Enhanced pushover procedure and inelastic demand estimation for performance-based seismic evaluation of buildings

A performance-based seismic evaluation of a building refers to achieving a predictable and controllable level of seismic performance for a predefined design event. An essential component of this evaluation process is structural analysis of the building. NEHRP Guidelines for Seismic Rehabilitation of Buildings (FEMA-273) has proposed simplified nonlinear static analysis procedures to estimate seismic demands. These procedures are based on the assumption that the building response is controlled by fundamental mode of vibration. To verify the applicability and limitations of this assumption, instrumented response of a number of buildings, which experienced strong ground shaking in the 1994 Northridge earthquake, are evaluated. This preliminary evaluation reveals that the nonlinear static procedure is inadequate and non-conservative.; A new enhanced adaptive modal pushover procedure is proposed, which accounts for the effect of higher modes and overcomes limitations of the FEMA procedure. The new methodology is applied to a variety of lateral force-resisting systems and response is compared with detailed nonlinear time-history response. It is demonstrated that the proposed procedure is able to reasonably capture important response attributes even for structures with discontinuities in strength and/or stiffness that only a detailed dynamic analysis could predict. The potential limitation of the proposed method for shear-critical systems is identified.; A number of the provisions in FEMA-273 have evolved from past research which examined the response of single-degree-of-freedom (SDOF) oscillators. Though limited studies were also conducted on the behavior of non-ductile and multi-degree-of-freedom (MDOF) systems, much of the work was based on simplified models with little or no correlation to real structures. Hence, a more comprehensive evaluation of both SDOF and MDOF systems subjected to seismic loads was carried out in this research to further investigate the basis of the FEMA-273 provisions. In addition to regular systems, complex hysteresis models, representative of highly degrading shear-critical systems, were also used in the analytical simulation study.; Finally, a detailed parametric study is carried out, investigating the response of realistic MDOF structures to a suite of ground motions, representative of the design events suggested in FEMA-273. The ground motions were derived from the SAC database and scaled to geometric mean spectral acceleration at the fundamental frequency of the respective structure. Results of the study indicate that inelastic demands in degrading MDOF systems can exceed the same for corresponding SDOF systems. This can render the commonly used philosophy of representing real buildings by equivalent SDOF systems questionable. The notion of cumulative plastic rotation demand is proposed as a more reliable parameter for establishing local acceptance criteria rather than plastic rotation currently suggested in FEMA-273.