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The equations found in NFPA standards can produce large exhaust requirements for tall atria. Building codes also require an equal amount of makeup air produced by mechanical or natural means or a combination of the two. Building codes limit makeup air to no more than 200 fpm of airflow toward the fire. While large quantities of exhaust air can be difficult to manage, tackling large quantities of makeup air is even more difficult. Buildings often have insufficient space to allow vents and louvers to introduce makeup air under the prescribed maximum velocity.

Makeup air must be introduced below the smoke layer. In tall spaces, multiple levels may be available to introduce makeup air. In smaller areas, wall space may be insufficient. In these cases, operable panels or doors in an exterior wall can be used to introduce makeup air by natural means. Makeup air also can be introduced by adjacent zones when multiple zones are present (Figure 3).

Makeup air can be difficult to provide in atria because of the physical constraints of the building design. Tall atria with walking surfaces located near the top can require exhaust rates exceeding 300,000 to 400,000 cfm. When makeup air is equal to the exhaust rate and limited to no more than 200 fpm, smoke-control systems can require up to 1,500 to 2,000 sq ft of free vent area. If grilles are used, the free area often is much smaller than the actual size of the grille. If panels or doors are used, a large quantity may be needed to meet the free-area requirements.

There are ways to limit the impact of smoke control in tall atria. The best method is to reduce the height of the highest walking surface in the atrium or provide a way to separate the walking surface from the atrium under a fire condition, such as with fire shutters. If the building design cannot accommodate either option, the fire size needs to be limited with sprinkler control and smaller fuel loads. Another approach is to use a computational flow analysis to determine actual exhaust quantities based on performance objectives. These types of analyses can help reduce NFPA-standard exhaust rates for exhaust-method systems or allow higher velocities in makeup-airflow rates.

If makeup air can be managed, benefits of the exhaust design method include the ability to open adjacent zones to one another. Exhaust-method smoke-control systems also are easier to commission because they do not rely on the balancing of pressure differences.

Conclusion

Smoke-control systems have been required by building codes for decades. For much of this time, design criteria were straightforward. For the last 16 years, smoke-control design has focused on performance criteria based on the physics of fire. Applying the design methods found in current codes has posed some unique challenges. Often, the prescribed approach does not fit within a building's constraints. In these situations, designers are required to adapt or modify the design so the systems can prevent smoke migration effectively. During the last 16 years, HVAC professionals have learned how to meet building-code requirements effectively and adapt a design to meet a building code's intent.

Current smoke-control systems can be very effective in meeting building-code design criteria. Using pressurization or passive design methods, smoke-control systems can be effective in preventing the spread of smoke throughout a building. The exhaust method can help smoke-control systems be effective in creating a tenable environment. How the systems are adapted to a respective building depends on whether a practical approach has been used. The more elaborate the system, the more chances for failure. With practical application and design, code requirements — both prescribed and intended — can be achieved in today's buildings.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan White at megan.white@penton.com.

Vice president of growth markets for jba Consulting Engineers, Allyn J. Vaughn, PE, FSPE, LEED AP, has more than 28 years of experience in fire-protection and smoke-control-system design and commissioning. He has been responsible for third-party testing of smoke-control systems in Las Vegas for more than 13 years. Vice president of engineering for jba Consulting Engineers, Brad R. Geinzer, PE, LEED AP, has more than 20 years of experience in smoke-control-system design and commissioning. He was involved in the design and commissioning of the first Las Vegas building to use the performance-based criteria of the 1994 edition of the Uniform Building Code.