Seismic Behavior Of Beam-to-Column Composite Connections In Low-alloy Steel Frame Structure

With the gradual application of middle-high strength steel in the steel frame structure, the improvement of the steel strength will has evidental impact on the seismic performance of the beam-to-column composite connection, which becomes the focus of the present researches. In the Northridge earthquake in USA and Kobe earthquake in Japan, it was found that most of the connection failures were due to the fracture at the weld between the bottom flange of the beam and the column flange, and the slab action is an innegligible reason of the stress concentration there. Thus, because of the increase of the steel strength, relevant researches about the specific change of the bearing capacity, stiffness, ductility and the slab effect of the connection in the steel frame structure are needed. But now the researchers have inadequate knowledge about its bearing capacity and failure mechanism. So the study on the performance of the beam-to-column composite connection with middle-high strength steel is significant for both theory and engineering practice.This dissertation, supported by the National Natural Science Foundation of China (No. 50578083 & 50708051), is concerned with the bearing capacity and seismic performance of the beam-to-column composite connection with middle-high strength steel. The main research and achievements of this thesis are as follows.In the help of the general-purpose finite element software ANSYS, the numerical model of the bolt-welded composite connection is founded and Elasto-plastic analyses are conducted. Using the models, some parameters such as the steel strength, the concrete grade, reinforcement ratio of the concrete slab and the slab thickness are analyzed to confirm their effect on the bearing capacity of the connection.Considering the impact of the reinforcement ratio and the loading mode, four middle-high strength specimens are tested under low-cycle reciprocal load. Their behaviors on the loading capacity, plastic deformation, stress distribution and failure mode are investigated, and the ductile characters and seismic performance are summarized, which is compared with corresponding ordinary steel composite connections. The results prove that the middle-high strength steel can improve the bearing strength of the connection without decreasing the ductility.By segmental linear fit method, the envelopes of the hysteretic curve are processed to derive the trilinear restoring force model of the connection. According to the failure modes of the four connections,"components method"is used for reference to combine the contribution of various components, and the design formula of connection strength and initial stiffness are derived. In this phase, the buckling of the steel beam and column, the local compression of the concrete are fully taken into account. The comparison with the available experiment data shows that the method can calculate the bearing capacity and the initial stiffness more accurately, and the overall accuracy can meet the requirements of the engineering design.The data, conclusion and method obtained from this paper can provide reference for the practice, and laid an experimental foundation for the further application of higher strength steel in the steel frame structures.