Inspection history of the I-35W bridgeby Art Hughes, Minnesota Public Radio
A few facts about the Interstate 35W bridge that collapsed in downtown Minneapolis Wednesday.
1967: Bridge opens to traffic. It's built by contractors Industrial Construction Co. and Hurcon Inc.
It is eight lanes wide, and 1,900 ft long. The main bridge deck is a 458 ft.-long span with hollow concrete pillars on each side of the Mississippi River.
1990: The bridge is classified as "structurally deficient" by the U.S. Department of Transportation, because corrosion of the bearings prevent sufficient movement. The classification is based on a score of four or lower on a nine-point rating system.
1993: Federal transportation officials increase frequency of inspections on the bridge from once every two years to every year. They find some corrosion of the steel in the bridge joints and fatigue cracks in the bridge approach spans.
1999: Swiss company Boschung fitted the bridge with an automated de-icing system installed on the bridge deck and railings.
2001: An independent study by University of Minnesota engineers deems the bridge "structurally deficient," but not in need of immediate replacement. (*Read excerpt of this report below)
2004-2007: Another independent study by URS Corp. explores the bridge's "fatigue potential." Transportation officials considered two alternatives: Add additional steel plates to critical members to reinforce trusses; or reinspect welds in the trusses to monitor the bridge's usability.
Transportation officials choose to reinspect the welds and begin the work in May 2007. They find no growth in any of the existing weld cracks. Summer 2007: $2.4 million in repair and maintenance is conducted on the bridge deck, lights, and joints by Progressive Contractors, Inc., headquartered in St. Michael, Minnesota.
Aug. 1, 2007: Bridge span over the Mississippi River collapses at 6:05 p.m.
*2001 Fatigue Evaluation of the Bridge Truss of Bridge 9340. University of Minnesota report commissioned by MnDOT.
Abstract: This 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 then developed two-and three-dimensional finite-element models of the bridge, and used the models to calculate the stress ranges throughout the deck truss.
The bridge's deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system.
The research helped determine that the fatigue cracking of the deck truss is not likely, which means that the bridge should not have any problems with fatigue cracking in the foreseeable future.
As a result, MnDOT does not need to prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project.
The research also has implications for other bridges. The project verified that the use of strain gages at key locations, combined with detailed analysis, help predict the bridge's behavior. In addition, the instrumentation plan can be used in other similar bridges.
(The Associated Press contributed to this report)