magnification factor of drift based on demand capacity ratio of columns for various performance levels in RC frames with intermediate ductility

Varying the modeling of RC columns in the Updated version of in ASCE41-06 concrete provisions has been resulted in reduction of performance level of structure. This leads to a more conservative safety level in Code 2800. In this study in order to make the design in proportion with the performance level the design itself, the design criteria on code 2800 are investigated. To have a better Proportionality between structure performance level and the Code 2800 performance targets, the parameter of inelastic drift of structure is regulated. In seismic design of structures, estimating maximum inelastic lateral displacement of the structure occurring in the sever earthquake is of great importance. in most seismic design provisions, maximum inelastic displacement of the structure is estimated by amplifying the lateral displacement computed from an elastic analysis with a displacement amplification factor (Cd). Reviewing several seismic design provisions indicates that in most of them Cd is only dependent on the earthquake force resisting system.
6 RC frames with intermediate ductility and 4.5.6.8.10 and 12 stories are used for this study. For determination of real drift occurring in major earthquake (inelastic drift), nonlinear time history analysis and pushover analysis using IDARC program is performed. For linear analysis, equivalent static procedure is employed using ETABS program. In nonlinear time history analysis, 7 earthquake ground motions consistent with soil type II of Standard no. 2800 are used. These records are scaled according to Standard no. 2800 directions, Also 4 types of lateral loading patterns are used in pushover analysis, and they are triangular distribution, generalized power distribution, uniform distribution and modal distribution. For linear analysis, equivalent static procedure is employed using ETABS program. Performance level of structure elements is obtained based on the mentioned analysis. Bending forces of each of columns are calculated and compared with the strengths calculated based on the ABA. The inelastic displacements which are computed by nonlinear analysis are then divided by elastic displacements and so Cd for each story of 6 frames is determined.
In most of investigating of frames, the pushover analysis and time history analysis, most of the columns are in the IO performance level and only a small percentage of them have reached the level of performance of LS. The results indicate that lower criteria limitations are needed to control the drifts and its effects on the structure level performance and response are investigated. In this research, Cd is considered as a function of R (structural behavior factor), like most researches and provisions,
Finally, in order to investigate the changes in 4TH edition of code 2800, the mentioned frame is designed again based on the 4Th edition criteria and then the structure response is investigated. The results indicate that in the frames design by 4TH edition of code 2800, demand capacity ratio of bending in the upper stories of frame increases while in the lower stories There was no significant change. Furthermore the reduction in drift criteria limitations, leads to reduction in section dimensions, damage concentration in a specified story, total structure damage index and also more usage of section capacity. The latter will result in convergence of structure performance level and Code 2800 performance target.