Posted: July 31, 2024
On June 11, 2023, a tanker truck carrying 8,500 gallons of gasoline crashed and caught fire on the Cottman Avenue offramp from northbound I-95 near Philadelphia. The offramp descended from I-95 and turned more than 90 degrees to pass beneath overpass bridges (consisting of steel plate girders and a concrete deck slab) which support the northbound and southbound I-95 traffic. The consequences of the fire came quickly: the 104-foot northbound bridge span collapsed. Although the adjacent southbound span did not collapse, it was severely damaged by the fire and was demolished and replaced.
Regionally, the nearly 160,000 vehicles that use this stretch of I-95 daily faced a lengthy detour around the fire-exposed overpass bridge. This type of bridge closure poses a potentially enormous economic consequence for drivers and for the state. Other recent highway fires illustrate the costs: a 2017 fire beneath a section of I-85 in Atlanta caused a 43-day closure and a $15 million replacement; a burning tanker truck in Oakland, California, caused the collapse of the McArthur Maze in 2007, requiring a 30-day closure and $90 million price tag. The Pennsylvania Department of Transportation (PennDOT) was able to repair I-95 relatively quickly, minimizing the economic impact of the collapse. However, PennDOT also wanted to find ways to reduce the likelihood of similar incident happening again.
PennDOT asked Lehigh University Professor Spencer Quiel to conduct a forensic analysis of the fire event and the response of the overpass bridge. Such an analysis involves figuring out what happened structurally to the bridge structure during the fire and developing recommendations for mitigating its effects. Quiel, who specializes in structural fire engineering, and his former postdoc, Zhedu Zhu, now working at CHI Engineers in New Jersey, had previously worked with PennDOT on several bridge fire research projects, making them a natural choice for the I-95 analysis. Quiel and Zhu’s objective was to figure out not just what happened to the various components of the I-95 span during the June 2023 fire by modeling it computationally, but also to recommend levels of passive fire protection (i.e. protective heat resistant coatings) that could minimize the consequences of a future fire event.
The design of the offramp itself was vulnerable to a truck crash and subsequent fire – as Quiel says, “it's a severe sweep of a turn, going downhill off I-95 and passing between two solid walls and under a low clearance as the truck decelerates – definitely a hot spot for a crash and fire.” To examine the fire event, Quiel and Zhu used PennDOT’s field observations from the fire as well as its drawings and measurements of the bridge and the site. This information was then used as input for computational models in two different software packages. The first was FLaME (Fire Load and Mitigation Evaluator), which was developed by Quiel and Zhu to calculate the heat exposure from larges fires to bridge and tunnel structures. That custom software package, Quiel says, allows them to “model the fire itself as a geometric model that emits heat energy. We can use that model to calculate the heat flux [or heat energy transfer, or for example the heat from a bonfire warming your hands] that would be transmitted from the fire to the structure. We then create a map of thermal exposure across the length and width of the bridge, and we use a thermal analysis to determine the temperature increase in the steel girder and concrete deck. Based on how those elements heat up, we then plug those results into a structural analysis.” Thermal and structural analysis of the bridge structure was performed using a modeling software called SAFIR, which allows researchers to model elements under fire such as beams and columns, slabs and walls, and different materials such as steel, timber, concrete, and others.
Quiel and Zhu found that their models came very close to matching the real-life response of the overpass to the June 2023 fire. This outcome allowed them to use the model to simulate other more generic fire hazards, in order to generally assess the vulnerability of the bridge to a range of fire exposures. In the second phase of their study, the models were used to develop recommendations for passive fire protection based on performance objectives, ranging from collapse prevention (though the bridge may be damaged by the fire) up to total preservation of functionality.
Passive fire protection involves the application of a coating or encasement material to the structural elements, particularly those vulnerable to fire-induced heating such as steel bridge girders. One coating option is intumescent paint, which Quiel says is “a little thicker than typical paint, a little bit more pasty and with a finished look that is typically less smooth.” When this coating is applied to a structure and exposed to heat, Quiel says that “it performs quite differently than your typical house paint” – it is chemically reactive and begins to char under high-intensity heating. As a result, it will expand to become several orders of magnitude thicker. That grayish-black char layer has a lot of thermal resistance and acts as insulation. Intumescent paint is weather resistant and can provide corrosion protection to coated steel. But unfortunately for engineers like Quiel and Zhu, the data on intumescent paint’s thermal performance is mostly proprietary, closely held by the companies who make the coatings. Because of this, it’s difficult to run calculations and make recommendations.
Instead, Quiel and Zhu turned to a more conventional passive fire protection coating called spray-applied fire-resistive material (SFRM) as a “stand-in” for intumescent paint when running their calculations. This lightweight, cementitious, gypsum-based material is non-reactive but thermally resistant – it is applied to structure elements at a certain thickness based standardized fire tests to mitigate their increase in temperature. An “hourly” fire resistance rating is then determined as the time at which a temperature increase milestone would be met – these ratings reflect the relative thermal resistance of a given passive fire protection application.
Thermal data on SFRM materials is much more widely published (including data produced recently at Lehigh by Quiel’s research group). Quiel and Zhu applied increasing thicknesses of a generic SFRM material to the steel girders in their model of the I-95 overpass bridge to see how the response would improve when subjected to the actual June 2023 fire event. Models with the same thicknesses of SFRM were then subjected to a “virtual” standard fire test to determine the equivalent hourly rating. These hourly ratings are useful, Quiel says, because “they are the language used by fire protection product vendors.” If PennDOT knew what hourly rating of passive fire protection was needed to improve the performance of their bridge, they could approach an intumescent paint vendor to request a quote for the application of that rated amount of their product.
Although an analysis of one overpass bridge on one specific highway might seem like a relatively small project, Quiel believes that this effort can advance the application of structural-fire engineering in the industry. It helps to, he says, “dial in on how you can demonstrate improvement and what’s possible. That’s one of the things that we really wanted to provide for PennDOT: useful, actionable information based on advanced analysis. By successfully capturing what happened during the fire event, the models can help us demonstrate the cost-benefit of providing mitigation.”
This month, Quiel’s research group started a new, two-year research project with PennDOT for large-scale fire testing of steel girders with intumescent paint. Modeling based on those tests will provide an additional basis for the calculating improvements in the response of steel girder bridges to severe fire.
Bridge fires might be relatively rare, but, Quiel says, “hazards that are low probability can have big consequences, so it’s appropriate for us to carefully consider our vulnerability to these hazards and what we can do to respond.” Keeping infrastructure intact and making it more resilient can have a big impact, not just on the traffic flow but also for the broader economy and the environment.