Several readers of the post titled ‘Smart Bridge across the Mississippi River’ have asked if the cause of the collapse of the old I-35W bridge has ever been officially determined. The answer is yes.
The National Transportation Safety Board is mandated by Congress to investigate transporation accidents and determine the probable causes. The NTSB issued its report (HAR0803) on the I-35W bridge collapse on November 14, 2008. It’s a detailed, 162-page, engineering study. Here’s an excerpt from the report’s conclusions (the italics are mine):
[T]he probable cause of the collapse . . . was the inadequate load capacity (bridge was not strong enough), due to a design error by Sverdrup & Parcel and Associates (the bridge designers) of the gusset plates at the U10 nodes (specific places within the bridge structure described in the report), which failed under a combination of (1) substantial increases in the weight of the bridge which resulted from previous bridge modifications, and (2) the traffic and concentrated construction loads on the bridge on the day of the collapse.
What exactly are gusset plates?
The collapsed bridge belonged to a class of bridge called truss bridges. These are bridges assembled from straight pieces of steel — girders, beams, angles, etc. — that are connected together in the form of triangles, and whose ends are tied together by gusset plates. The NTSB report defines a gusset plate as “A metal plate used to unite multiple structural members of a truss.
The I-35W bridge had a total of 112 nodes. The gusset plates at each node were 1/2 inch thick steel. According to the NTSB report (page 128), they should have been 1 inch thick. That was the design error. Catastrophic failure of two or more gusset plates in the central region of the bridge initiated the sudden collapse.
As noted in the report’s conclusions, there were two contributing factors:
(1) The bridge was initially constructed with 1.5 inches of concrete as the deck surface. To combat corrosion of the underlying steel, the layer of concrete was eventually increased to an average of 8.7 inches by the time the bridge collapsed. The weight of the additional concrete increased the dead load on the bridge by 13.4 percent (page 23).
(2) On the day of the collapse, deck renovations were underway. The additional weight of construction equipment as well as piles of sand and gravel for making cement were concentrated on one side of the bridge.