Stunning pictures of the collapse of this major eight lane bridge during the evening rush hour. In spite of the catastrophic failure of the bridge during evening rush hour, only 13 people were killed and 145 injured.

Background

The Mississippi River Bridge was an eight-lane, steel truss arch bridge that carried Interstate 35W across the Saint Anthony Falls of the Mississippi River in Minneapolis, Minnesota, United States. During the evening rush hour on August 1, 2007, it suddenly collapsed, killing 13 people and injuring 145. The bridge was Minnesota’s fifth busiest, carrying 140,000 vehicles daily.

The National Transportation Safety Board or NTSB cited a design flaw as the likely cause of the collapse, and asserted that additional weight on the bridge at the time of the collapse contributed to the catastrophic failure.

Just over a year later the replacement bridge was opened. Considering the span and width, this was no mean achievement.

 Previous problems experienced

The first indication of future problems arose in December 1985 when the temperature reached a chilly −34 °C. The vehicles driving across the bridge experienced black ice resulting in a major multiple vehicle collision on the bridge on the northbound lanes.

The culprit was identified as Black Ice

Finally in December 1996, the bridge was identified as the single most treacherous cold-weather spot in the Minneapolis-Saint Paul or the so-called Twin Cities freeway system. This arose due to the almost frictionless thin layer of black ice that regularly formed when temperatures dropped below freezing. A significant contributing factor to the icing problem was the bridge’s proximity to Saint Anthony Falls. This site was noted for frequent spinouts and collisions which characterised this type of problem.

By January 1999, the Department of Transport began testing magnesium chloride solutions and a mixture of magnesium chloride and a corn-processing byproduct to see whether either would reduce the black ice that appeared on the bridge during the winter months.

In October 1999, the state embedded temperature-activated nozzles in the bridge deck to spray the bridge with potassium acetate solution to keep the area free of winter black ice. Ultimately this system came into operation in 2000.

Even though there have been no more major multi-vehicle collisions since the automated de-icing system was installed, it has been raised as a possibility that the potassium acetate may have contributed to the collapse of the 35W bridge by corroding the structural supports.

What had maintenance and inspection revealed?

Since 1993, the bridge was inspected annually by the DOT. 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 report also noted a concern about lack of redundancy in the main truss system, which meant that 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 subsequent reports the bridge was again rated as “structurally deficient” and in possible need of replacement according to the United_States Department of Transportation’s National Bridge Inventory.

At 6:05 p.m. on August 1, 2007, with rush hour bridge traffic moving slowly through the limited number of lanes, the central span of the bridge suddenly gave way, followed by the adjoining spans. The structure and deck collapsed into the river and onto the riverbanks below, the south part toppling 25 m eastward in the process. A total of 111 vehicles were involved, catapulting their occupants and construction workers up to 35m down to the river or onto its banks. Northern sections fell into a rail yard, landing on three unoccupied and stationary freight train cars.

Bridge being built in 1967