Evaluation of Modal and FEMA Pushover Analyses: Vertically 'Regular' and Irregular Generic Frames

摘要

This technical note compared the seismic demands from FEMA-356 NSP, MPA procedure, and nonlinear RHA for 54 generic frames- 30 vertically "regular" frames and 24 irregular frames- subjected to an ensemble of 20 ground motions. The comparative svaluation based on the story-drift demands has led to the following conclusions: 1. If a sufficient number (2 or 3) of "modes" are included, the height-wise variation of story drifts determined by MPA is generally similar to the "exact" results from nonlinear RHA. The first "mode" alone, which is the basis for pushover procedures currently used in engineering practice, does not adequately estimate seismic demands. 2. MPA is almost always more accurate in estimating drifts in all stories of all frames than all of FEMA distributions. The envelope of the four estimates corresponding to the four FEMA force distributions is less biased than the individual estimates but is almost always more biased than MPA. Generally well below 30%, the bias in MPA exceeds this limit in the upper-story drifts in 15-and 18-story frames designed for a ductility factor of 6; however, this bias in estimating the largest story drift over building height is less than 30% for all the "regular" and irregular frames investigated. 3. The MPA procedure maintains its superiority for almost all of the irregular frames. The MPA is almost always more accurate compared to all of the FEMA force distributions and to the envelope of the four FEMA estimates. Neither of the approximate procedures, however, can estimate seismic demands accurately for vertically irregular frames with strong or stiff-and-strong first story or lower half; seismic demands for such systems should be determined by nonlinear RHA. 4. An approximate procedure is likely to be accurate in estimating the seismic demands due to an individual excitation if both its bias and dispersion are small. In this sense, the MPA procedure is more reliable than FEMA-356 force distributions, although pushover procedures are not recommended in conjunction with individual excitations.