Effect of transverse reinforcement on rectangular concrete columns with MTSTR

To better understand the confining effect of multiple-tied-spiral transverse reinforcement (MTSTR) for concrete with rectangular cross-section, eight rectangular concrete columns were fabricated and tested under axial compression; four columns used MTSTR, while the remainder were counterpart reference columns reinforced by conventional stirrups. The main variable considered was the confinement index of transverse reinforcement; this index includes the ratio of the confinement index of interior circular spirals to that of exterior rectangular hoops and the sum of the confinement indices. The results showed that rectangular concrete columns with MTSTR exhibited better ductility than reference columns with the same confinement index of transverse reinforcement; however, the peak load for reinforced columns with MTSTR increased depending on the ratio of the confinement index of circular spirals to that of rectangular hoops; the critical ratio of the confinement index was approximately 2.72. Moreover, compared with previous investigations, the influence of the yield strength ratio of interior circular spirals to exterior rectangular hoops on the load-carrying capacity of the concrete columns with MTSTR was linear. Finally, a simplified method based on the models of Mander et al. is proposed to predict the peak strength of rectangular concrete columns with MTSTR.