from press conference in Minneapolis at 2 pm CDT, August 2, 2007:
Gov. Pawlenty stated that MnDOT has hired Wiss, Janney and Elstner, forensic engineers who will conduct a parallel investigation to the one being conducted by the NTSB; this same firm investigated Boston's Big Dig. MnDOT will conduct emergency inspections on three other bridges in Minnesota with a similar steel truss arch design. MnDOT will also investigate all "structurally deficient" bridges: As of 2007, Minnesota has 13,026 bridges, and 1000 bridges in total that fall into the category of structurally deficient; 77,000 bridges are so designated across the United States. Inspections in 2005 and 2006 stated concerns about stress and fatigue, but did not indicate an immediate need for weight restrictions or immediate retrofit. "There was a view that the bridge was ultimately going to have to be replaced...in the future," Pawlenty said.
Dan Dorgan, MnDOT's chief bridge inspector, covered the 35W bridge's maintenance history. Construction on the Interstate 35 W Bridge began in 1964 and was completed in 1967. On average, 141,000 cars traveled over the bridge daily, making it the busiest in the state. In 1990 this steel truss arch bridge was classified as structurally deficient, initially due to corrosion of the bearings "so that they were not able to move as freely as designed. As the years have passed, there has been some corrosion around the joints of the bridge and in the approach spans there were fatigue cracks that were repaired. There were cracks in the tab welds of the bridge "since the day it was built, but those were not growing and they were stable. And for those reasons we thought the bridge was stable and fit for service," Dorgan stated. Two subsequent studies, one by a University of Minnesota associate professor of civil engineering Robert Dexter (an expert in steel fatigue who died in 2004) and one by the engineering firm URS Corporation, showed that the bridge would be serviceable until 2020, at which point either the deck or the entire bridge would have to be replaced.
"In light of what happened, I'd say we thought we had done all we could. Obviously something went terribly wrong," said Dorgan.
When asked if the freeze-thaw cycles of Minnesota contributed to the collapse, Dorgan noted, "Up until the late 1960s, engineers did not believe that fatigue was something you'd see in bridges.... Unfortunately that was a wrong assumption. Some of the construction practices used in that era, the way they welded members together, those engineers did the best they could, but they were not aware of the [fatigue] issue. It started to manifest itself in the middle 1970s, when we started to see problems across the country, which is why we went to fracture critical inspections where you get within an arm's length of what you need to inspect." He also pointed out that while temperature variance can add to fatigue, the main culprit is truck loads over 40 years.
As for the construction going on at the time of the collapse, Dorgan noted that while there was work being done on the deck joints, the collapse would almost certainly have been triggered beneath the bridge in the steel truss superstructure.
No word yet whether there were any strain gauges or other sensors embedded in the bridge that could have warned of potentially catastrophic fatigue.