Inelastic design approach for asymmetric structure-foundation systems

Current seismic provisions allow the seismic design of building structures to be based on static or dynamic analyses of damped, elastic models of the structure. The seismic base shear force is prescribed in terms of "elastic design spectra," or, more precisely, design spectra with no reductions due to inelastic behavior. However, the codes anticipate that structures will undergo inelastic deformations under strong seismic events. Such inelastic behavior is usually incorporated into the design by dividing the "elastic" spectra by a factor, R, that reduces the spectrum from its original "elastic" demand level to a design level. Current approaches for developing the R factor are based on true elastic spectra as references. In this thesis, a more consistent approach for developing inelastic design spectra based on inelastic unreduced spectrum that differs from true elastic spectrum is proposed. This inelastic unreduced spectrum can be obtained directly from inelastic analysis, or it can be constructed from elastic spectrum with higher value of critical damping ratio.; In areas where soil-structure interaction, SSI, can be significant such as in California and Mexico City, SSI extends the elastic period and also increases critical damping ratio of the entire building-foundation elastic systems. SSI has different effects in inelastic systems than in elastic systems. For inelastic systems, the inelastic spectra ordinates are greater than for elastic systems when plotted with SSI period. This implies that the reduction factor, R, which is currently not affected by SSI is incorrect. The R factor should be also a function of foundation and supported soil flexibility, lambda. Based on an approach to construct inelastic design spectra for fixed-base systems, a modified procedure to develop seismic inelastic design spectra for planar inelastic SSI system is proposed. This method is developed such that the inelastic reduction factor is also a function of their relative stiffness between the structure and the surrounding soil with the same format as inelastic spectra for fixed-base system.; After determining the seismic design base shear force from design spectra, this force is then distributed among vertical resisting elements via static or dynamic modal elastic analysis. The current force distribution provisions provide non-consistent seismic design force for each element. The current static method always under estimate the design forces for resisting elements on the unfavorable side of the structure while it may overestimate or underestimate the design forces for resisting element on the favorable side of the structures. The current modal dynamic method is only suitable for designing resisting element on the unfavorable side of the structure. However, it is not good to be used to design other resisting elements since it will underestimate the design forces for those other resisting elements. Consequently, a new modified static method is proposed in order to eliminate this inconsistency in the current force distribution provisions for both asymmetric fixed-base and SSI systems.; The maximum relative displacement with respect to the foundation and maximum total displacement of the entire systems are also important. In this thesis, simple and consistent rules to approximate these maximum displacements directly from the basic structural parameters and seismic design coefficient are proposed. These rules provide good approximations to the exact results.