The Leap-frog Effect: Protecting tall buildings from exterior fire spread

The proposed leap-frog test uses the same apparatus as ASTM E2307 and NFPA 285, with the same fixed window opening size, to create the fire exposure on the exterior side of the spandrel panel or curtain wall. The “opening” is 762 mm (30 in.) high and 1981 mm (78 in.) wide. The research report concludes the current size of the fire (time-temperature curve/burner) is sufficient to provide:

• incident heat flux of 35 kW/m² at a height of 0.91 m (3 ft) above the head of the window opening; and
• incident heat flux of 9 kW/m² at a height of 3 m (10 ft) above the head of the window opening.

In addition to the WPI research findings, testing of a typical aluminum curtain wall system with a 914-mm (36-in.) spandrel height was conducted at Southwest Research Institute, San Antonio, Texas, and witnessed by UL. Instrumentation was installed to provide a temperature profile and incident heat flux measurements vertically and horizontally during the test. The flame temperatures and heat fluxes were determined to be consistent horizontally across a 610-mm (24-in.) width, when measured at 305, 610, 914, and 1220 mm (12, 24, 36, and 48 in.) above the opening.

Based on the research findings, the ASTM task group agreed to utilize the approach of measuring the incident heat flux behind the exterior wall on the floor above the burn room. The choice of pass/fail criteria was selected based on the level of heat flux required for unpiloted ignition of easy-to-ignite combustible materials, and glass breakage. Those heat flux limits are consistent with the normal temperature rise limits imposed by ASTM E119 when determining fire resistance ratings of assemblies (i.e. a maximum average temperature rise of  121 C [250 F], together with a maximum individual temperature rise of 163 C [325 F]). These represent an incident heat flux of 1.8 to 2.6 kW/m² at the measurement location (Figure 4).

When considering floor-to-floor fire spread via openings (e.g. windows), the nature of exterior wall/curtain wall designs is a critical factor dictating the relative capability to resist floor-to-floor fire spread. Key factors impacting curtain wall resistance to vertical fire spread, which need to be evaluated by testing, can include:

• full or partial height vision glass or spandrel panel design;
• nature of the glass used to construct glazing system;
• the curtain wall components installed (e.g. framing, spandrel panels, rainscreen, and air gap);
• vertical or horizontal projections on exterior that may deflect or enhance flame behavior;
• building geometry at curtain wall—inclined, staggered, or sloped;
• operable windows/openings—size and orientation; and
• the vertical alignment of windows/openings.

Among its other functions, a spandrel containment system impedes the vertical spread of fire via exterior fire spread, from the floor of origin to the floor(s) above.

Conclusion
The industry’s understanding of exterior fires and their mechanism of spread in buildings has been researched and reported. However, the potential of fire spread, particularly as related to super high-rise buildings and their façades, presents unacceptable levels of risk to building occupants. Current code practices recognize the successful record of full sprinkler protected high-rise buildings, and only require the void space between the curtain wall and the floor slab be resistive to fire spread using a perimeter fire barrier system. However, as the desire to improve energy efficiency becomes increasingly urgent, more innovative ways to insulate buildings are changing the external surfaces of buildings with an increase in the volume of potentially combustible materials being applied. A number of significant fires have demonstrated the potential risks.

Building geometry and exterior projections of the curtain wall or building structural elements can have a beneficial or negative effect on flame length extension and heat flux exposure to curtain wall elements above the fire compartment. This can be particularly important if operable windows, ventilation openings, or inclined exterior wall designs are employed. Of course, any such condition can allow the unrestricted passage of flames and hot gases from a fire on a floor below into the space above. Also the position of the opening relative to the expected flame extension is important in assessing the risk of a leap-frog event.

The current code requirements focus on the fire testing of specific assemblies not necessarily consistent with the design goals of the architect; yet the larger concern is the associated risk of the fire’s leap-frog effect for high-rise buildings. A review of the history of significant high-rise fire losses where the leap-frog effect was evident shows that the hazard is real and can be catastrophic. Key factors impacting a curtain wall’s fire resistance are being addressed by the proposed ASTM standard. This regulation will be useful when there is a need to provide enhanced protection or evaluate a curtain wall assembly’s potential performance when subject to uncontrolled heat/flame exposure. Most importantly, the fire risk for high-rise, buildings requires the consideration of several factors including:

• the engineering design of the sprinkler systems;
• fire department response capabilities;
• the occupancies and associated fire loads;
• the building’s evacuation approach;
• compartmentation features; and
• security threat assessment scenarios.

With appropriate consideration and evaluation of these risk factors, it is possible to select a curtain wall design meeting both the aesthetic goals and fire-safety objectives for any building.

Based on several years of literature review involving high-rise fires, research reports, fire modeling, and actual fire test data developed for the ASTM task group, the author believes there is sufficient justification and information available to proceed with the development and implementation of the ASTM E5.11.20 “leap-frog” standard.

Tony Crimi, P.Eng., MASc., is a registered professional engineer and founder of A.C. Consulting Solutions Inc., specializing in building- and fire-related codes, standards, and product development activities in the United States, Canada, and Europe. Working with manufacturers and industry associations, he advocates for the approval and safe use of materials and products, and for their code recognition. Crimi has more than three decades of experience in the area of codes, standards, testing, and conformity assessment. He is an active participant in International Code Council (ICC), National Fire Protection Association (NFPA), ASTM, Underwriters Laboratories (UL), and International Organization for Standardization (ISO), and is the immediate past-chair of the National Building Code of Canada (NBC) Standing Committee on Fire Protection. Crimi can be reached at tcrimi@sympatico.ca.