NTSB says design flaw contributed to Minneapolis bridge collapse
The National Transportation Safety Board (NTSB) says a design flaw with gusset plates — steel plates used to connect beams — on the I-35W bridge in Minneapolis contributed to the bridge’s Aug. 1 collapse that killed 13 people. As a result of the ongoing investigation into the collapse, NTSB is urging the Federal Highway Administration to require owners of similar steel truss bridges to conduct “load capacity calculations to verify that the stress levels on all structural elements, including gusset plates, remain within applicable design requirements.”
The gusset plates in eight of 112 joints in the I-35W bridge were half the thickness required to provide a margin of safety, according to the NTSB report. At the same time, the board says it has no evidence that the bridge’s deficiencies “are widespread or even go beyond this particular bridge.” “The [NTSB] has issued this recommendation, at this time, to ensure that the original design calculations for other bridges of this type have been made correctly, before any planned modification or operational changes are accomplished affecting such bridges and before any additional stresses are placed on them,” said NTSB Chairman Mark Rosenker in a statement.
NTSB expects to have a final report on the bridge collapse ready before the end of this year. The interim report is available at http://www.ntsb.gov/Pressrel/2008/080115.html.