Seismic Behavior of Under Confined Square Reinforced Concrete Columns

Abstract

The role of confining reinforcement in dissipating a large amount of energy in reinforced concrete (RC) elements at the time of seismic events has been well demonstrated in previous studies. In India, this practice of providing special confinement in RC elements began to be followed after the ductile detailing code came into being in the year 1993. Hence, the structures built before 1990s in seismicaly active areas had no ductile detailing, as the design recommendation of pre 90s era do not consider adequate confinement requirements. The Present experimental study is a part of an ongoing project of upgrading these old reinforced concrete structures, which had no special confinement and are located in seismically active regions. Three full-scale columns with different amount of transverse reinforcement (volumetric ratio of 1.31, 0.33, and 0.22) were tested in a specially made testing setup. One specimen was designed as per the current Indian guidelines for ductile detailing (IS 13920:2016), while the other two specimens were designed as per the guidelines which existed before the Code on ductile detailing was implemented. The response of the columns under quasi-static reverse lateral cyclic loading (at constant axial load) was recorded in terms of lateral load v/s deflection, moment v/s curvature, energy dissipation, stiffness reduction and different ductility parameters. As expected, the results of the study show inferior seismic performance of under-confined columns as indicated by the key strength and ductility parameters. The results have though quantified the strength and ductility deficit of these under confined RC columns, which shall be of great help in designing suitable retrofit for the existing structures constructed with no ductile detailing features.