Research On Cross-shaped Concrete Filled Steel Tubes Subjected To Axial Compression

Nowadays cross-shaped concrete filled steel tubular(CFST)columns possess high load-carrying capacity,good ductility,which can be used in residential and official buildings to save architectural space at room corners,and have many advantages of structural behavior of common CFST as columns.Despite its wide application prospect,researches on the cross-shaped CFST columns are scarce,and no design method can be used.This dissertation concerns with the axial load behavior of cross-shaped CSFT columns.The research study presents the combination of experiments and finite element analyses to quantify the failure mechanism after ultimate compressive loads and investigate the parameters that affect the basic mechanical properties of cross-shaped CFST columns.The main contents of this paper are as follows:1)Experiments were performed on total of 6 cross-shaped CFST stub and slender columns in order to investigate their behavior under axial compressive loads.The key parameters considered in the experimental study were width-to-thickness ratio and slenderness ratio.The influence of these parameters on the failure modes,loaddeformation relationships,ultimate compressive load,were analyzed.The experimental results indicated that by decreasing the width-to-thickness ratio of the cross-shaped CFST stub columns both bearing capacity and ductility was improved.2)The finite element models of the cross-shaped CFST columns subjected to concentric loading were developed for simulation using the commercial software ABAQUS.The finite element model was validated using experimental results from current research and reasonable agreement was found between the experimental and finite element results.The initial stiffness,ultimate strength and the trend of the descending branches of load-deformation curves obtained from experimental and numerical analysis were identical.3)Further parametric analysis with ABAQUS was carried out to study the influences of width-thickness ratio,yield stress of steel tube and concrete strength on section capacity of cross-shaped CFST stub columns subjected to concentric loading.By decreasing the breadth-to-thickness ratio both section capacity and ductility was improved.For low breadth-to-thickness ratio,increase in yield stress improved the both ductility and section capacity of columns but for high breadth-to-thickness ratio it only enhanced the section capacity and was ineffective to increase the ductility.By increasing compressive strength section capacity was increased but columns showed less ductile behavior.4)Finally,a theoretical method for calculating ultimate strength was proposed based on a constitutive model of confined concrete and a simplification corresponding to this method was conducted.The experimental results were also compared with the results calculated by the ACI,EC4,GJB and CECS recommendations as well as by proposed theoretical method.The results showed that all design codes underestimated the axial compressive capacity of the specimens.The results based on the theoretical model were the nearest to the experimental results.