NTSB now says it's not ruling out the chance that officials overlooked evidence of the bridge's weakened state in 1999.
The National Transportation Safety Board has not ruled out the possibility that Minnesota transportation officials missed a potential clue to the impending failure of the Interstate 35W bridge, NTSB Chairman Mark Rosenker said Monday.
One year after the structure collapsed, killing 13 people, the federal agency is still studying whether photos of critical gusset plate connections taken by inspectors in 1999 should have prompted MnDOT to take action, Rosenker said. The photos showed bowing or warping of the plates.
"Should that be a signal or a symptom of an upcoming catastrophe? We don't know yet, but that's being analyzed as we speak,'' he said. "We believe we'll have that answer.''
Rosenker declined to disclose any new findings. But his comments were a shift from a controversial statement he made in January that there was no way for MnDOT to have known about the gusset plate defect.
Rosenker said the final probable-cause report on the bridge collapse will be complete within 90 to 100 days. The findings will be aired at a public meeting of NTSB board members that could last two days, he said.
The process should answer all questions about MnDOT's 40-year stewardship of the bridge, he said.
"You will have an excellent idea of what happened and why,'' Rosenker said.
Over the next "days and weeks,'' investigators will post certain findings in the case at www.ntsb.gov. Rosenker said the final docket of public information will include 19,000 pictures of the bridge and a computerized video model of how the structure fell.
Since early on in the investigation, the federal agency has linked the accident to weight on the bridge deck having overwhelmed steel gusset plates. The plates were originally designed in the mid-1960s to be 1/2-inch thick instead of the necessary 1 inch, the NTSB has said.
Rosenker said the final report also will address whether MnDOT officials approved the loading of heavy construction materials on the bridge. Weight loads from the repaving project have been studied as a possible contributing factor in the collapse.
Rosenker said other questions that will be covered in the final report include whether high temperatures or vibration from the repaving work contributed to the collapse. He said the report also will answer whether heavily corroded bearings on the bridge played a role in its failure.
Besides determining a precise probable cause for the disaster, the NTSB is likely to make new safety recommendations. For one thing, Rosenker said, it has contacted about one-third of all state transportation departments in an effort to find the best approach to creating, approving and implementing bridge engineering designs.
Action and diagnoses
In Minnesota, MnDOT has moved to fix flawed practices exposed during early scrutiny of the bridge disaster. For instance, MnDOT had a history of inaction when it came to repairing corrosion, locked bearings and other problems that were spotlighted year after year in bridge inspection reports.
MnDOT said in June that a new process has been implemented to ensure that inspection reports receive internal review by specially designated decisionmakers.
Two investigative reports related to the collapse have already been produced, and various parties to pending litigation have been forming conclusions about the disaster.
On Friday, for instance, the repaving contractor released findings from an engineering study that blamed the collapse on undersized gusset plates and "load creep'' unrelated to the repaving underway when the bridge collapsed.
Kyle Hart, the attorney representing Progressive Contractors Inc., said two previous construction projects ordered by MnDOT added 4.08 million pounds of concrete to the bridge. He said that additional weight, coupled with years of unchecked corrosion in the bridge's superstructure, left almost no margin of safety in terms of load capacity.
"If the bridge had been properly designed and maintained or repaired, the weight we placed on the bridge during construction would have been far below the amount of weight necessary to initiate collapse,'' Hart said.
During the early months of the NTSB investigation, Rosenker drew sharp criticism from U.S. Rep. Jim Oberstar, chairman of the House Transportation and Infrastructure Committee. The Minnesota Democrat said in January that Rosenker made rushed public remarks about the investigation that seemed to rule out corrosion, maintenance and metal fatigue as contributing factors. The congressman was irked by the NTSB's decision not to hold a public hearing midway through the investigation.
Late last week, Oberstar said he is confident that the final report will show multiple causes for the failure. He also said that he's seen no evidence of partisanship in the investigation.
"If the board does its job, it will cite corrosion, metal fatigue, under-design of the gusset, failure to conduct periodic, effective inspection of and maintenance of the bridge structure itself,'' Oberstar said. "A combination of those factors should be the principal elements of probable cause for failure of the bridge.''
He said the NTSB's final report on the bridge collapse will arrive in time to influence next year's congressional funding decisions on surface transportation across the country.
Tony Kennedy 612-673-4213