Load Transfer Mechanisms of Steel-Concrete Composite Columns with Circular Reinforced Composite Dowels

Concrete-encased steel (CES) composite columns with H-shaped steel cores are increasingly used in high-rise and heavily loaded structures because they combine high strength, stiffness, and fire resistance. In these members, load introduction zones are critical, as shear transfer between steel and concrete governs composite action. Current design provisions treat the steel-concrete bond very conservatively and rely on headed studs, which become inefficient in high-strength concrete and require dense arrangements to carry high interface shear.

This thesis investigates circular reinforced composite dowel (CRCD) connectors acting together with natural bond in CES columns, particularly when connectors are placed on both the web and the flanges of H-shaped profiles. CRCD connectors are formed by pre-perforating the steel element and placing a traversing rebar through the opening before concreting. The aim is to quantify the combined contribution of bond and CRCD connectors to shear transfer, identify the governing failure mechanisms and slip capacity, and develop predictive models, including reliability assessment, for load-introduction zones in CES columns.

To this end, 45 specific push-out tests were carried out on 15 configurations with HEB 200 and HEA 360 sections. The test matrix covered pure-bond specimens; single and multiple CRCD connectors on the web; connectors on the flanges; and combined web+flange layouts, using normal- and high-strength concrete and systematically varying the number of connectors, their spacing, the opening diameter, and the rebar diameter. A three-dimensional nonlinear finite element model in Abaqus was calibrated against these tests and used for a parametric study on connector geometry and layout. On this basis, a semi-empirical equation was derived for the connection shear resistance, calibrated by reliability analysis in line with EN 1990. Additional expressions were proposed to estimate the stiffness, associated with the adhesion, and the slip capacity, to support geometric optimisation of connectors on both the web and the flanges.

The results show that CRCD connectors markedly enhance shear transfer compared to pure bond, and that placing connectors on the flanges is mechanically feasible even with limited cover, enabling higher connector density over a given embedded length. In HEB~200 specimens, the web and flanges layout achieved up to 101% higher plateau resistance than a comparable web-only configuration. The finite element model accurately reproduced the ultimate and plateau resistance and captured the observed failure modes. The semi-empirical resistance model predicts test results with an average error of about 7% and supports a design partial safety factor of 1.62 within the investigated parameter range.