Breaking News: at 6:05PM Central Time on Wednesday, August 1, 2007, the I-35W bridge suffered structural failure and collapsed into the Mississippi River. The accident happened at the height of rush hour with an estimated 100 cars on the structure. There were many survivors, but tragically, a number of people were not so fortunate.

Update—as of August 21, 2007, all known victims of the bridge disaster have been recovered. The toll stands at 13 dead and 144 injured. The operation now shifts into the process of removing the old structure and preparing to build the new bridge. The NTSB remains on site working to figure out the cause of the bridge failure.

Claim to fame: was built with a single 458 foot long steel arch to avoid putting any piers in the water to impede river navigation.

Note, this page is for the old I-35W Bridge that failed in August, 2007. For the new I-35W St. Anthony Falls Bridge, please click here. For complete coverage of the bridge disaster, please click here.

This bridge features an anti-ice system. A series of PVC pipes carries a deicer fluid to outlets that are drilled into the deck of the bridge. When the temperature for ice is right, the deicer fluid is pumped onto the bridge deck. This system has proven to be successful enough that it is being installed on other bridges in Minnesota.

The National Bridge Inventory contains a report on this bridge from 2003. It reports the following items:

• Deck Condition: Fair.
• Superstructure Condition: Poor.
• Substructure Condition: Satisfactory.
• Scour: Foundations determined to be stable.
• Bridge Railings: Meets currently acceptable standards.
• Structural Evaluation: Meets minimum tolerable limits to be left in place as-is.
• Water Adequacy Evaluation: Superior to present desirable criteria.
• Bridge Sufficiency Rating: 50%

A University of Minnesota Civil Engineer in a report to MN-DOT recently noted that this bridge is considered to be a non-redundant structure. That is, if any one member fails, the entire bridge can collapse. A key factor is that there are only four pylons holding up the arch. Any damage to any one pylon would be catastrophic. The textbook example of a non-redundant bridge is the Silver Bridge over the Ohio River. It failed shortly before Christmas in 1967 resulting in 46 deaths. A single piece of hardware failed due to a tiny manufacturing defect. But that piece was non-redundant, and the entire bridge collapsed into the icy river. Today, bridge engineers design bridges so that any single piece of the bridge can fail without causing the entire bridge to collapse. It is tragic that the I-35W bridge was built a few years too early to benefit from that lesson.

Note—several Civil Engineers objected to me calling this an arch truss deck bridge. This was not a true arch, rather, it just happened to have an arch shape. A correct type is a three span continuous deck truss. A truss bridge has the latticework of steel beams, and a deck truss has the roadway on top.

Governor Tim Pawlenty has ordered inspections all such deck truss bridges in the state. There are two similar bridges located in the Twin Cities area, the Highway 23 DeSoto Bridge in St. Cloud and the MN-243 Bridge over the Saint Croix River in Osceola, Wisconsin. A third deck truss bridge is located on Highway 123 over the Kettle River in Sandstone, MN. The Sauk Rapids Bridge has now been added to the inspection list. The Sauk Rapids Bridge is unique in this list in that it is scheduled to be demolished soon due to a new bridge being built to replace it. In addition, the I-90 Dresbach Bridge over the Mississippi River main channel near La Crosse, Wisconsin, is a non-redundant style bridge that is on MN-DOT's watch list. MN-DOT has an active study to examine alternatives that include removing the Dresbach Bridge, which potentially has many years of remaining life, and replacing it with a bridge that has built-in redundancy.

The photo above shows the two piers on the south end of the I-35W bridge. Engineers are focusing on the metal joint just above the pier on the right side as the location of the failure. Below is a close-up of the southwest pier of the bridge. Notice the amount of rust. The triangle piece of metal is the top of the bridge bearing, a device that is supposed to allow the bridge to slide back and forth as the bridge expands and contracts during heating and cooling cycles. The bearings had failed on this bridge many years ago. This prevented the bridge from moving freely, so it had to flex and bind instead, putting additional loads on the structure.

Above, you can see the south end of the I-35W bridge under one of the main arches of the 10th Avenue Bridge. This view shows just how close these two structures were. The bottom picture is the top deck of the I-35W bridge, looking from the parking lot of the former Hardee's burger joint on the north east side of the bridge. The Metrodome is visible in the background. The hometown Minnesota Twins had just started a game when the I-35W bridge collapsed.