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The idea of CFD modeling is to divide the space of interest into a large number of cells and use a computer program to solve the governing equations for each cell. If that CFD modeling is done properly, it can simulate atrium smoke flow in realistic detail, including smoke-layer formation. Ideal for analysis of a minimum smoke layer, CFD modeling also can simulate a transition zone, which is important for tenability designs, which are discussed later. CFD modeling requires a level of specialized knowledge for proper use, and a CFD simulation can take many hours, even days, as discussed in past articles in HPAC Engineering.^2,7

The National Institute of Standards and Technology (NIST) has developed a CFD model specifically for fire applications: Fire Dynamics Simulator (FDS).⁸Traditionally, CFD models are expensive, but the FDS model, including documentation, can be downloaded free of charge from www.fire.nist.gov/fds.

DESIGN APPROACHES

The IBC requires that the bottom of a smoke layer be at least 6 ft above the highest walking surface that forms a portion of the required means of egress in an atrium. This means that above the highest required means of egress in an atrium, 6 ft plus space for at least the minimum smoke layer should be available. If this much space is not available, other approaches can be used: separation of the top floor(s), the design of a dummy top floor, minimization of foldback, and/or designing for tenability. Engineers are cautioned that alternate approaches need to be based on appropriate engineering analyses.

Separation of top floor(s)

The top floor(s) of an atrium can be separated from the rest of the building to meet the minimum-smoke-layer-depth requirement. This separation can be done with glass so the occupants can see into the atrium, but the required means of egress on the top level(s) are not inside the large open space of the atrium. While this approach may be unpopular with architects, it may be the only practical way to provide the essential space for the formation of a smoke layer.

Design of a dummy top floor

Some atria are designed with a dummy top floor that occupants cannot access. The only people who have access to the dummy top floor are maintenance people. Because these few people are very familiar with the building, they can leave the dummy floor quickly in the event of a fire. Therefore, the code can be interpreted to mean that the walkways on these floors are not part of the required means of egress. As such, the smoke layer can include the dummy floor. If this approach is desired, local code officials should be contacted early in the project to see if they concur with this code interpretation.

Minimization of foldback

As stated previously, when a ceiling jet reaches a wall, it folds under itself, forming the minimum smoke layer. For long, narrow atria, this foldback can be minimized by exhausting smoke from smoke reservoirs located at the ends (Figure 3). There generally are no accepted algebraic equations for analysis of this approach, and it is suggested that CFD modeling be used. Minimum smoke-layer depth in this case is about 10 percent of the floor-to-ceiling height, but a properly done CFD analysis can provide more accurate information. Also, the smoke-control system can become a tenability design.

Designing for tenability

All of the approaches mentioned are based on the idea that occupants need to be kept away from smoke, but another approach is to design systems that provide a tenable environment for occupants during evacuation, even though there may be some contact with diluted smoke. NFPA 92B recognizes such tenability designs. Section 909.1 of the IBC states that smoke-control systems are intended to provide a tenable environment for occupant evacuation or relocation. Tenability designs are an alternative approach to prescriptive code requirements, and local code officials should be notified early in a project that an alternative approach may be used.

A tenability design needs to be based on smoke-transport and tenability analyses. CFD modeling has the potential to realistically simulate smoke transport and is recommended for smoke-transport analysis. The smoke layer has relatively high concentrations of contaminants near the ceiling and lower concentrations in the transition zone. Many tenability designs take advantage of the lower concentrations near the bottom of the smoke layer. A properly conducted CFD analysis can simulate the formation of a smoke layer, including the transition zone between the smoke layer and the air below.

In addition to CFD simulations of smoke transport, a tenability design needs to include a tenability analysis that evaluates the effect of exposure to heat, toxic gases, and reduced visibility. Tenability analysis is too large a topic to deal with in this article, but is treated in “Principles of Smoke Management.”⁹

A realistic analysis of design fires is essential, and information about design-fire analysis is provided in various sources.^1,5,9,10 A design fire provides results in the heat-release rate of fires and the material(s) burned. If an atrium is designed to have almost no materials that can burn, a design fire still needs to account for transient fuels, which are items that may be in the atrium for a short period of time. Examples of transient fuels include trash waiting to be removed, painting solvents, construction materials during a renovation, and new items awaiting movement.

SUMMARY

Many atrium smoke-control designers do not know there is a minimum smoke-layer depth for atrium smoke-control systems. In most places in the United States, the codes require that smoke-layer depth be at least 20 percent of the floor-to-ceiling height, unless there is an engineering analysis. While this requirement is not popular with architects and owners, it is based on accepted research results. If a smoke layer is not of sufficient depth, occupants can be exposed to smoke during fire situations.

Engineering analysis of this minimum layer can be done by comparison with data from full-scale fire tests or by scale modeling, but the common method is by CFD modeling. A properly conducted CFD analysis can simulate the formation of a smoke layer in realistic detail. Such an analysis requires a level of expertise. For designs for which 20-percent space is not available at the top of an atrium, a number of approaches can be used.

REFERENCES

1. Klote, J.H., & Evans, D.H. (2007). A guide to smoke control in the 2006 IBC. Country Club Hills, IL: International Code Council.

2. Klote, J.H. (2008, December). State-of-the-art of atrium smoke control. HPAC Engineering, pp. 36-40.

3. Alpert, R.L. (1972). Calculation of response time of ceiling-mounted fire detectors. Fire Technology, 8, 181-195.

4. Alpert, R.L. (1975). Turbulent ceiling-jet induced by large-scale fires. Combustion Science and Technology, 11, 197-213.

5. SFPE. (2002). The SFPE handbook of fire protection engineering (3rd ed.). Bethesda, MD: Society of Fire Protection Engineers.

6. Peacock, R.D., et al. (2005). CFAST — Consolidated Model of Fire Growth and Smoke Transport (version 6) user's guide. Gaithersburg, MD: National Institute of Standards and Technology.

7. Klote, J.H. (2006 June). CFD: A new way to design atrium smoke control. HPAC Engineering, pp. 19-27.

8. McGrattan, K., et al. (2008). Fire Dynamics Simulator (version 5) — User's guide. Gaithersburg, MD: National Institute of Standards and Technology.

9. Klote, J.H., & Milke, J.A. (2002). Principles of smoke management. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

10. Klote, J.H. (2002, September). Design fires: What you need to know. HPAC Engineering, pp. 43-51.

A smoke-control consultant and member of HPAC Engineering's Editorial Advisory Board, John H. Klote, DSc, PE, developed and conducts a series of smoke-control seminars for the Society of Fire Protection Engineers. For 19 years, he conducted fire research for the National Institute of Standards and Technology. He is a co-author of the books “A Guide to Smoke Control in the 2006 IBC” and “Principles of Smoke Management.”

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