| Steel tubular structures are widely used in civil engineering fields such as offshore platforms, large span roofs, transmission line towers, and high-rise buildings for their advantages such as the graceful shape, structural efficiency, and short construction period. As an important part of the tubular structure, the tubular joint has been studied for a long time. Completed researches mainly focused on the static and high cycle fatigue properties of tubular joints, from which, static and fatigue design methods have been proposed. However, only a little research work on the seismic properties of tubular joints has been carried out, leading to many uncertainties in seismic performance assessment of in-service tubular structure and seismic design of new tubular structure. In view of those facts above and to improve the research on tubular structure and promote the application of tubular structure, it's necessary to investigate the seismic properties of steel tubular joints and structures thoroughly. In this dissertation, hysteretic properties of Y-type rectangular hollow section (RHS) joints including RHS-to-RHS joints and longitudinal plate-to-RHS joints were analyzed by experimental, finite element (FE), and theoretical methods. The details are as follows:(1) Quasi-static tests of ten Y-type RHS-to-RHS joints and eight Y-type longitudinal plate-to-RHS joints were carried out. For most specimens, the rapid crack propagation led to the test termination, soon after the penetrating cracks formed which located along the curves connecting key points on the chord surface. Thus the formation of the penetrating cracks along the connecting curves of key points could be considered as the symbol of entering unsteady working stage for the joint. And the hysteretic properties of the joint were assessed based on the performance in the steady working stage. The hysteresis curves of tested joints are plump. The influence of stress relief annealing treatment and geometrical parameters on the tested joints'hysteretic properties in the steady working stage was analyzed. It is shown that annealing significantly increases the ductility ratio and the accumulative energy dissipation ratio of the joint. With increasing brace-to-chord angle, the ductility ratio decreases slightly for annealed RHS-to-RHS joint, while the ductility ratio, the accumulative ductility ratio, and the accumulative energy dissipation ratio increase dramatically for plate-to-RHS joint. With decreasing width-to-thickness ratio of the chord, the accumulative ductility ratio and the accumulative energy dissipation ratio decrease for RHS-to-RHS joint. With increasing ratio of the effective thickness of plate to the chord width, the ductility ratio and the accumulative energy dissipation ratio increase for plate-to-RHS joint.(2) Using bilinear hardening model and simplified Lemaitre's damage model, a constitutive model considering mixed hardening and damage evolution was established and then incorporated into the ANSYS FE code through a user subroutine. Using material properties from the uniaxial tensile test, an approximate method for calculating the material damage parameters was proposed. Considering the material damage evolution using the constitutive model, the aforementioned experiments of the joints were numerically simulated. Corresponding to the cracking zones of the tested joints, key volumes and key areas of the three-dimensional FE model were defined. Based on the damage dissipation energy, the method for judging crack penetrating in key zones was proposed, by which the steady working life of the joint can be predicted. Results of simulating the tests show that the FE model considering material damage evolution can be used to analyze the hysteretic properties of Y-type RHS-to-RHS joint and Y-type longitudinal plate-to-RHS joint in the steady working stage.(3) The influence of loading program on the Y-type RHS-to-RHS joint's and the Y-type longitudinal plate-to-RHS joint's hysteretic properties in the steady working stage was analyzed using FE method. It is concluded that: 1) When symmetrical loads with constant amplitudes were applied, with increasing displacement amplitude, the steady working life of the joint decreases, the accumulative ductility ratio and the accumulative energy dissipation ratio of the RHS-to-RHS joint first decrease and then increase, the accumulative ductility ratio of the plate-to-RHS joint decreases, and the accumulative energy dissipation ratio of the plate-to-RHS joint first increase and then decrease. 2) Keeping the displacement amplitude constant, appropriate average amplitude in tension or compression leads to slight increase in the steady working life, the accumulative ductility ratio, and the accumulative energy dissipation ratio of the RHS-to-RHS joint. However, average amplitude in tension or compression leads to decrease in the steady working life, the accumulative ductility ratio, and the accumulative energy dissipation ratio of the plate-to-RHS joint.Using FE analysis, a range of geometrical parameters was given to obtain joints having superior hysteretic properties, which can be referenced in the engineering application.(4) Based on the results of the FE analysis, simplified restoring force model of the Y-type RHS-to-RHS joint and Y-type longitudinal plate-to-RHS joint was proposed, which was incorporated into the ANSYS FE code as a user-defined element through a user subroutine. The user-defined element was then used to simulate the experiments of joints carried out by the author. The calculated hysteresis curves agree well with the curves calculated by the three-dimensional FE model considering material damage evolution. In the FE analysis of the seismic behavior of the overall structure, it can significantly simplify the FE modeling and analysis to simulate the hysteresis behavior of the RHS joint using the user defined element instead of the three-dimensional FE model.(5) In view of the good hysteretic properties of the Y-type RHS-to-RHS joint and Y-type longitudinal plate-to-RHS joint, the RHS bracing system using these two types of joints to dissipate energy was proposed. A single-story, single-bay frame was chosen as the basic structure, which was retrofitted with the chevron RHS bracing system containing Y-type RHS-to-RHS joints. The retrofitting effect was investigated using FE method. Comparing to the properties of basic structures retrofitted with the steel plate shear wall (SPSW) with flat infill, the SPSW with corrugated infill, the single-diagonal RHS bracing member using steel studs to restrain brace against buckling, the single-diagonal RHS bracing member, the crossed-diagonal steel bracing members using steel studs to restrain braces against buckling, and the crossed-diagonal steel bracing members, the structure retrofitted with the bracing system containing RHS joints has better properties considering the deformation capacity, the energy dissipation capacity, and the economic performance synthetically, though it has lower initial stiffness relatively. Besides, the structure retrofitted with the bracing system containing RHS joints dissipated energy stably. Thus it can be seen that the application prospect of the RHS bracing system proposed by the author is bright.
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