A methodology for fragility analysis of buried water pipes considering coupled horizontal and vertical ground motions

With recent advances in computational capabilities as well as broad improvements in ground motion characterization and inelastic modeling of structural and geotechnical systems, large-scale direct models for underground structures-e.g., tunnels, water reservoirs, etc.-can now be devised with relative ease and deployed in engineering practice. In this study, a fragility-based seismic performance characterization for pipes is presented and demonstrated on two steel water pipes. Existing guideline documents and design codes are used for defining the performance criteria; and the fragility functions are developed through a Probabilistic Seismic Demand Analysis procedure for a variety of ground motion intensity measures. 800 nonlinear time-history analyses are carried out for computing the fragility functions. The analyses featured advanced numerical modeling techniques developed for soil-structure interaction problems, which included the Domain Reduction Method for injecting free-field motions into a truncated simulation domain, and the Perfectly Matched Layers for absorbing the scattered waves. The analyses are carried out using nonlinear models for both soil layers and the pipe, and coupled horizontal and vertical input ground motions. Finally, fragility surfaces of two steel water pipes, which consider the intensity measures in both horizontal and vertical directions, are devised for various soil layer profiles and engineering demand parameters.