Seismic Behavior of Joint Region–Enhanced Concrete-Encased Steel Tubular Beam-to-Column Connections

Concrete-encased steel tubular (CEST) structural systems combine the advantages of steel and concrete to provide excellent mechanical performance and good compatibility with prefabricated construction. To enhance the plastic deformation capacity of the connection region, a joint-enhanced CEST beam-to-column connection is proposed, incorporating advanced cementitious materials, such as ultra-high-performance concrete (UHPC) and ultra-high-toughness cementitious composite (UHTCC), into the joint. The seismic performance of these connections was evaluated through experimental, numerical, and theoretical investigations. Five full-scale specimens with different joint concrete materials, connector types, and steel beam configurations were tested under constant axial compression and lateral cyclic loading. The results showed that all specimens exhibited stable hysteretic responses with rotation capacities exceeding 4% and ductility coefficients above 3.0. Specimens with normal concrete (NC) experienced severe joint damage, whereas those using UHPC and UHTCC effectively protected the joint region and promoted hinge formation in the nonjoint beam segment. In addition, specimens equipped with Perfobond Leiste (PBL) connectors developed higher stiffness and strength and better resisted local buckling compared with stud-connected specimens. Refined finite-element (FE) models validated against test data were further employed to conduct parametric studies, revealing that the bearing capacity is governed primarily by steel beam strength and flange configuration, while concrete strength and cover thickness have limited influence. A simplified design formula was developed for evaluating the bending capacity of CEST beams, and comparison with experimental results showed accurate and slightly conservative predictions, demonstrating its applicability for engineering design.