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November 14th, 2008
THE DIG
I-35W Bridge Collapse: NTSB stresses connection between bad design and weight

The National Transportation Safety Board began its public, two-day Board Meeting yesterday to deliver a final report on the cause of last year’s I-35W Bridge collapse in Minneapolis. Federal investigators told the Board that undersized steel plates reinforcing the bridge’s gussets were the chief cause of the collapse. The investigators blamed the design firm of Sverdrup & Parcel for not calculating the proper size needed for the gussets when the bridge was designed in 1963. The plates were only half the necessary thickness to support the bridge. The design firm had also not calculated the additional weight that would be added to the bridge over its 40-year lifespan due to increased car traffic. Additionally, on the day of the collapse, the bridge was further strained by 287 tons of construction material piled in the center of the span.

At the time I-35 was built, the federal government and the state relied only on the seal of the engineer who signed off the project as proof of its integrity. But the investigation uncovered that the gusset plates did not meet engineering guidelines for 1967, the year the bridge was completed. Today’s Board meeting included discussion on the potential need for greater state and federal inspection of bridges. Both the Minnesota Department of Transportation (MnDot) and the Federal Highway Administration (FHWA) conducted only “condition inspections,” looking for rust and corrosion on bridges. NTSB Acting Chairman Mark V. Rosenker stated that the Board will be making recommendations that future bridge inspections should also contain “design inspections” to look for structural deficiencies as well.

The I-35 Bridge was categorized as a “fracture critical” structure prior to its collapse. A fracture critical design means that if one major component of the bridge fails, the entire bridge could collapse. However, a fracture critical bridge is not necessarily structurally deficient. As of 2008, there are 622 fracture critical bridges in the U.S.

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