Design method for checking tensile stresses under service loads in cracked prestressed concrete members with 2,400-MPa strands

The ACI 318-14 building code requirements specify that the net tensile stresses of prestressing strands under service loads (Δf_ps) shall be limited to 250 MPa for proper crack control of prestressed concrete (PC) members which are classified as Class C. However, as high-strength prestressing strands with a tensile strength of 2,400 MPa have recently been developed and applied to PC members, this requirement needs to be reviewed. In this study, experiments and analysis were carried out in order to investigate the Δf_ps of PC members with the recently developed 2400 MPa strands, and in the test, the strain distributions, flexural crack widths, and stress change in bonded prestressed reinforcement were measured in detail according to the applied loads. In addition, the minimum magnitudes of effective prestress to satisfy the stress limitation of the net tensile stress of prestressing strands specified in the current design code were estimated using a nonlinear flexural analysis model, and a simplified design equation was proposed for a simple calculation of the Δf_ps at the design stage.