Mechanical behavior of concrete filled double skin tubular circular deep beams

Concrete filled double skin circular tubular member (CFDST) has a hollowed section consisting of double concentric steel tubes with concrete fill in between their walls. The CFDST members are consequently lighter than ordinary concrete filled tubular members (CFT) which have solid sections. The purpose of this study is to investigate the mechanical behavior of CFDST deep beam under a combined action of shear and bending through a three points static loading test. Two testing parameters were considered, namely, the inner-to-outer diameter ratio (Di/Do) and the outer tube diameter-to-thickness ratio (Do/to). The results showed that the obtained failure modes were controlled by the parameter Di/Do. Their strengths for ratios of less than 0.47 fairly agreed with the estimation values proposed herein. Furthermore, biaxial elasto-plastic stress histories of both tubes under plane stress condition were also discussed.