Efficacy of R-factors in the performance based evaluation of RCC buildings

Abstract

The main characteristic of prescriptive building design codes is that they adopt force-based seismic design procedure for evaluating seismic force demand. The actual seismic force demand is reduced by the introduction of response reduction factor (R) to obtain design lateral seismic force. This diminution in real seismic force implicitly accounts for the energy dissipating capability of the structures when building is expected to undergo inelastic deformation during strong earthquakes. The code is silent about ductility value of the structures designed for a given value of R. Also, irrespective of the structures geometrical configuration and strength, the assigned R-values as per codal provisions are same in most of the cases. Hence, proper estimation of R-factor is of great importance to rightly design the structures with proper assessment of ductile deformation supplemented by correct reinforcement detailing. In the present study, effect of geometrical configuration on R-factor has been considered. The R-factor has been determined for buildings with and without plan irregularities. G + 3, G + 7 and G + 11 story square shaped and G + 7 story L-shaped and C-shaped buildings have been considered for the study purpose. All the buildings have been designed as per IS codal provisions. Material nonlinearity has been considered using plastic hinge model. Confinement effect in RC beam and column section has been considered using Mander’s model. Pushover analysis of the buildings has been done using FEMA440 equivalent linearization and displacement modification methods. The bi-linearization of pushover curve has been done using equal area method with initial tangent stiffness. It has been observed that the R-factor values for various performance levels are different and judgment of considering an R-factor for a building depends upon specific performance objectives. Also, it has been observed that the R factor is sensitive to both geometrical configuration and material strength of structures.