ABSTRACT
Reinforced Concrete (RC) columns carrying axial compressive loads with or without moment require enough ductile response to withstand an unexpected excessive-lateral deformation. The ductility of RC columns significantly depends on the lateral confining pressure provided by transverse reinforcement (ACI-ASCE, 1997). In particular, the supplementary ties are effective in preventing the premature buckling of unsupported longitudinal middle bars and widening the effectively confined area of column core (Sheikh & Khoury 1993). It is also commonly known (Ozcebe & Saatcioglu 1987; Sakai and Sheikh, 1989) that the selection of a proper configuration of supplementary ties is a more feasible approach than a reduction of spacing of peripheral ties to achieve the same level of ductility of columns. Based on extensive test result, ACI 318-11 provision (2011) specifies the consecutive crossties engaging the same longitudinal middle bars as a supplementary ties. Considering practical installation of crossties onto longitudinal bars, ACI 318-11 provision permits the hook angle of the crossties to be 90-degree at one end and 135-degree at the other end, although most experimental studies (Mander et al. 1988; Saatcioglu & Razvi, 1992) used 135-degree hooks or 180-degree hooks for extension of the crossties into the core concrete.
