The dynamic response of a bridge or beam to a pulsating load whose amplitude varies harmonically with time but at a different frequency is examined. Analytical solutions for the generalised force are given and they show that forces exist at sum and difference frequencies. Resonances can be excited if forces at any of these frequencies are equal to a natural frequency of the structure. Experimental verification of the theory is given by the use of a vibration shaker, slider crank mechanism and a cantilever beam.
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