Theoretical And Experimental Study On Novel Menbers Of Concrete-Filled Steel Tube

Concrete-filled steel tubes (CFT) have widely developmental foreground due to their excellent properties such as high strength, high ductility, light weight and construction convenience. In order to improve further the behavior of CFT members, changing the self-stress state and optimizing cross-sectional configuration of composite structure are the effective means. The first part in the dissertation proposes square expansive concrete-filled steel tubes (ECFT) by using the expansive concrete instead of the ordinary concrete to produce an initial confinement. The second part proposes the non-uni-thickness walled rectangular CFT by re-adjusting the thickness of steel tube to optimize cross-sectional configuration, based on the stress characteristic of rectangular CFT beams. Through the experiments and the theoretical analysis, the behaviors of two novel CFT members were investigated. The main work in the dissertation includes:Firstly, Mixture proportions of the expansive concrete used suitably to fill in square steel tubes are determined, through three series of laboratory tests. At the same time, the influences of the expansion agent addition, fly ash and silicon power on the mechanical properties, compressive strength and free expansion ratio of expansive concrete have been discussed and studied, and the free expansive model of expansive concrete is investigated. The results supply theoretical basis for the study on the behaviour of square ECFT membersSecondly, to investigate the mechanical properties of square ECFT columns under axially and eccentrically compressive load, laboratory tests of 20 axially compressed specimens and 18 eccentrically compressed specimens are carried out. Experimental results indicate that the ultimate strength of axially compressed square ECFT columns with appropriate expansion agent addition can enhance 7%~15% as compared with that of ordinary CFT columns; the elevated proportion of the ultimate strength of eccentrically compressed specimens is not as good as that of axially compressed specimens and can achieve 3%~12%. Moreover, the influences of the expansion agent addition, width-thickness ratio and eccentricity ratio on the behavior of square ECFT columns have been investigated. The experimental results are compared with predicted ones on current design codes and the ultimate strength of square ECFT columns is provided.Thirdly,taking into account the effects of material nonlinear constitutive relation and geometric nonlinearity, and taking into account the variation of Poisson's ratio as well as elastic modulus of concrete under compressive load by introducing stress field variables, the nonlinear finite element model for the axially loaded square CFT short column is built up. Meanwhile, according to the comparability in mechanical mode, the expansion effect of the expansive concrete is equivalent to that of ordinary concrete when subjected to a temperature field to obtain the full range load-deformation curve for square ECFT columns. The results compares well with experimental data. Moreover, the interactional mechanism between the core concrete and the steel tube is investigated. The analysis reveals that there is the big contact pressure between the core concrete and the steel tube before loading. The contact pressure reduces weakly when loading in the initial period, and the contact pressure increases rapidly when the axial force reaches the ultimate strength nearby. The average contact pressure of square ECFT columns is always bigger than that of ordinary square CFT columns in the entire process. The confining effect of the steel tube on the core expansive concrete has its greatest effect mainly at the corner regions. The centre region follows. The confining effect at the off-diagonal regions is the smallest. The load-bearing capacity of the square ECFT columns is provided mostly by the diagonal regions.Finally, an experimental investigation on the mechanical behaviour of non-uni-thickness walled rectangular CFT beams subjected to pure bending has been carried out. The results show that the non-uni-thickness walled rectangular CFT is a sort of optimized composite structural member with favorable ductile performance. The moment capacity for non-uni-thickness walled beam is superior to that of uni-thickness walled beam under the condition of the same overall size and the same materials consumption. Meanwhile, the full range load- deformation relation of the non-uni-thickness walled rectangular CFT beams are predicted based on the finite element method, which coincide well with experimental date. The propositional parameter of the steel tube thickness is provided to optimize the cross section for the rectangular CFT beam, since the bending moment capacity can be enhanced by more than 5%. Moreover, Theoretical formula for the moment capacity for non-uni-thickness walled rectangular CFT beam is presented. A comparison of the predicted results with experimental results indicates that the proposed method can attain to the engineering demand.In this paper, much research is published firstly in the above field and discovered the mechanical properties of square ECFT and non-uni-thickness walled rectangular CFT. The results may supply significant theoretical basis for the square ECFT and the non-uni-thickness walled rectangular CFT in practical applications, and provide useful information for the further research.