The National Transportation Safety Board (NTSB) says a design flaw brought down a major bridge in Minneapolis last August that killed 13 and injured 145.
It’s a design that was commonplace in bridges built 40 years ago and inspectors will be out in force to see whether the design may be present in thousands of bridges.
The flaw is a gusset plate, which is a half inch piece of metal that holds together a junction of several girders.
NTSB inspectors found broken gusset plates in eight out of 122 spots that held together steel beams in the 1,000 foot span of Interstate 35 W bridge.
The plates should be the strongest part of a steel truss and engineers now say that gusset plate should be one inch thick. They warn against loading more concrete or heavy equipment on top of bridges with this design flaw.
When the Interstate 35 W bridge collapsed into the Mississippi River, tons of repair equipment was on the bridge and additional repairs over the last decades had added load on the already stressed joints.
Instead of looking for rust and decay, bridge inspectors across the country will now be focused on the half inch gussets.
The I-35 bridge was a design that has fallen out of favor with bridge designers. Called a “fracture critical” it means there is less redundancy to create a lighter design. That also means that a failure in one area can cause an entire bridge to collapse.
The faulty gusset plates could be designed into more than 12,000 bridges across the country, though that won’t be known until inspections are complete.
The cost of repairing the collapsed I-35 span is estimated to be $400 million and that work is already underway. #