Research On Axial Compression Resistance Performance Of Steel-tube Confined Reinforced Concrete Column-RC Beam Knee Joints Stiffened By Steel Rings

Steel-tube confined reinforced concrete column is a new type of composite structure member which is rising in recent years. With high bearing capacity, good ductility and convenience in construction, it brings notable economic benefits. In present China, Steel-tube confined reinforced concrete column is mainly applied together with beam and slab of RC floor system. In this kind of structure, the beam-column joint frequently becomes the weakness because the steel-tube is disconnected in joint area, which causes confined effect decline on joint concrete. Based on the previous research, circular steel-tube confined reinforced concrete column-RC beam knee joint is selected as the research object, and a new construction method that strengthens the joint by additional steel rings in joint area is proposed. Compared with conventional construction method, the method is uncomplicated and unified. With adequate confinement effect for joint concrete provided, the steel rings can be placed flexibly in joint area according to beam longitudinal bars’ location and the spacing of two adjacent steel rings can be adjusted according to the number of rebar. Besides, the construction method requires no need of changing the shape of the joint and possesses a good prospect of engineering application.In the thesis, 152 circular steel-tube confined reinforced concrete column-RC beam knee joint finite element models stiffened with steel rings in joint area were established by ABAQUS software, basing on the validation of finite element method proved reasonable and reliable. Axial compressive loading tests of the joint models with different parameters have been simulated numerically. The results show that the stiffened joint has a better axial compression performance than common joint, due to higher bearing capacity and better ductility. Usually the concrete between the rings is the weakest part of the joint, so the thickness of the rings and the ratio of joint longitudinal reinforcement could be increased properly, in order to strengthen the joint more. It is proved that the bearing capacity of stiffened joint has approached or reached the level of steel-tube confined reinforced concrete short column by comparison. The joint stiffening method is feasible and effective.Through the parameter analysis results, it is concluded that the bearing capacity of the stiffened joint under compressive load increases with increasing concrete strength class, steel ring thickness and the overall height of rings in joint area. When the joint is under compression, the steel rings don’t deliver the axial force, only providing confinement effect on core concrete. Shear force within the beam end influences the bearing capacity of the reinforced joints too, so strengthening the beam part should be paid attention to while strengthening the joint itself.Based on the analysis results, the relative theory foundations and mechanics, formula for calculating the bearing capacity of the reinforced joints under compression was established, The formula fits the finite element analysis results well through comparison to FEM results. The formula can be a good reference for designing or optimization on circular steel-tube confined reinforced concrete column-RC beam knee joint.