Strength design of pretensioned flexural concrete members at prestress transfer

This paper presents a rational method for the design of pretensioned flexural concrete members due to the effects of prestress transfer. Conditions at prestress transfer often control the level of prestress that can be placed in pretensioned flexural members. The level of prestress significantly influences the maximum span capability. It is proposed that the flexural design of pretensioned, prestressed concrete members for the effects of prestress transfer be based on strength design criteria. In practice, the proposed method will generally lead to higher prestress levels than the empirical limit of 0.5 f(ci)' given in ACI 318-99 Building Code and AASHTO Standard Specifications for Highway Bridges. Satisfying the current 0.6 f(ci)' limit often results in excessive strand debonding or necessitates draping strand at member ends. Debonding results in weakening of shear strength and poses possible durability concerns. Another significant advantage of the proposed strength design approach is that it automatically and rationally allows for calculation of any top bonded reinforcement required to maintain strength at transfer with controlled tension cracking. The current method directs the design engineer to use an uncracked section analysis of an already cracked section to calculate the bonded reinforcement area.