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Undersized gusset plates — used to connect bridge beams — were discovered at eight of 112 joints on the main trusses of the Interstate 35W bridge that collapsed in Minneapolis last year, killing 13 and injuring 145. Plates on the Interstate Bridge appear to be the right size, but they’re deteriorating.
Gusset plates have since become the focus of special inspections at 25 Minnesota bridges that bear engineering similarities to the I-35W bridge, including the Interstate span, which was
closed Tuesday evening after inspectors found at least
10 rusted plates just two days into a five-day examination.
“This is really the first time gusset plates have been on the radar screen,” said Kristine Hernandez, a public affairs coordinator for MnDOT’s District 6, headquartered in Rochester. “There are different types
of inspections. The others focused on different things.”
An April 2007 MnDOT inspection of the bridge’s structural stability revealed no major problems. Neither did a routine inspection on Aug. 1 — the same day theI-35W bridge collapsed.
Inspectors gave the Interstate Bridge a “fair” rating in August, according to MnDOT reports. At the time, MnDOT officials acknowledged the World War II-era span was old and deteriorating but did not deem the structure dangerous.
That changed Tuesday when inspectors noticed visible holes in several rusted gusset plates.
“There is rust and corrosion on the gusset plates themselves, several of them,” Hernandez said. “There are holes as a result of the rust.”
Two inspection crews continued to examine the bridge Wednesday, using ultrasonic equipment “to see just how far the corrosion has gone,” said Donna Lindberg, a MnDOT spokeswoman.
Inspectors are likely to find more damaged gusset plates as the examination continues. The teams have yet to reach the main span of the 2,288-foot bridge, Lindberg said.
MnDOT inspectors discovered serious gusset plate problems at two other bridges this year. In March, the DeSoto Bridge in St. Cloud, Minn., was closed after inspectors found four bent gusset plates. Officials partially closed the Blatnik Bridge in Duluth, Minn., to reinforce its gusset plates.
A spotlight on gusset plates began to intensify in January after a preliminary National Transportation Safety Board report that found undersized gusset plates on the I-35W bridge. After the report, Gov. Tim Pawlenty ordered detailed inspections at the 25 Minnesota bridges that bear similar engineering features as the failed I-35W.
The Interstate Bridge is one of the first seven bridges to be examined in the new gusset plate inspections. Crews on the Interstate Bridge are expected to finish Friday, when MnDOT officials will begin at least a weeklong review of the data.
