Ep 57 I-35W Bridge Collapse in Minneapolis

Engineering News – Non Invasive Ultrasound Therapy (1:05)

This week's engineering failure is the I-35W Bridge Collapse (5:05). Designed and constructed (10:55) in the 60s, the bridge had been inspected (16:40) regularly over the years. But unfortunately, after 40 years it collapsed (30:30) and the City of Minneapolis rushed to replace the bridge (38:45).


Engineering News

I-35W Bridge Collapse

Episode Summary

Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole

And I’m Brian. And we’re both from Calgary, AB.

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This week in engineering news, non-invasive ultrasound therapy for the brain.

  • Team of engineers at the University of California San Diego

  • Developed a device to enable noninvasive, ultrasound based therapies for the brain.

  • One example is sound waves used in trials to treat epilepsy.

  • While they are trying to target specific areas of the brain with ultrasound waves, they tend to bounce around and over target some areas and under target others, which can lead to haemorrhaging or overheating in brain tissue.

  • They are using similar technology that concert hall designers use to make sure everyone hears the music perfectly, to diffuse the ultrasound waves and distribute them uniformly. These uniform waves then stimulate the areas of the brain that are sensitive to the waves and leave adjacent areas alone.

Now on to this week’s engineering failure; the I-35W Bridge Over the Mississippi River in Minneapolis Minnesota.

  • The bridge’s official name is Bridge 9340 and it’s located just west of Downtown Minneapolis.

  • The bridge was an eight lane, steel truss arch bridge that crossed the Mississippi River 875m downstream from St. Anthony’s Falls.

  • I was in Minneapolis in 2019 and there is a pedestrian bridge called Stone Arch Bridge that crosses the river between the falls and the replacement bridge. It’s a really cool area and although I had heard about the original bridge collapse, the location was a little lost on me and I didn’t realize that I was right there until I was looking up maps while researching this episode. Also, I thought that St Anthony’s Falls were man made, but they are in fact the largest natural major waterfall on the Mississippi which is pretty cool. Also, fun fact, the Mississippi River is the second longest river in North America, it runs over 3,500km from the northerns part of Minnesota all the way south to the Gulf of Mexico.

  • Bridge 9340 opened in 1967 and carried 140,000 vehicles per day.

  • It suffered catastrophic failure during afternoon rush hour on August 1, 2007 at 6:07pm, killing 13 people and injuring 145. The bridge was in use for 40 years at the time of the collapse. The bridge was also undergoing some lighting and railing upgrades, and while 4 lanes were closed, there were 260,000 kgs of supplies and equipment on the bridge.

  • The National Transportation Safety Board investigated the collapse and noted that the likely cause was a design flaw of a gusset plate that was too thin and ripped along a line of rivets.

    • A gusset plate is where beams or girders fasten to columns by way of bolts, rivets, or welding.

Design and Construction

  • The bridge had 14 spans extending 580m. The three main spans were deck truss construction, with the middle span being the longest at 140m over the 119m river crossing. Meaning that no piers were located in the water. Nine of the eleven approach spans were steel multi-girder construction and two were concrete slab construction. The roadway deck was approximately 35m above the water level.

  • Interesting story about pier 6, which is the pier at the south end of the longest span over the river. One of the original contractors who was going to build the bridge expressed concern that pier 6 couldn’t be built as planned and after trying to negotiate alternate options with the department of transportation that contractor backed out of the project altogether.

  • On December 19, 1985 there was a large pile up caused by black ice when the temperature reached -34C. Due to its proximity to St. Anthony’s Falls, the bridge was highly susceptible to an almost frictionless thin layer of black ice. In October 1999, Minnesota Department of Transportation embedded temperature-activated nozzles in the bridge deck to spray a potassium acetate solution and prevent ice from forming. Although not proven during the investigation, it’s thought by some that the potassium acetate may have contributed to the collapse by corroding some of the structural elements.


  • The bridge had been annually inspected since 1993. In 1990 the bridge was given a rating of “structurally deficient” due to significant corrosion of bearings. 75,000 other US bridges had this classification in 2007; and I’d wager that most of them are still in operation without significant repairs and several more bridges have been added to this list.

  • In 2001 the University of Minnesota’s civil engineering department studied the bridge.

    • Cracking had been discovered in the cross girders at the end of the approach spans

    • Resistance to motion at connection points where the main trusses connected to cross girders was causing the bridge to distort out of the plane and causing stress cracking.

      • To combat this they drilled the cracks to prevent them from spreading and added support struts to the cross girders to prevent more out of plane distortion

    • The report also flagged a lack of redundancy in the main truss system. We’ve talked about redundancy a lot in past episodes. In bridge design it means that failure of one member would not lead to total failure of the bridge; that there is redundancy in the structural members such that failure of one member transfers loads to the other member to carry it until the bridge can be repaired.

  • In 2005 the bridge was again rated as structurally deficient and in need of replacement. (s/) Which is shocking because they didn’t do anything to address it since 1990. (s/)

    • The sufficiency rating is a score out of 100. Bridge 9340 received a score of 50. Only 4% of heavily used bridges receive a score of 50 or lower. But since the bridge was deemed to have met “minimum tolerable limits to be left in place as it is” they didn’t close it.

