Research On The Basic Mechanical Property Of Concrete-Filled FRP And Steel Tube Composite Columns

Modern architectural structures is developing toward big-plan, high-rise, heavy load with the requirement of industrialized production and ability to prevent serious environment. In order to adapt the developing trend of constructional engineering, an innovative composite structure, named as concrete-filled FRP and steel tube column is proposed based on the concepts of concrete-filled steel tube, FRP-confined concrete and tubed reinforced concrete columns. This new type of composite column confines concrete-filled steel tube (CFT) with Fiber reinforced polymer (FRP) wraps to provide additional transverse confinement. FRP material has become increasingly popular as a jacketing material for civil infrastructure applications, as it has many advantages such as high strength-to-weight ratio, good corrosion resistance and non-magnetism. It has been successfully used in the field for retrofitting old structures and in new structures replacing steel or steel tubes. Using FRP to confine traditional CFTs can improve the performance and compressive strength of the core concrete and delay the local bulking of steel tube, and also prevent the steel tube from corrosion. Based on experimental and theoretical analysis, the properties of short and mid-height concrete-filled FRP and steel tube columns under axial compression are studied. The main contents details as follows:(1) Eleven concrete-filled FRP and steel tube short column specimens were tested under axial compressive loading to investigate especially the failure mechanism, the failure mode and the ultimate baring capacity. The axial behavior of columns with different factors including the type of FRP, the number of layers of FRP wraps, the thickness of steel tube and the unconfined concrete strength are analyzed. The experimental results show that the FRP wraps can greatly raise the bearing capacity and rigidity of CFT columns; the steel tube and the FRP wraps can work together excellently, and their strength are utilized sufficiently; columns confined with different types of FRP exhibited different failure modes; As more layers of FRP wraps are applied, the higher the ultimate bearing capacity and the better the ductility; With the increase in the thickness of steel tubes, the yield strength and bearing capacity increase greatly; Increasing the concrete grade may increase the bearing capacity to some extent. (2) By combining the experimental data and the related theories of concrete-filled steel tube and FRP-confined concrete structures, the bearing capacity of the axially compressed short concrete-filled FRP and steel tubular column is analyzed and derived theoretically. The formula for calculating the bearing capacity of the short columns is proposed. Furthermore, with confinable effect coefficient based on the unified theory, simplified empirical formula predicting the ultimate bearing capacity of the short composite column is proposed. The calculated values agree well with the experimental results. The whole processes of the composite columns under axial compression are simulated with numerical method, the numerical analysis results are shown to be in reasonable agreement with test results.(3) Fifteen concrete-filled FRP and steel tubular mid-height columns were tested under axial compression loading. The failure mode, failure mechanism and performance characteristics are studied. The bearing capacity, load-deflection relationship and load-strain relationship of mid-height columns with different factors including the slenderness ratios of columns, the type of FRP and the quantity of FRP are analyzed. The experimental results indicate that the composite columns exploit the advantages both of concrete-filled steel tube and FRP sheets, and the steel tube and the FRP sheets can work together very well; compared with concrete-filled steel tubular columns, the bearing capacity of the columns confined with FRP is enhanced greatly, and the ductility and the deformation ability are improved remarkably; the decrease in slenderness ratio as well as the increase in FRP quantity can all improve the bearing capacity and ductility of the columns. In the same condition, the utilization of material strength and the ductility of the columns confined with GFRP are a little better than that of those confined with CFRP.(4) Based on the experimental study and theoretical analysis, the formula for calculating the bearing capacity of the mid-height columns is proposed. The calculated results and experimental results agree well. The load-deflection curves and load-strain curves are calculated by ANSYS analysis software in finite element method. The calculated results agree well with the experimental results, verifying the applicability of using ANSYS to analyze this kind of composite structure.