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Fatigue Evaluation of the Deck Truss of Bridge 9340
Minnesota Dept. of Transportation
Final Report

Bridge 9340 is a deck truss with steel multi-girder approach spans built in 1967 across the Mississippi River just east of Downtown Minneapolis. The approach spans have several fatigue problems; primarily due to out-of-plane distortion of the girders. Although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and floor truss systems. Concern about fatigue cracking in the deck truss is heightened by a lack of redundancy in the main truss system. The detailed fatigue assessment in this report shows that fatigue cracking of the deck truss is not likely. Therefore, replacement of the bridge, and the associated very high cost, may be deferred."

Full report (pdf):

Two- and three-dimensional finite-elements models of the bridge were developed and calibrated based on the measured stress ranges. .. The peak stress ranges are less than the fatigue thresholds at all details. Therefore, fatigue cracking is not expected during the remaining useful life of the bridge. The most critical details, i.e., the details with the greatest ratios of peak stress range to the fatigue threshold, were in the floor trusses. Therefore, if fatigue problems were to develop due to a future increase in loading, the cracking would manifest in a floor truss first. Cracks in the floor trusses should be readily detectable since the floor trusses are easy to inspect from the catwalk. In the event that the cracks propagate undetected, the bridge would most likely tolerate the loss of a floor truss without collapse, whereas the failure of one of the two main trusses would be more critical.

This research has implications for bridges other than 9340. The research verified that the behavior of this type of bridge can be deduced with a modest number of strain gages at key locations combined with detailed analyses.

Model and detailed analyses, meet reality.

Originally posted to jmknapp on Wed Aug 01, 2007 at 06:56 PM PDT.

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Comment Preferences

  •  Some Engineer (4+ / 0-)
    Recommended by:
    jmknapp, annetteboardman, Kingsmeg, Lashe

    has got a lot of 'splainin' to do.

    Of course this report was 6 years ago,and in all likelihood, they already noted some cracks and that is why they were working on the bridge.  It just didn't happen fast enough.

    There are bagels in the fridge

    by Sychotic1 on Wed Aug 01, 2007 at 06:49:07 PM PDT

  •  so - (3+ / 0-)
    Recommended by:
    jmknapp, Lashe, william f harrison

    that's about the bridge that just collapsed? the very one?

  •  On I40 about 60 miles west of the AR line, (8+ / 0-)

    a bridge collapsed about 5 or 6 years ago in the middle of the night.  It was not in a major city and was caused by a tow boat striking the supports.  However, there are literally hundreds of bridges like the one in Minniapolis carrying a mass of trafic that they were never designed to carry and being inspected every few years, with replacement delayed because we have to spend billions of dollars for new weapons systems and billion dollar aircraft and new hydrogen bombs to defeat Osama ben Lordy and wib the war on terror.  Good plan.  

    A private gyn office offering full gyn services including abortion care to 18 weeks.

    by william f harrison on Wed Aug 01, 2007 at 07:19:00 PM PDT

  •  Hope somebody is hopping to it (3+ / 0-)
    Recommended by:
    jmknapp, LNK, Lashe

    This research has implications for bridges other than 9340

    "What did you say to the reporter?" "I don't know, but I'll find out and get back to you." Schumer/Gonzales

    by Catte Nappe on Wed Aug 01, 2007 at 07:19:24 PM PDT

    •  Who's that "someone"? (1+ / 0-)
      Recommended by:
      Catte Nappe

      Given that the Rethugs have been trying to cut taxes and drown the government, given Katrina and FEMA, who do you expect to "hop" to it?

      I hope the Dems tell the American people who is really responsible for lack of attention to infrastructure.

  •  This makes me wonder (2+ / 0-)
    Recommended by:
    jmknapp, MyBrainWorks

    How many other interstate bridges are out there that could collapse like this one? As most of these interstates were created in the 1940s and 1950s, they must all be nearing 50-60 years old. What is the real lifespan of these bridges?

    It makes me wonder how safe our bridges really are.

  •  In Boston... (7+ / 0-)

    ... one of the urgent reasons for building the Big Dig was fatigue cracking in the elevated Central Artery and Charles River Bridge (both I-93), compounded by two successive Quebecois trucks striking bridge uprights in the early '90s.    

    It's likely that there were undetected extended fatigue cracks in the 35W bridge that a series of factors (traffic, resurfacing construction, abutting rail vibrations, yadda, yadda, yadda) caused the collapse.

    The maintenance budget cuts by damn near every entity in this nation (hey, in Boston we have flying highway metal now!) is going to make this a recurring nightmare.  Y'all heard it here first.

  •  My first comment to my husband when I saw the (12+ / 0-)

    photos, was there is insufficient redundancy in the system and the members are too light for a 4 lane bridge. Thanks for posting the report and letting an old engineer know she still has it. I would place a side bet that this structure was originally designed as two lane and had been modified to four, probably by  removing pedestrian ways and center separation. Plus the first paragraph quoted, the last sentence contradicts everything proceeding it. I'll leave the rest of my commentary on current engineering graduates unsaid.

    •  Thanks... (3+ / 0-)
      Recommended by:
      Agathena, susie dow, dougymi

      for your professional assessment, raindrop, and DO post more commentary, since as of 11 pm EDT the TV news is reporting that inspections by the Minneapolis and Federal gov'ts found no real problems....

      As others have noted, 35 years of tax cutting and ignoring infrastructure have taken their toll.

      And thanks for this diary, mjknapp.  Keep posting the facts on this tragedy.

