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.’