Minneapolis Bridge Collapse: Cause, Lawsuits, and Memorial

The Minneapolis bridge collapse happened on the evening of August 1, 2007, when the Interstate 35W bridge over the Mississippi River gave way during rush hour, killing 13 people and injuring 145. The eight-lane steel deck truss span, which carried roughly 141,000 vehicles per day, dropped into the river and onto its banks at about 6:05 p.m., sending more than 50 vehicles into the water. Federal investigators later concluded the disaster came from a design error made four decades earlier: the bridge’s gusset plates had been built roughly half as thick as they should have been.¹

The bridge, designated Bridge 9340, had opened in November 1967. About 1,000 feet of it collapsed. The 456-foot main span fell 108 feet.² Roughly 120 vehicles carrying 160 people were on the span when it fell.³ A deck resurfacing project was underway, and construction crews had staged about 300 tons of sand, gravel, and other materials in the closed southbound lanes since roughly 2:30 that afternoon in preparation for a concrete pour scheduled for 7:00 p.m.¹

The Thirteen People Who Died

The collapse killed Julia Blackhawk, Richard Chit, Paul Eickstadt, Sherry Engebretsen, Peter Hausmann, Patrick Holmes, Greg Jolstad, Vera Peck, Christine Sacorafas, Hana Sahal, Sadiya Sahal, Scott Sathers, and Artemio Trinidad-Mena.⁴ Hana Sahal, two years old, was the youngest; she and her mother Sadiya were among those whose vehicle went into the river.⁵

Why the Bridge Fell

The National Transportation Safety Board issued its final report on November 14, 2008, after a 16-month investigation. The probable cause, the board found, was “inadequate load capacity, due to a design error by Sverdrup & Parcel and Associates, Inc., of the gusset plates at the U10 nodes.” At eight of the twelve main truss nodes, the gusset plates were half an inch thick when they should have been about one inch.⁶ Gusset plates are the steel connectors holding truss members together at each joint, and the U10 plates carried the combined forces of five intersecting members at a structurally sensitive inflection point.⁷

The undersized plates had held for forty years. Two categories of added stress pushed them past their limit. Renovations in 1977 and 1998 thickened the concrete deck by about two inches, raising forces on the trusses by roughly 30 percent.⁷ Then the concentrated construction loads staged near the U10 node that afternoon added another 10 to 15 percent on top of that.¹

The NTSB pointed to contributing failures beyond the original design mistake. Sverdrup & Parcel’s quality control procedures had not caught the miscalculation. Federal and state officials had conducted an inadequate design review in 1965. And across the industry, gusset plates were routinely left out of bridge load-rating analyses. Investigators ruled out corrosion, preexisting fatigue cracking, temperature effects, pier movement, and floor truss fractures.¹

Warnings That Missed the Real Problem

The bridge had carried a “structurally deficient” rating since 1990 because of corrosion in its bearings. That federal label means elements need monitoring or repair; it does not mean a bridge is unsafe or likely to fall. Its June 2006 inspection gave it an overall condition rating of 4 on a 0-to-9 scale.⁸

MnDOT had hired URS Corporation in 2003 to perform a fatigue analysis of the bridge’s support structure. Between 2004 and 2006, URS identified 20 “fracture critical” truss beams as the most likely locations for crack formation. In November 2006, URS recommended installing an acoustic emission monitoring system that could detect steel cracking in real time, at an estimated cost of $20,000 to $40,000. MnDOT declined, opting for less continuous ultrasonic testing and noting that no active fatigue cracking had been found.⁹ Recommendations to add structural redundancy to the bridge dated back to 1999. A 2006 URS report suggested a $13 million redeck for redundancy; MnDOT chose a $3.5 million deck overlay instead, deferring full replacement to 2017–2022.¹⁰

None of those missed opportunities involved what actually failed. The gusset plates were not part of any load-rating analysis or focused inspection protocol, and the NTSB itself stated that routine inspections would not have identified the original design error.¹⁰

The Replacement Bridge

Rebuilding moved fast. Congress authorized about $250 million in emergency relief on August 4, 2007, and President George W. Bush signed it into law two days later as Public Law 110-56.¹¹ MnDOT awarded a $234 million design-build contract to Flatiron-Manson on October 8, 2007, with FIGG Bridge Engineers as lead designer. Construction began November 1, 2007.¹²

The new I-35W St. Anthony Falls Bridge opened to traffic on September 18, 2008, about thirteen months after the collapse and roughly three months ahead of its December 24, 2008, deadline.¹² It is a post-tensioned concrete box girder bridge with four spans totaling about 1,218 feet, anchored by a 504-foot main span. It was designed for a 100-year service life, contains no fracture-critical elements, and has redundant load paths through its foundations, footings, columns, and twin box girders. A built-in “Smart Bridge” sensor system monitors its condition in real time.¹³

Compensation and Lawsuits

Payments to victims and their families eventually exceeded $95 million, coming through several channels.¹⁴

