I-35W Mississippi River Bridge (2007) Minneapolis, Minnesota, USA
The I-35W Mississippi River Bridge disaster occurred at 6.05 p.m., on August 1, 2007. The eight lane, 581 metre bridge collapsed into the Mississippi River killing 13 people and injuring 145.
Commonly called the ‘i-35 Minneapolis’ and opened in 1967, the bridge carried over 141,000 vehicles a day, on four northbound and four southbound lanes. Spanning the river at a height of 35 metres, the main, centre span was 140 metres long, of deck truss construction, as were the two other main spans. The support piers for these spans did not rest in the water but on opposite sides of the riverbank, leaving the river channel free for shipping. An additional eleven approach spans completed the bridge.
During its forty year life span the i-35 had undergone two separate modifications processes: in 1977 the deck thickness was increased and in 1988 the drainage was improved, an anti-icing system was installed, concrete traffic railings were upgraded, bolts were replaced, the piers were repaired and attention was paid to the cross girders. The overall effect was to increase the ‘dead load’ of the bridge. The i-35 gained weight.
At the time of the disaster another repair and renovation project was underway - repaving the bridge - and two northbound and two southbound lanes had been closed to traffic to facilitate the work in progress. Hard hatted work crews were preparing for a concrete pour of the southbound lanes - due to commence that evening - and in the course of the day construction equipment, sand and gravel had been delivered onto the bridge itself. Rush hour was underway and traffic was heavy.
A surveillance camera caught the moment when 305 metres of the deck truss collapsed with almost 140 metres of the main span falling to the Mississippi, which flowed at a depth of 4.5 metres below - it took 13 seconds. Of the 111 vehicles crossing the collapsed section 17 were taken from the water. An insufficient load capacity was identified by the National Transportation Safety Board as the probable cause of the collapse. In turn, they attributed this to a design flaw.
The Minneapolis i-35W was quickly rebuilt, with a post tensioned, box girder superstructure, supporting ten traffic lanes and opening on September 18th 2008.