08.17: Experimental behaviour of high‐strength thin‐walled concrete filled steel tubular stub columns

ABSTRACT Developed more than 80 years ago, the design procedure of concrete filled steel tubes (CFST) has several limitations that can still be found in most current international design standards. The two main limitations that present themselves are the lack of ability to achieve material enhancement when using high strength materials and restrictions on the geometry of the steel tube thickness to the overall column diameter. This geometric limitation is placed on the design to ensure that the steel tube remains ‘compact’. In complying with these constraints it is assumed in the design equations that local buckling of the steel tube will not take place before the ultimate global buckling load is achieved. With the introduction of higher strength materials, specifically when dealing with steel, the strength increase leads to the design of sections with increased slenderness. As the section becomes more slender, the ability to maintain the compact section limitations in the design codes becomes increasingly difficult. This paper looks at an experimental study on very thin‐walled steel tubes filled with high‐strength concrete and how localised buckling affects their compressive capacity. Adopting a diameter to thickness ratio of 200, 24 circular tubes were constructed using a high‐strength cold‐formed steel sheet, with yield stresses above 500MPa and filled with 100MPa concrete. Employing both thin‐walled sections and high strength materials, the effects of localised buckling in these composite members when subjected to compression has been studied. The experimental programme explored four main conditions; when both the steel and concrete were loaded and bonded together, when the concrete core inside the CFST was loaded, when the steel tube in the CFST was loaded and when the concrete core without the steel tube was tested. By studying the failure modes of both the concrete and the steel tube when acting compositely and individually, the experimental work shows that there is valid evidence to support the theory that even when using high strength materials in slender sections, confinement to the concrete core is provided. Finally, the results of the experimental work have been compared to the compressive design capacities predicted by current international design codes, showing that these provisions are conservative.