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Dynamic load environment of bridge-mounted sign support structures.
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2005-09-01
[PDF-1.14 MB]
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Edition:Final report.
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Abstract:An investigation was conducted into the failure of a welded aluminum truss sign support structure on an existing interstate highway bridge. The investigation was conducted in three main steps; 1) fatigue testing in the laboratory of surviving segments of the failed sign, 2) collection of dynamic response data of the identical replacement structure in situ, and 3) finite element modeling and simulation of the bridge and truss structural system. The welded aluminum space truss indicated a typical fatigue failure, with a fatigue crack initiating at a welded chord/diagonal connection detail (AASHTO fatigue category ET; CAFL = .44 ksi). Fatigue testing in the laboratory of surviving segments of the structure produced an identical fatigue failure at a similar location after 3,000,000 load cycles at a 1 ksi stress range. Field monitoring of acceleration data at three different locations of the in-situ truss was conducted in order to characterize the dynamic behavior of the truss and the bridge structural system. A finite element model of a segment of the multi-span bridge which included the mounting location of the sign support truss, was assembled. In the modeling of the truss a moving traffic load, consisting of a single truck, was considered. A modal time history analysis for moving vehicle loads was performed. The analysis results indicated that the failure was a classical fatigue rupture, induced primarily by the dynamic effect of moving truck traffic on the bridge. Even though inferred cyclic stress levels were well below the CAFL for the detail in question, the extremely high number of low amplitude traffic induced stress cycles (in the hundreds of millions), combined with the absence of an endurance limit for welded aluminum, resulted in the observed failure. (A typical truck passage resulted in roughly 75 stress cycles in the truss, due to the low damping and extended time of vibration decay.) The predicted lifetime of the replacement sign support structure is approximately that exhibited by the original structure, namely thirty to forty years.
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