3.0) (4.8) (7.8) (12.0) (19.0) 76.2 1.3 2.0 3.3 5.0 8.0 (250) (2.6) (4.1) (6.8) (10.3) (16.4) 91.4 1.1 1.8 2.9 4.4 7.1 (300) (2.3) (3.6) (6.0) (9.1) (14.5)
growing fire cross sectional area of 93 m2,
1000 ft2(930 m2, 10 000 ft2) Clear
Height Time (minutes)
m (ft) Fast Medium Slow
7.6 2.0 3.0 4.3 (25) [5.8] [8.8] [13.0] 15.2 1.8 2.7 3.7 (50) [6.2] [9.0] [13.0] 22.8 1.7 2.3 3.2 (75) [6.0] [8.7] [12.2] 30.5 1.5 2.0 2.8 (100) [5.8] [8.2] [11.3] 45.7 1.2 1.8 2.3 (150) [5.3] [7.3] [10.0] 61.0 1.1 1.5 2.2 (200) [4.8] [6.8] [9.0] 76.2 1.0 1.4 2.0 (250) [4.5] [6.2] [8.3] 91.4 0.9 1.3 1.8 (300) [4.3] [5.8] [7.7]
References
1. Milke J. A., Fire hazard assessment in atriums. The roundtable on fire safety in atriums – Are the codes meeting the challenge? National Fire Protection Association, Quincy, MA, 1988.
2. Sharry, J.A., An atrium fire, Fire Journal. National Fire Protection Association, Quincy, MA, 1973, pp. 39-41.
3. National Building Code of Canada, Associate Committee on the National Building Code, National Research Council of Canada, Ottawa, pp. 98 99.
4. NFPA 92B, Smoke management for malls, atria and large areas, National Fire Protection Association, Quincy, MA, 1990.
5. Kim, A.K. and G.D. Lougheed, The protection of glazing systems with dedicated sprinklers. Journal of Fire Protection Engineering, vol. 2, no. 2, 1990, pp. 49-59.
6. Hansell, G.O. and H.P. Morgan, Smoke control in atrium buildings using depressurization. Part 1: Design principles, fire science and technology. vol. 10, no. 1 and no. 2, 1990, pp. 11-26.
7. Hansell, G.O. and H.P. Morgan, Smoke control in atrium buildings using depressurization. Part 2: Considerations affecting practical design, fire science and technology. vol. 10, no. 1 and no. 2, 1990, pp. 27-41.
