Underdesigned Gusset Plates Cited by NTSB in I-35W Bridge Collapse Investigation

This article reports on the National Transportation Safety Board (NTSB)'s recommendations in informing the Federal Highway Administration (FHWA) of its interim findings in the investigation of the August 1, 2007 collapse of the Interstate 35W Bridge in Minneapolis, in which 13 people were killed and 145 injured. Under-designed gusset plates fractured early in the collapse, prompting the NTSB to ask that the FHWA require all owners and operators of bridges that are steel deck truss with no redundancies in their load paths to determine whether their gusset plates are sufficiently robust. The FHWA reports that there are roughly 465 bridges of this type in the U.S. The gussets were just 0.5 inches thick, roughly half the thickness required for the imposed loads. The NTSB is also examining whether extra loads from renovations and repairs to the bridge deck in 1977 and 1988 were a factor in the collapse. A key consideration in gusset plate evaluations is the ratio of demand to capacity (D/C), which is normally less than 1.0.