The I-35W Bridge Collapse: When Infrastructure Fails

At 6:05 p.m. on August 1, 2007, the Interstate 35W Mississippi River Bridge in Minneapolis collapsed into the river during the evening rush hour, killing 13 people and injuring 145 others. The bridge had carried 140,000 vehicles per day. The National Transportation Safety Board traced the cause to a design flaw in steel gusset plates that had been present since the bridge was built in 1967 — made fatally worse by the weight of construction equipment placed on the bridge that day. The I-35W bridge had been rated “structurally deficient” by federal inspectors for 17 consecutive years before it fell into the river.

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What Was the I-35W Bridge and Why Was It Flagged as Deficient?

The I-35W bridge — officially the Saint Anthony Falls Bridge — was a deck truss bridge, 1,907 feet long, spanning the Mississippi River just south of downtown Minneapolis. Built between 1964 and 1967 and opened in November 1967, it was one of the primary north-south routes through the Twin Cities metropolitan area and handled traffic loads that had increased substantially since its design.

Federal highway inspections had flagged the bridge as “structurally deficient” based on its deck and structural ratings since 1990, a designation that in federal terminology indicates the bridge has elements that need monitoring or strengthening but does not necessarily mean imminent danger. Minnesota Department of Transportation inspectors had also identified corrosion in the steel truss structure and fatigue cracks in several locations during routine inspections. The bridge had been the subject of engineering studies in 1999 and 2003 examining whether it needed reinforcement. ^1^

On August 1, 2007, construction crews were resurfacing the bridge deck. Equipment on the bridge that evening included two concrete mixer trucks weighing a total of approximately 220,000 pounds, a water truck, and other machinery, all concentrated near the center of the bridge’s span. Sand and aggregate stockpiled for the construction project added additional load.

What Caused the I-35W Bridge to Collapse?

The NTSB investigation, which took 18 months and produced a report in November 2008, identified the cause as undersized gusset plates at the U10 node — a connection point near the center of the bridge’s truss structure where multiple steel members met.

A gusset plate is a flat piece of steel used to connect structural members at a joint. The gusset plates at the U10 node had been specified at half the thickness that the bridge’s design loads required. This was an error made by the original design firm, Sverdrup & Parcel, during the 1960s design phase. The error had not been caught during the bridge’s design review, during its construction, or during 40 years of subsequent inspections — in part because the plates were in locations that made visual inspection difficult and because inspectors were not systematically checking plate thickness against design loads. ^2^

The undersized plates had been marginally adequate — or close to it — under the bridge’s original design loads. Forty years of increased traffic weight, the addition of a concrete median barrier in 1977 that added permanent dead load to the structure, and the construction equipment concentrated on the center span on August 1 pushed the U10 node past its capacity. The gusset plates failed, and a 456-foot section of the bridge dropped into the Mississippi River.

A School Bus Carrying 52 Children Was on the Bridge

The collapse occurred during peak rush hour. A school bus carrying 52 children from the Waite House summer program was on the bridge when it fell; the bus came to rest on a section of collapsed roadway above the water line. All 52 children survived, along with their driver, Jeremy Hernandez, who used the emergency exit to help evacuate the children before first responders arrived. ^3^

Thirteen people died in the collapse. Among the dead were Sherry Engebretsen, 60, who had been driving home from work; Peter Hausmann, 47, driving with his daughter, who survived; Artemio Trinidad-Mena, 29, a construction worker on the bridge; and Vera Peck, 55, and her husband Richard Enstad, 60, killed together when their car fell into the river. Five of the dead were construction workers employed by the firm working on the deck resurfacing. Recovery of the dead and collapsed structure from the Mississippi River continued for more than a month.

17 Years of “Structurally Deficient” Ratings and No One Checked the Gusset Plates

The NTSB’s 2008 report found that the Minnesota Department of Transportation bore some responsibility for adding loads to a structurally deficient bridge without engineering analysis of whether the structure could bear them. It also found that 40 years of inspections had failed to catch the undersized gusset plates because inspectors were not looking for original design errors — only for deterioration of existing conditions.

URS Corporation, which had been hired by MnDOT to study the bridge and had prepared a 2006 report recommending gusset plate repairs, was named in civil litigation. MnDOT itself settled with victims’ families for $62 million in 2010 — the maximum allowed under Minnesota’s cap on state liability. ^4^ The new bridge — also called the Saint Anthony Falls Bridge — was designed and constructed in 13 months at a cost of $234 million and opened on September 18, 2008, 414 days after the collapse. It includes sensors embedded throughout the structure that continuously monitor load and strain, and a design safety margin substantially higher than the bridge it replaced.

12 Percent of American Bridges Were Structurally Deficient in 2007

The federal “structurally deficient” designation that had applied to the I-35W bridge since 1990 is a rating that, at the time of the collapse, applied to approximately 77,000 bridges in the United States — about 12 percent of the national bridge inventory. The I-35W bridge was a case where a pre-existing design flaw combined with altered load conditions to push a monitored but not fully understood structural weakness past its limit. ^5^

The 13 people who died on August 1, 2007, died because a design error made in 1967 was never caught in 40 years of inspections, and because the agency responsible for the bridge did not check whether the construction loads placed on it that day were within the structure’s capacity. The accountability gap here — civil settlements, no criminal consequences — mirrors the Hyatt Regency collapse in 1981 and the Flint water crisis in 2014. The question of how many other bridges carry similar unchecked design errors from decades past is one that bridge inspectors across the country are still working through.

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Sources:

1. National Transportation Safety Board. Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007. NTSB/HAR-08/03, 2008.
2. Holt, Mark, and Frittelli, John. The I-35W Bridge Collapse: Issues for Congress. Congressional Research Service, 2007.
3. Star Tribune. Collapse of the I-35W Bridge: A Year of Reporting. Minneapolis Star Tribune Archives, 2008.
4. Associated Press. “Minnesota Settles Bridge Collapse Claims for $62 Million.” AP Wire, January 2010.
5. Federal Highway Administration. 2007 National Bridge Inventory Data. U.S. Department of Transportation, 2007.