Calibration of strength design of pretensioned flexural concrete members at release

Strength design method for pretensioned flexural concrete members at prestress release was developed in 2001 as a rational alternative to the working stress design method. Load and resistance factors for the developed strength design method were selected to match those used in similar applications (e.g., tied columns) without reliability-based calibration. The purpose of this paper is to present the calibration of resistance factors for the strength design of pretensioned flexural concrete members at release to achieve a target reliability index of 3.5. Resistance models were developed to capture material, fabrication, and professional uncertainty. Statistical parameters of load and resistance were obtained from the literature. Formulae for strength design at release were developed for rectangular and flanged sections in cases when top reinforcement is needed and when top reinforcement is not necessarily needed. Reliability analysis was conducted for several rectangular and inverted-T sections and resistance factors of 0.75 and 0.70 were recommended in light of the results for 5 ksi (34.5 MPa) and 8 ksi (55.2 MPa) concrete strengths, respectively. Moreover, a simplified design procedure was proposed to assist designers and producers in applying the strength design method at prestress release. Three design examples were also presented and compared against those designed using the working stress method.