The Innerbelt Bridge (CUY-90-1524) carries Interstate Route 90 over the Cuyahoga River Valley in Cleveland, Ohio. The bridge was designed in 1959 and is a cantilevered steel deck truss with 9 spans for a total length of 829 m (2,721 ft) and a main span of 122 m (400 ft). In 1984 the bridge was widened by adding a 3.05 m (10 ft) deep plate girder, cantilevered from the southern truss at the western end of the bridge, to accommodate a new eastbound entrance ramp. During routine inspections of the bridge, significant out-of-plane vibrations in the web of this retrofitted "widening" girder were observed. The added eastbound entrance ramp was subsequently closed, and a forensic investigation into the cause and severity of the vibrations was commissioned. Initially a finite element model was developed and analyzed. It was found from this analysis, that the web of the girder was quite likely elastically buckled under the dead load of the structure. Live load testing was then performed to quantify the magnitude of the vibrations and their structural implications. The live load testing consisted of instrumenting the web panel and supports with strain gages and accelerometers, and monitoring these instruments during passages of a test vehicle, both on the entrance ramp and the travel lanes of the bridge. This investigation determined that the observed vibrations were due to the dynamic variation of the amplitude of the buckled shape of the web panels under the impact of traffic-induced live loads. It was also determined that the induced stresses due to the out of plane vibrations were below the threshold of expected fatigue damage and that no corrective measures were required.
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