By Megan Mansell Williams  |  Photos By Peg Skorpinski

 

When the steel truss of the I-35W bridge in Minneapolis broke into three pieces and tumbled into the Mississippi River on August 1, Berkeley professor of civil and environmental engineering Abolhassan Astaneh-Asl began investigating. He booked a hotel room overlooking the wreckage and boarded a plane headed east. For nine days he took photos, interviewed survivors, consulted other engineers and studied the bridge’s designs and reports to reconstruct the event and prevent similar catastrophes in the future, whether benign or intentional.

“I’m trying to figure out how this bridge lasted so long,” Astaneh-Asl told the audience at the Society of American Military Engineers’ Brown Bag Technical Seminar in San Francisco in late August. “The bridge was fracture critical. By fracturing one member, the entire system collapsed like a house of cards. The public may not know it, but we have a lot of bridges like this.” According to California’s highway agency Caltrans, more than 800 raised spans in the Bay Area alone have been deemed “structurally deficient,” the same rating as the I-35W.

A leading expert in failed steel, Astaneh-Asl has assessed—among others—the destruction of the World Trade Center in 2001 (he recently released a study pinpointing what he says were money-saving design flaws that weakened the towers) and the April 29 collapse of an overpass in the East Bay’s MacArthur Maze, which fell onto the 880 roadway when a gasoline tanker carrying 8,600 gallons of fuel hit a guardrail and caught fire.

The I-35W bridge collapsed due to a “perfect storm” of accumulated problems, Astaneh-Asl concludes. There was a history of corrosion, poor maintenance, crude pre-1970s welding technology and fatigue cracks exacerbated by brutal winters and administration of de-icing agents. Inspection of key areas was blocked because openings had been covered up to prevent pigeons from nesting inside. But the final straw, he says, may have been construction on the span at the time of the collapse, resulting in cuts in the roadway and added weight from heavy trucks and materials.

The National Transportation Safety Board won’t release a final report on the accident for many months, but Astaneh-Asl says it is critical to identify the nation’s weakest bridges now and pursue appropriate retrofitting measures, especially for urban interstate bridges traveled by large numbers of commuters.