Bowing of gusset plates was visible in photos taken of the 35W bridge by inspectors more than 4 years before the collapse.
Two crucial gusset plate connections on the Interstate 35W bridge were visibly deformed at least four years before the structure collapsed, according to photographs newly released by the National Transportation Safety Board.
The NTSB's Office of Highway Safety confirmed Friday that the bowing of steel gusset plates depicted in the June 2003 photos is part of the investigation into why the bridge collapsed last Aug. 1. The deformed plates were located in the bridge's U-10 connections -- nodes of connecting beams that federal investigators believe were among the first points of failure.
The photos came to light last week when the NTSB released more than 100 pages of new information from its investigation. The agency has provided five official updates and will continue to release additional documents leading up to a final report by the end of the year.
Minnesota Department of Transportation officials declined to answer questions from the Star Tribune about the U-10 gusset plate deformations. One of the questions MnDOT was asked was whether knowledge of gusset plate bending on the I-35W bridge contributed to its decision Thursday to close the Hwy. 23 bridge in St. Cloud after inspectors found that gusset plates on that span were bending in four locations.
The St. Cloud bridge is closed indefinitely and may have to be replaced.
"Since some of your questions are directly relating to NTSB published information, we are unsure if we will be able to answer them,'' MnDOT spokeswoman Lucy Kender said in an e-mail. "We will let you know next week.''
There is no record in MnDOT's I-35W bridge inspection reports of anyone ever fixing the out-of-plane bending on the U-10 gusset plates. One of the questions the Star Tribune posed to MnDOT was when the state first became aware of the abnormality.
Plates were under-designed
NTSB Chairman Mark Rosenker has said repeatedly that the U-10 gusset plates, along with more than a dozen other gusset plates that fractured on the bridge, were under-designed 40 years ago -- they were only one-half inch thick. Rosenker has not commented on pre-collapse bowing in the U-10 gusset plates as depicted in the photos.
The pair of U-10 nodes were above the Mississippi River, directly opposite each other in the steel superstructure that was holding up the 40-year-old bridge.
NTSB spokesman Terry Williams said on Friday that the bowing is among "the many things that we are looking at as part of this investigation.''
One of the photos is stamped June 10, 2003. It shows slight bowing in two gusset plates at U-10 West, underneath the bridge deck. A second photo, stamped June 12, 2003, shows one gusset plate in the U-10 East connection bending in the same upstream direction as the plates in the U-10 West connection.
On the day that the I-35W bridge collapsed, tons of sand and gravel were piled on decking in the vicinity of the U-10 connections and the NTSB is investigating whether the construction loads overwhelmed the bridge. Thirteen people were killed, including one of the construction workers, and 145 people were injured in the disaster.
The pre-collapse condition of the U-10 gusset plates is being scrutinized by lawyers representing victims of the collapse.
Phil Sieff, a Minneapolis attorney helping to represent 96 of the victims on a no-compensation basis, said Friday that he wants to know when MnDOT bridge officials first discovered the deformations and what, if anything, they did about them.
"Anyone in the business would know to look into that problem,'' Sieff said.
He said the load-bearing strength of a gusset plate could be compromised by a deformation. He said he doubted that the plates were bent during the initial construction during the 1960s.
Since then, MnDOT had added significant weight to the bridge by adding a layer of concrete to the deck in 1977 and by installing concrete barriers in 1998.
James Schwebel, who is representing another group of victims, said the June 2003 photos contained in the documents the NTSB released are not the only evidence of gusset plates problems beyond the original thickness of the steel.
"We're convinced there was substantial evidence of gusset plate bending that was visible to the naked eye'' before the bridge collapsed, Schwebel said.
The two photos of the U-10 nodes are believed to have been taken by URS Inc., the San Francisco consulting firm that was hired by MnDOT from 2003 through 2007 to study the structural integrity of the I-35W bridge.
Williams, the NTSB spokesman, said he was working to confirm the origin of the photos.
Staff Writer Paul McEnroe contributed to this report. Tony Kennedy • 612-673-4213