Finite Element Simulation And Calculation Methods Of CFST Member Under Torsion Moment

Although a lot of research on CFST structure focus on mechanical behavior under axial compression, eccentric compression(compression and bending) and pure bending, there were a few research concerning torsional behavior of CFST structure, until now no available calculation methods of CFST structure subjected to pure torsion were mentioned in Standard. With the request of large-span and diversity of structural forms in structure, the need of research on torsional behavior is becoming increasingly important. Circular CFST member was taken as research object in this paper and finite element model was set up to simulate torsional behavior under whole loading path and a simplified calculation method was proposed to predict torsion resistance. The main research content, results and conclusions in this paper were given as following:(1) A FEM model was set up to simulate mechanical behavior and failure mechanism of CFST members under torsion moment, which shows that, restrained torsion was different from free torsion, the difference was not obvious under high slenderness ratio, otherwise, the torsional capacity of CFST member under restrained torsion is larger than the one under free torsion,that is to say, the influence of the slenderness ratio on torsional resistance due to restrained torsion shear effect of the fixed end can not be ignored.(2) Analysis on parameters concerning torsional strength of CFST members under restrained torsion,namely steel yield strength, concrete compression strength, steel ratio and slenderness ratio, were carried out systematically, the influence of slenderness ratio to restrained torsion should be considered especially. In order to analyze the influence of varied slenderness ratio to restrained torsion, the influence coefficient considering of restrained torsion shear effect was proposed.(3) Based on ultimate equilibrium theory and plasticity theory in classical Material Mechanics, the data from test and FEM model were analysed and fitted, the simplified calculation method to predict the torsion resistance of CFST member under restrained torsion was proposed and its applicable condition was given. In addition, the simplified calculation formula has compared with other calculations, the calculation given in this paper has higher accuracy and smaller data scattering.(4) On the basis of the above design formula, the torsional distribution of steel and concrete was calculated. It shows that, in the early stage of loading path, shear stress and shear strain of steel is lager than core concrete because of its outer edge position, so the most of torsion moment beared by outer steel tubular. Then in the later stage of loading path, concrete cracking was restrained by outer steel tubular, making the phenomenon of helical-effect, making the core concrete in the condition of compression and torsion instead of pure torsion, that is to say, the condition changed from cracking in tension to complicated three-dimensional stress state, thus enhancing the torsional capacity of core concrete. While the loading condition of steel tube transferred from pure torsion to tension-torsion, making little improvement in its torsion resistance.