Design of Deep Beams Using Strut-and-Tie Models--Part I: Evaluating U.S. Provisions

This paper evaluates the strut-and-tie modeling provisions of ACI 318-05 and AASHTO Load and Resistance Factor Design (LRFD). The evaluation used a database of 596 tests from the literature of reinforced concrete beams with shear span-depth ratios less than 2. This compilation of data was used to examine the strain energy in various truss models that could be used to model the test specimens. Results showed that single direct strut between the load and one of the reaction points was the favored mechanism. The findings also indicate that the use of neither ACI 318-05 nor AASHTO LRFD produces levels of safety that meet a 5% exclusion rule. The rather large increase in web reinforcement in required AASHTO LRFD compared with ACI 318 did not appear to significantly increase the percentage of safety estimates of efficiency factor.