Earthquake motions as felt by structures along three orthgonal directions are, in general, statistically correlated. Here the effect of this correlation on structural response is studied. Assuming the existence of principal axes of excitations, along which the components of motion are uncorrelated, the mean square and design responses are obtained in terms spectral characteristics of these uncorrelated components. These responses depend upon the relative orientation of the structure with respect to the principal excitation components. For each response quantity, there exists a set of three orthogonal directions along which if principal excitations are applied they will cause a maximum response. Identification of these directions is necessary to obtain the worst case response for design purposes. A methodology to obtain such a maximum response of a quantity of design interest is outlined.
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