Impact of New ACI 318 Flexural Resistance Factor on Bond Failures

Abstract

Monte Carlo simulation is used to quantify the reliability of reinforced concrete cantilever beams without stirrups designed using ASCE 7-98 load factors with strength reduction factors given in Appendix C of ACI 318-99 and the main body of ACI 318-02. Limit states corresponding to flexural failure neglecting strain hardening of the reinforcement, flexural failure considering strain hardening, and bond failure are investigated for various dead-to-live load ratios, reinforcing bar sizes, and flexural reinforcement ratios. Development lengths for the simulated beams were computed using ACI 318-02 Eq. (12-1). Computed mean-value, first-order, second-moment reliability indices confirm the simulation results, but in all cases slightly overestimate the reliability. Bond failures are more probable than flexural failures, and more probable for beams with small diameter bars than for beams with large diameter bars. Flexure and bond reliabilities are lower for the beams designed to ACI 318-02 than for the beams designed to ACI 318-99 for reinforcement ratios less than 0.023. Flexural reliability indices are more consistent for the beams designed to ACI 318-02, but bond reliability indices are more variable. To improve the consistency and magnitude of bond reliability indices for beams designed to ACI 318-02 the reinforcement bar size factor γ could be increased from 0.8 to 0.85 and development lengths could be increased by 22 to 27% for small diameter bars and up to 19% for large diameter bars. The bar size factor may have to be increased further to accommodate bars with confining reinforcement and also reinforcement detailed using the simplified equations in Section 12.2.2 of ACI 318-02.