Behaviour of CFST columns with external confinement under uni-axial compression

Abstract

This thesis presents both experimental and theoretical studies on the structural behaviour of externally confined concrete-filled-steel-tube (CFST) short columns under uni-axial compression. External confinement in the form of tie bars, steel rings, spirals and jackets is proposed. From the study, it can be concluded that the external confinement is highly effective in improving the uni-axial behaviour of CFST columns by improving confining stress and suppressing inelastic local buckling.

The first part of the thesis covers the experimental behaviour of concentrically loaded confined CFST columns. The experimental programme involved testing a total of 208 specimens that consisted of 46 hollow-steel-tube columns with or without external confinement and CFST columns: 28 unconfined, 25 tie-confined, 77 ring-confined, 24 spiral-confined and 8 jacket-confined columns. The test results are summarized and discussed. From analysing the obtained experimental results, a better understanding of the structural behaviours, such as steel-concrete interface bonding, stress state of steel tube, behaviour of core concrete of CFST columns can be obtained. Besides, the test results obtained are useful for deriving and verifying theoretical models in this study.

In the second part of the thesis, a theoretical stress-strain model is proposed based on the obtained test results. This model is obtained through a four-step procedure. As the first crucial step, a new and accurate hoop strain equation is proposed and verified based on experimental results. Having validated the assumption of stress path independence of the core concrete in CFST columns under monotonic uni-axial compression, an actively confined concrete constitutive model is adopted to predict the axial stress-strain response of the core concrete. Next, with the aid of Generalized Hooke’s Law and Prandtl-Reuss theory, the complicated stress-state of steel tube is accurately modelled. Lastly, by using the newly proposed hoop strain equation together with the interaction models among the core concrete, steel tube and external confinement, the uni-axial behaviours of CFST columns can be predicted. The validity of this model is subsequently verified with the author’s and published experimental results of CFST columns and FRP-confined concrete.

Finally, a parametric study using the proposed model is conducted in this thesis, involving a wide range of parameters, i.e. steel tube yield strength, concrete compressive strength, steel ratio (steel tube outer diameter / thickness) and external confinement (types and arrangements). From the parametric study, two sets of critical steel ratios based on two levels of ductility at different steel yield and concrete compressive strengths are established. Moreover, the maximum column strengths of CFST columns obtained from the model and previous experimental studies are compared with the design strengths calculated using different design codes and the results indicate that the design codes cannot give reasonably accurate predictions. Therefore, new design equations are proposed in this thesis to predict the maximum strength of the CFST columns.