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PREFACE:

Estimating seismic demands at low performance levels, such as life safety and collapse prevention, requires explicit consideration of inelastic behavior ofthe structure. While nonlinear response history analysis (RHA) is the most rigorous procedure to compute seismic demands, current structural engineering practice uses the nonlinear static procedure (NSP) or pushover analysis in FEMA-273 [Building Seismic Safety Council, 1997]. The seismic demands are computed by nonlinear static analysis of the structure subjected to monotonically increasing lateral forces with an invariant height-wise distribution until a predetermined target displacement is reached.

 Both the force distribution and targetdisplacement are based on the assumption that the response is controlled by the fundamental mode and that the mode shape remains unchanged after the structure yields. Obviously, after the structure yields both assumptions are approximate, but investigations [Saiidi and Sozen, 1981; Miranda, 1991; Lawson et al., 1994; Fajfar and Fischinger, 1988; Krawinkler and Seneviratna,1998; Kim and D’Amore, 1999; Maison and Bonowitz, 1999; Gupta and Krawinkler, 1999, 2000; Skokan and Hart, 2000] have led to good estimates of seismic demands. However, such satisfactory predictions of seismic demands are mostly restricted to low- and medium-rise structures in which inelasticaction is distributed throughout the height of the structure [Krawinkler and Seneviratna, 1998; Gupta and Krawinkler, 1999].

None of the invariant force distributions canaccount for the contributions of higher modes to response, or for a redistribution of inertia forces because of structural yielding and the associated changes in the vibration properties of the structure. To overcome these limitations, several researchers have proposed adaptive force distributions that attempt to follow more closely the time-variant distributions of inertia forces [Fajfar and Fischinger, 1988; Bracci et al., 1997; Gupta and Kunnath, 2000]. While these adaptive force distributions may provide better estimates of seismic demands [Gupta and Kunnath, 2000], they are conceptually complicated and computationally demanding for routine application in structural engineering practice.

The principal objective of thisinvestigation is to develop an improved pushover analysis procedure based on structural dynamics theory that retains the conceptual simplicity and computational attractiveness of the procedure with invariant force distribution, but provides superior accuracy in estimating seismic demands on buildings. First, we show that pushover analysis of a one-story system predicts perfectly peak seismic demands. Next we develop a modal pushover analysis (MPA) procedure for linearly elastic buildings and demonstrate that it is equivalent to the well-known response spectrum analysis (RSA) procedure. The MPA procedure is then extended to inelastic buildings, the underlying assumptions and approximations are identified, and the errors in the procedure relative to a rigorous nonlinear RHA are documented. Finally, the seismic demands determined by pushover analysis using three force distributions in FEMA-273 are compared against the MPA and nonlinear RHA procedures.