Evaluation of Inelastic MDOF Effects for Seismic Design

The main objective of this research is to develop procedures to quantify seismic demands on Multi Degree of Freedom (MDOF) systems for use in conceptual design using response spectral representations of input ground motions. The emphasis is to provide comprehensive, yet simple tools for conceptual design. A design methodology. which incorporates demand/capacity concepts explicitly. is used as the framework for the development and incorporation of research results.

A statistical evaluation of the results of inelastic dynamic analyses is used as the basis to derive empirical rules (denoted as MDOF modifications) that permit the estimation of seismic demands on MDOF systems using elastic fmelastic spectral values for a Single Degree of Freedom (SOOF) system with a period equal to the first mode period of the MDOF system. A comprehensive evaluation of the inelastic dynamic response of two types of lateral load resistant systems is performed; Moment Resisting Frames (MRFs) and Structural Walls (SWs). In MRFs the inelastic response is controlled by story shears. whereas for SWs it is desirable to control inelastic response by overturning moments. Suitable parameters are identified to quantify seismic demands for each strucnJra1 system. and the effect of different modes of deformation and "failure" mechanism at limiting the inertia forces transmitted into the structure is evaluated.

Three types of failure mechanisms are investigated in the study on MRFs. The results on deformation demands show that interstory drift (ductility) demands depend strongly on the initial period. strength and failure mechanism of structures, but that roof drift demands are insensitive to the yield IDC'd Janism Quantitative information is developed to estimate roof displacement demands and energy demands for MRFs using spectral displacements and energy spectra.

The results of the study on SWs indicate that higher modes have a significant effect on the base shear demands and the distribution of story shear demands over the height. In systems with constant flexural strength over the height, higher mode effects lead to flexural yielding over a significant height of the wall. Plastic hinge rotation demands at the base are used as a measure of structural damage, and empirical relationships are provided to emmate these demands from spectral displacements of the first mode SDOF system.