Failure Analysis of a Bridge-Mounted Sign Support Truss

An investigation was conducted into the failure of a welded aluminum truss sign support structure mounted on an 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 sign truss structural system. The welded aluminum space truss indicated a typical high-cycle fatigue failure, with a fatigue crack initiating at an AASHTO category ET tubular chord to tubular diagonal welded detail. Fatigue testing in the laboratory of surviving segments of the structure produced an identical fatigue failure at a similar welded detail after 3,000,000 additional load cycles, at a constant amplitude nominal stress in the diagonal of 6.89 MPa (1 ksi). Field monitoring of acceleration data at three different locations of an identical in-situ truss was conducted in order to characterize the dynamic behavior of the sign support truss/bridge structural system. A finite element (FE) model of a segment of the multispan bridge, which included the location of the sign support truss, was also assembled for use in the investigation. Modal analysis, both analytical and experimental, indicated that the lowest natural frequencies of the bridge/sign system were high enough to rule out wind-induced vibration as a likely cause of the fatigue failure. Experimental observation as well as FE time history analyses for moving vehicle loads were performed for the bridge/sign system. The results indicated that the failure was a traffic-induced, extremely high-cycle fatigue failure (with the number of cycles running well into the hundreds of millions), even though the indicated stress amplitudes were only half of the constant amplitude fatigue limit (CAFL) for a category ET detail. The predicted lifetime of the replacement sign support structure is approximately that exhibited by the original structure, namely thirty to forty years.