Investigation on allowable compressive stresses in pretensioned concrete members at transfer

Modern structural concrete members have become longer and slender, and the prestressed concrete method is an efficient way for sufficiently satisfying the strength and serviceability requirements of such members. Particularly, pretensioned concrete members are typically produced as a precast system that can reduce the construction time and waste significantly by minimizing on-site work. Thus, the pretensioned precast concrete structure is an effective method that can meet both environmental and economic demands in international society. Productivity of the pretensioned members, however, is significantly influenced by the timing of releasing prestress, because prestressing forces can be transferred when the target compressive strength of early age concrete is attained. Unfortunately, the allowable compressive stress at transfer specified in current design code provisions may not be reliable in some practical situations, and those stress limits were determined by empirical methods without a theoretical basis. To overcome such problems, a reasonable estimation equation for allowable compressive stress at transfer was proposed in this paper using extensive analyses based on the Strength Design Method (SDM), in which the eccentricity ratio of the tendon, the sectional size and shape, the level of prestress, and the self-weight moment were considered. The allowable releasing compressive stresses specified in international design codes and based on the proposed equation were compared to test results collected from literature. These comparisons indicated that the allowable stresses specified in current design codes provided unconservative evaluation results for pretensioned members with low eccentricity ratios and conservative results for those with high eccentricity ratios, while the proposed equation reasonably evaluated the allowable compressive stresses of pretensioned members with rectangular, tee, and inverted tee sections.