Structural, regulatory, and human errors contributed to Washington bridge collapse

Benekohal recommended that other states adopt the Illinois Department of Transportation’s policy of reporting the lowest vertical clearance of a bridge, rather than the highest, and its periodic use of LIDAR to verify clearance numbers, which can be affected by repavement, snow or other factors.

The pilot car was the source of the human error factors detailed in the paper. Oversized vehicles have pilot cars to guide them, intended to help thwart the kind of scenario that led to the I-5 collapse. The pilot car has an antenna that is supposed to alert the driver if it hits a bridge or other structure, indicating that the clearance is too low for the following oversized vehicle to continue. Then the pilot driver must call the truck driver to tell him or her to adjust course.

“In this case, the pilot car either did not impact the bridge or the driver didn’t hear the impact. They never called the truck, so that part of the safety mechanism failed,” Stark said. “One solution we suggest is a sensor at the top of the pole that automatically contacts the oversized vehicle if it hits an object. This eliminates the drivers having to communicate quickly so the oversized vehicle can change course. Also, the pilot car antenna was not straight, so it was not accurately measuring the full height.”

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

“I think one of the interesting things about this failure is that the initial damage of where the truck hit was not a primary support, it was a cross beam, and the damage cascaded, causing the entire collapse,” LaFave said. “We then looked at ways to reinforce bridges with this design, to increase the capacity and reduce the chance of this kind of failure occurring. We can selectively add supports so there are ways to redistribute the impact load, so the structure can remain stable and stay standing even if there’s damage to a particular area.”

U of I says that the researchers hope that their recommendations can help address the differing factors that contributed to this incident, so that it will not be repeated. Updating databases to reflect minimum heights, automated reporting between the pilot car and the oversized vehicle, and structural reinforcement could prevent accidents and collapses like this one, increasing safety and preventing costly repairs, they said.

“A structural failure is obviously not desirable. However, the positive outcome is that we’re taking the opportunity to learn from a failure,” Fahnestock said. “We want to understand what happened so that we can be a part of preventing something like this from happening in the future, and thus provide a safer and more reliable infrastructure.”

— Read more in Timothy D. Stark et al., “I-5 Skagit River Bridge Collapse Review,” Journal of Performance of Constructed Facilities (2016) (DOI: 10.1061/(ASCE)CF.1943-5509.0000913)