Design criteria for smoke-control systems have changed over the years. Instead of ventilating an affected area, smoke-control systems now must prevent the spread of smoke to non-affected areas and/or provide a tenable environment in the area of incident. How effective are current smoke-control systems? Can building-code requirements for smoke-control systems be met reasonably? How can applications not described in building codes be addressed?

Smoke-control-system requirements have been in place for years, and smoke-control-system design has had to adapt to building-code-requirement changes. This article will address some practical design applications of these systems as well as how building-code requirements can be applied. The article also will address ways to adapt a design to meet building-code intent.

History of Smoke-Control Requirements

Smoke-control systems have been utilized in high-rise buildings since the mid-1970s. In early building codes, mechanical systems were required to provide a specific exhaust rate to ventilate a space; building codes later required specific air-change rates, typically six air changes per hour (ACH).

The 1994 edition of the Uniform Building Code (UBC) required smoke-control systems to be designed using a performance-based approach according to one of four basic design methods: pressurization, passive, airflow, and exhaust. These methods were tied to National Fire Protection Association (NFPA) standards for smoke-control systems. Design criteria for the pressurization, passive, and airflow methods included maintaining smoke in the zone of origin to prevent its spread throughout a building; the exhaust method's criteria included maintaining the smoke layer above the highest walking surface in an effort to maintain a tenable environment.

Since the adoption of the 1994 UBC, other model codes have incorporated these design requirements. The International Building Code (IBC) and International Fire Code (IFC) include language similar to that in the 1994 UBC. Refinements have been made to the UBC to stay current with NFPA standards, but the criteria have remained much the same as when the four design methods were introduced in 1994.

The IBC requires smoke-control systems for atria, three-story covered malls, and underground buildings, but offers them as an option in lieu of smoke venting. Although the IBC does not mandate smoke-control systems for high-rise buildings, certain jurisdictions have amended the code to require them, similar to what was required by previous editions of the UBC. Since performance-based design criteria were introduced, many buildings have been outfitted with smoke-control systems using this approach.

Pressurization Method

Smoke-control systems using the pressurization method maintain a pressure difference between the zone in alarm and adjacent zones. To maintain a minimum pressure difference of 0.05 in. w.c., a mechanical system typically is configured with 100-percent exhaust, with no supply air introduced into the zone. Zone barriers are required to be intact and closed; doors and dampers must close and seal off the zone. This design method can be effective in preventing the spread of smoke to other areas of a building (Figure 1).

The pressurization method is most widely used for areas that have the ability to be sealed, such as individual floors and back-of-house spaces. However, barriers need to be maintained to ensure the zone envelope is intact. Otherwise, the system cannot maintain the required pressure difference. Maintaining smoke-zone barriers over long periods of time can be difficult, in part because alterations to spaces adjacent to the barriers can cause leaks. Alterations also can result in work on smoke-barrier walls by personnel unfamiliar with the walls' requirements. Without proper documentation of the location of smoke-barrier walls, alterations can render these systems useless. A smoke-control system designed with the pressurization method can be effective if adjusted properly and maintained.

Sizing exhaust fans for pressurization systems can be challenging. The exhaust required to achieve a desired pressure difference depends on construction leakage. Building codes provide leakage rates that can be used in choosing an exhaust fan. However, a design may need to accommodate construction that is tighter than building codes allow, which could result in overpressurization. Selecting fans to operate effectively under various ranges of operation can help minimize field changes or equipment replacement during initial commissioning.