Evaluation of modal pushover analysis using vertically irregular frames

[ynopum]

Evaluation of modal pushover analysis using vertically irregular frames

Chintanapakdee, Chatpan; Chopra, Anil K.

[13WCEE Secretariat?], [Vancouver, British Columbia, Canada?], 13th World Conference on Earthquake Engineering: Conference Proceedings, Vancouver, British Columbia, Canada, August 1-6, 2004, Paper No. 2139, 2004, pdf (CF 96)

Recently, modal pushover analysis (MPA) has been developed to improve conventional pushover procedures by including higher mode contributions to seismic demands. This study compares the seismic demands for vertically irregular frames determined by the MPA procedure and the rigorous nonlinear response history analysis (RHA) using an ensemble of 20 ground motions. Forty-eight irregular frames, all 12 stories high with strong-columns and weak-beams, were designed with three types of irregularity--stiffness, strength, and combined stiffness and strength--introduced in eight different locations along the height using two modification factors. Next, the median and dispersion values of the ratio of story drift demands determined by modal pushover analysis (MPA) and nonlinear RHA were computed to measure the bias and dispersion of MPA estimates leading to the following results: (1) the bias in the MPA procedure does not increase, i.e., its accuracy does not deteriorate, in spite of irregularity in stiffness, strength, or stiffness and strength provided the irregularity is in the middle or upper story; (2) the MPA procedure is less accurate relative to the reference "regular" frame in estimating the seismic demands of frames with strong or stiff-and-strong first story; soft, weak, or soft-and-weak lower half; stiff, strong, or stiff-and-strong lower half; (3) in spite of the larger bias in estimating drift demands for some of the stories in particular cases, the MPA procedure identifies stories with the largest drift demands and estimates them to a sufficient degree of accuracy, detecting critical stories in such frames; and (4) the bias in the MPA procedure for frames with soft, weak or soft-and-weak first story is about the same as for the "regular" frame.

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