WASHINGTON — The National Transportation Safety Board will offer an update on its investigation into the collapse of the Interstate 35W bridge in Minneapolis last summer, which killed 13 people.
NTSB Chairman Mark Rosenker will speak today at a 12:30 news conference. However, a final conclusion on what caused the bridge to fall is not expected for 12 to 18 months.
The New York Times reported late Monday that investigators have told it that the bridge came down because of a flaw in its design. Designers specified a metal plate that was too thin to serve as a junction of several girders, investigators told the Times.
Federal officials first indicated a week after the Aug. 1 collapse that they were looking at a possible design flaw in some of the steel gusset plates that tie steel beams together under the bridge, the Star Tribune reported at the time. The officials warned that added weight from construction work on the bridge may have been a factor in its collapse.
At that time, U.S. Transportation Secretary Mary Peters advised states to consider the additional stress placed on bridges during construction projects. Two months later, she said the NTSB had a “working theory” of a poorly designed gusset plate and a heavy load of construction materials.
The gusset plate
The Star Tribune reported in mid-October that NTSB investigators had intensified their inspection of a long-corroded gusset plate in the section of the bridge that fell first. Authorities also were analyzing what role the 91-degree heat on Aug. 1 might have played in increasing stress on the already weakened L-11 gusset plate, which connected four steel beams located near the bridge’s south end, the Star Tribune reported.
In 1993, a state inspector found that the half-inch gusset plate had lost nearly half of its thickness in some spots because of corrosion along an 18-inch line, but no repairs were ordered, according to Minnesota Department of Transportation records combed by the Star Tribune.
The bridge was designed in the 1960s. But like most other bridges, it gradually gained weight as workers installed concrete structures to separate eastbound and westbound lanes and made other changes, adding strain to any weak spot. At the time of the collapse, crews had brought tons of equipment and material onto the deck for a repair job.
Help for warm-weather inspection
The information released today will be important to highway departments planning 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,” one investigator told the New York Times. “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-of thing. But you don’t know that.”
The NTSB may advise highway departments to reanalyze the design of bridges before carrying out major work on them, the Times said. Previous practice has been to assume the design was sound, but to inspect for age-related deterioration.
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 gradually fallen out of favor with highway departments.
The New York Times, the Associated Press and staff writers contributed to this report.