Seismic evaluation of R/C framed building using shear failure model

Abstract

Prediction of nonlinear shear hinge parameters in RC members is difficult because it involves a number of parameters like shear capacity, shear displacement, shear stiffness. As shear failure are brittle in nature, designer must ensure that shear failure can never occur. Designer has to design the sections such that flexural failure (ductile mode of failure) precedes the shear failure. Also design code does not permit shear failure. However, past earthquakes reveal that majority of the reinforced concrete (RC) structures failed due to shear. Indian construction practice does not guaranty safety against shear. Therefore accurate modelling of shear failure is almost certain for seismic evaluation of RC framed building.
A thorough literature review does not reveal any information about the nonlinear modelling of RC sections in Shear. The current industry practice is to do nonlinear analysis for flexure only. Therefore, the primary objective of the present work is to develop nonlinear force-deformation model for reinforced concrete section for shear and demonstrate the importance of modelling shear hinge in seismic evaluation of RC framed building. From the existing literature it is found that equations given in Indian Standard IS-456: 2000 and American Standard ACI-318: 2008 represent good estimate of ultimate strength. However, FEMA-356 recommends ignoring concrete contribution in shear strength calculation for ductile beam under earthquake loading. No clarity is found regarding yield strength from the literature. Priestley et al. (1996) is reported to be most effective for calculating shear displacement at yield whereas model proposed by Park and Paulay (1975) is most effective in predicting the ultimate shear displacements for beams and columns. Combining these models shear hinge properties can be calculated.
To demonstrate the importance of modelling shear hinges, an existing RC framed building is selected. Two building models, one with shear hinge and other without shear hinges, are analysed using nonlinear static (pushover) analysis.
This study found that modelling shear hinges is necessary to correctly evaluate strength and ductility of the building. When analysis ignores shear failure model it overestimates the base shear and roof displacement capacity of the building. The results obtained here show that the presence of shear hinge can correctly reveal the non-ductile failure mode of the building.