Statistics of SDF-system estimate of roof displacement for pushover analysis of buildings

Chopra, Anil K.; Goel, Rakesh K.; Chintanapakdee, Chatpan

PEER-2001/16, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 2001-12, 58 pages (400/P33/2001-16)

This report investigates the basic premise that the roof displacement of a multistory building can be determined from the deformation of a SDF system. For this purpose, the response of both systems is determined rigorously by nonlinear response history analysis, without introducing any of the approximations underlying the simplified methods for estimating the deformation of a SDF system (see, e.g., FEMA-273 or ATC-40 guidelines). The statistics of the SDF-system estimate of roof displacement are presented for a variety of building frames and six SAC buildings subjected to ground motion ensembles. Two sets of structural systems and ground motions are considered. The first set is generic one-bay frames of six different heights: 3, 6, 9, 12, 15, and 18 stories designed for ductility factor mu = 1, 1.5, 2, 4, and 6 subjected to 20 large-magnitude, small-distance records. The second set is six "SAC" buildings--9- and 20-story model buildings designed according to Los Angeles, Seattle, and Boston codes--subjected to 20 ground motion records representing 2% probability of exceedance in 50 years. The statistics of two roof-displacement, mu subscript r, ratios are presented. The data presented include histograms of the 20 values, range of values, median value, and dispersion measure. These data for generic frames indicate that the first-"mode" SDF system overestimates the median roof displacement for systems subjected to large ductility demand mu, but underestimates for small mu. Similar data for SAC buildings demonstrate that the bias and dispersion in the SDF estimate of roof displacement increases when P-delta effects (due to gravity loads) are included. The SDF estimate of roof displacement due to individual ground motions can be alarmingly small (as low as 0.312 to 0.817 of the "exact" value for the six SAC buildings) or surprisingly large (as large as 1.45 to 2.15 of the exact value for Seattle and Los Angeles buildings), especially when P-delta effects are included. The situation is worse than indicated by these data because they do not include several cases where the first-"mode" SDF system collapsed but the building as a whole did not. This large discrepancy arises because for individual ground motions the SDF system may underestimate or overestimate the yielding-induced permanent drift in the "exact" response determined by nonlinear RHA. While this discrepancy is not improved significantly by including higher "mode" contributions, the MPA procedure has the advantage of reducing the dispersion in the roof displacement and the underestimation of the median roof displacement for elastic or nearly elastic cases at the expense of increasing slightly the overestimate of roof displacement of buildings responding far into the inelastic range.

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.

Inelastic deformation ratios for design and evaluation of structures: single-degree-of-freedom bilinear systems

Chopra, Anil K.; Chintanapakdee, Chatpan

UCB/EERC-2003/09, Earthquake Engineering Research Center, University of California, Berkeley, 2003, 81 pages (400/E177/2003-09)

The relationship between the peak deformations of inelastic and corresponding linear single-degree-of-freedom (SDF) systems is investigated. Presented are the median of the inelastic deformation ratio for 232 ground motions organized into thirteen ensembles of ground motions, representing large or small earthquake magnitude and distance, and NEHRP site classes B, C, and D; near-fault ground motions are also included. Two sets of results are presented, one for systems with known ductility factor and the other for systems with known yield-strength reduction factor.

Can anybody here can give technique,tips or correct procedure in preparing a manual mesh in slab, in a fast way using etabs software.

Thanks in advance.

Hii friends,

I was designing steel structures using staad ,
I used both BS5950 and IS 800 LSD as the code.
There are a number of parameters in the steel design to BS or IS.
The explanation to them in Staad is not adequate, it would be wonderful if we share the knowledge we have of these parameters, their uses and references.
Thanks hoping for a positive responce