EXPERT ADVISORY

A bridge on Pennsylvania’s I-95 collapsed Sunday morning after a tanker truck caught fire beneath it. A University of Michigan expert is available to comment on why a collapse like this occurs and what the path to repair looks like.

Sherif El-Tawil, the Antoine E. Naaman Collegiate Professor of Civil and Environmental Engineering, studies failure modes of built structures through computer modeling.

“Bridges in the U.S. are not designed for fire rating like buildings; for example, they typically don’t have fireproofing. They also don’t benefit from the protection of sprinkler systems like many buildings. So, they are more vulnerable to fire than buildings,” he said. “However, fire on or below bridges is a rare event, and the lack of protection is therefore considered justifiable by many bridge owners given its high cost.

“It appears from the pictures that the bridge girders were steel. These are susceptible to heat from fire in two ways: 1) the rise in temperature causes a reduction in stiffness, and 2) a reduction in strength. Both contribute to increasing sag as the fire progresses. Since these beams are likely simply supported (sitting on supports that can allow lateral movement for normal thermal effects), they may have a tendency to fall off the supports, precipitating collapse. I believe this is the failure scenario in this case.

“In terms of repair, this will likely be a several weeks long project. All the old components must be removed and new ones installed. The time may be cut if accelerated bridge construction techniques are used, but still, it will be a while.”

Contact: eltawil@umich.edu