Behavior and design of concrete-filled cold-formed lean duplex stainless steel structural members

Abstract

Lean duplex stainless steel (LDSS), as a relatively new material, has recently gained significant attention due to its exceptionally good mechanical properties both in strength and ductility, good corrosion resistance and lower cost compared to the duplex stainless steel type. Concrete-filled cold-formed LDSS members comprise two parts: an outer LDSS tube and the inner infilled concrete. From the perspective of structural performance, the infilled concrete provides confinement that could offer more efficient and economic use of the stainless steel and concrete materials. In addition, concrete-filled LDSS members possess great flexural resistance and good constructability, with the stainless steel tube acting as permanent formwork. However, as an innovative composite structure, concrete-filled LDSS tubular section has not been covered in any design specification. To acquire a comprehensive understanding on the structural behavior of concrete-filled LDSS members, extensive experimental and numerical investigations have been carried out.

Both material behavior tests and member behavior tests were conducted. In terms of material behavior, tensile coupon tests, including flat and corner coupons, as well as stub column tests, were carried out. In terms of member behavior, four-point bending tests about either major axis or minor axis were employed to study the flexural performance of concrete-filled LDSS beam members subjected to bending. In addition, pin-ended column tests and beam-column tests were carried out to investigate the member behavior under combined axial compression and bending. A total of 157 tests, including 18 coupon tests, 16 stub column tests, 35 pin-ended column tests, 32 four-point bending tests and 56 beam-column tests, have been conducted.

The experimental results were used to calibrate the finite element models, and then comprehensive parametric studies were carried out to investigate the influence of key variables on the structural behavior of the concrete-filled LDSS members and to generate extensive numerical data. The confined concrete model was modified, and the concrete-steel tube interface was carefully modeled with the conventional Coulomb friction model associated with shear strength limit. A total of 709 finite element analysis results, including 123 stub columns, 160 pin-ended columns, 192 beams under four-point bending and 234 beam-columns, have been generated.

The test strengths and finite element analysis results were compared with the design strengths predicted using the current North American, Australian and European design provisions for cold-formed stainless steel structures and composite carbon steel structures, and the applicability of current international design specifications was carefully examined. Finally, recommendations and modifications on existing design provisions are proposed and examined.