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A novel composite structural member, UHPC-encased concrete-filled steel tubular (UC-CFST) column, is proposed in this study to address the shortcomings of the ordinary concrete-encased CFST (OC-CFST) columns. A total of 22 specimens subjected to axial compression loading were tested, including nine UC-CFST stub columns, nine OC-CFST stub columns, and four CFST stub columns. The experimental results showed that the encasing UHPC of UC-CFST columns was less damaged than the encasing ordinary concrete of OC-CFST columns and did not spall when the UHPC was crushed. The UC-CFST stub columns reached their ultimate load-bearing capacity when the encasing UHPC attained the peak compressive strain, which was much greater than that of the OC-CFST stub columns. Compared to the OC-CFST stub columns, the ultimate load-bearing capacity of UC-CFST stub columns improved markedly by 75%–289% and the compressive stiffness increased by 22%–49%, while the ductility decreased by about 50% on average. Furthermore, increasing the area ratio of the encased CFST in the UC-CFST stub columns was unfavorable for the compressive stiffness and load-ultimate bearing capacity. Finally, by comparing the different calculation methods for ultimate load-bearing capacity of OC-CFST columns recommended by Chinese standard CECS 188, American standard AISC 360, Japanese standard AIJ-SRC, and European standard Eurocode 4, it was found that the calculation methods applicable to OC-CFST columns would underestimate the ultimate load-bearing capacity of UC-CFST columns to varying degrees. Thus, a modified calculation formula based on the CECS 188 standard was proposed for accurate prediction of the ultimate load-bearing capacity of the UC-CFST stub columns.

Previous studies have proven that CFST (concrete-filled steel tube) battened built-up columns (CFST-BBCs) exhibit good hysteresis energy dissipation capacity and deformation capacity under lateral cyclic repeated loading. To further understand the elastoplastic seismic response characteristics of such composite structures under strong earthquakes, two pseudo-dynamic tests of 1/8-scaled CFST-BBC pier specimens were first performed. The acceleration records obtained in the 1995 Kobe Earthquake and the 2008 Wenchuan Earthquake were used to investigate how ground motion intensity, ground motion characteristics, and aftershocks affect the seismic response of the CFST-BBC pier. Meanwhile, an extensive time-history analysis of 17 fiber-based finite element models (FFEMs) under 20 different ground motions was performed to reveal the influence of the ground motion intensity, the same-intensity aftershock, and the main structural parameters on the elastoplastic seismic response of the CFST-BBC pier. Finally, a two-stage checking method suitable for the seismic performance of the CFST-BBC pier under the E1-level frequent earthquake and the E2-level rare earthquake was proposed. Verified by experimental and FFEM analysis results, the proposed checking method can accurately and effectively evaluate the strength and deformation capacity of the CFST-BBC piers under strong earthquakes.