Seismic properties of joints composed of CFST columns and RBS composite beams

Some design recommendations or standards, such as FEMA 350 and Eurocode 8 Part: 3, recommend reduced beam sections (RBS) to be used in earthquake-prone zones and provide practical design guidelines. These recommendations, however, are mainly based on research conducted on joints without floor slabs. In reality, steel beams are often connected to reinforced concrete (RC) floor slabs by shear connectors. Thus, it is important to explore the performance of the joint with both RBS steel beams and RC slabs. Nowadays, concrete-filled steel tubular (CFST) columns are gaining popularity all over the world due to many advantages. The major benefits are: 1) Steel tube acts as a permanent and integral formwork for infilled concrete, which also improves the strength of concrete core because of the confinement effect provided by steel tube; 2) Infilled concrete prevents local buckling of the steel tube wall and steel tube provides confinement to the concrete. Therefore, in this thesis a composite joint is proposed and investigated, which is composed of concrete-filled steel tubular (CFST) column and RBS composite beam. Firstly, an experimental study to investigate the cyclic behaviour of the proposed composite joints was carried out. Six small-scale specimens of CFST column joint utilising through-diaphragms welded to RBS composite beam were tested under cyclic loading. The sectional configuration of the beam, slab and the dimensions of RBS were variables, and the strength, ductility, stiffness degradation and energy dissipation capacity of joint were analysed to reflect the seismic behaviour of the composite joint. The experimental results showed that the flexural strength and stiffness of the joint under sagging moment is improved due to the presence of the RC slab, and the RBS composite beams can promote the plastic hinge formation in the reduced region, which possibly prevents column failure. Overall, this type of composite joint has a good prospect in seismic zones. Secondly, parametric studies using finite element (FE) analyses were used to make-up for the limited number of specimen tests. The FE models considered both geometric and material nonlinearity, interaction and connection between steel and concrete, etc. After verifying the accuracy of the modelling technique of FE models by comparisons of computer results with existing experimental data, extensive FE models were developed and utilized to undertake parametric studies on the seismic behaviour of the proposed composite joint covering a wide range of parameters. It is found that the cyclic behaviour of the joint is obviously affected by the thickness of RC slab, RBS and steel strength of the beam. Furthermore, a new design recommendation for RBS composite beams was also proposed based on the results of the parametric study. Finally, a new mechanical model was developed to predict the moment-rotation relationship of the through diaphragm joints with RBS composite beams, and this model was validated by existing experimental results. Besides, an application of this proposed model was introduced, which can be used in the analysis of the frames with RBS composite beams using SAP2000