Composite Action and Confinement Effects in Tubular Steel-Concrete Columns

Composite columns composed of concrete-filled steel tubes (CFT) have become increasingly popular in structural applications around the world. This type of column can offer many advantages, for instance, high strength, ductility, and large energy absorption capacity, as well as increased speed of construction, positive safety aspects, and possible use of simple standardized connections. Furthermore, today's possibility to produce concretes with higher compressive strengths allows the design of more slender columns, which permits more usable floor space. The aim of this research project was to improve the current knowledge of the mechanical behavior of CFT columns to make a more efficient use of high-strength concrete possible. The main topics of interest were to investigate: the efficiency of the steel tube in confining the concrete core, the demand for bond strength to ensure composite action, and various means of load application to the column. To achieve this, experiments and nonlinear finite element analyses were used in combination. <p />The results obtained from tests and FE analyses on CFT columns show that it is possible to use high-strength concrete and still achieve a structural ductile behavior. However, a thicker steel tube is needed for high-strength concrete compared with normal-strength concrete if the aim is the same ductility. Although the slender CFT columns in this study did not exhibit the beneficial effects of confinement by means of increased concrete strength, the ductility was positively influenced, since the concrete core continued to carry high stresses even long after the unconfined compressive strength had been reached. The effect of enhanced concrete strength due to confinement is most pronounced for short columns in concentric loading, and the effect decreases with increasing slenderness and eccentricity. Furthermore, the behavior of the columns was greatly influenced by how the load was applied to the column. To ensure composite action, it seems not enough to rely on the natural bond strength when the load is applied only to the steel tube or concrete core. Instead, the connection design should force the entire section to undergo the same deformations by blocking action. Thus, the bond strength will be of minor importance. This is of special importance when high-strength concrete is used, since a need for higher shear stress transfer can be expected