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National Transportation Safety Board concludes investigation into I-35W bridge collapse
Cites design error in original plans as probable cause
ST. PAUL, Minn. – The National Transportation Safety Board today released the results of its investigation into the cause of the Interstate 35W Bridge collapse on Aug. 1, 2007 and made recommendations for changes in industry practice to prevent a similar tragedy from occurring on the nation’s other bridges.
During a two-day public hearing that began Thursday, Nov. 13, the NTSB cited an error in the original design of the gusset plates as the probable cause of the collapse. Construction on the bridge began in 1965 and it opened to traffic in 1967.
“The Minnesota Department of Transportation appreciates the thorough investigation that the NTSB undertook during the past 16 months, and we respect the professional manner in which the NTSB investigative staff performed this very difficult assignment,” said Tom Sorel, Mn/DOT commissioner.
“It was critical to have a definitive and complete review of the accident and history of the bridge. Only through such a process can we learn and make improvements,” he said.
As the NTSB released information during the last year, Mn/DOT took actions in response to their findings, including:
- Requiring peer review of major bridge designs – The NTSB identified the cause of the I-35W collapse as a design error in the original 1965 plans. Mn/DOT implemented a policy requiring a peer review of the design of major bridges by an independent consultant. The new I-35W Bridge in Minneapolis that opened in September included a peer review. Future replacements such as the Hastings Bridge, Lafayette Bridge in St. Paul and others will require a similar review.
- Reviewing gusset plate designs on Mn/DOT bridges –In July 2008, Mn/DOT completed the review of all truss bridges on the state system and confirmed that the original design of the gusset plates and other bridge members met design requirements. The deficiencies that existed in the original I-35W design were not present in other bridges.
Mn/DOT also reviewed the condition of these gusset plates in the field. Gusset plates on three bridges—Blatnik Bridge in Duluth, the Winona Bridge and the Hastings Bridge—were strengthened after deficiencies were found from corrosion or added weight to the bridge. These bridges were restored to the full safety factor required. The Desoto Bridge in St. Cloud, upon field inspection, was found to have several bent gusset plates. Mn/DOT decided to accelerate the already planned replacement of the Desoto Bridge to better coordinate with other road construction projects in St. Cloud.
Information regarding Mn/DOT’s review of these truss bridges is on the department’s Web site at www.dot.state.mn.us by clicking on Special Statewide Bridge Inspections.
“The actions we have taken reflect our commitment to improve our processes and protect the safety of the public,” Sorel said. “The loss of 13 people and injury of so many others will forever remain in our thoughts as we perform our mission to deliver transportation services to the state of Minnesota.”
The Federal Highway Administration and American Association of State Highway and Transportation Officials will be reviewing the NTSB recommendations for adoption nationwide.
To view the NTSB report, see http://www.ntsb.gov/events/boardmeeting.htm#upcoming.