Study of the I-35W Highway Bridge Collapse Mechanism

Abstract

The Deck truss portion of the I-35W Highway Bridge in Minneapolis, Minnesota collapsed on August 1, 2007 while roadwork was underway on the bridge. The entire truss was recovered from the river to study the causes of failure. The National Transportation Safety Board attributes the collapse to inadequate load carrying capacity of the steel gusset plates connecting the main truss members at four specific nodes. Permanent deformations of the members in proximity to these nodes were documented and a surveillance video camera recorded the collapse event in a major section of the structure.

The inelastic behavior of the deck truss during the collapse event is studied in this research by performing nonlinear structural analysis on a simplified two-dimensional model. Nonlinear behavior is discretized at specific locations starting with buckling of the critical gusset plates and continuing with yielding in members where the internal forces increased at a higher rate during the post-buckling behavior.

The analysis results show the sequence of failure events that lead to the formation of a collapse mechanism in the center span of the deck truss, which is the first to fall into the river. Comparison between the available evidence and the analysis results validate the conclusions drawn in this research.