Contradicting the National Transportation Safety Board’s report that blamed too-thin gusset plates for the Aug. 1, 2007, collapse of the Minneapolis Interstate 35W bridge, an independent analysis has concluded that rusted, frozen roller bearings prevented thermal expansion and caused a truss chord to fail, triggering the gusset-plate failure. The analysis by Thornton Tomasetti, New York City, using forensic bridge information modeling, was presented to victims’ families and will be deployed in an anticipated lawsuit against URS Corp., San Francisco, and Progressive Contractors Inc., Minneapolis, in the next few months, according to Chris Messerly, one of the pro bono lawyers with Robins Kaplan Miller & Ciresi LLP, Minneapolis.

Photo: Associated Press
New finding blames rusted bearings, not gussets.

“Our experts’ findings confirm that URS, the engineering company hired by the state to ensure the safety of the bridge, and PCI, the construction company who chose to pile its materials on the bridge rather than keeping them just off the bridge, are responsible for this catastrophe,” says Messerly.

Lawsuits on behalf of 117 families of the 13 people killed and 145 injured will be filed in state court within the next few months, he says. “This is a negligence claim. URS knew these things: The design was obsolete; there was no redundancy in the bridge; some gusset plates were bent; the roller bearings were corroded and would not move with broad temperature changes....As they were hired to assess the safety of the bridge, they had a duty to see that [it] was safe. They failed in this duty.”

URS declined to comment. A lawyer for PCI was unavailable to comment due to a medical emergency. Thornton Tom-asetti (TT), which was not allowed to comment, consolidated more than 50,000 paper documents on the collapsed bridge into a forensics information modeling program made available by NTSB. The team created an interactive computer graphic model of the bridge using shop drawings, photos, videos and maintenance records.

Messerly says there is no written report to issue. “We have not asked TT for a report, and I am not sure we will, as they have already given us a very comprehensive presentation of why the bridge fell.”

The issue is essentially one of maintenance versus design. NTSB’s report had blamed the quality-assurance procedures of original bridge designer Sverdrup & Parcel and Associates Inc., St. Louis, for the gusset plates on the main truss. Sixteen of the gusset plates were 1/2 in. thick rather than the required 1 in., according to NTSB Chairman Mark Rosenker.

“We stand by our investigation,” says Terry Williams, NTSB spokesman, adding the safety board has no comment on the TT presentation. Bruce Magladry, the NTSB investigator who was instrumental in alerting state departments of transportation about possibly underdesigned gussets before the NTSB report was released, said in a recent speech, “Forty years ago a design mistake was made that led to construction of a bridge that was in danger of collapsing the day it was completed....It took 40 years for the right circumstances to come about.”

Officials with Northbrook, Ill.-based Wiss, Janney, Elstner Associates Inc., the forensic engineer that discovered the gusset-plate problem, declined to comment on the new findings, citing the ongoing litigation. But its engineers considered the effects of bridge bearings, thermal expansion and construction loads in great detail in their investigation.

Its 164-page report, commissioned just after the collapse by Minnesota Gov. Tim Pawlenty (R) and released along with the NTSB findings, allows that previous URS inspections indicated that the roller bearings at Pier 6 were locked up. However, Wiss, Janney notes that it was unable to determine whether or not they were in motion at the time of collapse.

It speculates, however, that locked bearings may have actually increased the bridge’s capacity, while operational ones would have forced an earlier collapse as the bridge heated up from summer temperatures. “Compromised roller bearings and the associated restraint forces could have resulted in a substantial increase in the bridge’s ability to carry superimposed loads at the time the construction material was being placed on the deck,” it says.

The report continues, “If the bridge structure was still realizing a net heat gain at the time of collapse, the buildup of restraint forces in the Pier 6 bearings could have caused them to slip.” Either way, it dismisses the bearings and points to the gussets as the trigger.

One source close to the NTSB investigation says about the new theory, “I don’t think it’s well-founded,” adding that personal-injury attorneys are “getting a little bit nervous” and pointing to new defendants in an effort to discourage the victims’ families from taking state settlement money, the offer for which expires this month. “I don’t think that things are always driven by facts,” the source adds. “They are driven by dollars.”

State lawmakers last year passed a $38-million bill to compensate victims, up to $400,000 each. Messerly testified in favor of creating the fund. “By taking the money from the fund, they do waive any claim against the state,” Messerly says, adding , “The statute specifically allows the victims to bring claims against all other responsible parties—including URS and PCI.”