.jpg)
Preventing High-Rise Façade Fire Spread through Adequate Fire Testing
By Gaurav Agarwal, PhD, Richard Davis, PE, and Yi Wang PhD
In recent times, the world has seen an influx of new building products that enhance the energy efficiency, weatherability, and esthetics of high-rise buildings. Unfortunately, such products can be combustible and have led to several instances of severe fires.1-2
Wall assemblies of aluminum or metal composite materials (ACMs/MCMs) are examples of such combustible building products. These cladding assemblies can contain multiple combustible components, including an ACM cladding, continuous insulation (CI), or weather-resistive barrier (WRB), as shown in Figure A. These cladding assemblies are characterized by an air cavity gap to facilitate rainwater seepage and façade ventilation behind the cladding. They give rise to the question of whether building codes have adapted to cover realistic fire hazards posed by such wall assemblies.
Fire Scenarios and Building Code
For an exterior wall assembly with an air cavity gap, the system must have adequate resistance to two fire scenarios:
- A direct fire exposure to the cladding system, which can either be due to spill plumes from a window as in a post-flashover compartment fire (e.g., the Grenfell fire, London, UK, 2017),2 or an exterior fire caused by combustible elements external to the building such as a dumpster, vehicle, balcony, etc. (e.g., Knowsley Heights, UK, 1991; Munich, Germany, 1996; Dijon, France, 2010; Grozny, Russia, 2013; Torch Tower, Dubai, 2017; Istanbul, Turkey, 2018).3 The extreme scenarios for an external fire source can also be a city conflagration (after an earthquake) or wildland-urban interface fires.
For this scenario, most building codes use a large-scale fire test to simulate a post-flashover event to determine the fire performance of a wall assembly. Despite exterior fire events being severe in nature and highly relevant in terms of statistics, most building codes do not include a fire test to simulate such hazards. For instance, the NFPA-285 fire test used in the IBC explicitly states that it covers only post-flashover fires of interior origin. With most present-day high-rise buildings being sprinklered, the post-flashover fire hazard will become statistically less likely to occur than exterior fires. Thus, there is a need for a robust test that can evaluate the hazard of realistically severe exterior fires.
- A fire that may initiate or penetrate the concealed space of the air cavity within the cladding system and spread due to the combustible components. Possible ignition sources for such a scenario are welding sparks, electrical fires, flying embers, etc.; and fire penetration to the cavity space through an inadequately protected window or other openings, such as plumbing/HVAC vents. For instance, the 2007 Water Club hotel fire in Atlantic City, New Jersey USA,3 was a result of welding sparks falling from the roof into the air cavity of its façade and igniting the exposed CI. Such cavity fires have the potential to go undetected until they become uncontrollable due to their concealed source.
Current building codes do not have an appropriate fire test, replicating the end installation of cavity-based cladding systems, to protect against such cavity fire hazards.
Façade Fire Tests and Relative Severity
The relative severity of either post-flashover or exterior fires is defined in terms of the heat flux exerted on the exterior façade. A robust fire test would apply a sufficiently high heat flux to reveal the vulnerabilities of the wall system, as expected in a realistic fire scenario. Multiple large-scale façade fire tests exist globally to test wall systems,3 and their heat flux exposure vary from as low as 40 kW/m2 in the NFPA-285 test to up to 110 kW/m2 in the ANSI/FM 4880 test, as shown in Figure B.
Several literature studies4-8 have shown that heat flux to a single-wall façade on the order of 40 kW/m2 is not representative of a realistic post-flashover fire hazard; instead, 70–80 kW/m2 heat flux exposure is more realistic. Other studies9-10 have revealed that the heat flux to single-wall façades can be 100 kW/m2 or higher if the interior has high combustible loading. Oleszkiewicz’s9 research shows that a corner near a window may result in heat fluxes of up to 150 kW/m2, due to re-radiation from the adjacent wall. Similarly, Alpert and Davis11 have demonstrated that the heat fluxes in corner fire scenarios for exterior fires are on the order of 110 kW/m2. Therefore, heat fluxes of 100 kW/m2 or higher offer a more-realistic representation of these fire hazards, regardless of whether the test simulates exterior or post-flashover fires.
Recent Research Using ANSI/FM 4880
Recently, FM Global used ANSI/FM 4880 to conduct research12 to examine the fire hazards of ACM wall assemblies. Traditionally, ANSI/FM 4880 was based on 25-ft and 50-ft–high corner fire tests; in these tests, a wood fire source imposes a 110 kW/m2 heat flux on the wall surfaces and the extent of fire propagation is measured. Since the 1990s, research has been conducted to develop a new test that would correlate with the corner tests and be comparatively cost-effective. Correspondingly, the 16-ft–high parallel panel test (16-ft PPT) was developed13 and included in ANSI/FM 4880 as an alternative to corner fire tests.
The test consists of two 16-ft–high and 3.5-ft–wide wall panels, mounted in parallel at 1.75-ft separation; the dimensions and the placement of the two parallel panels are carefully designed to trap the convective heat effectively and achieve the same radiation view factor as in the corner fire tests. A well-controlled propane sand burner provides 110 kW/m2 heat flux to the lower portion of both wall panels, and the whole assembly is placed under a calorimeter to measure time-resolved heat release rate (HRR).
