March 25, 2008

National Transportation Safety Board releases update on Interstate 35W bridge-collapse investigation

Minneapolis

More than half the weight on the main river span of the Interstate 35W bridge was concentrated on a narrow, 115-foot stretch of roadway that straddled the point where the Aug. 1 collapse began, according to documents released by federal investigators.

Eight piles of rocks and sand that together weighed more than 191 tons were placed over a critical connection plate that was half as thick as it should have been, according to schematics drawn up by the National Transportation Safety Board, the agency charged with determining the cause of the disaster.

As part of an ongoing road resurfacing project, fine and coarse aggregate were placed on a short stretch of roadway cordoned off from traffic and no more than 16 feet wide, directly above where the NTSB found a design flaw that sent tons of concrete and steel crashing into the Mississippi River, killing 13 and injuring 145.

In addition, close to 100 additional tons of cement mixers, water trucks and other construction equipment was located near the piles of aggregate, according to a report in the St. Paul Pioneer Press.

The information was released as part of an update on the ongoing investigation. The NTSB has not reached a final conclusion on the cause of the collapse but expects to wrap up its investigation by the end of the year.

“Significant progress continues to be made in the investigation,” NTSB Chairman Mark Rosenker said in a statement.

On Jan. 15, the NTSB said it was focusing on a design flaw in the bridge: a handful of massive gusset plates — the steel plates that hold the truss bridge’s steel beams together — were one-half-inch thick and insufficiently designed. It also noted that several inches of concrete had been added to the bridge’s roadway over the years, adding to the weight on the bridge.

As a result, the Federal Highway Administration asked states doing work on “fracture critical” steel-truss bridges to recalculate the capacity of gusset plates before doing modifications.

DCN News Services

Print | Email | Comment