Cyclic Behavior of Reinforced Concrete Columns with Unstressed Steel Strands as Longitudinal Reinforcement

The use of unstressed Grade 1860 MPa seven-wire steel strands as longitudinal reinforcement has the potential to reduce steel congestion in reinforced concrete members. However, no studies which use unstressed steel strands as the longitudinal reinforcement in concrete columns have been reported in the literature. Thus, in this study, five large-scale column specimens: four of which used unstressed steel strands and one which used conventional Grade 420 MPa deformed bars as longitudinal reinforcement were subjected to cyclic loading under constant axial load to investigate the effectiveness in using strands. Columns with unstressed steel strands as longitudinal reinforcement exhibited drift capacities ranging between 4.96% and 5.70%, which is well over the conventional expectation of 3.50% drift requirement for columns in special moment frames. Also, the specimens exhibited lower energy dissipation but enhanced recentering capability owing to the fact that the strands have significantly higher tensile yield capacity compared to deformed bars. A reduction in effective elastic stiffness was also observed as the postcracking stiffness of specimens with strands dropped due to the low longitudinal reinforcement ratio of 0.78% provided. The test results indicated the relative ineffectiveness of strands in compression than in tension, where the strand bulged and unwound once the surrounding confinement deteriorated. Despite this, strands did straighten up and continued to sustain tensile stresses under load reversal. Based on the experimental observations, two constitutive models, one which considers strain hardening and the other which assumes an elasto-perfectly plastic response in tension was proposed for strands. The constitutive model with strain hardening was able to produce reasonably accurate estimates of flexural strength when used in a detailed moment-curvature analysis. Both constitutive models when used in a simplified analysis, which used equivalent stress block for concrete, was able to consistently produce conservative estimates of flexural strength, which is suitable for design purpose.