Smoke control system design

 

Smoke control systems keep building occupants safe from smoke generated during unwanted fires. Requirements for smoke control systems are given in Section 909 of the 2007 and 2010 International Building Code (IBC), the primary model building code used in the United States. For atrium smoke control, IBC 909 refers to NFPA 92B, "Standard for Smoke Management Systems in Malls, Atria, and Large Spaces" for design of smoke control systems.

Smoke control system design in high-rise buildings is accomplished using the pressurization method (IBC 909.6). Mechanical ventilation systems are used to create small pressure differences (minimum of 0.05 inches of water column) across smoke barriers, a special type of fire-resistant construction defined in IBC 709 and IBC 909.5. Maximum pressure differences are limited by door opening forces, which must remain less than 30 lb. During a fire, small pressure differences created by smoke control systems keep smoke confined to one smoke control zone. Smoke control systems based on the pressurization method are designed to prevent smoke from spreading to adjacent smoke control zones, but they are not intended to maintain a tenable environment in the smoke control zone where the fire originates. Instead, that is the purpose of smoke control systems based on the exhaust method -- widely used for atrium smoke control.

Atrium smoke control
Atrium smoke control, and smoke control in similar large volume spaces, is achieved using the exhaust method (IBC 909.8). Exhaust inlets located near the ceiling remove smoke at a rate that is greater than or equal to the rate at which it is generated, or at a rate that maintains a tenable environment during evacuation. Due to the prescriptive requirements of IBC 909.8.1, smoke control systems in an atrium are often designed to maintain the smoke layer 6 ft above the highest occupiable walking surface by exhausting smoke at a rate that is greater than or equal to the smoke production rate. For reasons discussed below, this can lead to unnecessary overdesign of atrium smoke control systems.

The smoke production rate (and the required exhaust rate) for a fire in an atrium increases as the heat release rate of the design fire increases. Therefore, establishment of the design fire is the most critical step in design of atrium smoke control systems. IBC 909.9 requires the design fire to be determined by a "registered design professional" (meaning a licensed Professional Engineer) based on a "rational analysis". Once the design fire has been established by a licensed Fire Protection Engineer, the required exhaust rate (cubic feet per minute, or cfm) must be calculated.

For smoke control system in "small" or "simple" atriums, the required exhaust rate can be calculated by applying the algebraic plume entrainment equations presented in NFPA 92B. However, these equations have a limited range of applicability and there are several common situations where NFPA 92B's algebraic equations are not appropriate for design of atrium smoke control systems, either because they lead to unnecessarily high exhaust rates, or their inherent assumptions break down. Examples include:

  1. Tall spaces (> 5 stories). The algebraic equations in NFPA 92B assume that all air entrained into a fire plume instantly becomes smoke, and the total amount of smoke produced increases super-linearly with height above the fire. Application of NFPA 92B's algebraic equations to determine exhaust rates in tall atria will lead to unnecessarily large exhaust requirements. This will result in an over-designed atrium smoke control system, which adds significant construction costs but does not improve occupant safety over an appropriately-sized system.

  2. Very large-volume atriums. The algebraic equations in NFPA 92B do not account for any effect of smoke dilution, which can be significant in large-volume spaces. The NFPA 92B algebraic equations would require the same exhaust rate in an indoor sports arena and a a 10 ft by 10 ft building shaped like a church steeple, provided the design fire and height of the highest occupiable level are the same! However, it is common sense that the sports arena should require less exhaust because the smoke is spread out over the large volume of air contained in the arena. However, this is not recognized by the NFPA 92B algebraic equations, and application of these equations to very large-volume spaces will lead to unnecessary overdesign of atrium smoke control systems.

  3. Atriums and similar large-volume spaces that do not have a single central floor opening with a large plan area. The algebraic axisymmetric and balcony spill plume equations in NFPA 92B are not applicable to design of atrium smoke control systems unless the plume is not affected by upper balconies or walls. Thus, their range of applicability is limited to spaces having a well-defined central opening, and inappropriate use of these equations can lead to erroneous design.

  4. Spaces where the smoke layer depth cannot be maintained at a minimum of 20% of the floor-to-ceiling height. When designing an atrium smoke control system where the highest occupiable walking surface is close to the ceiling, it is sometimes not possible to simultaneously meet NFPA 92B requirements for minimum smoke layer depth (20% of the floor-to-ceiling height) and simultaneously maintain the smoke layer 6 ft above the highest occupiable walking surface as required by the International Building Code. Specifically, NFPA 92B requires that the minimum design depth of the smoke layer shall be twenty percent of the floor to ceiling height or "based on an engineering analysis". When these prescriptive requirements cannot be met simultaneously, NFPA 92B's algebraic equations cannot be used to size atrium smoke control systems, and an engineering analysis (often entailing computer fire modeling) is required.

For cases where the NFPA 92B algebraic equations for smoke exhaust rate do not apply, or would lead to unnecessarily large exhaust rates, computer fire modeling is usually applied to determine the required exhaust rate. This is accomplished by performance-based design, or an ASET/REST analysis (Available Safe Egress Time vs. Required Safe Egress Time) which is usually addressed through IBC 104.11 "Alternative materials, design and methods of construction and equipment" or California Building Code Section 108.7 "Alternate materials, designs, tests and methods of construction".

Applying techniques such as computer fire modeling, ASET/RSET analyses, and performance-based design to atrium smoke control systems can often lead to significant construction cost savings and make innovative designs possible. Since makeup air (or supply air) is usually provided at 85% to 95% of the exhaust rate, reduction of the exhaust rate can lead to major reductions in makeup air requirements. From an aesthetic and cost standpoint, this is often more significant than reducing the exhaust rate because exhaust fans and inlets can usually be incorporated in the ceiling construction, but low-level makeup air inlets are often problematic in atria.

Reax Engineering, Inc. has considerable expertise in sizing and design of smoke control systems, including application of NFPA 92B's algebraic equations, computer fire modeling, ASET/RSET analyses, and performance-based design. For inquiries related to smoke control system design or atrium smoke control, please contact David Rich.

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            Fire Modeling
            Design of Smoke Control System
            Performance based design
            Structural Fire Engineering
            ASET/RSET analysis
 
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