    •  Thank you raindrop (0+ / 0-)

      It seems to me a non-engineer, that everything came together in one fatal moment: rush hour, 4 lanes full, among the vehicles a tractor trailer, a school bus, increased erosion of the bridge supports from recent heavy rainfall in May, June, July.

      If your bet is correct that would explain much.

      This above all: to thine own self be true...-WS

      by Agathena on Wed Aug 01, 2007 at 09:06:34 PM PDT

      [ Parent ]

    •  Two separate authors? (1+ / 0-)
      Recommended by:

      Plus the first paragraph quoted, the last sentence contradicts everything proceeding it. [raindrop]

      This struck me as well, as though someone else read the report and decided "replacement of the bridge, and the associated very high cost, may be deferred." What's that about? First part sounds like written by a structural engineer, last part by a manager.

    •  Engineering graduates (0+ / 0-)

      My degree is in electrical engineering, but we did have to take strength of materials as a core course, a part of which involving analyzing the forces in truss bridges. It was a lot of work to analyze even fairly simple designs--page after page of calculations. I recall one student asked the prof: "do civil engineers really go through all this calculation?" The reply was, without evident sarcasm, "Oh no, they just look at the blueprints and say 'that should hold.'"

      Maybe there was too much faith in the calculations (the finite elements computer model described) and not enough engineering judgment in this case.

  •  Infrastructure Report Card (2+ / 0-)
    Recommended by:
    susie dow, homo neurotic

    Here it is: the American Society of Civil Engineer's 2005 Report (can't locate 2006).  Doesn't matter though, every year the ASCE issues these we get a "D".

  •  The Big Picture (0+ / 0-)

    If investigation reveals that the bridge collapsed first at that rusty-looking metal "foot" on top of the concrete pillar at the mid foreground of the picture, then someone in the Minneapolis/Minnesota engineering oversight office has to go to jail.

    And it's evident that this bridge was designed in such a way that such a collapse would collapse all the sections between all the supports, in both driving directions. An adequate "failsafe" design would allow a section to fail without dragging down all the sections around it. Looking at the collapsed bridge, it looks like all six segments - both directions supported by two sets of supports - collapsed. If that rusty support failed, then at most the pair of segments directly resting on it should have collapsed: 1/3 the damage. A reliable design would have redundant supports, so if one failed, the other could support the road long enough for it to be cleared and closed.

    How many other bridges were designed and maintained that way? Thousands of ticking timebombs?

    I'm no civil engineer. But even all that is obvious to me with a glance. Who among the people who designed, maintained and inspected the bridge will admit they could have, should have known? And even if they don't admit it, who will be called to account?

    "When the going gets weird, the weird turn pro." - HST

    by DocGonzo on Wed Aug 01, 2007 at 08:40:21 PM PDT

  •  Republican Builders (2+ / 0-)
    Recommended by:
    eugene, zorba

    I note that this bridge was built during the reign of Harold Levander, a rare Republican Minnesota governor (the rest of the era were typically Democrats or Democratic-Farmer-Laborers).

    He was preceeded by a DFL governor, under whom presumably the project was planned and cleared.

    That scenario is similar to that in which Boston's Big Dig was planned by Democratic Governor Dukakkis, but executed by Republican successor William Weld. The Big Dig has been plagued by problems since it has been in service.

    How many major public projects are lethal catastrophes when planned and funded by Democrats, but executed by Republicans and their crony contractors?

    "When the going gets weird, the weird turn pro." - HST

    by DocGonzo on Wed Aug 01, 2007 at 08:48:30 PM PDT

    •  This is a plausible angle to take (1+ / 0-)
      Recommended by:

      but it's only plausible.  Interstate highways are Federal projects.  I strongly doubt that there was anything about the design or the construction of that bridge that was not completely typical for the projects of that era.  In any case, the design would have to pass muster with the Federal Highway Administration.

      The more interesting question is whether the Minnesota DOT had complete responsibility for highway maintenance.  I bet that they did, but don't know that for a fact.

  •  Why not wait? (1+ / 0-)
    Recommended by:

    Why not wait until the collapse is assessed and at the very least preliminary reports are made as to the cause of the collapse?  There might be other causes (defects or problems not related to fatigue) for the collapse and to blame people for screwing up  without knowing what happened is just as bad as Frist making a diagnosis of Schiavo from a few video tapes.

  •  2001- They said fix this Minnesota bridge (0+ / 0-)

    Long before 2005 they were saying fix the New Orleans levees. We need to get around to fixing our infrastructure sooner.  That seriously needs to be a higher priority. Or else more people will die when stuff collapses.

    •  I think that you are misconstruing the report (2+ / 0-)
      Recommended by:
      dufffbeer, Kingsmeg

      It reads like a document written by a consultant who knows that whether he gets the next job depends on how much money he saves the agency that hired him.  Telling them why they don't need to spend money on bridge maintenance ordinarily would be a good career move-- except if bad advice catches up with you.

  •  Wait 'til the dams start bursting (1+ / 0-)
    Recommended by:

    While all states except Alabama now have laws or regulations establishing dam safety programs, enforcement is spotty, largely because of the paucity of inspectors. In Texas, for example, there are only six state employees to inspect nearly 7,500 dams. One Texas official noted that with the current staff level "some dams would not be examined for three centuries."

    Let's do the math. Two of my teenaged grandchildren live in Texas. If we count 30 years for each generation, that means all the dams in Texas will be inspected by the time my grandchildren's great-great-great-great-great-great-great-great-great-grandchildren ring in a new year in 2306. Reassuring, isn't it?

    That's from an CSM article from 1/06, but I'm betting that's nothing's changed, unless of course, we now have less than 6 inspectors....

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