Shear response of concrete filled tubes part 1: Experiments

Abstract

Concrete-filled steel tubes (CFSTs) and internally-reinforced CFSTs (RCFSTs) are most commonly used as columns/piers or piles or shafts for deep foundations for buildings and transportation structures. Accurate engineering properties of CFSTs are needed but, in many cases, these expressions have been derived from reinforced concrete (RC) or structural steel design expressions and do not accurately capture the strength of CFSTs. In particular the steel tube without the concrete is prone to premature buckling and the shear reinforcement of RC components is discontinuous; clearly neither is an appropriate starting expression for CFTs with restrained and continuous steel. Although prior research has focused on the strength of CFSTs under axial and flexural loading, in comparison with flexural studies, very few research studies have evaluated the shear strength of CFSTs or RCFSTs. Most specifications compute the shear capacity of CFST by either assuming that either the steel alone contributes to the shear strength or by treating the CFST as a reinforced concrete member reinforced by the steel tube (analogous to spiral). Comparison of these existing provisions with prior test results performed on small-diameter tubes (approximately 150 mm (6 in.) in diameter) shows that these current expressions greatly underestimate the shear capacity of CFST. To develop a more accurate expression, a research program was undertaken to experimentally and analytically investigate the shear capacity of large-scale (508 mm or 20 in. diameter) tubes. A range of design parameters including shear-span aspect ratio, diameter-to-thickness ratio (D/t), concrete compressive strength, tube type, tail length and interface condition were studied. The tests were used to determine the response mode (flexure, shear or bond) and a simple numerical expression was developed to separate flexural and shear response modes. For specimens failing in shear, the effects of the parameters which impacted the behavior are described. The research experimental results are combined with prior research results and used to evaluate different shear expressions for CFST and RCFST. The final design expression is presented in the companion paper; this expression was validated using these results as well as those from the experimentally-validated parametric study conducted with high-resolution numerical models. The resulting shear strength is approximately twice current expressions.