Why bridges fail

A University of California, Berkeley civil engineer has criticised the design of the bridge that 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,’ claimed Berkeley professor of civil and environmental engineering Abolhassan Astaneh.

The I-35W bridge over the Mississippi River collapsed during rush hour on August 1, killing at least four people. An estimated 20 to 65 people are missing.

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–Martinez Bridge, a truss bridge that he inspected with Caltrans engineers about 15 years ago. The Benicia-Martinez Bridge, also known as the George Miller Jr. Bridge, crosses the Carquinez Strait just west of Suisun Bay, linking Benicia, California in the north with Martinez, California to the south.

The inspection turned up early signs of fatigue - microcracks in plates welded to the top of the members - that Caltrans, the state agency responsible for highway, bridge, and rail transportation planning, construction, and maintenance. was able to cut out and replace.

‘As soon as the cracks developed, it was fixed and everything was fine,’ said Astaneh.

Astaneh has led National Science Foundation investigations into the collapses of the World Trade Center and MacArthur Maze, a complex of freeway interchanges located near the eastern end of the San Francisco-Oakland Bay Bridge in Oakland, California.