MINNEAPOLIS – More than 191 tons of construction material was piled over the weakest areas of the Interstate 35W bridge shortly before the span collapsed into the Mississippi River, federal investigators said.
The piles of rock and sand, to be used in resurfacing the bridge, were placed over steel gusset plates, connectors joining bridge beams, that were thinner than they should have been, the National Transportation Safety Board said in an update on its investigation Monday.
The Aug. 1 collapse near downtown Minneapolis killed 13 people and injured 145. The NTSB said it expects to produce its final report on the cause of the collapse by the end of the year.
The NTSB had previously cited the too-thin gusset plates and construction project weights as factors in the collapse, but Monday's update included drawings and tables that pinpointed the locations of those heavy loads on the structure.
The NTSB calculated that the bridge was carrying a total load of about 630 tons at the time of the collapse, with a little more than half of that sitting on the center span.
The agency also said it had gotten "archival information" from an engineering company that may help explain how the bridge ended up with thin gusset plates when it was opened in 1967.
The NTSB had said in January that some of the steel plates were too thin and were fractured. Chairman Mark Rosenker called the plates "the critical factor" in the failure but stopped short of saying they caused the collapse, saying investigators couldn't find the original design calculations.
However, the NTSB now says the Jacobs Engineering Group, which acquired the company that was the original design consultant for the bridge, has provided information "to help investigators better understand what type of system of checks and balances would have been in place when the bridge was designed back in the 1960s." The company acquired by Jacobs was Sverdrup & Parcel.
The NTSB update gave no details on that new information, and NTSB spokesman Terry Williams wouldn't elaborate. Representatives of Jacobs Engineering didn't return a call seeking comment Monday.
When the NTSB spotlighted the gusset flaw in January, it asked the Federal Highway Administration to require bridge owners to check their spans to make sure they were designed correctly for the loads they were carrying. The highway administration is continuing to work with state transportation departments to refine its guidance on calculating load rating for steel truss bridges, the NTSB said.