A 2013 bridge collapse on Interstate 5 near Mount Vernon, Wash., likely stemmed from incomplete record-keeping by the state's department of transportation, an analysis by civil engineers at the University of Illinois at Urbana-Champaign has found.

The collapse was precipitated when an oversized trailer—which had a permit to cross the bridge—clipped the top of the second cross-frame on the bridge. The opening of the bridge was curved, however, so the clearance over the far lanes was lower than the clearance in the center lanes. According to the study, the Washington Department of Transportation keeps only the maximum clearance in its bridge database, which is used to issue permits for oversized vehicles.

The posted height was the maximum in the center, not the lower section above the outer lanes, which the truck hit. Image credit: Tim Stark.“Civil Engineering Professor Tim Stark says that “even though this accident occurred three years ago, it’s still very important because many bridges have this same design, not only in Washington but in other states.”

The analysis found other contributing factors, both structural and human. But the key issue, says Stark, was the variable vertical bridge clearance.

“Many bridges have a square opening, so the clearance is the same across all lanes," Stark says. The problem with this bridge was that it curved down over the edge lanes. The oversized trailer was 15 feet 9 inches tall. The database said the bridge was 17 feet 3 inches, which was in the center. That was almost two feet higher than the edges, which is where the oversized trailer was traveling.”

The structural analysis revealed that the impact to the second cross-frame, rather than the first, caused so much damage because it pulled down the top of the bridge truss, which caused the entire structure to fail.

“The damage cascaded, causing the entire collapse,” says Civil Engineering Professor Jim LaFave.

The researchers also say that the truck's pilot car—whose role was to guide the oversized vehicle—was also in error. The car has an antenna that is supposed to alert the driver if it hits a bridge or other structure. The pilot driver then must alert the truck driver to adjust course.

“In this case, the pilot car either did not impact the bridge or the driver didn’t hear the impact," says Stark. "They never called the truck, so that part of the safety mechanism failed.” He says one solution may be a sensor at the top of the pole that automatically contacts the oversized vehicle if it hits an object.

Updating databases to reflect minimum heights, automated reporting between the pilot car and the oversized vehicle, and structural reinforcement could help prevent accidents and collapses like this one, the researchers say.