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.”