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Underground segmented pipeline with flexible joints is considered as a periodically piecewise homogeneous rod. Longitudinal vibrations of the rod are investigated by utilizing E.A. Ilyushina's method which reduces this problem to vibration study of separation boundaries between homogeneous parts of the rod. In this way, the averaged differential equations of motion and the average wave velocities are obtained for inhomogeneous rod when its cell of periodicity contains two, three, and more elastic components. It is shown that the average velocities are the same as the ones calculated through effective static moduli of the composite rod. The iron/steel pipelines with rubber and lead rings at joints are considered as examples. In these examples the average wave velocities in segmented pipeline (calculated by the method mentioned above) are significantly less than the wave velocity in the iron/steel pipe itself. The last result justifies the necessity of supersonic case study (the case in which wave velocity in the pipe less than seismic wave velocity in the soil) when earthquake response of buried pipelines is investigated. Usually this case is ignored assuming that the wave velocity in the pipe exceeds the wave velocity in the soil. The results of this paper clearly demonstrate that for segmented pipeline this assumption may not be true.

LONGITUDINAL SEISMIC VIBRATIONS OF A SEGMENTED PIPELINE CONSIDERED AS THE PERIODICALLY INHOMOGENEOUS ROD