Smoke Management in Large Arenas

Prior to the 2010 National Building Code of Canada (NBC), designing a code-conforming indoor stadium or arena with more than about 2,000 seats was a challenge (some would say a nightmare). Obtaining a building permit was an equal or even greater challenge.

Subsequent to the 2010 NBC, life was a little easier if you had knowledge and expertise in applying the National Fire Protection Association (NFPA) 101 Life Safety Code — assuming your office even had a copy of this code and the referenced NFPA 92B Standard for smoke management.

Essentially, the 2010 NBC (and the current 2007 Ontario Building Code) deals with public assembly spaces as either indoor assembly occupancies or outdoor occupancies. The indoor requirements are practical for facilities without fixed seats or a limited number of fixed seats. But the requirements don’t work well for stadium-style cinemas or for arenas with more than about 2,000 seats.

The problems in terms of the fire protection design are with respect to the seating layouts and the required exit capacity. In the case of the stadium-style cinemas that were constructed in Canada beginning in the 1990s, an “equivalency” was required typically based on the NFPA 101 Life Safety Code. Similarly, arenas with more than about 2,000 fixed seats also relied on the NFPA Life Safety Code.

The advantage of NFPA 101 is that it recognizes alternative seating layouts based on the actual required width for the number of occupants who would use an aisle or other egress route. It also acknowledges that large indoor arenas can provide protection to allow longer egress times, similar to outdoor venues.

The Ontario Building Code (OBC) was revised to address stadium-style cinema seating, but it still does not address large indoor arenas, which means alternative solutions are required. In the case of the previous 2005 NBC, it was recognized that significant code changes would be required to address alternative seating layouts and indoor arenas with fixed seats. Due to time and manpower constraints, it was determined that the best solution for the current 2010 NBC was to reference NFPA 101 rather than delay providing requirements to address these types of buildings. Therefore the current NBC permits the use of NFPA 101 for assembly seating without being considered an “equivalency” but rather as a code-conforming design.

Allowing longer egress times

The first task for a designer of such facilities is to obtain a copy of NFPA 101. The second objective is to obtain a copy of NFPA 92B Standard for Smoke Management Systems in Malls, Atria and Large Spaces, which is referenced by NFPA 101.

NFPA 101 includes requirements applicable to “smoke protected assembly seating.” These requirements allow facilities to have less exit capacity and reduced row and aisle widths based on providing smoke protection that will allow longer times for people to exit the building, similar to what is permitted for outdoor stadiums. The basic performance criteria for permitting longer egress times is that systems are available that will maintain the layer of smoke originating from a fire more than 6 feet above the floor level, including 6 feet above the highest seats, during the time required for evacuation.

Calculating the smoke layer

NFPA 92B provides the formulas for calculating the smoke layer from a fire. However, there are a number of different formulas and it must be determined which is the correct one based on the size and location of the fire and ceiling heights.

The most important public areas of an arena facility for fire protection purposes are the concourse and seating bowl. A typical concourse provides the primary circulation to the seating bowl and has the concessions and washrooms. The height of a concourse generally ranges from about 13 to 20 feet. A seating bowl on the other hand may range from 50 feet to more than twice that height.

In order to design the smoke management system it is first necessary to determine the size and type of fire. Most engineers cannot envisage what a 1mW, 5mW or 10mW fire might look like, or a fast t-Squared fire versus an ultrafast t-Squared fire. NFPA 92B does provide some guidance on the fuel load associated with certain types and quantities of combustible material. In an arena, the concourse would have a fuel load associated with concession stands, such as counters or stored boxes of styrofoam cups.

The ice surface would have a very low combustible load during a hockey game but a much more significant load when a combustible stage is constructed for a music concert, for example, or when the arena is filled with fiberglass boats for a boat show.

As well, in the case of the concourse, the fire would be controlled by sprinklers and therefore the fire size would be limited. But the effectiveness of sprinklers in the bowl performance space would be questionable due to the ceiling height. Therefore it would be necessary to design the smoke exhaust system based on a fire that involves the entire performance stage or other venue structure in the space that has a high combustible fire load.

Containing smoke

in the high space

Smoke management is required for the unenclosed egress route, which generally includes the seating bowl and concourse. The seating bowl’s very high ceiling is both good and bad. The height provides a very large volume to contain smoke, but on the other hand, as smoke rises it expands and its volume becomes much larger, although less concentrated. In the concourse, based on its lower ceiling height and having sprinklers operating, less smoke volume is produced. However, the reservoir to contain smoke above the occupants’ heads is smaller.

Typically, the engineer will design the smoke management system as two zones: the bowl and the concourse. In addition to determining the amount of smoke exhaust required it is necessary to determine the quantity and location of the make-up air. Again there are two aspects to make-up air. One is to make sure the required smoke exhaust rate will be achieved, and the other is to make sure the smoke will not spread out of the respective zone.

Detection is tricky

The other challenge in arenas is providing a system for detecting a fire and activating the smoke management system. Typically, in the case of the bowl automatic detection is not practical. For example, the heat and smoke may not be sufficient to activate sprinklers or smoke detectors located at the very high ceiling in the seating bowl. It is more likely that occupants will have left the bowl before the detector is activated automatically and therefore it would be necessary to activate the bowl smoke management system manually. However, it is possible to estimate the time for automatic detection in this high space by further analyses of the heat and smoke produced by a fire.

Smoke detection is also problematic in arenas due to the different uses and temperatures of the space. For example, normally smoke detectors are located on the underside of the roof, but some tests in an arena have suggested that smoke may stratify well below the roof and never reach a detector.

In the case of a concourse it is necessary to confirm that the smoke layer will not have descended too low before it is detected and the smoke management system has been activated. Where there is a high concourse ceiling there is an inherent large smoke reservoir volume above occupants’ heads so there is more time available to activate the smoke management system. However, where the concourse ceiling is lower, early detection and fast activation of the smoke management system is necessary, which means these spaces often require a smoke detection system. The analysis of the smoke layer must also take into account the time between the detection of a fire and the time that the smoke exhaust fans reach full speed and maximum exhaust capacity.

In summary, there are many design advantages for the use of NFPA 101, especially for indoor
arenas where the designer can use its provisions for a smoke protected building. Naturally architects will enjoy the benefits of the seating and egress routes in their designs, and it will be up to the engineer to design a smoke management system that supports their designs.

At the same time, as with all designs there are cost implications. In the case of arenas these usually end up as a comparison of the cost of the smoke management system versus more seats.cce

Jonathan Rubes, P.Eng. is principal of Rubes Codes Consultants in Toronto. He has over 30 years experience on more than 5,000 projects and has served on National Building Code of Canada committees for 25 years. E-mail jrubes@rubescodes.com