Inelastic analysis techniques in seismic evaluations

The analytical methods available to the design engineer today are either dynamic time history analyses, or monotonic static nonlinear analyses, or equivalent static analyses with simulated inelastic in- fluences. This paper suggests in addition to the inelastic time history analyses, some simplified analyses techniques based on equating the seismic demand expressed in terms of response spectra with the inelastic capacity described in terms of force-deformation characteristics. The inelastic response in terms of accel- erations and maximum displacements can be evaluated accurately, for a single-degree-of-freedom (s.d.o.f.) system, or approximately for a multi-degrees-of-freedom (m.d.o.f.) system. Such method is similar with the proposed technique for evaluation of response by NEHRP, 1996 to be released in the future codes In an effort to develop design methods based on performance it is clear that the evaluation of the inelastic response is required. The methods avail- able to the design engineer today are either dynamic time history analyses, or monotonic static nonlinear analyses, or equivalent static analyses with simu- lated inelastic influences. Although the inelastic time history analyses (ITHA) are becoming more cost effective, the static monotonic nonlinear analyses (push-over type) provide sufficient insight in the expected behavior for design purposes. The simplified techniques currently available seem to produce rational results, without an apparent theory behind. This paper suggests in addition to the ine- lastic time history analyses, some simplified analyses techniques based on equating the seismic demand expressed in terms of response spectra with the inelastic capacity described in terms of force-deformation characteristics. The response demands obtained from the proposed method can be used along with a composite energy-displacement spectrum to determine the seismic performance of a structure in terms of local, or global, damage levels. The response spectrum analysis is a well recog- nized method to evaluate the seismic response of structures. Traditionally the inelastic response spectra are obtained either from inelastic analysis using equal ductility demands or from the elastic response using ductility based adjustments (New- mark and Hall, 1982) Such spectra are usually concerned with the acceleration response, or force demands, only. The inelastic deformations seem to more valuable in evaluationg the inelastic response. A simultaneous quantification of deformation and force responses are valuable for interpreting the response and for the design process. Recently methodologies based on so called ca- pacity spectrum were developed (Dierlein et al, 1991, Freeman, 1994). The response is estimated using the elastic response spectrum presented in terms of acceleration spectrum in conjunction with a curve representing the monotonic nonlinear resis- tence of a structure. The inelastic capacity function (or the monotonic force and displacement resistence envelope mentioned above) was used by Free- man (1994) in conjunction with a combined elastic acceleration and displacement response spectra to determine response both accelerationand displace- ment demands. The inelastic hysteretic effects were considered through an increased period. This paper surveys several techniques for the evaluation of inelastic response and suggests to find simultaneously the inelastic deformation and accel- eration demands for simple structures defined by single (s.d.o.f.) and by multiple-degree-of-freedom (m.d.o.f.) models through a spectral approach.