by Tony Crimi, P.Eng., MASc.
Building owners and occupants often take fire safety for granted. They assume that buildings are constructed with fire safety in mind and significant attention has been paid to building codes. Nevertheless, there exists one particularly critical juncture frequently overlooked in fire-safe design—the void space between an exterior curtain wall and the edge of the floor. This area can be addressed by perimeter fire barrier systems.
Unlike some fire safety elements addressed primarily through design and specification decisions, perimeter fire barrier systems require careful attention to design, specification, and installation to work properly. Consequently, they demand close collaboration by the architect, specifier, and general contractor to ensure each link in the chain is appropriately addressed.
This article provides a background on the importance of perimeter fire barrier systems, as well as actionable guidance for architects, specifiers, and general contractors to ensure they deliver the level of fire safety their customers have come to expect.
Overview of fire and life safety
According to National Fire Protection Association (NFPA) statistics, there is one structure fire in the United States every 63 seconds. From 2009 to 2013, U.S. fire departments responded to an estimated average of 14,500 reported structure fires in high-rise buildings annually. (This comes from the NFPA’s November 2016 publication, “High-rise Building Fires Report,” by M. Ahrens. Visit www.nfpa.org/news-and-research/fire-statistics-and-reports/fire-statistics/fires-by-property-type/high-rise-building-fires.)
During this same time period, high-rise building fires caused an annual average of 40 civilian deaths and 520 injuries, along with $154 million in direct property damage (i.e. not including reputation damage or litigation costs). Five property types account for three-quarters of high-rise fires:
- apartments or other multifamily housing;
- facilities offering care for the sick; and
- office buildings.
In the early 1970s, the construction industry began to recognize fires in buildings with curtain wall construction were reaching through windows and traveling from floor to floor. Major fires in the United States and Mexico prompted suppliers, code officials, and model code groups to seek passive systems that could contain a fire at the building’s perimeter. Various insulating materials were developed in an attempt to solve this challenge.
The intersection of the exterior wall and the floor assembly provides a number of different paths that may allow a fire to spread. Each of these paths is addressed by different test standards. The International Building Code (IBC) and NFPA codes establish different requirements for each potential path and addresses the means to protect the paths or to prevent the spread of fire based on each separate one.
As with all joint firestops, the intent is to confine a fire to the room of origin and prevent propagation through the floor, ceiling, or walls. With ineffective curtain wall and perimeter void fire protection, fire can spread through the space between floors and walls, through the window head transom and the cavity of the curtain wall, or through broken glass or melted aluminum spandrel panels.
Conceptually, the easiest way to look at the three paths for the fire to spread to adjacent floor levels at the exterior wall is:
- through the void spaces created between the edge of the floor and an exterior curtain wall—these are protected by perimeter fire barrier systems per ASTM E2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-scale, Multi-story Test Apparatus, and ASTM E2393, Standard Practice for Onsite Inspection of Installed Fire-resistive Joint Systems and Perimeter Fire Barriers;
- via the voids or cavities within the exterior curtain wall, with fire spreading by a path within the concealed space of the exterior wall, or along the outer surface of the exterior wall—these are protected by assemblies compliant with NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-loadbearing Wall Assemblies Containing Combustible Components; and
- by leapfrogging (i.e. spreading to the exterior and then impinging on an opening in an upper level)—this mechanism is currently addressed prescriptively, using spandrel panels or sprinkler protection, with a new ASTM test method still under development.
The perimeter fire barrier system is a unique building construction detail installed to protect against the passage of fire, hot gases, and toxic smoke through the voids between the floor slab edge and a nonrated exterior wall (usually a curtain wall). Perimeter fire barrier systems are used to resist interior propagation of fire through the gap between floor and exterior wall for a period equal to the floor’s fire-resistance rating. Additionally, a building’s perimeter fire barrier system should accommodate various movements, such as those induced by thermal differentials, seismicity, and wind loads.