The National Transportation Safety Board (NTSB) has identified a potential design flaw in the collapsed Minneapolis Bridge. Because of this flaw, the added weight put on the bridge during construction could have played a part in last Wednesday’s Minneapolis Bridge collapse that killed five people and left eight missing. This finding caused Secretary of Transportation Mary Peters, to issue a national alert to bridge engineers to “carefully consider the additional weight placed on bridges during construction or repair projects.”
Earlier this week, construction crews who had been working on the I-35W Minneapolis Bridge reported that they had felt the structure “wobble” in the days leading up to the disaster. Now, the NTSB is taking a close look at the design of steel connecting plates called gusset plates that were used on the Minneapolis Bridge. Gusset plates are usually fabricated from lower quality steel. When the Minneapolis Bridge was built in the mid 1960s, such plates were usually welded to trusses. This type of welding makes the plates more prone to fatigue. Standard bridge design today calls for gusset plates to be bolted to trusses.
The Minneapolis Bridge was being resurfaced prior to the collapse. Traffic on the 8-lane bridge had been reduced to four and several large trucks were parked on the structure during the construction. At a news conference yesterday, Minnesota Department of Transportation (MNDOT) construction engineer Liz Benjamin said that workers had dumped large sand and gravel piles on the I-35 W Bridge at 2:00 p.m., just hours before the disaster. The sand and gravel piles could have weighed as much as 100 tons. Even so, they still would have been lighter than the trucks parked on the Minneapolis Bridge.
The NTSB would not say exactly what type of design flaw investigators were looking at, only that it involved the gusset plates. An unidentified source told the Minneapolis Star Tribune that the NTSB is looking at the thickness of those plates. Despite Wednesday’s alert, the NTSB has said that the gusset plates are just one area that the agency is investigating, and has not ruled out other causes of the Minneapolis Bridge collapse.
Meanwhile, Navy and FBI divers continued to search the Mississippi River for victims. Eight people are still missing in the Minneapolis Bridge Collapse, and they are presumed dead. Four empty cars were removed from the water yesterday, but no more victims were found. Rescue officials said that the divers were doing a hand –over-hand search, and they delayed the removal of heavy debris in order to give the divers more time. The FBI had to stop using an unmanned submarine to search the river. It was too big to maneuver among the twisted wreckage of the I-35 W Bridge. The FBI is planning to try a smaller sub, although that vehicle might have problems with the swift currents of the Mississippi. Officials at the site said they expected to start removing the heavy debris sometime this week.
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