Probabilistic Calibration Of The Strength Reduction Factor For The Design Of Rectangular Short Concrete Columns Reinforced With Frp Bars Under Eccentric Axial Loading - Update Of Aci 440 Rules

The design of concrete columns reinforced with fibre-reinforced polymer (FRP) bars is currently debatable. The present study proposes a reliability-based approach with the objective of calibrating the optimal strength reduction factor for the design of short rectangular FRP-RC columns under eccentric axial loading according to the ACI-440 guide. The reliability approach relies on a fast hybrid first-order reliability method (FORM)-based response-surface (RS) method. The results of such a hybrid approach have been validated using a Monte Carlo importance sampling (MC-IS) technique. The uncertainty of geometrical, material and loading variables is considered through various dedicated statistical distribution laws. Several design parameters have been considered: load eccentricity, aspect ratio of the concrete section, reinforcement ratio, live to dead load ratio, strength of concrete, and grade of FRP bars. In addition, Top-Bottom and Uniform reinforcement configurations, which are preferred for high- and small-load eccentricities, respectively, are considered. To ensure the generalization of the approach, full combinations between the design parameters are performed, resulting in more than 105 structural classes. Thus, calibrated strength reduction factors can cover a wide range of design situations. After demonstrating the possible inappropriateness of ACI-440 rules, the study proposes a novel approach to choose the strength reduction factors based on a quadratic function of the live-to-dead load ratio and load eccentricity. Eventually, the study presents recommendations for the selection of reduction factors distinguishing small and high load eccentricity and concludes with a working example.