Experimental investigations on the partially concrete-filled steel tubular slender beams

This paper focuses on the flexural behaviour of partially concrete-filled steel tubular (PCFST) beams having slender cross-sections (D/t > 150). PCFST beam is developed by optimising the tension zone concrete in a concrete-filled steel tubular (CFST) beam. Twelve specimens were investigated under a four-point load to understand the flexural behaviour and load transfer mechanism of PCFST slender beams. Parameters considered for the tests are concrete compressive strength, shear-span-to-depth ratio, and concrete wall thickness. The failure mode, load-deflection, strain distribution and moment-rotation data of the test specimens are reported and compared. In general, PCFST beams exhibit the same flexure behaviour as CFST beams in terms of flexural resistance and stiffness, and the weight reduction is around 40 %. However, composite action is lost in PCFST without tension zone concrete, and the beam behaves like an empty steel tube. Therefore, a minimum concrete wall thickness (t(c,min)) in the tension zone is required in PCFST and an equation for t(c,min) is proposed based on the test results. Concrete compressive strength is found to have lesser significance in the PCFST beam, whereas a decrease in the shear span-to-depth ratio from 2 to 1 can alter the mode of failure from flexure to shear. A numerical model is developed using ABAQUS to support the test results. This study finds the PCFST beam an optimised replacement for CFST beams in flexure-predominant structural systems. Additionally, a flexure capacity equation is proposed using the plastic stress distribution method, and made good predictions with the experimental and finite element results.