9/11 Blazes Debunk Code Assumptions About Fire Behavior in Open-Plan Offices
By Nadine M. Post, Engineering News Record
Structural fire engineering is heating up in the U.S. and Europe, thanks in large part to the “traveling fires” observed on Sept. 11, 2001, at the World Trade Center. Structural and fire-protection engineers, aware that current design assumptions do not reflect the behavior of large fires in open-plan office spaces, are developing tools to prevent unprotected structures from collapsing under extreme fire loads.
“I very much believe that certain structures should be analyzed for fire exposure” when there are specific threats or there is a high consequence of failure, says Kevin J. LaMalva, a structural and fire-protection engineer with Simpson, Gumpertz & Heger, Waltham, Mass.
As a member of the American Society of Civil Engineers fire-protection technical committee, LaMalva is co-authoring “Performance-Based Design Procedures for Fire Effects on Structures.” The PBD guide is intended as a non-binding appendix for the 2016 edition of the standard “ASCE/SEI 7: Minimum Design Loads for Buildings and Other Structures.”
The proposed appendix and commentary set forth performance criteria and evaluation methods for structural systems exposed to significant fires, such as traveling fires or other fires large enough to threaten the structural system.
Structural fire engineering—the interface between fire dynamics and structural engineering—is a relatively young discipline. This would be the first time fire is considered as an explicit load, like wind or seismic, in a U.S. standard.
“There is no such guidance for structural engineers in the U.S.,” says Therese McAllister, a research structural engineer at the National Institute of Standards and Technology, Gaithersburg, Md., and LaMalva’s co-author. Building officials need guidance to help early adopters with the approval of proposed PBDs, she adds.
When it comes to large-floor-plate open-plan office space, the 9/11 fires revealed that model building codes’ prescriptive provisions are flawed. Traditional methods for specifying fire load on the structure erroneously assume uniform burning and homogenous temperature conditions throughout a compartment, regardless of its size.
That’s not the condition in a traveling fire, in which the “flame front” spreads around the floor plate, toward openings such as broken windows, to oxygen. As it travels, the fire burns out as it consumes flammable contents, but there is no cooling behind the flames. Smoke, ahead of and behind the flames, actually preheats and post-heats the structure, causing it to lose strength.
In the U.K., researchers at the University of Edinburgh, sponsored by a $100,000 grant from multidisciplinary engineer Arup, developed a PBD method to keep large-compartment structures standing, even in an unfought fire. The work, finished in 2010, was inspired by the fires set by the highjacked plane attacks on the World Trade Center (WTC).
The fires traveled, defying design assumptions in place for 100 years. “We were surprised and at first thought it was an anomaly,” says Guillermo Rein, who led the Edinburgh research and is currently a professor of mechanical engineering at Imperial College, London.
The WTC fires were troubling, adds Rein. Large-compartment fires last longer and produce more heat, though they burn over a limited area at any one time.
Due to this behavior, conventional design approaches are not necessarily conservative, as assumed, says Rein. The research indicates that the worst-case scenario would be a fire traveling with a size 10% to 25% of the floor area.
The traveling-fire methodology uses simple analytical calculations coupled with a finite element model. Computational fluid-dynamics modeling confirms the results, says Rein. Using a PC or a laptop, engineers trained in the methodology can model the behavior of a two- to three-hour fire over an entire floor in two or three days. A computer cluster can accelerate the modeling to several hours. More costly cloud computing can almost model in real time, says Rein.
Five people are trained in applying the methodology: Rein and four engineers at Arup, including his two former Edinburgh Ph.D. students-researchers, Angus Law, currently at Arup Leeds, and Jamie Stern-Gottfried, currently at Arup Berlin.
Rein is seeking funds to produce a guide that explains the methodology. He figures it is a three-year project.
Each analysis is building-specific. This can result in a more cost-effective design, with protection tailored to the threat, says Rein. The method compliments the traditional method, he adds.
Arup has applied the approach to a half-dozen projects. It recommends contacting the authorities having jurisdiction as early as during conceptual design. The analysis is more involved and costs more than a prescriptive approach, says Arup, but it potentially increases the flexibility of the architecture while increasing the structure’s robustness.
Morgan J. Hurley, technical director for the Society of Fire Protection Engineers, Bethesda, Md., has some issues with the traveling-fire method. It is difficult to predict the way a fire will travel because travel is based on the number of openings, such as broken windows, and that is an unknown, he explains. Similarly, the distribution and type of fuel—building contents—influence fire behavior.
In response, Law says the method allows Arup to predict a range of possible scenarios for any particular building.
The Edinburgh research has been peer-reviewed, but it hasn’t gone through the full standardization process. An effort to include it in the Eurocode is beginning. Once that happens, the method will be transferable to the U.S. Says Hurley, “The fire doesn’t care if the building is in the U.K. or Spain or New York City.”
This summer, ASCE’s fire committee chair, Maria Garlock, expects to present the proposed PBD appendix to the main ASCE 7 committee for balloting. Its future is uncertain, says Garlock, a professor of civil and environmental engineering at Princeton University. Seeing potential liability, structural engineers, not typically trained in fire engineering, may resist even a non-mandatory appendix.
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