Mechanical Performance of High Strength Concrete Filled Tubes with High Diameter to Thickness Ratios

In both the structural and civil disciplines, engineers are increasingly faced with growing loading demands to achieve taller, wider and longer designs. Unfortunately, as the demand on structural members has increased over the past few decades, the member supply capacity as determined by the design codes has remained somewhat constant. With the current design Standards internationally being taken to their limitations to achieve higher capacities, a glass ceiling has developed which is preventing the design of more impressive structures. Composite structural members provide a solution to this problem by utilising the strength properties of different materials. When dealing specifically with vertical load resisting systems, Concrete Filled Steel Tubes (CFST) harness the excellent confined compressive strength of concrete which is provided by the surrounding steel tube which acts in tension. This thesis aims at improving the understanding of the mechanical behaviour of thin-walled high-strength concrete filled steel tubular columns under concentric compressive loading. With minimal research having been conducted into the performance of circular CFST columns that utilise both high-strength concrete and high-strength steel in combination with high diameter to thickness ratios, this work specifically deals with these types of structures to improve and extend the current body of knowledge in this field. Starting with an experimental investigation of high-strength stub CFST’s to determine the mechanical response, a detailed numerical model has been validated to predict the true behaviour of these composite columns. Due to the lack of sophistication in the available concrete constitutive models when under triaxial loading, this thesis then derives and implements a novel constitutive model for concrete, based on the work done in cemented granular materials. Utilising the new material model, the influence of different diameter to thickness ratios, column length, and concrete strengths has then been analysed using the developed numerical modelling techniques. Using the results of the experimental and numerical investigations, it has been shown that the steel tube regardless of thickness will always provide confinement to the concrete core; thus enhancing its strength. Finally, a full probabilistic reliability analysis was then conducted on the current design methodology found in the Australian Standards for CFST’s demonstrating that if thin-walled steel tubes prone to localised buckling are considered, the current design equations specified can be adopted even though they are currently limited to lower grades of steel and concrete then those investigated. This conclusion has been shown to be independent of both the concrete and steel strengths