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Dynamic Strain Measurements on a Complex Freeway Bridge

Dynamic strain measurements were made near the exit end of a complex freeway bridge. The bridge is called “complex” because it is supported by six piers. The piers support I-beams, which, in turn, support the bridge pavement. Dynamic strain produced by vehicles was measured through changes in resistance and reluctance in the steel I-beams through the use of a complex-reluctance bridge (CRB) as a sensor. The strain patterns were measured solely on the exit span of the bridge. However, strain patterns produced by vehicles as much as three spans away were detected in these preliminary experiments. Strain patterns from bridge oscillations were always present, and a crude averaging technique was used here to emphasize vehicle strain behavior. The height of compressive peaks occurring in the exit span when the vehicles were in the next-to-last span fell into two distinct groups. One of these groups seemed to indicate loaded vehicles, and the other seemed to indicate unloaded vehicles. The heights of the tensile peaks generated when articulated vehicles (those having a hinge or pivot connection) were in the exit span did not exhibit such a clear division. Another characteristic that separated the two groups was the occurrence in one group of what appeared to be a large compressive spike superposed in the middle of the tensile peak when a loaded vehicle was in the exit span. Speculation here is that this spike resulted from a coupling of bridge oscillations with vehicle oscillations, which would also be influ-enced by vehicle velocity, although no velocity meas-urements were made. As the experiment was being set up, a loud sonic pulse was generated by a vehicle passing over the exit span, which was consistent with part of the exit span on the concrete block rising off the block and coming back down. Unfortunately, that did not happen again during the period when data were obtained, so the strain signature associated with the sonic pulse could not be registered. However, the compressive spike in the middle of the tensile peak appears to predict behavior consistent with this unusual event. In the following sections, a brief description of the CRB sensor and its behavior appropriate to measuring the transition between static and dynamic strains is presented. This, in turn, is followed by the display of dynamic strain patterns of automobiles, nonarticulated vehicles (single-unit vehicles), and articulated vehicles (tractor trailers). Finally, speculation on the possible origin of the loud sonic pulse is presented.


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