WASHINGTON, DC (Nov. 15, 2008) — Failures on the part of bridge designers and inspectors led to the I-35 W bridge collapse in Minneapolis, Minn., that killed 13 people, injured 145 and left a chartered school bus with 52 children stranded last year, the NTSB determined in its final investigation report released yesterday.

According to the report, on Aug. 1, 2007 the eight-lane highway bridge experienced a "catastrophic failure" in the main span of the deck truss, sending a 456-foot section of the bridge into the Mississippi River 108 feet below.

In its initial findings, the NTSB found the bridge had an inadequate load capacity due to design error by Sverdrup & Parcel and Associates, Inc. Plates that tied steel beams together failed under increased weight of the bridge due to expansions to the bridge and culminating with the heavy traffic on the day of the collapse. Investigators said 24 of these plates were half the size they should have been based upon the weight the bridge was supposed to bear. The "generally accepted" practice among federal and state transportation officials of giving inadequate attention to this component further contributed to 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 to improve bridge design, review procedures, inspection procedures, inspection, training and load rating evaluations.