During a fire event in a large building like the MGM Grand, it is more likely that an occupant will be exposed to the smoke and gases than the actual flames from a fire. Many occupants are unaware at how quickly smoke and gas can accumulate and travel — both horizontally and vertically. In addition to the many passive and active fire protection systems installed in modern era buildings, which were lacking in the MGM, a smoke control system is another way to protect occupants from the effects of fire.

WORKING UNDER WITH PRESSURE

Even before the MGM fire, discussions about use of purpose-built or shared systems that could effectively manage the smoke generation in a building had been taking place within and between multiple organizations including NFPA, ASHRAE (and its predecessor - the American Society of Refrigeration Engineers), National Bureau of Standards (now NIST), GSA, and code consulting firms.  

NFPA’s first smoke control document, NFPA 92A, “Recommended Practice for Smoke-Control Systems” was published in 1988. During the next 21 years, NFPA 92A matured into a “standard” and NFPA 92B, “Standard for Smoke Management Systems in Malls, Atria, and Large Spaces” was published. In 2012 both standards were merged together into one, NFPA 92, “Standard for Smoke Control Systems.” 

NFPA 92 covers two types of smoke control systems: smoke management systems and smoke containment systems. Smoke management systems work to maintain tenable environments in the means of egress from large volume spaces or prevent the movement of smoke into surrounding spaces. Smoke containment systems keep smoke from entering specific areas using pressurization. One of the most important design objectives for both smoke management systems and smoke containment systems is to maintain a tenable environment for evacuees for a specified time period. 

In large volume spaces, a smoke management system is often designed to keep or maintain the smoke layer 6 ft above the highest walking surface for the duration of time it takes to evacuate the building. This can be accomplished by providing a large unoccupied volume of space for smoke to accumulate in or by using mechanical equipment to exhaust the smoke to the outside or some unoccupied space in the building. The mechanical equipment can also be used to prevent smoke from entering communicating spaces. NFPA 92 provides methods for calculating smoke temperature, density, and flow rate. The smoke production properties are used to determine the required size of the smoke reservoir or the size/quantity of fans that should be installed.

Smoke containment systems use similar fans and mechanical equipment to create pressure differences across smoke barriers. Smoke containment systems are used in stairwells, elevators, and refuge areas. Upon activation, positive pressure is introduced into these areas to prevent smoke from entering.

In order for a smoke control system to be effective, it needs to coordinate with a number of other building systems including the fire alarm and HVAC systems. Automatic activation of the system occurs when the system receives signals from the fire alarm control panel that a detector has activated. The control panel is able to identify which detector has activated and which smoke control function should operate. The panel also can shut down the HVAC system upon detection of smoke — usually when detected in the supply air portion of the system.

IMPROVEMENTS IN PRACTICE ON THE STRIP

If given the chance to visit Las Vegas as a fire protection enthusiast (or tourist), be sure to check out the Luxor. Something you might not notice are the eight faux rock structures tucked into the corners of the pyramid. Each of these structures houses a fan that is actually part of the smoke management system. The Luxor’s 820,000 m³ atrium is open to staggered balconies lining the interior of the pyramid, which posed a unique challenge for the fire protection engineers. Smoke movement in this type of atrium could compromise multiple stories, affecting many hundreds of occupants. To keep smoke away from the balconies, the engineers used multiple fans positioned in a way to create a vortex, directing smoke towards to center of the pyramid. This one of a kind design uses the principles of NFPA 92 to achieve life safety.

Not all systems require camouflaged fans and vortexes to contain or manage the smoke. NFPA 92 provides detailed requirements for the design, installation, and testing of smoke control system. Find additional information at www.nfpa.org/92. ES

References

 Best, R. (1982). Investigation Report on the MGM Grand Hotel Fire. Quincy: National Fire Protection Association.