An alternating force (for example, due to unbalance in a locomotive driving apparatus) moves across a symmetrical two-span beam (or equivalent bridge) with uniform velocity. The stress-time equations for the three time eras of the problem have been derived by classical methods. The experimental part of the investigation employs a mechanical system with wire resistance strain gages. Resonances from the first through the sixth mode have been investigated. There is good agreement between theory and experiment in so far as number and location of resonances are concerned. The discrepancy in amplitude is to be expected due to the omission of damping in the theory. The most important observation is the “multiplicity of resonances” associated with “each” natural mode.

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