Retrofitting Of Beam-Column Junction By Gfrp Wrap

Abstract

Fiber reinforced Polymer Composites (FRPCs) have become popular for civil engineering applications, especially in structural up gradation due to their sound engineering properties. FRPC wraps are successfully employed as external confinement for improving the strength and ductility of existing concrete beamcolumn joints. The present study is focused on the mechanical response of exterior beamcolumn junction wrapped with GFRP composite sheets. Beam-column junction plays a very important role in moment resisting framed structures, when it is subjected to seismic loading. Amongst all the junctions, exterior junctions are considered as the most critical due to asymmetric loading under seismic action. To obtain accurate experimental results, it is essential to choose proper boundary conditions and loading actions during testing. Based on this criteria one independent exterior junction with three different conditions as given below are compared with exterior junction in G+1 moment resisting frame. 1. Column with fixed boundary conditions and 30% axial force of its load carrying capacity. 2. Column with hinged boundary conditions without any axial force. 3. Column with hinged boundary conditions and 30% axial force of its load carrying capacity. This study is carried out with help of “ETABS 8 Nonlinear”. From the results, 3rd alternative of testing set-up proves compatible with actual condition. To study the behavior of external beam-column junction wrapped with GFRP, eleven ½ scale specimens are prepared having cross section of 150mm * 200mm for beam as well as column & with 0.9% of tension and compression reinforcement in beam and 2.26% compression reinforcement in column respectively. Reinforcement for these specimens is divided in to two major categories, i.e. Non-ductile and Ductile. From these two categories, one specimen each has been provided with curvature in reinforcement at joint to study the effect of curved reinforcement. Thus in totality five non-ductile, four ductile, one non-ductile with curved reinforcement and one ductile with curved reinforcement i.e. total eleven specimens are cast and overall thirteen specimens are tested under un-strengthened, wrapped before failure and wrapping after failure conditions respectively. For these tests, cyclic loading is applied considering displacement variation of ± 5mm and applying 100 kN axial load to the column with hinge boundary conditions. The structural response has been measured in terms of displacement, load at tip of the beam and strain at the joint. LVDT and load cell are used to measure displacement and load respectively. Electronic and mechanical strain gauges are used to evaluate strain induced at different locations for the beam-column junction. By compiling the results, maximum load under corresponding displacement, energy dissipated, energy applied, specific damping capacity (SDC) and stiffness have been calculated. Improvement in behavior of specimens wrapped before and after failure also has been computed and discussed. Analytical models of non-ductile and ductile control specimens have been prepared in ANSYS considering similar boundary conditions as used in experimentation. Incorporating behavior of specimens only up to the linear stage, load has been applied to the model. Theoretical values of failure load have been calculated for control as well as strengthened specimens. To calculate failure load, Limit State method is used for control specimens, whereas maximum strain observed in GFRP during testing is utilized for the strengthened specimens. Experimental results in terms of displacement and strain are compared with results evaluated by ANSYS. Model is found stiffer compared to specimen tested in the experiment due to linear material property assigned to the model. This comparison encourages need of incorporating material non-linearity in the model. Also analytical and experimental failure loads have been compared here. This comparison has elaborated the need of finding contribution of concrete, steel and FRP in resisting the total load by obtaining strains in concrete, steel and FRP respectively using electronic strain gauges.