When modal analysis technique is used to determine spectral densities of responses of continuous linear damped mechanical structures subjected to stationary random excitation, a summation of all modal spectral densities (both direct- and cross-) should be performed. A very common textbook recommendation is that the modal cross-spectral densities may be neglected under certain conditions. In this paper that recommendation will be discussed. Using a simple example (a simply supported beam), it is demonstrated that the modal cross-spectral densities may contribute considerably tot he response although the textbook conditions are satisfied.
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