Atriums and Plugholes

Atriums are popular in commercial and residential buildings as they provide high marketing value with their environmentally controlled, naturally lit spaces. Such spaces, however, present a challenge for fire protection engineers. They may have a large number of occupants and the compartment geometries may be complex.

By interconnecting a number of floor spaces, an atrium violates the concept of floor-to-floor compartmentation which is intended to limit the spread of fire and smoke between floors. A fire on the floor of an atrium or in any space open to it can cause smoke to fill the atrium and connected floor spaces. Elevators, open stairs and egress routes that are within the atrium can also become smoke-laden.

Because of these difficulties, building codes place restrictions on the use of atrium spaces. Some of the code requirements commonly applied include:

the installation of automatic sprinklers throughout the building,

limits on combustible materials on the floor of an atrium,

the installation of mechanical exhaust systems for use by firefighters to vent smoke,

the provision of smoke management systems to maintain tenable conditions in egress routes.

The use of smoke management systems to maintain tenable conditions in egress routes in atriums has become common in recent years. One approach is to use a mechanical exhaust system to maintain the smoke layer above the highest egress route. Design information for these systems is available in engineering guidelines such as National Fire Protection Association (NFPA) 92B, “Guide for Smoke Management Systems in Malls, Atria, and Large Areas,” 1995, and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) “Design of Smoke Management Systems,” by Klote, J.K. and Milke, J.A.,1992.

A number of situations may have an impact on the effectiveness of a mechanical exhaust system used for atrium smoke management. One concern, raised by many designers and researchers, is the possibility of fresh air being pulled into the exhaust inlet for systems in which the “headroom,” where smoke is to accumulate above the highest egress route, is minimal. This “plugholing”of the exhaust inlet by the fresh air can decrease the efficiency of the smoke exhaust system and result in a deeper layer of smoke to which occupants may be exposed (see figure). The term plugholing originated in British technical literature and will be defined in the 2000 edition of NFPA 92B to describe the plugging of the exhaust inlet by air.

In a recently completed research project sponsored by ASHRAE, the National Research Council of Canada used full-scale physical model studies combined with computational fluid dynamics (CFD) modelling to investigate this issue.1

The results demonstrated that a design approach similar to one used in the United Kingdom to prevent plugholing in gravity venting systems (natural smoke venting through vent openings in a building’s roof) could be applied to an atrium mechanical smoke exhaust system. A design approach was developed and has been approved by the NFPA Smoke Management Committee for inclusion in the next edition of NFPA 92B.

To minimize plugholing, multiple inlets should be used for the mechanical smoke exhaust system. Also, the maximum mass (volumetric) flow rate through each exhaust inlet must be limited depending on the depth of the smoke layer below the exhaust inlet. Using mass flow rate, the design criterion is given by:

where

mmax = maximum mass rate of exhaust without plugholing, kg/s;

Ts = absolute temperature of the smoke layer, K;

To = absolute ambient temperature, K;

d = depth of the smoke layer below the exhaust inlet, m;

b = exhaust location factor (dimensionless);

C = 3.13 (constant).

Based on limited information, suggested values for b are 2.0 for a ceiling exhaust inlet located near a wall, 2.0 for a wall exhaust inlet located near the ceiling, and 2.8 for a ceiling exhaust inlet far from any walls.

In addition to limiting the maximum flow rate through each exhaust inlet, the designer should ensure that there is a minimum separation between inlets to minimize interaction of the smoke flows. Design criteria for spacing the exhaust inlets will be provided in the next edition of NFPA 92B.

The plugholing of fresh air into an atrium smoke exhaust system has the potential to expose occupants to smoke during evacuation. Selecting the appropriate number of exhaust inlets can minimize these problems and improve the efficiency of the exhaust system. Such considerations are particularly important in retrofits and, as mentioned above, in other applications in which the headroom above the highest evacuation route in the atrium is minimal.CCE

Drs. G.D. Lougheed. and G.V. Hadjisophocleous, P.Eng., are senior research officers in the fire risk management program of the National Research Council’s Institute for Research in Construction in Ottawa.