In modern industrial facilities there is extensive multi-span piping for process pipelines and sprinkler systems. Seismic analysis of such multi-span piping is complex and designers often resort to approximate methods of analysis. The approximations involved include neglecting the effect of adjacent spans and ignoring the contribution of higher modes. However, vigorous analysis of typical multi-span piping indicates that these results may be in error by over 600 percent. This paper describes two case studies of typical multi-span pipelines (with equal spans) in a large industrial facility. In each case, the stresses in the pipes due to horizontal seismic load are computed using both conventional approximate methods as well as a more accurate finite-element approach. Results of the study show that the approximate techniques can err both on the conservative side as well as the un-conservative side and the magnitude of error can exceed a factor of 6.0. To help develop a better approach, the effects of factors like number of spans, end restraints and pipe diameter are studied. This leads to development of models of simply-supported single-span beams with equivalent lengths which encompass the behavior of the multi-span pipes. These bounding models are then used to develop an approximate analysis approach which is usually conservative with much lower errors than the conventional approach. The proposed approach requires marginally more computations than the conventional approach. The proposed approach is utilized to analyse the pipes in the earlier case study and errors are found to be small.
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