Behaviour and design of concrete-filled double skin stainless steel members

Abstract

Concrete-filled double skin tubular (CFDST) sections comprise two metal tubes—an outer and inner tube—with concrete sandwiched between the tubes. CFDST sections share the constructability benefits of traditional concrete-filled tubular (CFT) sections, with the steel tubes acting as permanent formwork, but would typically be lighter due to the absence of the inner concrete core. Previous studies on CFDST members have focused mainly on circular and square hollow sections of mild carbon steel infilled with concrete of compressive strengths up to 72 MPa. Investigations into CFDST stub columns with austenitic stainless steel outer tubes are very limited in the literature, while the behaviour of CFDST members with lean duplex or ferritic stainless steel outer tubes remains unexplored.

The structural performance of concrete-filled double skin stainless steel members was investigated . Extensive laboratory testing programme was conducted as part of the study, including details of test specimen fabrication, material coupon tests, stub column tests, four-point bending tests, and three-point bending tests. Tests were carried on cross-sections consist of rectangular, square and circular hollow sections (RHS, SHS and CHS respectively) outer and inner tubes. A total of 168 tests, including 69 stub column tests, 70 simply supported four-point bending tests, 29 simply supported three-point bending tests, as well as tests on material properties, have been conducted.

A numerical modelling programme, which was run in parallel with the laboratory testing programme, was performed. Following successful replication of experimental behaviour, comprehensive parametric studies were undertaken to investigate the influence of key variables on the structural response of the CFDST cross-sections and to generate further numerical results in the areas unexplored experimentally. The confined concrete model was modified and found applicable to predict the structural response of CFDST members. The properties of concrete-metal tubes interface were carefully examined. The conventional Coulomb friction model associated with shear strength limit was assessed. Recommendations were proposed that accurately capture the slip response of the beam model. Upon validation, the FE models were used to conduct parametric studies, and hence generated a large number of numerical results: 816 CFDST stub column simulations and 774 CFDST beam simulations.

Assessment of current European, Australian and North American design provisions for composite carbon steel members to the design of the studied CFDST cross-section is presented. Comparisons are made with all the experimentally and numerically results. Design modifications to the local buckling coefficient and effective concrete strengths are proposed, and improved design predictions were achieved through the modified design rules.