Numerical and Experimental Study on the Effect of External Box Stiffeners for Rigid Connections of I-Beams to Concrete-Filled Circular Columns

This study introduces an innovative connection to improve the seismic performance of I-beam–to–concrete-filled circular column joints. The concept employs a steel box with optimized internal and external stiffeners, eliminating continuity and doubler plates to simplify construction. Calibrated finite-element analyses were first conducted to select three configurations for experimental testing under cyclic quasi-static loading, measuring energy dissipation, stiffness, ultimate moment, panel-zone rotation, and strain distribution. The best-performing specimen was then identified, followed by a numerical parametric study varying beam and column dimensions to determine the minimum steel-box thickness beyond which further increases offer negligible benefit and to assess its effect on connection rigidity. Experimentally, stiffeners aligned with beam flanges significantly improved moment capacity, stiffness, and energy dissipation. Based on parametric analyses, connections with appropriate box-to-flange thickness ratios achieved over 95% of the maximum flexural strength and stiffness, confirming the reliability of the proposed non-dimensional design approach. Numerical analyses showed that the proposed non-dimensional thickness ratios accurately predict connection behavior, where appropriate flange-to-box proportions ensure over 95% of maximum flexural strength and stiffness, leading to stable and rigid joint performance. Overall, the proposed detailing offers a constructible alternative to conventional plate-intensive solutions while achieving superior cyclic behavior.