Effects of Increasing Allowable Compressive Stress at Prestress Transfer

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This paper evaluates the benefits and weaknesses of increasing the allowable compressive stress at prestress transfer in prestressed concrete members. A historical background of the allowable release stress in compression is provided. A simple design example is used to quantify potential production and design benefits of increasing the allowable stress and the applicability of these benefits is discussed. Test results of 36 pretensioned beams that were subjected to a range of maximum compressive stresses at release are reported and analyzed. The results suggest that the compressive stress limit at prestress transfer must govern the design of the pretensioned member to achieve significant production or design benefits from increasing the allowable stress, but that production and design benefits cannot be maximized simultaneously. The findings indicate that although increasing the allowable release stress in compression at midspan of a member to 0.65fci' or 0.70fci' is possible, members subjected to midspan release stresses in excess of 0.70fci' displayed premature flexural cracking. Before design codes are changed, however, additional testing on full-scale specimens is recommended.