Pitch spacing effect on the axial compressive behaviour of spirally reinforced concrete-filled steel tube (SRCFT)

• Reinforced concrete significantly improves the axial compressive behaviour of CFTs.
• The shear reinforcement has a significant effect on the axial behaviour of RCFTs.
• The pitch spacing has a significant effect on the post-yield behaviour of SRCFTs.
• SRCFT columns show different behaviour in the failure mode compared to CFTs.

Concrete-filled steel tubes (CFTs) exhibit superior performance under static and dynamic loads, due to composite action. Factors, such as the thickness of the steel tube and the concrete core condition, have a significant effect on the structural behaviour of CFT columns, particularly in the post-yield behaviour. Studies show that if the concrete core is reinforced with steel bars, the new composite member has better characteristics compared to CFT columns. In this study, the axial compressive behaviour of reinforced concrete-filled steel tube (RCFT) columns using spirally reinforced concrete (SRCFT) was investigated and compared with CFTs. The main variation was the pitch spacing of the spiral shear reinforcement. Fifteen specimens, including three CFTs and twelve SRCFTs, were tested in five groups. The test results indicated that a SRCFT column has much better post-yield behaviour than a CFT column. A reduction in the pitch spacing rate further improves the post-yield behaviour of the SRCFTs. A comparison of the measured strength of the specimens with corresponding values predicted by two international codes (ACI 318-11 and EC4-1994) shows a good prediction of EC4 and a conservative estimation of ACI.