How an Unchecked Assumption Brought Down a Bridge

By ThinkReliability Staff

On August 1, 2007, the I-35 bridge over the Mississippi River in Minneapolis, Minnesota collapsed during evening rush hour, killing 13 and injuring at least 145.  During the National Transportation Safety Board’s investigation, it was discovered that the gusset plates (the riveted metal plate that joins several structural members) were designed with inadequate load capacity.  At the time of the bridge collapse, the load on the gusset plate that failed was higher than usual, due to construction materials and equipment concentrated on the deck over the location of the gusset plate and rush hour traffic slowed by the construction.  In addition to these weights, the dead load (weight of the bridge structure) had increased by more than four million pounds due to improvements made to the bridge since it opened in 1967.

Bridges are inspected regularly, and go through a design review process . . . so how did the gusset plate design error get missed?    The design for the gusset plates was apparently supposed to be a preliminary design, which neglected shear stress.  Although the firm that designed the bridge required a review of all calculations before the final design, the procedure did not ensure that all calculations were rechecked, so the gusset plate calculations that ignored shear stress were overlooked.

The design was reviewed by the government, but their design review did not apply to gusset plates.  The gusset plate capacity was not calculated as part of the load rating calculations.   Gusset plates were not listed as a separate element to be inspected during a bridge inspection.  And, the training for bridge inspectors continued very little information about gusset plates.  Why?  Because it was widely assumed that gusset plates are stronger than the members they join and so can be neglected in calculations in order to simplify the analysis.  In most cases, this assumption is true.  However, since the gusset plates were designed incorrectly, and so were much weaker than typical, allowing this assumption to go unchecked, on several different occasions, proved disastrous.

Thanks to this tragedy, it’s unlikely the same problem will happen again.  Structural design and bridge inspection training material is being rewritten to include the lessons learned from this bridge collapse, and inspections are now considering the strength of gusset plates as part of their evaluation.  Assumptions are made all the time, but these assumptions need to be verified.

Click on download PDF to see the NTSB’s root cause analysis investigation results visually displayed in a Cause  Map.  A  Cause Map can capture all of the causes from an investigation in a simple, intuitive format that fits on one page.

Click here for another example of a case where a minor item caused some major issues.

Learn more about the I-35 Bridge collapse.