[glatt]

Originally Posted by tw

Questions that we should expect an engineering analysis to answer.

A story in this morning's paper contains information leaked by someone on the investigation. Apparently it was a design flaw, not rust or poor maintenance, that caused the failure of the Minnesota bridge. The gusset plates were not thick enough.

The National Transportation Safety Board is expected to announce today that investigators have traced the failure to steel structures known as gusset plates that held together beams on the Minneapolis bridge, according to sources who spoke on condition of anonymity because they were not authorized to discuss the board's findings.

Some gusset plates on the bridge, which carried eight-lane Interstate 35W across the Mississippi River, snapped during evening rush hour on Aug. 1, leading the bridge to crumple, according to the sources.

Gusset plates are common on steel bridges across the nation, including in the Washington area. They hold together angled beams on the bridge's frame.

The sources said the fault in the Minneapolis span stemmed from the bridge's design and would not have been discovered during detailed state inspections.

When the bridge was built in the 1960s, its gusset plates were not thick nor strong enough to meet safety margins of the era, the sources said. Over decades, renovations added weight to the span. It was undergoing a construction project with heavy equipment and material at the time of the collapse.

The sources said investigators were not sure what role those projects played in the incident. But investigators have speculated that the weight from equipment and materials may have triggered the plates' failure, two of the sources said.

During the construction projects, the sources said, state officials and contractors did not recalculate how extra weight might affect the gusset plates. They said it was not standard procedure to do such studies.

The NTSB has not uncovered similar flaws in other bridges, the sources said.

The safety board is expected to recommend at a news conference today that federal and state authorities conduct more rigorous engineering studies of gusset plates before beginning renovation projects on bridges in the future, the sources said.