Because state law capped Minnesota’s tort liability at $1 million total, the Minnesota Legislature created a special $36.64 million compensation fund, signed into law by Governor Tim Pawlenty on May 8, 2008. A special master panel chaired by attorney Susan Holden administered the fund. All 179 claims filed by the October 15, 2008, deadline were settled by April 16, 2009, with individual payments ranging from $4,500 to more than $2.2 million.¹⁵¹⁶

Civil suits were consolidated in Hennepin County District Court before Judge Deborah Hedlund. More than 100 people sued URS Corporation, alleging it had missed warning signs during its inspection work. In August 2010, URS agreed to pay $52.4 million to settle, with $48.6 million going to victims, $1.5 million designated for a memorial, and $5 million paid to the state. URS admitted no fault.¹⁷ Progressive Contractors Inc., which had been performing the resurfacing work, reached a separate confidential settlement with victims and paid $1 million to the state.¹⁸

The hardest fight involved Jacobs Engineering Group, which had acquired Sverdrup & Parcel’s successor firm in 1999. Jacobs argued it was shielded by a Minnesota law limiting construction liability to 15 years, but the compensation-fund statute had included a provision nullifying that statute of limitations for the I-35W case and applying the change retroactively. Jacobs challenged the provision on due process and contract impairment grounds. The Minnesota Supreme Court upheld it, acknowledging the change “may be economically unfair” but finding nothing in the due process clause to preclude it. The U.S. Supreme Court declined review in May 2012.¹⁹ In October 2012, Jacobs settled with the state for $8.9 million, admitting no wrongdoing. That closed out all litigation.²⁰

No criminal charges were filed against any officials, engineers, or contractors in connection with the collapse.¹

What Changed in Bridge Inspection Policy

Within days of the collapse, U.S. Secretary of Transportation Mary Peters ordered inspections of all 756 steel arch truss bridges in the country and directed the DOT Inspector General to review the national bridge inspection program.¹¹

On January 15, 2008, the Federal Highway Administration issued Technical Advisory T 5140.29, directing state highway officials to include gusset plates when calculating load capacity for the nation’s roughly 13,000 steel truss bridges. States were required to recalculate gusset plate effects every time a bridge’s weight, capacity, or condition changed.²¹ The FHWA later published detailed load-rating guidance for gusset plates, and the NTSB classified its safety recommendation on gusset plates as “Closed—Acceptable Alternate Action” in November 2013.²²

In Minnesota, a 16-member joint legislative committee was formed on August 14, 2007, to investigate MnDOT’s decision-making. MnDOT reviewed all truss bridges on the state system by July 2008 and confirmed the specific gusset plate deficiencies in the I-35W bridge were not present elsewhere. The department also adopted a policy requiring independent peer reviews for all future major bridge designs.²³

The Memorial

The 35W Bridge Remembrance Garden was dedicated on August 1, 2011, the fourth anniversary of the collapse, by Governor Mark Dayton and Minneapolis Mayor R.T. Rybak. It sits near Gold Medal Park on a bluff overlooking the Mississippi River. Thirteen I-beams rise from the site, each 81 feet long in reference to the August 1 date, each inscribed with the name of one victim. A granite wall with water flowing over its surface carries the names of 171 survivors.²⁴

1
NTSB. Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007
2
Minnesota Historical Society. I-35W Bridge Collapse
3
Fire Engineering. US Fire Administration Releases Report on I-35W Bridge Collapse in Minneapolis
4
Times Leader. 13 Victims of Bridge Collapse Recalled
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Minnesota Public Radio. Bridge Collapse Victims
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Engineering News-Record. NTSB Cites Gussets and Loads in Collapse
7
University of Minnesota. Forensic Investigation of I-35W Bridge Collapse
8
FHWA. Fact Sheet: I-35 Bridge Inspections
9
Minnesota Public Radio. MnDOT Rejected Bridge Safety Recommendations
10
Engineering News-Record. Report: State DOT Ignored I-35W Bridge Recommendations
11
Eno Center for Transportation. Federal Bridge Policy: Past and Future
12
Minnesota Legislative Reference Library. Minnesota Issues: Bridges
13
Post-Tensioning Institute. Reconstruction of the I-35W Bridge
14
NPR. Minneapolis Bridge Collapse Lawsuits Settled for $52.4 Million
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Minnesota Judicial Branch. I-35W Bridge Collapse Fund Claims Settled
16
OHS Online. Minnesota Makes Public Payments to Bridge Collapse Victims
17
NBC News. Minneapolis Bridge Collapse Lawsuits Settled
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Star Tribune. Firm Settles I-35W Bridge Suit
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Minnesota Public Radio. Court Rulings on I-35W Bridge Rattle Builders
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Minnesota Public Radio. Minn. Settles Last Case in I-35W Bridge Disaster
21
FHWA. US Secretary of Transportation Calls on States to Recheck Gusset Plate Strength
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FHWA. Load Rating Guidance and Examples for Bolted and Riveted Gusset Plates in Truss Bridges
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MnDOT Newsline. NTSB Releases I-35W Bridge Investigation Findings
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MinnPost. New 35W Bridge Memorial Honors Those Who Died