Seismic design of foundations includes analyses of both the overall stability of the foundation system and the dynamic stiffness and damping of individual foundation elements. Stability considerations include both bearing capacity and translational (sliding) stability. Overall stability analyses are generally performed using pseudo-static analysis. Important considerations with respect to overall stability include the selection of the seismic coefficient, load combinations (e.g., seismic plus wind loads), the appropriate value for the dynamic shear strength of the soil, allowable local yielding of foundation elements, and the reliance on passive earth pressure in evaluation of sliding stability. The residual shear strength of the soil may also be an important consideration in evaluating post-earthquake overall stability for foundations on potentially liquefiable soils. Evaluation of dynamic stiffness is generally based upon closed form elastic solutions for shallow foundations and p-y and/or t-z curves for deep foundations. Evaluation of the dynamic modulus, including consideration of confining pressure and strain softening effects, is a key consideration in evaluating the dynamic stiffness of shallow foundations. Evaluation of group effects is a key consideration in p-y and t-z analyses of deep foundations. In some cases, evaluation of the lateral resistance of piles in liquefied soil may also be an important consideration in evaluation of deep foundation seismic response.

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