The difference in ground motion along a lifeline, the incoherent motion, is an essential component of the input. A long, straight, segmented pipe, with each link attached to the ground via a spring and dashpot is subjected to incoherent ground motion caused by a phase delay. The equations governing the axial response of the system are developed. Modal decomposition is used and closed form expressions are given for the natural frequencies and mode shapes. Examples are given showing the center joint displacement time history when the lifeline is subjected to earthquake loading. Spectral techniques can be used to bound the motion with the Interference Response (IR) spectrum. This spectrum is the maximum difference in motion (response) of two adjacent points which are excited by a difference in ground input. It is seen that the IR spectrum is a useful tool in the dynamic analysis of lifelines over a broad range of parameters.

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