Evaluation of Response Reduction Factor of Irregular Reinforced Concrete Framed Structures

Background/Objectives: Most of the seismic codes used today, incorporate the nonlinear response of a structure by the provision of an appropriate response reduction factor ‘R’ so that a linear elastic force based approach can be used for design. The value of R factor varies from 3 to 5 in IS 1893 for RC moment resisting frames, but it does not provide information on the components of R factor. This study focuses on the evaluation of the actual value of R for stepped buildings. Methods/Statistical Analysis: Three dimensional models with varying number of storeys in each step were created, consisting of both ordinary RC moment resisting frame and special RC moment resisting frame having 3, 6 and 9 storeys. Nonlinear static pushover analysis is carried out on the analytical models using finite element analysis software SAP 2000. The R factor components such as ductility and over strength factors were computed from the results obtained from the nonlinear static pushover analysis and finally the response reduction factor is calculated for all the models. The pattern of variation in R factor with the increase in the number of stories is investigated for both OMRF and SMRF in this study. Findings: The actual value of the R factor was found to be less than the value assumed during the design process and its value was found to decrease with the increase in the number of storeys. A certain percentage reduction in the response reduction factor has to be considered for irregular buildings. Application/Improvements: This study is focused only on the influence of height on the value of response reduction factor. Further research is needed considering a wider set of parameters.