No mystery about Minneapolis bruidge collapse

Published 16 August 2007

Expert says truss-arch bridges are like a linked chain: If one link fails, the entire chain collapses; Minneapolis’s I-35W bridge was such a bridge, in which “Local damage immediately means total collapse”

A University of California, Berkeley civil engineer has criticized the design of the bridge which collapsed earlier this month in Minneapolis. “Truss-arch” structures, such as the collapsed Minneapolis freeway bridge, are susceptible to fatigue and collapse, making them “a very bad system,” says Berkeley professor of civil and environmental engineering Abolhassan Astaneh. The I-35W bridge over the Mississippi River collapsed during rush hour on 1 August killing at least four people.

Astaneh compares truss-arch bridges to a linked chain. If one link fails, the entire chain collapses. When heavy vehicles such as trucks pass over such bridges, the steel members — lengths of metal arranged vertically in a triangular pattern — are pushed down and spring back. “That going down and coming up is called cycle of loading,” said Astaneh, “and if you do it millions of times the steel develops hair cracks. They’re very small but they can end up propagating and the crack starts moving and you get more and more cracks, and eventually you lose a member.” Most structural systems or bridges can support the loss of one member. “But not truss bridges and not arch bridges, and this bridge was actually a combination of the two,” he said. “Local damage immediately means total collapse.”

Astaneh says there are no truss-arch bridges in the San Francisco Bay Area and none that he knows of in California. The closest local approximation is the Benicia