A series of nonlinear seismic time history analyses was conducted in LS-DYNA to evaluate the seismic performance of buried reinforced concrete fluid storage structures. This study is aimed at presenting a sophisticated modeling procedure in time domain for soil-structure (SSI) and fluid-structure (FSI) interactions with limited focus on FSI. The current state-of-practice for modeling SSI and FSI was reviewed, including the use of discrete analogs representing the far field effects as well as nonlinear springs for the near field effects. The properties of analogs are usually computed through a frequency-domain FE program with equivalent linear properties. It is shown that the performance of embedded structures during MCE events on the U.S. West Coast can be most efficiently and accurately estimated through nonlinear time domain analyses. The availability of computational resources has made the sophisticated modeling of structural components and explicit modeling of the continuum soil and fluid domains feasible in today’s engineering practice.
A special attention is devoted to the modeling of soil materials. A consistent approach is proposed to model the nonlinear and pressure-dependent behavior of the soil which covers the small strain stiffness degradation and the large strain response up to the soil shear strength. The site response analyses performed in LS-DYNA were compared and validated with other conventional tools. Additionally, it is shown that the pore water pressure as well as construction staging should be modeled to accurately capture the soil state of stress and strain.
Unlike most conventional techniques, which operate in the frequency domain, material and geometric nonlinearities can be included in the analysis of reinforced concrete structures. Fiber-based integrated beam and composite shell finite elements were used to model the structural members. The material model used for concrete accounts for cracking in tension, crushing in compression, and post-peak strain softening. Finally, the interface between the soil and structure was modeled with contact surfaces to allow separation and sliding.
This study demonstrates that such detailed analyses can be efficiently performed within the demanding schedule and budget of modern infrastructure projects. Such numerical simulations provide invaluable insight into the complex interaction of the structure with the surrounding soil and fluid inside as well as vital inputs to the design of the structure.

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