Unlike other building systems, it is virtually impossible to test an atrium smoke-control system to design conditions. This primarily is because design conditions involve large design fires that can damage an atrium. Design conditions also can include wind, for which systems are nearly impossible to test. It is essential that an atrium smoke-control system be designed properly and tested to verify it operates as intended and that system components be inspected to ensure they function as specified.

This article discusses atria smoke-control methods for a variety of large open spaces, such as enclosed shopping malls, arcades, sports arenas, exhibition halls, and airplane hangers.

Smoke-control technology has made significant advances in recent years. Design analysis of these systems commonly is accomplished via one or more techniques, such as algebraic equations and zone-fire and computational-fluid-dynamics (CFD) modeling.¹ A detailed mathematical treatment of these techniques is beyond the scope of this article; however, state-of-the-art atrium smoke-control technology is addressed.

BASIC CONCEPTS

When a fire occurs, smoke rises in a plume. As the plume rises, it pulls air from the surrounding space, which causes the plume's mass flow to increase and its temperature to decrease. When the plume reaches the ceiling, it spreads out, forming a layer. An atrium smoke-control system exhausts smoke from that layer, providing a relatively smoke-free environment (figures 1 and 2).

Plume dynamics have been studied extensively, and algebraic equations have been developed to calculate the mass flow and temperature of a plume based on plume height and fire size. For steady-state conditions, exhausted smoke equals the mass flowing from a plume into a smoke layer. Thus, equations can be used to calculate the temperature and flow rate of smoke exhaust.

A book recently published by the International Code Council (ICC)² focuses on the requirements of the 2006 International Building Code (IBC),³ including the equations needed for system analysis. Intended for smoke-control designers and code officials, the book addresses all aspects of smoke control, including pressurization systems; atrium, stairwell, and elevator smoke control; design fires; smoke-control equipment; and inspection and commissioning. The book also details the 2006 IBC's adoption of National Fire Protection Association (NFPA) 92B: Standard for Smoke Management Systems in Malls, Atria, and Large Areas.⁴ A book on smoke control published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)5 includes derivations of many of the equations used in smoke-control design.

The algebraic-equation approach is based on an idealization in which a smoke layer's temperature is the same throughout and the bottom of the smoke layer is a horizontal plane, called the smoke-layer interface. In this idealization, “smoke-free” air is present 0.001 in. below the interface. During a real fire, a transition zone actually exists between the smoke layer and the air below. However, the algebraic-equation approach is useful for smoke-control design.

Smoke filling, an alternative approach to smoke exhaust, requires that occupants evacuate from or through the atrium as smoke fills the space. Smoke filling applies only to atria that have very large volumes above their highest walking surfaces, which create filling times that are sufficient for evacuation, including the amount of time occupants need to become aware of a fire and prepare for movement to an exit. Smoke-control equipment is not required for smoke filling.

Smoke-filling time can be calculated via algebraic equations and CFD and zone-fire models. Filling-time algebraic equations can be found in the previously mentioned book published by the ICC.² Over the last three decades, many zone-fire models have been developed, the most sophisticated of which is Consolidated Model of Fire and Smoke Transport (CFAST), developed by the National Institute of Standards and Technology (NIST).⁶ CFAST is available for free at http://fast.nist.gov. CFD modeling is addressed later in this article.

Because few atria are large enough to rely on smoke filling, the remainder of this article deals only with atrium smoke-exhaust systems.