Experimental and analytical investigation of damaged concrete beam-column joints strengthened by CFRP composites

Authors
H. Shariatmadar
E. Zamani Beydokhti
Ferdowsi University of Mashhad
Abstract
The scope of this study is to investigation of the rehabilitation of concrete beam column joints retrofitted by use of carbon-fibre-reinforced plastics (CFRPs) to achieve a safe, economic and practicable level of seismic damage. This paper investigates analytically the efficiency of the strengthening technique at improving the seismic behaviour of damaged structures. 4 beam-column connections are tested under reversed cyclic load. The connections have none-seismic detailing of rebars, i.e. no transverse rebar and seismic stirrups are used in the joint core and beam and column critical end zones, respectively. The joints are damaged in different levels and then retrofitted by carbon fibre reinforced materials (CFRP sheets). The strengthened joints were tested again to reach the ultimate drift capacity. The experimental results show that the beam column joints could be retrofitted by external bonding of FRP sheets until a limited level. This level determined for tested joint approximately equal to 1.5% story drift. The specimens initially damaged until 1% and 1.5% drifts showed the capacity increase up tp 5% and 3%, respectively. If the damage level is higher than this repair-ability level, other rehabilitation methods may be useful. Then, to simulate the behaviour of joints, a numerical model was developed in the OpenSees framework version 2.4.0. The tested joints such as reference joint and retrofitted joints are analyzed by Opensees nonlinear software. The open source Opensees software has several models for concrete and reinforcement rebar materials possible for considering reloading / unloading stiffness deterioration and hysteretic energy dissipation during reversed cyclic loads. Also nonlinear beam-column elements with spread or concentrated plasticity make this nonlinear software capable for high accurate simulation. The analytical models are used to assess the efficiency of the CFRP rehabilitation to set an optimum level of damage that the seismic behavior parameters could be compensated, safely, economically and practicable. The results of joint analysis are compared with experimental behavior of specimens. The hysteresis curves of the modeled beam column joints had a high level of accuracy in terms of stiffness degradation, moment carrying capacity, capacity degradation and energy dissipation. So, the model is calibrated for each level of damage intensities. The results showed that the model had a good accuracy in terms of load carrying capacity, secant stiffness, energy dissipation and joint ductility and the error was less than 10% between analytical and experimental results. Then, the effct of some variables such as column axial load and existence of transverse slab connected to the beam was analytically investigated. The results showed that increasing the axial load on the column increased the load carrying capacity and stiffness from 5% to 12% (related to initial damage intensity of the joint), but it had negligible effect on dissipated energy. Also modeling of transeverse slab revealed an increasing effect on the capacity, stiffness and energy. The positive effect was higher in absence of gravity loads on the slab. So, existence of transeverse slab with gravity load had negative effect on secant stiffness in specimens with initial damage higher than 1.5% story drift.

Keywords

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Modares Civil Engineering journal
Volume 16, Issue 5
November and December 2016
Pages 65-77