Axial Compression Behavior of L-Shaped CFST Columns Enhanced by Fully Bolted Threaded-Rod Confinement: An Experimental Assessment

Special-shaped concrete-filled steel tube (CFST) columns are increasingly adopted as efficient vertical load-carrying members in integrated residential structural systems. However, their intrinsically nonuniform confinement promotes early local buckling and bulging of tube plates and limits deformation stability under axial compression. This study presents an experimental assessment of an L-shaped CFST column enhanced by a fully bolted threaded-rod transverse tie (RT) system, which is intended to strengthen confinement delivery and delay tube instability. Two 1500 mm-high specimens with identical cross-sectional dimensions (400 mm × 200 mm legs; 6 mm wall thickness) were fabricated using Q235 steel and C30 concrete: one conventional baseline (L1) and one RT-improved column (L2) with pre-drilled bolt holes at 150 mm spacing and installed threaded rods (10 mm nominal diameter) to provide a distributed transverse restraint. Monotonic axial compression tests were conducted under staged load control while recording the axial shortening, mid-height lateral deflection, and longitudinal and transverse steel strains. The RT detailing postponed the onset of visible local buckling, tightened the lateral deflection envelope, and increased the measured peak axial resistance from 4354 kN (L1) to 5354 kN (L2), corresponding to an increase of approximately 23%. The combined deformation and strain evidence indicates that the RT system improves the confinement effectiveness by stabilizing the tube dilation and promoting a more controlled instability evolution. Overall, the fully bolted RT approach offers a practical and fabrication-compatible pathway for enhancing the axial strength and deformation performance of L-shaped CFST columns.