Mechanism of load introduction and transfer within steel-encased concrete-filled steel tube connections

This paper studies the load introduction and transfer mechanisms for steel-encased concrete-filled steel tube (CFST) K-type connections experimentally and numerically. Six K-type CFST connections with encased built-up latticed angles were tested. The finite element (FE) models were established and verified by the experimental results. Effectiveness of the reinforcing plates, i.e., direct bearing in an extended range was evaluated based on the parametric analysis. The upper chord length of 2D mm is sufficient to transfer the required load from the chord wall to inner concrete via the direct shear interaction only. Though employing insufficient confinement provided by the chord wall, the chord with non-compact and slender sections cannot fully transfer the required load, even if the reinforcing plates are adopted. In general, due to the non-uniformed load introduction from single-side braces in K-type CFST connections, the reinforcing plates cannot effectively provide direct bearing to the inner concrete due to the localised stress effect and plying force. Furthermore, the ability of load transfer in the interface of inner concrete and encased angles should be ensured to achieve full utilisation of angle strengths. Finally, the equations for the lengths of load introduction between the tube-concrete and concrete-angle interfaces were proposed according to the numerical results. The comparison with the numerical results demonstrates that the proposed equations can predict load introduction lengths accurately and reliably.