Flexural behaviour and design of concrete-filled metallic tubular members

Abstract

Concrete-filled steel tubular (CFST) structures have become popular because of their marvellous structural performances with high strength, high ductility, excellent fire resistance and seismic resistance as well as low construction costs and fast construction time. However, current design standards provide little information about the flexural behaviour of CFST as there have been insufficient studies and these design standards are specified for carbon steel only. This study focused on the structural performance of concrete-filled metallic tubular (CFT) beam members, namely concrete-filled high strength steel tubes (CFHSST), concrete-filled aluminium alloy tubes (CFAAT) and concrete-filled stainless steel tubes (CFSST). These CFT beams were examined through experiments and finite element analysis (FEA). Design recommendations are provided in this study. A total of 90 four-point bending tests was conducted to determine the structural behaviour of CFHSST, CFAAT and CFSST under pure bending. Amount of these four-point bending tests, 53 tests involved SHS/RHS and CHS high strength steel tubes, 18 tests involved SHS/RHS aluminium alloy tubes, and 19 tests involved SHS/RHS lean duplex stainless steel tubes. Cold-formed high strength steel of nominal 0.2% proof stresses 700 MPa (H-series), 900 MPa (V-series) and 1100 MPa (S-series), 6061-T6 heat treated aluminium alloy of nominal 0.2% proof stress 240 MPa, lean duplex stainless steel of nominal 0.2% proof stress 450 MPa, and nominal concrete cylinder compressive strengths of 40, 70 and 110 MPa were used in the experiments. Material properties were obtained from tensile coupon tests and concrete cylinder compression tests. Generally, the moment capacities of concrete-filled beams were found to be 7-45% larger than those of hollow beams. The ductility of composite beams was improved significantly. Finite element models (FEM) for CFHSST and CFAAT beams were developed and validated against the experimental flexural strengths and load-deformation history data. Extensive parametric studies were conducted on CFHSST and CFAAT beams using the validated FEM. In total, 384 numerical specimens were simulated, consisting of 144 square/rectangular CFHSST beams, 120 circular CFHSST beams, and 120 square/rectangular CFAAT beams. A wide range of cross-section dimensions and thicknesses was considered for the outer tube sections of the numerical specimens. The experimental and numerical results were used to assess different design approaches for CFT beams, including American specification (AISC 2016), Eurocode 4 (2004) and theoretical methods. Alternative design methods have been proposed to predict the bending moment of square/rectangular and circular CFHSST beams and square/rectangular CFAAT beams. The current design rules and proposed design rules were evaluated using reliability analysis. The proposed design methods were found to be conservative and reliable. (416 words)