NTSB releases report on I-35W bridge collapse

Findings cite design errors and excessive construction loads as factors

November 17, 2008

The National Transportation Safety Board (NTSB) Nov. 14 released its report on the August 2007 I-35W bridge collapse in Minneapolis. The report cited that the probable cause was the inadequate load capacity, due to Sverdrup & Parcel and Associates Inc.’s design error of the gusset plates at the U10 nodes, which failed under a combination of 1) substantial increases in the weight of the bridge, which resulted from previous modifications, and 2) the traffic and concentrated construction loads on the bridge on the day of the accident.

Two contributing factors to the error were the company’s lack of quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the bridge, and inadequate design review by federal and state transportation officials, including inadequate attention paid to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analysis.

During its investigation, the NTSB learned that 24 under-designed gusset plates escaped discovery in the original review process and were included in the design and construction of the bridge. The plates were about half the thickness of properly sized gusset plates.

On Aug. 1, the day of the collapse, roadwork was under way on the bridge. In the early afternoon, construction equipment and construction aggregates were delivered and positioned in the two closed inside southbound lanes. The equipment and aggregates were staged for a concrete pour of the southbound lanes that was to be performed about 7 p.m. and positioned toward the south end of the center section of the deck truss portion of the bridge near node U10 and were in place around 2:30 p.m.

Later that evening, a lateral instability at the upper end of the Li/U10W diagonal member led to the failure of the U10 node gusset plates. Because the deck truss portion was considered non-load-path-redundant, the total collapse of the deck truss was unavoidable once the gusset plates failed.

The NTSB examined other possible reasons for the accident—including corrosion damage on the gusset plates at the L11 nodes and elsewhere, fracture of a floor truss, pre-existing cracking in the bridge deck truss or approach spans, temperature effects and shifting of the piers—and determined that none were factors in the collapse.

As a result of its investigation, the NTSB made nine recommendations to the Federal Highway Administration and the American Association of State Highway and Transportation Officials, concerning improving bridge design review procedures, bridge inspection and inspection procedures, training and load rating evaluations.

For more information on the report, see the December issue of Roads & Bridges.