The paper describes the response spectrum methods which are being used currently in the industry for the calculation of seismic design response of piping systems supported at several points of primary structures. In actual practice, the seismic input motions at the piping supports are correlated to varying degrees. To include the effect of this correlation between different support motions in the calculation of piping response, several methodologies with different simplifying assumptions have been proposed. This paper evaluates the effect of these assumptions on the accuracy of the calculated piping response. For this evaluation the results obtained by some of the currently used approximate approaches have been compared with the results obtained by an analytically rational and a more accurate approach. The comparison shows large differences in the two sets of results. For the example problem, the response values calculated by the approximate approaches are rather on the overly conservative side. This conservatism is, however, neither uniform nor assured. The use of these approximate approaches is, therefore, not recommended.

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