Aug 2, 2007 9:10 am US/Central
Reports Differ On Pre-Collapse Condition Of Bridge
Minneapolis (WCCO) ―
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The bridge was currently under construction when the collapse occurred. MnDOT said they were not doing any structural work on the underside or bridge supports.
Photo from Tony Webster
In 2001 the Minnesota Department of Transportation commissioned the University of Minnesota Department of Civil Engineering to conduct a study of the Interstate 35W river bridge that collapsed in Minneapolis Wednesday.
Gov. Tim Pawlenty said the bridge was inspected by the Minnesota Department of Transportation in 2005 and 2006 and that no immediate structural problems were noted. "There were some minor things that needed attention," he said.
"They notified us from an engineering standpoint the deck might need to be rehabilitated or replaced in 2020 or beyond," Pawlenty said at a news conference Wednesday in Minneapolis.
The 40-year-old bridge was rated as "structurally deficient" two years ago and possibly in need of replacement, the Star Tribune reported. The newspaper said that rating was contained in the U.S. Department of Transportation's National Bridge Inventory database.
"We've seen it, and we are very familiar with it," Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, said of the 2005 assessment of the bridge.
Aamodt noted that many other bridges around the country carry the same designation that the I-35W bridge received. She declined to say what the agency was going to do to address the deficiencies found in 2005.
The 2001 research project resulted in a new, accurate way to assess fatigue cracking on Bridge 9340 on I-35W, which crosses the Mississippi River near downtown Minneapolis.
The research involved installation on both the main trusses and the floor truss to measure the live-load stress ranges. Researchers monitored the strain gages while trucks with known axle weights crossed the bridge under normal traffic. Researchers developed models to calculate stress ranges throughout the deck truss.
Researchers determined the bridge's deck truss at that time had not experienced fatigue cracking, but it had many poor fatigue details on the main truss and floor truss system. The research helped determine that the fatigue cracking of the deck truss was not likely, which meant the bridge should not have had any problems with fatigue cracking in the near future.
As a result, MnDOT determined it was not necessary to prematurely replace the bridge because of fatigue cracking, avoiding the high costs associated with such a large project.
Two recommendations were made by the researchers, however. Three members of the main truss with the highest stress ranges were to be inspected every two years. Also, lower chords and diagonals of all the floor trusses had high stress. They were to be inspected every 6 months since they could be inspected easily from the catwalk.
The bridge was currently under construction when the collapse occurred. MnDOT said they were not doing any structural work on the underside or bridge supports.
Click here to see the entire MnDOT report from 2001.
(© MMVII, CBS Broadcasting Inc. All Rights Reserved.)
Special Section: I-35W Bridge Collapse
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