Nonlinear Seismic Analyses of Median Ductile RCMRF Structures Based on Variation of Columns Acceptance Criteria

Based on experimental evidence and empirical models the proposed supplement to ASCE 41- 06 is developed for the purpose of updating provisions related to existing reinforced concrete structural buildings. Several experimental research programs have demonstrated that many older-type columns are capable of sustaining limited plastic deformation due to flexural yielding prior to shear failure. This proposed supplement concentrates on this failure mode and includes the categorization of columns based on failure mode, the selection oftarget probabilities of failure for each failure mode and revisions to modeling parameters and acceptance criteria for reinforced concrete columns. In this research, the effect of new provisions on seismic evaluation of reinforced concrete moment resisting frame is investigated. In this regard three medium ductile MRCRF structures with 4, 8 and 12 stories and two direction median moment resisting frame systems are considered. These structures initially have been analyzed and designed according to ‘Iranian Standard 2800, for seismic design of buildings’ and ‘Iranian concrete code of practice’. Then nonlinear static analysis and nonlinear time history analysis methods have been utilized to evaluate the seismic performance of these structures. In nonlinear static analysis there are several methods for determining target displacement among them some reliable methods are displacement coefficient method (FEMA-356), Capacity spectrum method (ATC-40), and equivalent linearization method and modifies coefficient method (FEMA-440). The target displacement with these methods are compared with maximum displacement in nonlinear time history analysis. It is observed that capacity spectrum method given by ATC-40 reports target displacement values higher than time history analysis. Furthermore, results obtained from the equivalent method and the modified coefficient methods suggested by FEMA-440 are closer to time history analysis values. The performance levels of these structures have been evaluated based upon target displacement of nonlinear static analysis that obtained from FEMA-440 methods and maximum displacement in nonlinear time history analysis. The effect of the variation of reinforced concrete columns modeling parameters and acceptance criteria on performance levels of reinforced concrete structure is investigated. For this purpose in nonlinear seismic design, the modeling parameters and acceptance criteria have been considered with those from FEMA-356 for columns “controlled by flexure” and then with those from proposed supplement to ASCE 41-06 for flexure failure (flexural yielding without shear failure) and flexure-shear failure (shear failure following flexural yielding). The obtained results indicate that these structures are to some extant conservative in their seismic performance due to the modifications of ASCE41-06.