    • I think what sometimes happens is that we as engineers think something is in bad shape and it should be replaced, but it's ok for a little while longer until you can get all of your ducks in a row. And all the reader sees is “it’s ok for a little while longer” and they don’t react with the urgency that they really ought to. I tend to go a little overboard on my language when talking about failing systems to stress to the reader that something needs to be done. Sometimes you have to say “omg this is going to fail tomorrow” for them to react and fix something.

    • The bridge was again inspected in 2006 and was expected to be inspected in the fall of 2007. I’m going to keep going on the collapse itself and we’ll circle back to the 2006 report in a bit.

  • The day after the collapse, the Governor said the bridge was scheduled to be replaced in 2020. Which is at least 13 years too late.

  • Interestingly, a reinforcement project was planned in December 2006 but cancelled a month later after engineers realised that drilling into the bridge would actually weaken it.

  • Also interesting, during the investigation, internal documents within Minnesota’s Department of Transportation talked about the risk of the bridge collapsing and they were worried they might have to condemn it, but they didn’t.

    • And to an extent, I kind of get it. Based on what this bridge serves, there would be a complete uprising if they closed this bridge without immediate plans to replace it. And I think it’s easier to just ignore it and hope it makes it through. You know, being an engineer is not easy. I am often the bearer of bad news, I am often protecting people from themselves, and I’m often asked to confine my designs into spaces that are smaller than required.


  • The central span gave way first, followed by the adjoining spans. The south part of the span shifted 25m east or downstream on the river as it collapsed.Looking at photos of the collapsed bridge it looks like every span collapsed; it wasn’t a case of some spans collapsing and breaking away from the bridge. This is the definition of catastrophic failure; every single piece gave way. The collapse was also caught on camera from a nearby security camera.

  • The bridge wasn’t submerged into the river when it collapsed, again looking at photos you can see nearly the entire deck sticking out of the water. Only a few vehicles were submerged, but even though their cars stayed on the deck and above water, people were still severely injured and access to them was challenging.

2006 Report

  • As promised, there was an extensive inspection done in 2006, months before the collapse, and it outlined a number of concerning items that needed to be immediately addressed.

  • The bridge was designed in accordance with 1961 standards, which was revamped in 1974 to include a completely different fatigue design method.

  • The report provided the following recommendations:

    • Five main truss members in one half of each truss, representing twenty members in the bridge, were identified as fracture critical; meaning that a fracture would result in failure of the member and with a lack of redundancy failure of a member would also likely result in failure of the bridge. These members should have been retrofitted with steel plating to add internal redundancy to the member and would have reduced the possibility of failure of the member if a fracture were to develop.

    • Further review of some of the weld locations should have been further inspected with access hole cover plates removed. This meant that the covers were not removed as part regular inspections and who really knows how long it had been since they were inspected.

    • The deck should have been replaced with a lighter, continuous deck through the main truss spans and a composite truss system to reduce the live load on the truss system and improve redundancy. To keep symmetrical loading during deck replacement, the report also recommended a more detailed analysis be done on the loading impact before deck replacement proceeded.

    • Corrosion was found in localised areas near the deck joints.

    • The 2006 report, not any other inspection report over 40 years on over 700 bridges of similar construction, had not noted the issues with the gusset plates, which was that at 13mm thickness they were undersized.

      • Reminder that gusset plates are large steel sheets where beams or girders fasten to columns by way of bolts, rivets, or welding.

    • This is especially interesting because a photo from 2003 showed that some of the gusset plates were bowed. They were also corroded.

    • It's important to note that they were oversized for 1967 loading and the loads on the bridge had increased over time as usage changed. Including 50mm of concrete added to the road surface which increased the static load of the bridge by 20%.


  • A $38 million fund was set up for victims in May 2008. And in August 2010 the lawsuits were settled for $52.4 million.

  • The company that designed the bridge, well the company that bought that company, was also sued by the state of Minnesota and they ended up settling out of court for $8.9million. In Canada, liability is typically 10 years, but under extenuating circumstances such as these, I’m not surprised they were sued 40 years later. I assume the engineer of record is no longer practising.

  • A replacement bridge was built in the same place and opened September 18th 2008. A little over a year after the collapse, which is pretty impressive as these projects usually take years. The bridge was done as a design build, which simply means that the contractor hires the consultant team and they operate as one team to design and construct the bridge. These often reduce a lot of paperwork and red tape and allow for fairly effective communication between the designers and contractors. The replacement bridge finished three months ahead of schedule and received an award for the “Best Overall Design-Build Project Award” from the Design-Build Institute of America.

So there you have it, the collapse of Bridge 9340 in Minneapolis. A design flaw, changing needs over 40 years of operation and bridge inspections that could have been better led to the collapse of the bridge during rush hour on a summer day in 2007. The US specifically, but really every country, needs to take better care of their ageing infrastructure to prevent catastrophic failure.

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Bye everyone, talk soon!