Seismic Demands for SDOF and MDOF Systems

Earthquake resistant design has the objective of providing structural capacities that exceed the demands imposed by severe earthquakes by a sufficient margin of safety. It is anticipated that future seismic codes will incorporate capacity I demand concepts explicitly in order to make the design process transparent and permit designs with a well defined and consistent level of protection. This research is concerned with the assessment of seismic demand parameters that are needed to implement a capacity / demand based seismic design approach. The objectives of the study are : (a) to assess the importance of different demand parameters, (b) to evaluate patterns in demand parameters that will improve our understanding of the physical phenomena involved in seismic response of structures, and (c) to provide statistical information on demand parameters that can be utilized to assess the performance of structures designed according to existing codes and implement the proposed capacity / demand design approach.

A comprehensive evaluation of seismic demand parameters is performed for bilinear and stiffness degrading Single Degree of Freedom (SDOF) systems. A less comprehensive but much more elaborate study is also performed on three types of Multi-Degree of Freedom (MDOF) structures. The purpose of this aspect of the study is to evaluate the modification that must be applied to strength demand parameters derived from simplified SDOFmodels in order to account for multi-mode effects in real structures.

In the SDOF study, the inelastic strength and cumulative damage demands are evaluated statistically for specified target ductility ratios. Such a statistical study can be attempted only for ground motions with similar frequency characteristics, such as rock and firm soil motions recorded not too close and not too far from the fault rupture. Strength demands are represented in terms of inelastic strength demand spectra or spectra of strength reduction factors. Expressions are developed that relate the strength reduction factor to period and target ductility ratio. In the MDOF study, it is found that the required strength for specified target ductility ratios depends strongly on the type of failure mechanism that will develop during severe earthquakes. Quantitative information is developed on relative strength requirements for three types of MDOF structures, showing the disadvantage of structures in which story mechanisms develop, and particularly the great strength capacities needed to control inelastic deformations in structures with weak stories.