Strength Reduction Factors for ACI 318 Strut-and-Tie Method for Deep Beams

The strut-and-tie approach has gained importance in reinforced concrete design practice in the United States in the last two decades. This method has proven suitable for designing shear-critical structural members where beam theory is not applicable. However the strength reduction factors specified for the ACI 318 strut-and-tie method have not been calibrated based on the structural reliability approach. Therefore, the reliability of members designed according to these provisions is unknown. In this study, the reliability of deep beams designed using the strut-and-tie method according to ACI 318 building code requirements for structural concrete was determined. Statistical parameters employed for loads, material uncertainty, and fabrication uncertainty were based on published literature. The uncertainty in the analytical model was characterized based on available test results. The findings indicate that current design practice using the strut-and-tie method promotes the likelihood of a nonductile failure mode relative to a ductile failure mode. Inconsistencies in reliability with respect to concrete strength are highlighted. The following reliability-based strength reduction factors are suggested: phi = 0.65 for struts and nodal zones and phi = 0.90 for ties.