Minimum Effective Length and Modified Criteria for Damage Evaluation of Continuous Buried Straight Steel Pipelines Subjected to Seismic Waves

It is impractical in the dynamic analysis of pipelines to consider the whole length of the pipe in a numerical model. Therefore, a minimum length is desired for the pipe model so that its midpoint responses are not affected by its end conditions. This length can be called the minimum effective length (MEL). In this study, at first, based on extensive nonlinear dynamic analyses, MEL values were determined for various types of soil using the equivalent bilinear springs for consideration of the soil-pipe-interaction. Then, by solving the dynamic equilibrium equation, assuming a linear behavior for the pipe and soil, it was shown that for very long buried straight pipes, the frequencies of all lateral modes are the same, and therefore the participation of higher modes is not negligible in long pipes. To take the higher modes into account, a relation was established between the unburied pipe frequencies and the MEL of the buried pipe. Then, assuming the local buckling of the pipe wall due to axial force and bending moment as the most probable damage of buried pipes subjected to seismic waves, a damage evaluation was performed to determine which conditions led to the reduction of the hollow-section area of the pipe and the loss of its efficiency. Finally, by analyzing various pipes subjected to three-component records, it was shown that the created strain could not lead to local buckling of the pipe. Consequently, other effective factors should be studied more thoroughly in evaluating the damage to buried continuous straight steel pipelines subjected to seismic waves.