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I35W Bridge Collapse - Potential Flaw

  1. Aug 9, 2007 #1

    Astronuc

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    This belongs under Civil/Structural, but is a Mechanical Engineering issue as well.

    Potential Flaw Seen in Design of Fallen Bridge, NYTimes, Aug 9
    http://www.nytimes.com/2007/08/09/us/09bridge.html

    The use of Gussets is standard in steel bridges. They form the joints shared by various members. The objective is to design them so that stress is well below a critical failure stress, but the entire bridge system also needs to be designed to distribute the load so as to prevent stress concentrations that would lead to failure. That apparently may not be the case.

    It would seem necessary at this time to re-evaluate all such bridges (truss) in order to identify those which might be in danger of failing. We now have software and hardware not available in the 1960's or 70's, which can do a very good job of identifying potential problems.
     
  2. jcsd
  3. Aug 9, 2007 #2

    FredGarvin

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    I heard that too this morning. The only thing the NTSB is doing right now is warning states of construction traffic weight. This does make a bit more sense since the focus has been on the amount of removed concrete.

    I do find it very difficult to label a 40+ year old bridge design "flawed" though. There are always circumstances that can not be thought of at the design stage.
     
  4. Aug 9, 2007 #3

    Astronuc

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    It may be more the case of a 'flawed system' that allowed a bridge to deteriorate to the point of catastrophic failure.

    It could be that the maintenance was insufficient.

    And - I have to wonder if the deck was simply upgraded with overlay, in which case the dead load was increased - and then there is the potential for increased live load. All of that should have been evaluated to see what the loads on the structure were doing to the stress in the key locations.

    But then again - to save money - that kind of analysis was likely not done.
     
  5. Aug 10, 2007 #4

    FredGarvin

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    That's the way I see it too. The area and loads gradually build up over time and creeped up on them. The DOT in that area continued to upgrade surrounding freeways and did nothing to the bridge.

    I can't help but wonder if the removal of the road surface for the construction had a major effect on the bridge's stiffness.
     
  6. Aug 22, 2007 #5

    Astronuc

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    Pigeon Poop?

    Pigeons Led To Bridge Collapse?
    http://www.time.com/time/nation/article/0,8599,1655309,00.html
    One has to design a system with the environment in mind, and then the users of the system have to monitor the environment and maintain the system.
     
  7. Aug 22, 2007 #6

    Chris Hillman

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    Oh right, blame the birds

    A good example of a violation of a hidden assumption in a mathematical model. I guess the next edition of a program which someone mentioned in another thread, Beam, will have a button marked "poop" :rolleyes:

    Birds are getting blamed for a lot of trouble these days. The Audobon Society has expressed concern that even a limited outbreak of avian flu in North American might lead to some kind of mass slaughter of migrating waterfowl.
     
  8. Aug 22, 2007 #7

    Astronuc

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    In the process and nuclear industries, we have to deal with 'chemical' intrusion which were unforseen or unanticipated because they weren't supposed to happen, or some alloy is added to the system and there is an unanticipated electrochemical (i.e. corrosion) problem.

    The engineer HAS to anticipate the environment.


    We now see problems because 40 years ago, the designers did not have 40 years of experience on many materials and systems.
     
  9. Aug 22, 2007 #8

    russ_watters

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    I see it as more a matter of upkeep. If properly painted, it will take much, much longer to rust.
     
  10. Aug 22, 2007 #9

    Chris Hillman

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    I agree of course that engineers have a responsibilty to try to think outside the box, to look for those hidden assumptions which could cause serious problems during the expected lifetime of the structure.

    In olden days, structures were often overdesigned, in part because engineers recognized they didn't know enough to do anything other than "play it safe". These days, structures are often designed to be much lighter because engineers have such powerful simulation methods available. Maybe it's time to reassess whether they might have been cutting it too fine.

    The NYT story read in part "raises the possibility that the bridge was structurally deficient from the day it opened. It does not explain, however, why the bridge stood for 40 years before collapsing." I wonder how many in this forum know about an unexpected flaw in the Brooklyn Bridge? (As built, not as designed.) I have heard some say that it's not quite clear why that bridge is still standing.

    Russ, have there been suggestions that the I35W bridge wasn't kept properly painted? I haven't seen that.
     
    Last edited: Aug 22, 2007
  11. Aug 23, 2007 #10

    Astronuc

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    I would agree that maintenance may be a key issue here. In addition to painting, and that could be a factor if corrosion is a key/critical factor, keeping the bridge clean and free of bird guano and de-icing compounds is probably another key/critical factor.

    We'll have to wait until they determine the primary failure sites.

    I would imagine 40 years ago, they would have used safety factors like 2 or 4 or more. Certainly corrosion would change that after 40 years. Interestingly, my company has done analysis of the Brooklyn Bridge. I'll have to query the structural guys.

    I take it that Russ is referring in general to proper maintenance of which painting is a big part, particularly in the north from Minnesota, across Wisonsin, Illinois, Indiana, Ohio, Pennsylvania, NY and NE where use of salt on roadways is common in the winter. NY has a lot of corrosion on some of its highway bridges, which are mostly girder or I-beam.
     
  12. Aug 23, 2007 #11
    There have been many reports about bridge collapses. Some may be because of traffic accidents, some may be because of design/construction problems, etc. Once such accidents occur, it will be hard to track who should pay the greatest responsibility. The design engineers would say we designed it according to our standand /code and if it is still our problem then it is the experts (who originally specified or edit the standard) that should be responsible. Since the standards are taken not to be problematic,then it seems the problem in construction (this is the most difficult and very problematic process for a bridge project). The construction company would say it is the third party which is responsible for the check and accept the quality of the construction process. Then The third party can not get rid of its main responsibility. After the bridge is complete, the maintenance/servicing department may be responsible for looking for potential problems existing (or that may exist) in the bridge. Who should be responsible for the bridge (the bridge is usually so big a project, mostly over several billion dollars), no department (maybe insurance can, but insurace may find somebody who has to share the responsibility) can afford such responsibility.

    The bridge has stood there for 40 years, about half of the expected bridge life in USA standard?. I must say the design and construction must be good enough. Many unexpected problems (the civil engineering object itself or by the external environmental or wotever) may occur.And this unexpected problems, as the word means, is unexpected by the original designers and construction engineers. e.g. when an earthquake comes (even if you can predict it beforehand), you may do nothing to stop it, and even though a building/bridge has been designed to resist earthquakes itself , the environmental foundation around the building is not what you can improve.

    There was a story about 911. It was said the building was designed to resist explosion (with airplanes).According to this theory, then the enemy's explosion (and/or airplanes) must be stronger than those expected by the designers. So it means that the enemy must be cleverer?

    Blaming the birds might be a good idea and let the government share some responsibility?

    Can I have a look at the picture? I cannot enter the website Astronuc gave.
     
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