NTSB Acting Chairman Mark V. Rosenker, in a speech today before the Transportation Committee, National Conference of State Legislatures, discussed the Safety Board's investigation and findings of the collapse of the I- 35W bridge in Minneapolis, Minnesota.
"The collapse of the I-35W bridge was one of the largest and most important highway accidents investigated by the Safety Board since its inception in 1967," said Rosenker. "During the 16-month investigation, there was much speculation about the cause of this tragic accident involving a 40-year-old bridge. We have learned many lessons to prevent this type of accident from occurring again."
On Wednesday, August 1, 2007, about 6:00 p.m. Central Daylight Time, the eight-lane, 1,907-foot-long I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota, experienced a catastrophic failure in the main span of the deck truss. As a result, 1,000 feet of the deck truss collapsed, with about 456 feet of the main span falling 108 feet into the 15-foot-deep river. As a result of the bridge collapse, 13 people died, and 145 people were injured.
The NTSB immediately launched a go-team to start the on- scene investigation, which would last more than three months. "Understandably, this accident raised many questions in the minds of Americans about the condition of the nation's bridges," Rosenker said.
Thousands of tons of steel from the collapsed bridge were removed from the Mississippi River. The collapsed portion of the bridge was a steel deck truss, which was considered "fracture critical." Therefore, a failure of any one of the major structural elements in the bridge could cause a collapse of the entire bridge. During the wreckage recovery, fractured gusset plates were discovered. The damage patterns and fracture features uncovered indicated that the collapse of the deck truss portion of the bridge was related to the fractured gusset plates, in particular originating with the failure of the gusset plates at node U10.
The NTSB investigation included the examination of original documents on the bridge design and discovered that the final design plans specified that the U10 gusset plates be 1/2 inch in thickness, rather than at least one inch thickness as required. The Board discovered that the design error was the result of the failure of the bridge design firm, Sverdrup & Parcel, to perform all the necessary calculations. Furthermore, the investigation revealed that Federal and State procedures for reviewing and approving bridge design plans and calculations were insufficient.
"As part of this investigation, a complex computer model of the entire bridge and key gusset plate connections was developed," Rosenker said. "The investigation also included visits to 14 other States to analyze their bridge assessment programs, and to make sure an extra set of eyes looked at what we were looking at, we took the unusual step of asking Sandia National Laboratories, one of the nation's premier laboratories, to conduct a peer review of our methodology and our conclusions," he said.
The Safety Board determined that the I-35W bridge was flawed from the beginning. Also, the Board discovered that load rating inspections do not address gusset plate capacity and computer programs used to identify the weakest link in the bridge did not include gusset plates. Additionally, the cumulative weight from two previous construction modifications, plus the concentrated weight of the construction materials and vehicles, and the traffic weight on the day of the accident exceeded the capacity of the U10 gusset plates.
"The I-35W bridge collapsed due to inadequately designed gusset plates," Rosenker said. "The gusset plates were too thin to provide the margin of safety expected in a properly designed bridge such as this. If the gusset plates had been designed correctly, the bridge would not have collapsed under the loads on August 1."
During its Board meeting last month on the I-35W bridge collapse, the Board issued recommendations on better quality control on bridge design, expanded inspection guidelines for gusset plates, the creation of guidelines for the placement of construction materials, and the use of technology to better identify corrosion on bridges.