Experimental and numerical evaluation of the behavior of composite column - reinforced concrete beam joints subjected to cyclic loads

Multistorey structures are formed by frame systems including connection columns and beams together. In composite column – reinforced concrete frames, the joint is formed by striking the reinforced concrete beam with the composite column and the steel profile in the column continues to carry compression and tensile loads, even if the concrete at the joint is cracked due to seismic load. In other words, the steel core encased in the column carries the shear force by providing ductility and prevents sudden collapse. Composite column – reinforced concrete beam joints behave better than reinforced concrete column – beam joints in terms of both load carrying capacity and deformation capacity. The steel profile core in the composite column mostly damps the deformations. Under the influence of increased loads, the steel profile core will not reach the yielding point, despite the concrete around steel profile is crushed and the longitudinal reinforcement yields. However, these local damage in the column has significant impact on the behavior of the joint, and thus the level of damage must be determined. The thesis work carried out consists of two stages. In the first stage, the frame elements were produced and then these elements were subjected to reversible repeated loads. In the second stage, finite element models (FEM) of the test elements were constructed by the ABAQUS program then the nonlinear analysis was performed. In the present study, a method for analysis of joint which is consisting of composite column–reinforced concrete beam is proposed. In the proposed method, nonlinear stress–strain relationships of materials are taken as basis. Besides, the experimentally obtained stress – strain relationships under compression and tension of the concrete were used. In the scope of the study, five 1/2 scale test elements were produced. Two reference elements were selected including a reinforced concrete column – beam connection and a composite column – reinforced concrete beam connection. The other three test elements were produced by welding the steel profile up to 50 cm in to the beam along the zone in which stirrups are more closely spaced according to weak and strong axes of profile. The finite element analysis of the joint elements was carried out by the ABAQUS package program. Plastic concrete damage model (CDP) was used for defining concrete material property and damage behavior. The steel assumed was to be elasto – plastic material. The C3D8R element feature was used as the finite element mesh structure. The results obtained from the numerical analysis were compared with the experimental results. The plastic hinge was shifted from beam ends to the end portions of the steel profile that is the end of the zone including closely spaced stirrups. By using also steel profile encased in concrete, the shear capacity of joint increased due to higher effective shear area.