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Weight-gain caused bridge collapse

Investigators said on Monday that the Interstate 35W bridge in Minneapolis, which collapsed into the Mississippi River on Aug. 1, killing 13, came down because of a flaw in its design.

The designers had specified a metal plate that was too thin to serve as a junction of several girders, investigators say.

The bridge was designed in the 1960s and lasted 40 years. But like most other bridges, it gradually gained weight during that period, as workers installed concrete structures to separate eastbound and westbound lanes and made other changes, adding strain to the weak spot. At the time of the collapse, crews had brought tonnes of equipment and material onto the deck for a repair job.

The National Transportation Safety Board had scheduled a news conference for yesterday to discuss its investigation.

The information released will be important to highway departments across the northern US, which are now planning their warm-weather inspection and repair programs. Usually they inspect for corrosion and age-related cracking, but that was not the problem in the Minneapolis collapse, investigators now say.

"This is not a bridge-inspection thing," said one investigator, "It's calculating loads and looking at designs." The investigator spoke on condition of anonymity.

Saying it was not clear whether other bridges might have the same flaw, the investigator said, "This could well be a one-off thing. But you don't know that."

The safety board may advise highway departments to re-analyze the design of bridges before carrying out major work on them. Previous practice has been to assume the design was sound, but to inspect for age-related deterioration. Early in the investigation of the Minneapolis case, the safety board identified the plate, called a gusset plate, as a possible cause. The board will not reach a formal conclusion for some time.

The I-35W bridge was of a type called "fracture critical," meaning that the failure of any major member would cause a collapse, because it had no redundancy. The design is lighter and less expensive to build, but has fallen out of favor with highway departments.

Investigators in the Minneapolis case said it took time to locate all the design information on the bridge, partly because of its age. Structural analysis is easier now than when it was built because of the improvement in computers.
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