Few bridges made to resist fire
The tanker truck full of gasoline lost control and burst into flames on a California highway exit ramp, blasting the bridge above it with intense heat.
The bridge was supported by girders made of steel — the reliable workhorse material of modern infrastructure, used to support skyscrapers and bridges the world over.
Yet it lasted all of 17 minutes.
That fire, in Oakland, California, in April 2007, was eerily similar to what happened in Northeast Philadelphia on Sunday morning, when a tanker fire brought down an overpass on Interstate 95.
Investigators have just begun to analyze what caused the 105-foot span to collapse onto the highway below. But as a review of the findings into the Oakland fire makes clear: Steel is more vulnerable to heat than one might think.
Steel melts at temperatures above 2,500 degrees Fahrenheit, depending on the type. But it starts to weaken well below that level, said Amir Farnam, a Drexel University associate professor of civil, architectural, and environmental engineering.
The metal loses about 20% of its strength at 750 degrees, and about half of its strength at 1,000 degrees, he said.
“You have a drastic decrease in the yielding strength,” he said.
LIKE A BLACKSMITH’S FORGE
As steel heats up, the individual molecules of iron and carbon shift in position, and the material begins to bend and deform, said Andrew Bechtel, chairman of the civil engineering department at the College of New Jersey.
“You can think of a blacksmith forging something,” he said. “They heat it up, and it becomes pliable.”
In addition to weakening the metal, heat also makes steel girders vulnerable in another way. They expand, said Gregory S. Rohrer, a professor of materials science and engineering at Carnegie Mellon University.
At 1,000 degrees, a steel beam expands in length by less than 1%, he said. But on a 105-foot span, that amounts to several inches — enough to cause failure if it is secured to other components that expand by different amounts.
Bechtel, the TCNJ engineer, said the weakening of the steel probably played the biggest role in the collapse. But the rapid expansion from heat could have contributed.
“Bridges expand and contract with temperature on a normal day, and engineers account for this,” he said. “But this is just a lot of heat. It’s abnormal.”
FIRES CAUSE ONE BRIDGE FAILURE PER YEAR, ON AVERAGE
The 2007 Oakland fire engulfed a tangle of highway exit ramps nicknamed the MacArthur Maze. Investigators reviewing the blast later estimated that the temperature of the overpass reached 1,560 to 1,830 degrees.
The heat caused the steel girders to fail at multiple points. One portion of the I-580 overpass collapsed within 17 minutes, and a second portion sagged and partly collapsed 20 minutes later.
Most bridges are not required to withstand fire, as the cost is considered prohibitive for an uncommon event, said Thomas Gernay, an assistant professor of civil and systems engineering at Johns Hopkins University. Yet the risk is real.
Fire caused 30 U.S. bridge failures from 1980 to 2012 — roughly one per year — compared to just 20 failures from earthquakes, according to an analysis by University of Buffalo engineers. Of the 30 bridges that failed due to fire, 13 were made of steel.
Bridges made from concrete are less vulnerable to fire, as the material is more heat resistant than steel. But concrete can fail in a fire, too, accounting for five of those 30 failures in the Buffalo study. (The rest were made of wood.)
That’s because concrete is brittle, and far less able to expand and contract than most metals, said Bechtel, the TCNJ engineering professor.
What’s more, concrete girders contain steel. So if it gets hot enough to melt the steel, the concrete gives way.
That’s what happened in another highway bridge failure, in 2017 in Atlanta, when stored construction materials caught fire beneath.
Excessive heat caused the steel inside the concrete to weaken, causing the girders to “delaminate” and crack, according to an investigation by the National Transportation Safety Board.
“Those things are supported by steel strands inside,” Bechtel said. “If those strands snap, then it goes.”