The flexural behaviour of concrete-filled hollow structural sections

Abstract

In Standard CAN/CSA S16.1-M89, the contribution of the concrete to the flexural capacity of concrete-filled hollow structural sections is acknowledged as an alternative approach, but no method of assessing it is given. Preliminary studies had indicated that the concrete increased the ultimate moment capacity, the initial flexural stiffness, and the ductility, and delayed local buckling of the steel, thus enhancing the behaviour considerably. A series of four flexural tests on rectangular and square cold-formed hollow structural steel sections and twelve on concrete-filled sections were undertaken to assess the general behaviour of these composite sections. The test specimens were selected to examine the effects of different ratios of depth to width and therefore of the proportions of steel and concrete in compression, and of different values of shear span to depth as related to the transfer of forces from one to the other when no direct means is provided for this transfer. The tests showed that the ultimate flexural strength of the concrete-filled sections is increased by about 10–30% over that of the bare steel sections, depending on the relative proportions of steel and concrete. The stiffness is also enhanced. In all cases, slip between the steel and concrete was not detrimental, even though shear-span-to-depth ratios as low as 1 were tested. Models are developed to predict the flexural strength of the composite section. Fully plastic stress blocks with the concrete at its maximum strength are used. The models are in excellent agreement with the test results. Key words: composite beams, concrete-filled, flexural behaviour, hollow structural sections.