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On This Date In History...

...two years ago today, August 1, 2007, the bridge carrying I-35W over the Mississippi River in Minneapolis, Minnesota collapsed at 6:05 PM, during the Twin Cities' rush hour. Because of repair work on the bridge, lane restrictions had traffic moving stop-and-go or crawling. Construction vehicles, parked on the bridge, would later figure into the failure investigation.

Rail
2004 satellite Photo of the I-35 bridge.
The I-35 bridge is on the far left,
the bridge to the right is the 10th Avenue Bridge.
(Public Domain photo by US Geological Survey.)

The central span collapsed first followed by the adjoining spans. Most of the truss structure and deck fell into the river and on to the river banks. The northern section fell onto rail yards and crushed several freight cars that were parked on the tracks.

An amazing set of images was captured by a Corps of Engineers security camera, located at the entrance of the parking area of the Lower St. Anthony Falls Lock & Dam, showing the collapse of the bridge.

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A security camera captured these
sequential images of the collapse.
The image is public domain as a part
of the Corps of Engineers facility.

About 100 vehicles were involved in the collapse. Cars and construction workers, who were on the bridge, fell the 115 feet to the surface. 93 people were rescued from the bridge and an additional 20 people were pulled from the icy waters of the Mississippi River. Thirteen people perished in the collapse. A school bus, which you can see in the photo below, rested against a concrete barricade, near the edge of the bridge remnants, in a most precarious position. 60 students were aboard the bus, on their way home from a field trip. Jeremy Hernandez was a 20 year old advisor to the group. He kicked out the emergency door and aided the children to escape the bus. One staff member was seriously injured as were 10 of the students, but all survived the ordeal.

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Vehicles left on the deck. The numbers on the windshields were for investigative purposes.
(Public domain photo by USCG.)

The National Transportation Safety Board investigated the collapse. On January 15, 2008, the NTSB announced that the collapse had been caused by the failure of a gusset plate that was undersized for the bridge. In addition, over the years, deck resurfacing added layers of concrete, increasing the dead load by 20%. Construction vehicles, parked on the bridge at the time of the collapse, added 578,000 pounds of sand and water on the vehicles themselves. All were parked over the weakest spot on the bridge at the time of the collapse.

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The NTSB released this photo of a gusset plate from the failure investigation. Note the cracks in the plate that ultimately caused its failure.
(Public domain photo taken by the US Government.)

The replacement for the collapsed structure, the St. Anthony Falls Bridge, opened on September 18, 2008, months ahead of schedule and under budget.

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The new St. Anthony Falls Bridge, carrying I-35W over the Mississippi River.
(Photo used under rules of GNU Free Documentation License.)

Note: All the images used in this article are either public domain, taken by employees of the US Government during the course of their work, or used freely under rules of the GNU Free Documentation License.

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