I35 Bridge update: Did heat, rusted plates doom bridge?

Did heat, rusted plates doom bridge?

Excerpt:

Ten weeks into their probe of the Interstate 35W bridge collapse, National Transportation Safety Board investigators have intensified their inspection of a long-corroded gusset plate that was located in the section of the bridge that fell first.

In addition, authorities are analyzing what role the 91-degree heat on Aug. 1 might have played in increasing stress on the already-weakened L-11 gusset plate, which connected four steel beams located near the bridge's south end.

In 1993, a state inspector found that the half-inch gusset plate had lost nearly half of its thickness in some spots due to corrosion along an 18-inch line, but no repairs were ordered, according to Minnesota Department of Transportation records.

Two structural engineers who have viewed the wreckage said in interviews this week that the L-11 gusset plate is one of three closely situated connections that could hold the secret of what caused the bridge to collapse. All three joints appear to have been damaged by some primary force -- not from secondary impacts sustained during the collapse, the engineers said.

Comment: See earlier CFG posting: Problems with the Gusset joints?. Star Tribune article has photo and a graphic that are worth viewing.