Ultimate Resistance of TSOB Connections Strengthened with Internal Stiffeners in Hollow-Section Columns

The T-head square-neck one-side bolt (TSOB) provides a cost-effective and convenient fastening solution for hollow-section column (HSC) connections. However, its ultimate resistance, particularly when internal stiffeners are adopted, has not yet been sufficiently clarified. In this study, finite element simulations combined with theoretical derivation are used to evaluate the ultimate strength of TSOB bolted connections incorporating inner stiffeners. The results indicate that strain hardening considerably increases the connection resistance, with the ultimate strength being approximately 50–70% higher than the corresponding yield strength, indicating potential for optimization of material efficiency. The parametric analysis further reveals that for H-steel stiffeners, the ultimate resistance is mainly governed by the stiffener length, flange width, and flange thickness. Specifically, the length and flange width exhibit quadratic influences, whereas the flange thickness shows an approximately linear effect. For octagon-steel stiffeners, the resistance is primarily controlled by the stiffener length and wall thickness, with the length following a quadratic trend and the wall thickness showing a linear relationship, while the width of the connecting face has only a marginal influence. Based on the observed failure mechanisms, a unified calculation approach is developed by introducing reduction factors for the effective punching area, with values ranging from 0.67 to 0.78. The proposed model is verified against finite element results, and the average prediction error is less than 9%. The findings provide a practical basis for strength evaluation and configuration optimization of internally stiffened TSOB-bolted HSC connections.