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NTSB Reports: Construction Materials Contributed to I-35W Bridge Collapse
Jennifer Roy, Contributing Editor -- Design News, March 18, 2008
Minnesota's I-35W bridge collapsed under the weight of tons of sand and construction materials that were stored at two of the structure’s weakest points, according to reports released Monday by the National Transportation Safety Board.
The reports contain photos of the wreckage, photos of the bridge before the disaster, several witness statements and diagrams detailing the location of each car, truck and construction vehicle on the bridge when it plunged into the Mississippi River Aug. 1, 2007.
The diagrams also estimate the weight of everything on the bridge, including the manufacturer’s weight for each vehicle. It determines each adult weighed about 200 lb and each child weighed between 50 and 100 lb, depending on their age. More than a dozen people were killed and 145 injured when the bridge collapsed.
Earlier reports released by the Federal Highway Administration (FHA) state stress at one of the two weakest points of the bridge was 83 percent more than it could handle, according to the Associated Press.
Though the reports issued Monday state the construction loads were much heavier than certain portions of the bridge could hold, a final report detailing the official cause is not expected until the end of the year, NTSB officials say.
“We have gotten and continue to receive excellent cooperation from the Federal Highway Administration and the Minnesota Dept. of Transportation and have collected a large body of evidence at this point,” NTSB Chairman Mark V. Rosenker wrote in a press release this week. “I applaud the team for its expeditious, thorough and tireless investigation thus far.”
An NTSB spokesman did not immediately return calls for comment.
NTSB officials in January announced serious design flaws in the size of several gusset plates used in the main truss of the 40-year-old bridge, which Rosenker, at the time, said was “a critical part” of the failure.
He said undersized plates were found at eight of the 112 joints on the main truss of the bridge. The 16 plates, two at each joint, were about half the required thickness and were too thin to provide the margin of safety expected in a properly designed bridge.
Following the announcement, the NTSB issued a recommendation to the FHA calling for states to conduct load capacity and stress tests on each of its steel-truss bridges.
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