A two-day meeting to adopt the final report of NTSB's investigations concluded on Friday.
The 581m-long highway bridge over the Mississippi River experienced a catastrophic failure in the main span of the deck truss on 1 August 2007. A total of 111 vehicles were on the 305m-long portion of the bridge that collapsed: 13 people died and 145 were injured.
Contributing to the design error, says the board, was the failure of Sverdrup & Parcel's quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the bridge and the inadequate design review by federal and state transportation officials.
Also contributing was the generally accepted practice among federal and state transportation officials of giving inadequate attention to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analysis.
During its investigation, NTSB learned that 24 under-designed gusset plates escaped discovery in the original review process and were incorporated into the design and construction of the bridge.
The final report incorporates a series of recommendations including a new quality assurance/ quality control programme, procedures for identifying locations where non-destructive inspections of truss bridges should be used to identify problems that would not otherwise be visible, modifications to the training of bridge inspectors and the development of guidelines to ensure that bridges are not overloaded during construction or maintenance work.
"Bridge designers, builders, owners, and inspectors will never look at gusset plates quite the same again, and as a result, these critical connections in a bridge will receive the attention they deserve in the design process, in future inspections, and when bridge load rating analyses are performed," said NTSB acting chairman Mark V Rosenker. "By addressing all three areas in our recommendations, we are hopeful that industry and government bodies will take appropriate action and the American people can continue to have confidence in the safety of our nation's bridges," he added.
A synopsis of the Board's report, including the probable cause, conclusions, and recommendations, is available on the NTSB's website, www.ntsb.gov . The Board's full report will be made available on the website in several weeks' time.
