The Minneapolis Bridge Collapse

In case you have not heard about it yet, an eight lane highway bridge over the Mississippi River in Minneapolis collapsed during rush hour yesterday evening. As with the Concorde bridge collapse last January here in Quebec, this hits close to home. Because this is what I do on a daily basis. Not design bridges, but design buildings and other structures in order to ensure the safety of those who use them.

I35W_Bridge
Image of the bridge
before collapse courtesy of Wikipedia.

Plus those twisted steel shapes brought back memories of another incident that I had worked on. On July 14, 1999 a crane carrying a large section of the operable roof for the future Miller Park, collapsed killing three iron workers. Our office was brought in by the owner to assess the damage and to determine the cause of the accident. It was early in my career so I was working with the team assessing the damage. But regardless, it was quite an experience.

mp20

Image of the roof piece being maneuvered into place, courtesy of JSOnline.

About a third of the roof structure had tumbled into the bowl of the stadium that was about 80% complete. The twisted steel members were boggling to the mind. Members made of some of the largest sizes available and strongest steel available had been twisted like playdough and left in a heap. And yet this amazing sight was surrounded by parts of the stadium that were barely touched by the immense impact and destruction created by the collapse. On the other side of the stadium a clear hole was ripped through two solid layers of concrete by the ‘headache ball’ connected to the hook of the crane. It had been ripped from the crane and loose of the piece it was carrying and whipped across the field only to stop in the basement concourse.

MP
Image courtesy of JSOnline.

But this incident in Minneapolis is different. In Milwaukee the cause was directly related to human action. The foreman decided to lift a large piece when the winds were stronger than the limits provided. In Minneapolis, it is very very early to pinpoint what the cause could be without obvious signs. But since bridges are relatively simple structures, there are some common modes of failure.

Member failure: One member’s capacity is exceeded and if there is no redundancy, there is nothing else to carry it’s load. This will overload other members to their failure until at some point there are members that can carry the load or the bridge completely collapses. This type of failure would likely create a progressive collapse and by preliminary accounts this collapse happened all at once. So it’s possible this was not the cause.

Fatigue: If you take a paper clip and bend it (just enough so that it doesn’t permanently deform) and do this repeatedly, eventually it will bend under less force. This is fatigue and is a significant concern for steel bridges due to the traffic that passes over it repeatedly. During design, this phenomenon is taken into account and the bridge members are sized appropriately. Though a life expectance for the bridge is estimated along with the estimated loads. This bridge is forty years old and had bumper to bumper traffic at the time of the collapse.

Temperature: Materials expand in heat and contract in cold. This is why one end of bridge structures are allowed to slide. If this is not accounted for, the entire bridge would be compressed in heat and possibly buckle. Or in cold it could contract and tear loose of it’s support. This is also not likely the case since it is standard practice to allow for this. Though it is possible that one member had seen the immense force created by temperature expansion and was overloaded.

Settlement: As with any structure, the foundations are designed to carry the load. But our understanding of soils is not an exact science and was even less understood back in the time of this bridges construction. In addition this is at a river and anchored in the old river bed which can prove difficult for constructing foundations. There is also the possibility that the river had undermined the foundation causing the bridge pier to list to one side.

Human error: They were doing construction on the road surface which should not affect the structural integrity below. But since it is all connected, work above may have exposed and aggravated a weakness below.

And lastly, it could be some combination of these where one existed and the introduction of another was enough to bring it down. I’m amazed at these images on television. How three spans of the bridge are flat to the ground and draped over the supports. It is still speculation, but that suggests a lack of redundancy since the other spans did not remain standing after the initial span collapsed.

I’ll likely have more to say tomorrow. If there is anything above that is unclear or possibly incorrect, let me know and I’ll address them. It’s late and after watching over two hours of CNN coverage, I’m exhausted.

UPDATE the next morning: A video of the collapse taken by a security camera was shown this morning on CNN. It shows that the collapse was progressive and started from the south end of the main span. The other spans to the north fell due to either a lack of redundancy or movement created by the force of the main span collapse. Another thing that is evident from the photos available this morning is that the support on the south end of the main span fell to the east instead of straight down. All of the reasons above could be the reason for failure of that support, if it originated there and not more to the right out of view of the video. But reports now suggest that the lack of redundancy and fatigue are strong possibilities.

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~ by Frank on August 1, 2007.

7 Responses to “The Minneapolis Bridge Collapse”

  1. Thanks for that background info and perspective. It’s useful to me as I’m a big fan of bridges but I’m fully a lay person and am not well informed on the physics and other factors that go into their construction.

    What a catastrophy! It’s particularly salient for we Montrealers in light of all we’re hearing lately about the collapse in Laval last year.

  2. Ah the Big Blue collapse. I saw the documentary on it. Wasn’t the problem that nobody considered the wind forces in determining the load limits of the crane?

  3. […] America’s aging infrastructure is hitting a tipping point.”  In an excellent post, Frank, an architect, suggests that fatigue was likely a contributing factor, while civil engineers warn […]

  4. I gotta say, when I look at various “before” pictures of the I35W bridge in Minneapolis I’m struck by how flimsy it looks. There’s really nothing to it — even the platform looked very thin. I would expect that people would be extra dilligent when assessing such a structure for fatique, but I guess not.

    Your story about the crane collapse is also a reminder, at least to me, to not be complacent about these things. Whenever I see a crane hoisting stuff around town I always steer clear of it. But I’m amazed at how many people walk right under these things without even looking up. Their blind faith in the infallibility of machines and infrastructure is kinda scary.

  5. blork: Yeah, I’ve always been fascinated with bridges along with all built structures. Even before I got into the business.

    Fagstein: I’d be interested to see that documentary even though I know the incident very well. It’s always interesting to see the difference between what is reported and how things actually happen. I’ll have to track that down someday.

    There was a specified wind speed limit for the piece that they were lifting. The wind gusts were exceeding that limit, but the foreman decided to go ahead with the lift possibly because they were badly behind schedule. The ironworkers actually had a major argument with him about it, but it went ahead anyhow.

    Here is a video of the actual collapse.

    blork: Actually when I first looked at the original bridge, I had the same feeling. It’s possible our eyes are accustomed to the hefty members of the Jacques Cartier Bridge. Given that this bridge had been double-checked by students at the U of Minnesota, I’m reassured that the member sizes are correct. I’ve had it happen before where I design something and it works on paper, but looks awfully slim in place. But after double-checking everything it works out.

    And I agree about not walking under things. When I’m on construction sites, I try to keep track of what the crane is doing over my head.

    I’m starting to suspect that the cause was more due to age like fatigue or a shift in the foundation. But that’s purely conjecture.

  6. Thanks for shedding some light on the tragedy with such an informative post. Of course as a Montrealer and urban enthusiast the news immediately got my attention, and it’s great to get an understanding of the potential factors at play here.

  7. Bruno, Let’s just hope these are isolated incidents and not part of a trend.

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