Many smoke-control designers do not know there is a minimum smoke-layer depth for atrium smoke-control systems. Section 4.4.1.1 of NFPA 92B, Standard for Smoke Management Systems in Malls, Atria, and Large Spaces, requires that the minimum design depth of a smoke layer be 20 percent of the floor-to-ceiling height, unless an engineering analysis is performed. This 20-percent requirement significantly limits what architects and owners can do with an atrium. In this article, the term “atrium” is used in a generic sense to mean any large-volume space, including stadiums, arenas, and airplane hangars.

It should surprise no one that the 20-percent requirement is not popular with architects and owners. A few years ago, I met a senior engineer who erroneously thought an atrium smoke-control system could be designed for any smoke-layer depth he wanted, even as little as 1 in. Was he ever wrong! The smoke layer that forms under a ceiling has a minimum depth for reasons discussed later in this article. Designs that do not allow the 20 percent mentioned previously may result in exposing building occupants to smoke in a fire situation.

The minimum smoke-layer-depth requirements of NFPA 92B apply to almost all places in the United States because the standard has been adopted by the International Building Code (IBC). A book published by the International Code Council (ICC)¹ presents discussions of IBC smoke-control requirements and extensive engineering information to meet those requirements. Regardless of the codes, if the smoke layer is not of sufficient depth, occupants can be exposed to smoke during fire situations.

The December 2008 issue of HPAC Engineering included an article (State-of-the-Art Atrium Smoke Control) that provided an overview of smoke-control technology, including basic concepts, smoke stratification, makeup air, plugholing, and minimum smoke-layer depth.² Minimum smoke-layer depth, including the reason for the 20-percent requirement, engineering analyses, and design approaches, is explained in greater detail in this article.

WHY 20 PERCENT?

A smoke plume rises above a fire. When the plume reaches the ceiling, smoke flows away from the point of impact in a radial direction, forming a ceiling jet. When the ceiling jet reaches a wall, the smoke flow goes around and under the ceiling jet (Figure 1). The ceiling jet has a depth of about 10 percent of the floor-to-ceiling height, as does the smoke flow under the ceiling jet. This means the smoke-layer depth is about 20 percent of the floor-to-ceiling height. The smoke may descend lower than this minimum smoke-layer depth. Designs need to have sufficient room for the smoke layer to form.

FIGURE 1: Formation of the minimum smoke layer.

During the 1970s, fundamental research about ceiling jets was conducted by Ron Alpert of Factory Mutual.^3,4 Based on his research, Alpert developed correlations for the thickness, temperature, and velocity of ceiling jets. These variables are functions of the height of the ceiling above a fire and the radial distance from the axis of the smoke plume. In most situations, the 10- and 20-percent values mentioned previously are conservatively large. For a detailed treatment of ceiling jets, including Alpert's research, see Chapter 2 of Section 2 of the SFPE Handbook.⁵ While an engineering analysis could rely on these correlations, there are advantages to using computational fluid dynamics (CFD).

ENGINEERING ANALYSIS

An engineering analysis of minimum smoke-layer depth can be performed with data from full-scale fire tests, scale modeling, or CFD analysis. Full-scale and scale-model tests are relatively expensive, which explains why they are not used often for this kind of analysis. Because zone fire models, such as Consolidated Model of Fire Growth and Smoke Transport (CFAST),⁶ are well-known and easy to use, some engineers may be tempted to try to use one to analyze minimum smoke-layer depth, but zone models are inappropriate for this application.

FIGURE 2: Atrium smoke flow.

In a room or an atrium with a fire, smoke rises and forms a smoke layer as previously described, and there is a gradual transition between the smoke layer and the air below (Figure 2[a]). Zone fire models consider the smoke layer to have uniform properties throughout. In zone fire modeling, the bottom of the smoke layer is considered to be a horizontal plane, so that at 0.01 in. above this plane, the concentration of contaminants is the same as everywhere else in the smoke layer (Figure 2[b]). While zone fire models are useful for many applications, they routinely predict smoke layers of unrealistically small depth in the early stages of fire development. For this reason, zone fire models must not be used to analyze minimum smoke-layer depth.