Thin steel plates appear to be the cause of the bridge collapse, according to results given by safety investigators. The bridge had been overweighed with construction materials when the plates gave way. 13 people were killed and 145 were injured on Aug. 1, 2007, when the bridge collapsed during rush hour.
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It is beyond belief that there has not been more visible public objection to the report that found that “thin gusset plates” were the “cause” of the bridge collapse, and I cannot believe that professional engineers with real accident investigation experience would have signed off on such a report. It’s just too easy to blame long-gone designers for this event.
Two things are clear, even from way out here. One is that the immediate cause of the collapse was overloading by extra decking and construction materials and machinery, combined with the vibration energy put into the structure by construction. The second is that the gusset plates were the “mode” of failure, not the “cause” of failure.
The gusset plates may have had a lower safety factor than the other elements in the structure, but that safety factor must be reckoned in relation to the original design load for the bridge, not the loads later imposed. When a decision was made to add lanes to the bridge, that decision presumably would have been made on the basis of an analysis of the as-built structure. We do not know whether that analysis happened. The decision may have been made in reliance on the safely factors presumably provided in the original design.
When the gusset plates were observed to be bending, that was a clear indication that the structure was beginning its failure. Gusset plates can’t bend unless the structural elements to which they are riveted have moved relative to each other.