Thursday, August 02, 2007

Structural Deficiencies In Collapsed Minneapolis I-35W Bridge Flagged By Inspectors As Early As 1990

According to a story published this afternoon (August 2nd, 2007) in the Anchorage Daily News, federal inspectors identified possible structural deficiencies in the collapsed Minneapolis bridge as early as 1990, but Minnesota officials thought a "patch and pray" strategy of targeted fixes and stepped-up inspections would be sufficient. This post also draws information from another story posted on the website. Here are highlights from both stories.

Questions about the cause and preventability of the collapse began surfacing today as authorities began shifting from rescue to recovery mode, searching for bodies that may be hidden beneath the river's swirling currents. As of press time, the official death count now stands at four (AP reports seven), with another 79 injuries. But police expect the death count to grow because bodies had been spotted in the water and as many as 30 people were still reported missing.

In resposne to the allegations about the 1990 findings, state bridge engineer Dan Dorgan said "We thought we had done all we could. Obviously something went terribly wrong." Most engineers seem to be now focusing on the possibility that sustained daily heavy traffic day might have contributed to the collapse.

In 1990, the federal government gave the I-35W bridge a rating of "structurally deficient," citing significant corrosion in its bearings. That made it one of 77,000 bridges in that category nationwide, 1,160 in Minnesota alone. The designation means some portions of the bridge needed to be scheduled for repair or replacement, and it was on a schedule for inspection every two years. Subsequent inspections during the '90s revealed fatigue cracks and corrosion in the steel around the bridge's joints; specifically, a 1993 entry noted 3,000 feet of cracks. Those problems were repaired, and the frequency of inspections upgraded from every other year to annually.

Dorgan said the bearings could not have been repaired without jacking up the entire deck of the bridge. Because the bearings were not sliding, inspectors concluded the corrosion was not a major issue.

When conducting inspections, inspectors get within an arm's length of various components of a bridge. If they spot cracks, that leads to more hands-on testing to determine the depth and extent of the fissures.

However, in a 2001 report from the University of Minnesota's Department of Civil Engineering, inspectors found some girders had become distorted. Engineers also saw evidence of fatigue on trusses and said the bridge might collapse if part of the truss gave way under the eight-lane freeway.

In 2005, after evaluating Minnesota's infrastructure, the American Society of Civil Engineers (ASCE) issued their landmark report highlighting problems in Minnesota's road network, but did not separately evaluate bridges.

The collapsed bridge's last full inspection was completed June 15, 2006. The report shows previous inspectors' notations of fatigue cracks in the spans approaching the river, including one 4 feet long that was reinforced with bolted plates

Gov. Tim Pawlenty responded late Thursday (August 1st) by ordering an immediate inspection of all bridges in the state with similar designs, but said the state was never warned that the bridge needed to be closed or immediately repaired. The governor indicated that while he was aware that the bridge was slated for replacement in the long term (officially in 2020), nothing indicated a need to accelerate the process.

And Congress is already reacting. Rep. James Oberstar (D-MN) said he'll press next year to increase the nation's annual funding for bridge construction and repair from $2 billion to $3 billion. He blamed President Bush for slashing Congress' highway spending bill last year by nearly $90 billion

The I-35W bridge over the Mississippi River in Minneapolis has a non-redundant truss design, meaning that the failure of any component of its steel superstructure or the buckling of one of its four concrete piers could cause a collapse.

The 40-year-old bridge was erected shortly before the Silver Bridge over the Ohio River, another non-redundant truss bridge, collapsed due to the failure of a single piece of hardware around Christmas in 1967, killing 46 people. That disaster led to changes in the design of truss bridges, although investigators have yet to determine whether the two collapses had similar causes.

However, Shankar Nair, a nationally known structural engineer who specializes in bridge design, said that even non-redundant truss bridges have performed well and shouldn't provoke major concerns "with modern steels and modern design techniques".

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