The emphasis in traditional seismic design codes and guidelines for structures in the United States has been on collapse prevention, and on structural analyses which empirically reduce elastic earthquake response forces to reflect inelastic behavior of structural elements. These traditional approaches also treat foundation elements as rigid, or assume they deform elastically and check that structural load demands are less than the capacity of foundation elements. However, the significant damage to structures from earthquakes in recent years has led to the recognition that improved analysis methods are needed to better evaluate structural performance in terms of levels of damage, and that traditional force based design procedures can lead to unnecessarily high retrofit costs, both for structural and foundation elements. New seismic retrofit design guidelines in the United States for both buildings and bridges document alternative nonlinear analysis procedures, with structural displacement being the measure of seismic demand and acceptable performance. ln these new procedures, the load-deformation and capacity of foundation elements play an important role in reducing the costs of seismic retrofit. The concept of allowing mobilization of ultimate capacity of foundations during earthquakes represents a major change in conventional design philosophy for geotechnical engineers. The nonlinear modeling of shallow foundations for buildings and piled footings for bridges are the focal points of this paper. ln the case of shallow footing foundations for building structures, emphasis is placed on discussions of moment-rotation behavior and modeling, including the effects of rocking uplift andprogressive settlement arising from mobilization of moment capacity. The development of a nonlinear moment-rotation SPRING model is described, which is suitable for use by structural computer codes. Application of the model to (a) evaluate settlements associated with pile capacity mobilization, and (b) illustrate reductions in inelastic demand on bridge columns are provided.

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