| Concrete-filled double skin steel tubular (CFDST) columns consist of twoconcentric steel tubes with concrete sandwiched between them. Its inner and outersteel tubes replace reinforcements and restrict the sandwiched concrete. Comparedwith the traditional concrete-filled steel composite component, the especialadvantages of CFDST include: lighter self-weight, stronger bending stiffness,favorable seismic behavior and reasonable fire resistance. The behaviors of CFDSTcolumn and concrete-filled steel tubular joint have been systematically investigatedby the researchers at home and abroad, but the research on CFDST joints has juststarted. The rational theoretical analysis model and the calculation method of jointsneed to be further studied. In view of the above mentioned situations, the T-stubsemi-rigid connection with stronger rigidity is introduced in composite joint in thisdissertation. The new form of joints, which are composed of CFDST column andH-shape steel beams with reinforced concrete slab using through high strength boltswith equal-length in both ends and T-stub connectors, have been reserached. Based onthe traditional research methods of steel-concrete composite joints, in the dissertation,the behaviors of new joint have been extensively investigated by experimental study,finite element simulation and theoretical analysis. The design methods of this newtype joint have also been discussed. The main achievements in the dissertation are asfollows:(1) Based on the wide range of the domestic and foreign literatures, thecomprehensive review and summary have been carried, including the present researchsituations of CFDST column, steel-concrete composite beam, T-stub connection andsteel-concrete composite joint. A systematic classification and study method ofsemi-rigid connections are analyzed. The models of moment-rotation relationship ofjoints have been discussed.(2) The pseudo static experiments on the behaviors of six crisscross joints of amoment-resisting frame consisting of CFDST column and steel-concrete compositebeam have been carried out. The test parameters varied included the axial load levelon the column, the size of T-stub, the stiffener, the thickness of slab and thereinforcement ratio in slab. To investigate the mechanical behaviors of the compositejoints, the failure modes, hysteretic behavior, shear strength, bearing capacitydegradation, stiffness degradation, energy dissipation and strain were evaluated in detail. To simulate the earthquake action, the antisymmetric loading forms at the endsof beams were adopted. The experimental results indicated that the shear failuremodes were arisen, but the new type joint were provided with higher carrying capacity,bigger initial rotational stiffness, good ductility and energy dissipation ability, whichindicated that the new type composite joints possess excellent seismic behavior.(3) Taking the test joints as prototypes, on the base of the geometricalnonlinearity, material nonlinearity and contacting nonlinearity of component of joints,the entity modeling and fine meshing of joint have been developed using the finiteelement software ABAQUS, which can reasonably simulate the experimentalboundary conditions, loading modes and select solver, so the entire loading process offinite element analysis has been conducted. The results of ABAQUS are good agreedwith the tests. The axial pressure ratio of CFDST columns, the preload of highstrength bolts and the composite effect of reinforced concrete slabs,which how toinfluence the mechanical behavior of composite joints, have been evaluated. Throughthe finite method analysis, lots of details can be got, which were indiscoverable intests, such as: the stress condition at panel zone sandwich concrete, the contact stressdistribution at T-stub flanges, the stress distribution and the main stress tracedistribution characteristics in reinforced concrete slab.(4) Founded on the analysis on force transmission mechanism of crisscross joint,the possible failure modes can be made. On this basis, the calculation formula of jointcracking moment has been deduced using the elastic method. The calculationformulas of joint moment capacities were established by contrasting the maximummoment capacity determined by components of joint and the composite beam itself.Base on the three broken lines calculation model of shear resistance in panel zonewebs of inner and outer steel tubes, the calculation model of moment resistance inpanel zone flanges of inner and outer steel tubes and the calculation model of diagonalcompression bar in panel zone concrete, the shear capacities of the joints have beenestablished using the superposition principle. A method has been developed forpredicting the initial rotational stiffness of the joints in both negative and positivemoment by the component method. On this basis, the trilinear skeleton curves ofmoment-rotation of the joints have been gotten. The three broken lines calculationmodels of load-displacement hysteretic loops of the joints have been establishedconsidering the slippage effect or not.(5) The moment capacities and shear capacities of semi-rigid T-stub joints havebeen proposed on the base of achievements of aforementioned the pseudo static tests, the finite element simulations, the theoretical analyses and existing relative researchachievements. The design or calculation method and construction requirements ofCFDST columns, reinforced concrete slabs, headed stud shear connectors, throughhigh strength bolts, T-stub connectors, fillet welds and stiffeners, et al.The research results show that: the proposed bolted T-stub compositeconnections exhibit good mechanical behaviors, meeting the design requirements ofcurrent codes for seismic resistant frames or frame-core wall structures. The test andtheory results of this dissertation can be used as the basic experimental data andtheoretical bases when the new type joints are applied in practical engineering. Thesuggested design methods can provide some beneficial supplements and reference torevision of future codes．... |
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