Simplified strength design method for allowable compressive stresses in pretensioned concrete members at transfer

Abstract

The allowable compressive stress of the pretensioned precast concrete members at transfer is the most important factor for determining its bed staying time at the Precast Concrete (PC) factory, and it consequently has a large influence on the productivity of pretensioned PC members. The current design code provisions have specified the allowable compressive stresses of concrete at prestress transfer in an empirical manner, and it was simply expressed only as the function of concrete compressive strength at the time of transfer. In contrast, the Strength Design Method (SDM) can theoretically determine the allowable compressive stresses of pretensioned concrete members considering the effect of influential factors, such as eccentricity ratio of tendons, sectional types, level of prestressing forces, and size of concrete section. In this study, a SDM based approach was proposed to simplify the calculation process of the SDM for the each type of concrete cross-section. In addition, four rectangular pretensioned concrete specimens were fabricated and tested to investigate their allowable prestress. It appeared that ACI318-11 and EC2-02 code models provided unsafe estimations on the allowable compressive stress of pretensioned concrete members with the low eccentricity ratios but excessively conservative results for those with high eccentricity ratios. The SDM and the simplified SDM proposed in this study showed reasonable accuracy and safety margin on the allowable compressive stresses of the pretensoined concrete specimens at prestress transfer.