I-35W Mississippi River Bridge - Maintenance and Inspection

Maintenance and Inspection

Since 1993, the bridge was inspected annually by Mn/DOT, although no inspection report was completed in 2007, due to the construction work. In the years prior to the collapse, several reports cited problems with the bridge structure. In 1990, the federal government gave the I-35W bridge a rating of "structurally deficient," citing significant corrosion in its bearings. Approximately 75,000 other U.S. bridges had this classification in 2007.

According to a 2001 study by the civil engineering department of the University of Minnesota, cracking had been previously discovered in the cross girders at the end of the approach spans. The main trusses connected to these cross girders and resistance to motion at the connection point bearings was leading to unanticipated out-of-plane distortion of the cross girders and subsequent stress cracking. The situation was addressed prior to the study by drilling the cracks to prevent further propagation and adding support struts to the cross girder to prevent further distortion. The report also noted a concern about lack of redundancy in the main truss system, which meant the bridge had a greater risk of collapse in the event of any single structural failure. Although the report concluded that the bridge should not have any problems with fatigue cracking in the foreseeable future, the bridge instrumentation by strain gauges and continuous structural health monitoring had been suggested.

In 2005, the bridge was again rated as "structurally deficient" and in possible need of replacement, according to the U.S. Department of Transportation's National Bridge Inventory database. Problems were noted in two subsequent inspection reports. The inspection carried out June 15, 2006 found problems of cracking and fatigue. On August 2, 2007, Governor Tim Pawlenty stated that the bridge had been scheduled to be replaced in 2020.

The I-35W bridge ranked near the bottom of federal inspection ratings nationwide. The scale used was a "sufficiency rating" which ranges from the highest score, 100, to the lowest score, zero. In 2005 the bridge was given a rating of 50, indicating that replacement may have been in order. Out of over 100,000 heavily used bridges, only about 4% scored below 50. On a separate measure, the I-35W bridge was rated "structurally deficient," but was deemed to have met "minimum tolerable limits to be left in place as it is."

In December 2006, a steel reinforcement project was planned for the bridge. However, the project was canceled in January 2007 in favor of periodic safety inspections, after it was revealed that drilling for the retrofitting would in fact weaken the bridge. In internal Mn/DOT documents, bridge officials talked about the possibility of the bridge collapsing and worried that it might have to be condemned.

The construction taking place in the weeks prior to the collapse included joint work and replacing lighting, concrete and guard rails. At the time of the collapse, four of the eight lanes were closed for resurfacing, and there were 575,000 pounds (261,000 kg) of construction supplies and equipment on the bridge.