Issue 97: Fire Hazards of Exterior Wall Assemblies Containing Combustible Components
By Amanda Kimball, P.E., Fire Protection Research Foundation
Many combustible materials are used today in commercial exterior wall assemblies to improve energy performance, reduce water and air infiltration, and allow for aesthetic design flexibility. These assemblies include Exterior Insulation Finish Systems (EIFS or ETICS), Metal Composite Material (MCM) claddings, high-pressure laminates, foam plastic in cavity walls, and water-resistive barriers (WRB). The combustibility of the assembly components directly impacts the fire hazard. For example, the insulation component of EIFS and MCM claddings is combustible foam which exhibits rapid flame spread upon fire exposure. There have been a number of documented fire incidents involving combustible exterior walls, but a better understanding is needed for the specific scenarios leading to these incidents to inform current test methods and potential mitigating strategies.
The Fire Protection Research Foundation undertook an international project with CSIRO (Commonwealth Scientific and Industrial Research Organization, Australia's national science agency) and FireSERT, the Institute for Fire Safety Engineering Research and Technology at the University of Ulster, to gather information on fire incidents involving combustible exterior walls, compile relevant test methods and regulations globally on the use of combustible materials in exterior walls, and identify the knowledge gaps for future work that would develop guidance on evaluation, testing, and fire mitigation strategies for these wall assemblies. A full report was published in June 2014 entitled Fire Hazards of Exterior Wall Assemblies Containing Combustible Components (White and Delichatsios, 2014).
Available statistics were compiled to determine the extent of fires that involve exterior walls. The statistics for the United States were based on information from the National Fire Incident Reporting System Version 5.0 (NFIRS 5.0) for 2007-2011 as well as the findings of the NFPA’s annual survey of fire department experience. The building types included in the analysis were assembly, educational, health care, residential (excluding one-or two-family homes), mercantile, offices, laboratories and data centers, manufacturing, and selected storage properties. For all building types analyzed, exterior wall fires accounted for 3% of all structure fires, 3% of civilian deaths and injuries, and 8% of property damage (White and Delichatsios, 2014).
The majority of exterior wall fires in the U.S. during the period studied were in low rise buildings. Of all building types, 79% of fires were in buildings one to two stories (White and Delichatsios, 2014). In addition, it was found that while sprinkler systems provide protection, there are still a significant number of fires occurring in buildings protected by sprinklers. For example, in 39% of exterior wall fires in buildings six to ten stories, an automatic extinguishing system was present (White and Delichatsios, 2014).
Statistics from other countries were also examined to determine the extent of fires involving exterior walls. For example, annual fire statistics from New South Wales Fire Brigade, one of the largest fire brigades in Australia, indicate that fires starting in wall assemblies/concealed wall spaces make up 0.5% of total fires and fires starting on exterior wall surfaces make up 1.3% of total fires (White and Delichatsios, 2014).
One issue identified from the examination of available statistics was that there was limited information on the types of exterior walls involved in fires, ignition details, and mechanisms of fire spread. Therefore, the study included a literature review of incident case studies. While few documents with detailed investigations of fire incidents were found, information presented in journals, newspaper articles, and from other sources were examined.
Based on the statistics, exterior wall fires are low frequency events, but the potential for loss can be high. The major fire incidents outlined in the case studies appear to have predominantly occurred in countries with poor regulatory controls on combustible exterior walls at the time of building construction or where the construction was not in accordance with regulations. The most common ignition scenarios are internal fires that then spread to the exterior wall. And it was found that exterior wall configurations with re-entrant corners and/or vertical channels may increase fire spread.
One of the other major components of the project was a review of current regulations and test methods globally. The regulations vary greatly between countries. While many require exterior wall assemblies to pass a full-scale test for buildings of a certain height and/or close to a property boundary, in some cases exceptions allow the use of materials based on results from small scale tests.
In addition, quite a large variation between the full scale tests was noted. Most simulate the scenario of an internal post-flashover fire that spreads via openings (i.e. windows), but the configurations can vary from a single wall surface to a re-entrant corner "L” arrangement, and the fire sizes and test durations are not constant.
Based on the research, the fire safety issues relating to exterior wall assemblies with combustible components can be summarized into four parts (White and Delichatsios, 2014):
- "Specification of fire exposure scenario and the heat flux distribution both inside the enclosure and from the façade flames originating from the fire in the enclosure. This requirement is prerequisite for the following parts.
- Fire resistance of the façade assembly and façade-floor slab junction including structural failure for non-combustible and combustible façade assemblies.
- Fire spread on the external surface of the façade assembly if combustible due to the flames from the enclosure fire.
- Fire spread and propagation inside the façade insulation, if combustible, due to the enclosure fire.”
To address these fire safety issues, the recommendations for future work are not to develop a new full-scale test, but to instead conduct further research to validate the existing full-scale and small-scale tests, develop a more affordable and dependable intermediate-scale test, investigate the effect of vertical channels on fire spread, and development of façade flame spread models. The specific recommendations for future work can be found in the full research report.
References
White, Nathan and Delichatsio, Michael. "Fire Hazards of Exterior Wall Assemblies Containing Combustible Components”. Fire Protection Research Foundation, June 2014.
Related Articles:
4th Quarter 2011 – The Monte Carlo Exterior Façade Fire – Jesse J. Beitel, Hughes Associates, Inc., and Douglas H. Evans, P.E., FSFPE, Clark County, Nevada
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4th Quarter 2011 – Complex Curtain Wall Geometry and Material Selection for Passive Fire Protection – Ajla Aksamija, PhD, and Bruce Toman, Perkins + Will
Curtain wall systems have evolved to more complex and customized solutions, driven by architectural design aspirations and technical capabilities. Innovative building forms are imposing new challenges in terms of facade structural stability, fire protection, and material selection compared to traditional flat facades and standard curtain walls. This article uses examples of actual buildings to show how challenges were overcome. READ MORE
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