Background:
Beam-to-box-column connections are critical regions of steel moment-resisting frames due to limitations associated with installing internal continuity plates. Their seismic performance strongly depends on the reliability of construction details. This study investigates the effect of the installation method of horizontal continuity plates on the cyclic behavior of beam-to-box-column connections and their ability to develop the desirable weak-beam–strong-column mechanism.
Methods:
Two full-scale beam-to-box-column connection specimens were designed, fabricated, and tested under cyclic loading according to the AISC 341-22 loading protocol. Both specimens had similar geometry and structural characteristics, with the primary difference being the method used to connect the internal continuity plate to the column wall. Electroslag welding (ESW) was used for specimen N01, whereas shielded metal arc welding (SMAW) was employed for specimen N02. Following fabrication, weld quality in the connection region was inspected, and the hysteretic response, envelope curves, failure mechanisms, and cumulative energy dissipation capacity were evaluated.

Results:
The results indicated that, despite weld quality control, premature damage occurred in the connection region of specimen N01. Fracture of the continuity-plate weld at a drift ratio of approximately 1.5% caused damage concentration and prevented the development of a significant plastic hinge in the beam. Consequently, specimen N01 reached only approximately 0.7 times the nominal plastic moment capacity of the beam (0.7Mp). In contrast, specimen N02 developed substantial plastic deformations in the beam and reached approximately 1.3 times the nominal plastic moment capacity (1.3Mp). The post-yield stiffness ratios of specimens N01 and N02 were approximately −1% and 7.8%, respectively. The cumulative dissipated energy of specimen N02 was approximately 232 kJ, about 1.8 times the 128 kJ recorded for specimen N01.

Conclusion:
The findings demonstrate that satisfying the required strength of connection components and confirming initial weld quality do not necessarily ensure satisfactory seismic performance of beam-to-box-column connections. Internal construction details can significantly influence damage development and prevent effective utilization of the beam’s plastic capacity. Therefore, connection details with improved constructability, quality-control capability, and reliability are essential for enhancing the seismic performance and ductile behavior of box-column connections.