Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Poor design led to I-35W bridge collapse?

  1. Aug 3, 2007 #1
    This image shows the ground supports of the bridge before the collapse:


    taken from:

    Nation's Bridges Face Immediate Inspection.
    Fifth Victim Found; President Bush To Visit Minneapolis
    UPDATED: 9:20 am EDT August 3, 2007

    A video of the collapse is also available on this page.

    Note the ground supports are slender concrete columns. Note also the
    steel arch only extends to the top of the support. It does not extend
    down into the ground.
    In contrast note the arches of the 10th Avenue bridge next to the
    collapsed bridge extend into the ground:

    10th Avenue Bridge.

    The strength of an arch extends from its curvature. Note that an arch
    of the I-35W bridge not extending into the ground means the arch is
    shorter which necessarily makes the arch straighter, and therefore
    The 10th Avenue Bridge also has supports in the middle of the river
    while the I-35W bridge did not. Compared to the supports of the 10th
    Avenue Bridge, the ground supports of the I-35W bridge can only be
    described as flimsy.
    This is a bridge that carries the most traffic in the state of
    Minnesota. Moreover the 10th Avenue bridge only has to carry 2 lanes,
    while the I-35W carried 8.
    A professor at Northwestern argues the failure was likely due to the
    joints connecting the bridge to the concrete supports:

    Investigators in bridge collapse focus on chilling video.
    By Jon Hilkevitch | Tribune transportation reporter
    9:41 PM CDT, August 2, 2007
    "The bridge must have been near a state of collapse for some time, and
    the construction might have contributed to its failure," said Zdenek
    Bazant, a professor of civil and environmental engineering at
    Northwestern University. Bazant said he suspects there may have been a
    hairline crack or fatigue in the steel joints near bridge supports,
    leading to the buckling"

    This page on the I-35W describes it as a truss bridge:

    I-35W Mississippi River bridge.

    These are among the cheapest and flimsiest of bridges. They lack the
    redundancy of many other types of bridges:

    Investigators in bridge collapse focus on chilling video.
    By Jon Hilkevitch | Tribune transportation reporter
    9:41 PM CDT, August 2, 2007
    "Other engineering experts said that the 1960s-design of steel-arched
    bridges did not contain structural redundancies, meaning that if one
    component fails, the whole structure is in jeopardy because the weight
    does not shift to other points on the bridge.
    "We know that we would not build a bridge like this today,'' said Kent
    Harries, an assistant engineering professor at the University of

    Bob Clark
  2. jcsd
  3. Aug 3, 2007 #2

    You can't extend a steel column down into a river. It would have corroded 30 years ago if they'd done that.

    Since this bridge stood up to 40+ years of continuous loading/unloading, I don't think this was a design flaw. It sounds more like a corrosion/maintenance problem that was not addressed as it should have been.

    Concrete is much more durable than steel and doesn't require as much maintenance to prevent corrosion. I wouldn't call steel truss bridges "flimsy" though.
  4. Aug 3, 2007 #3
    There are bridges that cross spans longer than this one without supports within the river. According to the reports they did not want supports in the river for this one so as not to interfere with river traffic.
    Given that, the ground supports should have been at least as strong as the the 10th Avenue Bridge supports, not flimsier.

    Bob Clark

    Attached Files:

  5. Aug 3, 2007 #4

    You missed my point. Of course you can make longer spans without supports in the river.


    You said commented about how the "steel support doesn't extend into the ground." You don't want to extend exposed steel down into the ground... especially on a riverbank (which is where the supports for this bridge were). That is the reason for the concrete pier. The cause of the collapse is much more likely to be corroded or fatigued steel. Concrete is very durable and much more weatherproof.
  6. Aug 4, 2007 #5

    The arch could be extended into the ground within the concrete supports.

    Bob Clark
  7. Aug 4, 2007 #6


    User Avatar
    Science Advisor

    Fatigue and buckling are not indications of poor design by themselves. Like has already been mentioned, they are most likely the sign of neglect.

    How about waiting for an official release of cause and then talk about design points? Or is it more "interesting" to straddle the conspiracy theory line and Monday morning quarterback the original designers?
  8. Aug 6, 2007 #7
    Amen. I love how everyone becomes an expert in structural engineering after something like this happens. Reminds me of all the "bombs in the world trade center" talk.

    Maybe Bob slept at a Holiday Inn Express last night? :biggrin:
  9. Aug 6, 2007 #8
    Here is a page with links to descriptions and images of river crossing bridges in the area:

    The Bridges And Structures Of The
    Major Rivers Of Minneapolis And St. Paul.

    Here's the link from that page for the I-35W bridge:

    I-35W Bridge
    Historic I-35W Mississippi River Crossing
    Minneapolis, MN.

    This mentions other truss type bridges in Minnesota that have been
    ordered to be inspected in the wake of the I-35W collapse. Note that
    all of these do have supports within the river.
    The page also has close-up images before the collapse of the bridge
    supports, suspected as being where the failure occurred. Notice the
    joints connecting the steel beams to the concrete supports in the next
    to last image on this page. Why are these joints so tiny on the top?
    It's almost like they are coming to a point? This clearly would result
    in a greater deal of pressure on this portion of the joint, like the
    pressure placed at the bottom of stiletto heels.
    Compare to the width at the top of the joints of the DeSoto bridge, a
    similar truss type bridge, in the next to last image here:

    Desoto Bridge
    Mississippi River Highway Crossing
    St. Cloud, MN

    I'ved attached zoomed in images of the I-35W and Desoto bridge joints below.

    Bob Clark

    Attached Files:

    Last edited: Aug 7, 2007
  10. Aug 7, 2007 #9
    The I35W bridge was not a steel arch, it was a three span steel truss bridge.
  11. Aug 9, 2007 #10


    User Avatar

    Staff: Mentor

    Yes, it would, and there is nothing wrong with that. In fact, it is done on purpose for flexibility. If you bolt them, they can't flex and that creates a tremendous amount of stress.

    We have standards of quality in the engineering forum as well as the science forums. Speculation on the cause of the collapse is fine, but it has to be grounded in real engineering. Idle speculation is not useful or helpful, especially when a lot of this stuff is easily verified with a Google or a Wik.

    Thread locked.
    Last edited: Aug 9, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Poor design led to I-35W bridge collapse?
  1. Good bridge design? (Replies: 2)

  2. Bridge Design (Replies: 4)