Seismic Performance of Space-Saving Special-Shaped Concrete-Filled Steel Tube (CFST) Frames with Different Joint Types: Symmetry Effects and Design Implications for Civil Transportation Buildings

Special-shaped concrete-filled steel tube (CFST) frames can be embedded in partition walls to improve space utilization, but their frame-level seismic behavior across joint types remains under-documented. This study examines six two-story, single-bay frames with cruciform, T-, and L-shaped CFST columns and three joint configurations: external hoops with vertical ribs, fully bolted joints, and fully bolted joints with replaceable flange plates. Low-cycle reversed loading tests were combined with validated ABAQUS and OpenSees models to interpret mechanisms and conduct parametric analyses. All frames exhibited stable spindle-shaped hysteresis with minor pinching; equivalent viscous damping reached 0.13–0.25, ductility coefficients 3.03–3.69, and drift angles 0.088–0.126 rad. Hooped-and-ribbed joints showed the highest capacity and energy dissipation, while replaceable joints localized damage for rapid repair. Parametric results revealed that increasing the steel grade and steel ratio (≈5–20%) improved seismic indices more effectively than raising the concrete strength. Recommended design windows include axial load ratio < 0.4–0.5, slenderness ≤ 30, stiffness ratio ≈ 0.36, and flexural-capacity ratio ≈ 1.0. These findings provide symmetry-based, repair-oriented guidance for transportation buildings requiring rapid post-earthquake recovery.