Vertical flame spread theory and experimental results show that the peak HRR correlates well with the flame spread extent, and can be used as a reliable measurement and objective pass-fail criterion of the test.13
The 16-ft PPT method was used in this research to evaluate ACM assemblies. The tested ACM assemblies were constructed with various combinations of ACM, CI, and WRB products. Figure D shows the HRR profiles for the 16-ft PPTs,12 and the thresholds for unlimited-height and 50-ft limited-height installation pass criteria per ANSI/FM 4880. Tests #1–3 used thermoplastic-core ACMs and consequently failed; Tests #4 and 5 used fire-retardant-core ACMs with no combustible CI and passed the unlimited-height installation criteria; and, Tests #6 and 7 used fire-retardant-core ACMs backed by two types of combustible CI, and both passed the 50-ft limited-height installation criteria. The results demonstrate that this HRR-based protocol in the 16-ft PPT provides an objective and robust means of evaluating the fire-performance of external wall assemblies.
Figure E shows the fire at peak propagation in Test #1 (fail), Test #4 (pass unlimited-height), and Test #6 (pass 50-ft limited height). The ACM assembly in Test #1 decisively failed the 16-ft PPT with accelerated propagation and flame heights extending higher than 25 ft within 4 minutes of the test. The same assembly, which has passed the 30-minute NFPA-285 fire test using a significantly lower heat flux of 40 kW/m2, is allowed for high-rise installations in the USA per IBC. These research test results12 demonstrate the importance of using a large-scale fire test with a realistic fire exposure to evaluate the fire performance of ACM assemblies.
Cavity Wall Test
Presently, there are no consensus fire tests to evaluate the fire hazard in concealed spaces of cladding assemblies. However, the FM 4411 standard includes a cavity wall fire test, shown in Figure F, which uses two 8 ft high x 4 ft wide panels. One panel is the cladding; the other is either the CI or WRB behind the cladding. The two panels are separated by the cavity depth, replicating the end installation of the cladding system. A propane burner provides 40 kW/m2 heat flux to both panels, which is representative of fires in concealed spaces. The performance of the cladding assembly is measured by HRR and fire propagation elements during the tests.
Path Forward
Based on the research results, the FM 4411 standard has been recently revised to include two fire tests for cavity wall façade systems: the 16-ft high PPT with realistic fire exposures to the façade for post-flashover and exterior fire scenarios, and a cavity wall test to evaluate the fire hazard in concealed spaces of cladding assemblies. Both tests will be proposed for adoption as the testing methods in corresponding building codes.
Gaurav Agarwal, PhD, Richard Davis, PE, and Yi Wang, PhD are with FM Global.
References
1Dubai Tower Burns a 2nd Time, and Flammable Cladding Is Again Under Scrutiny. (2017). https://www.nytimes.com/2017/08/03/world/middleeast/torch-tower-dubai-fire.html.
2London Fire: A Visual Guide to What Happened at Grenfell Tower. (2017). http://www.bbc.com/news/uk-40301289.
3White N, Delichatsios MA. (2014). Fire Hazards of Exterior Wall Assemblies Containing Combustible Components. The Fire Protection Research Foundation, Quincy, MA.
4Empis CA. (2010). Analysis of the Compartment Fire Parameters Influencing the Heat Flux Incident on the Structural Façade. University of Edinburgh.
5Hakkarainen T, Oksanen T. (2002). Fire Safety Assessment of Wooden Facades. Fire Mater 26 (1):7-27. doi:http://dx.doi.org/10.1002/fam.780.
6Mikkola E, Hakkarainen T, Matala A. (2013). Fire Safety of EPS ETICS in Residential Multistory Buildings. VTT.
7Peng L, Ni Z. (2016). Experimental study of window-ejected flame and plume on glass curtain walls. MATEC Web of Conferences 46:05009.
8Ondrus J. (1985). Fire hazards of facades with externally applied additional thermal insulation. Full scale experiments. LUTVDG/TVBB-3021-SE 3021.
9Oleszkiewicz I. (1990). Fire Exposure to Exterior Walls and Flame Spread on Combustible Cladding. Fire Technol 26 (4):357–375. doi: http://dx.doi.org/10.1007/bf01293079.
10Su J, Lafrance P-S, Hoehler M, Bundy M. (2017). Cross-Laminated Timber Compartment Fire Tests for Research on Fire Safety Challenges of Tall Wood Buildings – Phase 2. NRC, Canada and NIST, USA.
11Alpert RL, Davis RJ. (2002). Evaluation of Exterior Insulation and Finish System Fire Hazard for Commercial Applications. J Fire Prot Eng 12 (4):245–258. doi: http://dx.doi.org/10.1106/1042391031317.
12Agarwal G. (2017). Evaluation of the Fire Performance of Aluminum Composite Material (ACM) Assemblies using ANSI/FM 4880. FM Global, Norwood, MA. doi: https://www.fmglobal.com/research-and-resources/research-and-testing/research-technical-reports.
13Nam S, Bill RG. (2009). A New Intermediate-scale Fire Test for Evaluating Building Material Flammability. J Fire Prot Eng 19 (3):157–176. doi: http://dx.doi.org/10.1177/1042391508101994.