Investigation of the Dayton IR 75 sign truss failure of 9/11/06.
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Investigation of the Dayton IR 75 sign truss failure of 9/11/06.

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    • Abstract:
      Based upon a combination of in-situ field monitoring of traffic-induced bridge

      vibrations at the location of the failed sign support truss, finite element simulation of the

      expected dynamic response of the original truss in such an environment, the length of

      service of the truss at the time of its failure, the volume of truck traffic on the bridge in

      question during that time of service, and metallurgical examination of the failed

      components of the sign truss, the conclusion of the investigation is that extremely high-

      cycle fatigue of the chord/web diagonal welded connection was the cause of the truss

      failure. The in-service effective stress range of the AASHTO Category ET connection in

      question was most likely below the currently specified AASHTO constant amplitude

      fatigue limit (CAFL) for the detail, but the enormous quantity of response cycles

      (approaching or even exceeding 1 billion cycles) such a bridge mounted sign on a

      heavily traveled route accumulates over a service lifetime of 30 or 40 years, exceeds

      anything currently considered in the design codes for such structures. The implications

      of this fact on the current inventory of such structures in similar installations and of the

      same age range, i.e., installed with the original interstate routes, largely in the 1960’s,

      are obvious.

      The one somewhat puzzling aspect of the lower chord fracture of this structure,

      namely that the fracture passed near the weld toe but not actually contacting the weld

      toe, does not appear to be particularly significant. The fracture did indeed pass close to

      the weld and certainly within the heat affected zone of the weld. The fracture surfaces

      were too abraded to positively identify the initiation point; small cracks, however, were

      also identified within the weld under microscopic examination. The original flaw which

      eventually propagated and produced the rupture, apparently in this case, just happened

      to be near the weld rather than in the weld.

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