NTSB analysis shows the gusset plate fractured because of corrosion and mistaken design, not the force of the 35W bridge collapse.
A critical gusset plate connection on the I-35W bridge fractured partially along a line of corrosion that had gone unfixed by state transportation officials since at least 1993, according to evidence released Tuesday by the federal agency investigating the collapse.
The laboratory report from National Transportation Safety Board (NTSB) doesn't draw conclusions, but it said a corroded plate in the L-11 East gusset node broke apart from compression forces in the bridge truss, not from secondary impacts sustained during the collapse.
Gusset plates have been a primary focus of the federal investigation because many of the plates that were used to sandwich and join the bridge's beams were mistakenly designed to be one-half inch thick, rather than one inch. According to NTSB Chairman Mark Rosenker, the agency has not ruled out whether Minnesota officials missed potential clues to the impending failure.
In 1993, a state bridge inspector found that the half-inch gusset plate at L-11 East had lost nearly half of its thickness in some spots due to corrosion along an 18-inch line. No repairs were ever ordered.
Figure 3 in the newly released lab report is a close-up photo of the gusset plate taken after it was recovered from the Mississippi River. Arrows highlight the cleanly broken, rusted edge.
"Figure 3 shows the line of corrosion in the west gusset plate adjacent to and just below the fracture between the tension diagonal and the lower chord,'' the lab report said.
Kyle Hart, a lawyer for Progressive Contractors Inc., the firm that was repaving the bridge when it fell, said the newly released lab results are important to pinpointing the specific location of initial collapse. While PCI has been on the defensive for piling heavy construction loads onto the bridge on the day it collapsed, the company has maintained that the structure was improperly maintained by state workers and was ready to fail.
Hart said the latest NTSB evidence surrounding L-11 and other gusset plate connections is a breakthrough for NTSB outsiders, including lawyers for the 13 people who died and the 145 others who were injured in the collapse. The agency has largely kept the evidence confidential but has begun to negotiate terms for lawyers in bridge litigation to do their own inspections.
Coupled with Tuesday's disclosure of the lab report, the NTSB released a frame-by-frame study of videotape of the west side of the bridge taken by a surveillance camera at the Lower St. Anthony Falls Lock and Dam. According to the study, some of the first signs of movement were around the L-11 and nearby U-10 gusset plate connections on the downtown Minneapolis end of the bridge.
The center span, the segment arching over the river, collapsed flat into the water in about three seconds, the study said.
The 40-year-old bridge collapsed at 6:05 p.m. Aug. 1. Nine hours earlier, between 9 a.m. and 9:20 a.m., the surveillance cameras detected two or three people walking on lower members of the steel truss. According to the NTSB study, two electrical workers and two public works employees of the City of St. Paul were working on the bridge's anti-icing system when the video was taken.
Tony Kennedy • 612-673-4213