The I-35W (Interstate 35 West) bridge over the Mississippi River in Minneapolis opened to traffic in 1967. Thousands of vehicles drove across it every day, but no one imagined that a mistake in the bridge’s design, made over forty years prior, would have such disastrous consequences on one summer evening in August of 2007.
The I-35W bridge, given the designation “Bridge 9340,” was designed by a civil engineering company called Sverdrup & Parcel in the early 1960s. Construction on the bridge began in 1964, towards the end of the national highway construction boom that aimed to dramatically develop the roadway infrastructure of the United States. The bridge opened to traffic in late 1967.
Bridge 9340 consisted of fourteen spans: nine spans were of steel multi-girder construction, two were of concrete slab construction, and the main three were of deck truss construction. The Minnesota Department of Transportation, also known as MnDOT, was tasked with taking over annual bridge inspection beginning in 1993. Prior to this, it was federally inspected every other year.
Inspection reports by MnDOT often indicated significant corrosion, rusting, warped plates, and other structural issues with the bridge. It was noted that the lack of redundancy in the main truss design meant that the bridge was vulnerable to a collapse if a single critical piece in the truss were to fail. Subsequent inspection reports expressed concern about the bridge’s structural integrity, labelling it a “fracture critical bridge,” but no motion to close or drastically reinforce it was ever made.
On the evening of August 1, 2007, slow-moving traffic descended on the I-35W bridge during peak rush hour as it had done countless times before; however, at precisely 6:05 pm CDT, the unthinkable happened. Drivers on the bridge recalled hearing a beam snap, followed by a loud clank! Then, they felt the bridge begin to tremble. Within seconds, 1,000 feet of the bridge collapsed, and about 456 feet of the main span fell 108 feet down into the Mississippi River and its banks. Thirteen people were killed and 145 were injured as a result.
Emergency responders arrived on the scene within six minutes of the collapse. Only a handful of vehicles fell into the water, while many were trapped atop pieces of the broken bridge. A school bus with fifty-two children and nine adults was among the vehicles stranded on the wreckage. Twenty-year-old Jeremy Hernandez, a staff member onboard the bus, was critical to getting the children to safety.
Panic and fear spread across the country as the news broke out, and observers wondered how this could have happened. Within twenty-four hours of the bridge collapse, investigators from the National Transportation Safety Board (NTSB) arrived on the scene to figure out the root cause of the tragedy. Investigators looked at the weather, traffic patterns, and even the type of solvent used to melt ice that built up in wintertime. Ultimately, they determined that a deciding factor in the collapse had to do with nearly 300 tons of construction equipment placed right over a critical flaw in the bridge’s design—gusset plate U10. The bridge’s deck truss design included fourteen gusset plates, large steel sheets meant to connect girders to the truss structure. Plate U10, which was half an inch thick when it should have been one inch thick, was calculated as being the weakest of the fourteen.
This determination by the NTSB meant that a mistake made over forty years before the collapse led to the bridge’s ultimate failure in 2007. The critical design flaw was missed by over forty years of inspections, both state and federal, because checking for such design flaws after the fact was not standard practice.
The tragedy that unfolded in Minneapolis on that summer day in 2007 caused a rift that reached every corner of the nation. Government officials everywhere felt urged to ramp up inspections and funnel money towards renovating and maintaining their traffic infrastructure; but, as was proved by the Minneapolis bridge collapse, local government often lacks the funds to deal properly with crumbling infrastructure. It has been estimated that over a trillion dollars are needed to make repairs. This money has been difficult to attain, and local governments across the country continue to struggle to keep up with the cost of maintenance and safety.
