This paper develops a modal pushover analysis-(MPA) based approximate procedure to quantify the collapse potential of structural systems. The computationally demanding incremental dynamic analysis (IDA) of the structural system is avoided by MPA of the structure in conjunction with empirical equations for the collapse strength ratio for the first-mode single-degree-of-freedom (SDF) system; higher modes of vibration play essentially no role in estimating the ground motion intensity required to cause collapse of the structure. Presented are collapse fragility curves for 6-, 9-, and 20-story regular special momentresisting teel frames computed by the exact and approximate procedures, demonstrating that the MPA-based approximate procedure requires only a small fraction (1% in one example) of the computational effort inherent in exact IDA and still achieves highly accurate results.(IDA) [15], which requires nonlinear response history analysis (RHA) of the structure for an ensemble of ground motions, each scaled to many intensity levels, selected to cover a wide range of structural response-all the way from elastic behavior to global dynamic instability.Recognizing that IDA of practical structures is computationally extremely demanding, the developers of IDA have devised the approximate method SPO2IDA [16]. Restricted to first-mode dominated structures, this procedure defines the force-deformation curve in initial loading of the SDF system to match the static pushover (SPO) curve of the structure, and determines the peak deformation of the nonlinear SDF system by Excel-based software [17]. Because the SPO curve depends on the choice of the height-wise distribution of forces, SPO curves are determined for several force distributions and the 'worst' SPO is chosen, but the choice is not unique. This procedure also requires estimating the elastic stiffness of the SDF system from the IDA curve, a task that is complicated because several vibration modes contribute to the responses plotted in the IDA curve.Modal pushover analysis (MPA) [18], an approximate analysis procedure rooted in structural dynamics theory [19], avoids the preceding issues concerning SPO2IDA. The MPA has been utilized to estimate seismic demands instead of nonlinear RHA. Encouraged by the accuracy of MPA in estimating IDA curves [20], this paper explores the potential of MPA in investigating the collapse of buildings. Collapse here is synonymous with dynamic sideway instability in one or several stories of the structural system. COLLAPSE FRAGILITY CURVESThe ground motion intensity that causes collapse of a building can be determined by IDA. This procedure requires a series of nonlinear RHA of the structure for an ensemble of ground motions, and each record is scaled to several levels of intensity to encompass the full range of structural behavior: from elastic to yielding that continues to spread, finally leading to global dynamic instability. The results of these analyses for one ground motion lead to one IDA curve. This is a plot of the ground motion i...

No abstract

No abstract

In performance based earthquake engineering, it is important to accurately predict the seismic demand and capacities of structures. One recent estimation method is incremental dynamic analysis (IDA), which requires a series of nonlinear response history analyses (RHA) of the structure under various ground motions, each scaled to multiple levels of intensity, selected to cover entire range of structural response from elasticity, to yield and finally global dynamic instability. The implementation of IDA requires intensive computation and detailed knowledge of the nonlinear RHA of structures. In response to the complexity of IDA, an approximate method based on modal pushover analysis (MPA-based IDA) was developed. In MPA-based IDA, seismic demands are computed using the nonlinear RHA of the equivalent SDF systems instead of using nonlinear RHA of MDF systems. The objective of this study is to develop a simpler MPA-based IDA procedure that can avoid nonlinear RHA of equivalent SDF systems. For this purpose, MPA-based IDA employs the empirical equation of the inelastic displacement ratio (C R ), defined as the ratio of peak displacement of the inelastic SDF system to that of the corresponding elastic SDF system given the strength ratio R, and that of the collapse strength ratio (R c ), which is the ratio of collapse intensity to yield strength. The proposed procedure is verified by comparing the seismic demands and capacities of 6-, 9-, and 20-story steel moment frames as determined by the proposed method and exact IDA.

No abstract