Strength of Struts in Deep Concrete Members Designed Using Strut-and-Tie Method

Abstract

Results from an experimental investigation aimed at evaluating the adequacy of the strength factors for concrete struts in strut-and-tie models given in Appendix A of the 2002 ACI Building Code are presented. The main design variables considered were: the angle between primary strut-and-tie axes, amount of reinforcement crossing the strut, and concrete strength. A total of 12 deep beams were tested, eight with normal strength concrete and four with high-strength concrete. The ratio between experimentally obtained failure loads and the strengths predicted using the strut strength factors given in Appendix A of the ACI Code ranged between 1.00 and 1.22, and between 0.91 and 1.02 for normal and high-strength concrete beams, respectively. Inconsistencies were found in the provisions for minimum reinforcement crossing a strut in Sections A.3.3 and A.3.3.1 when applied to the test specimens, with the former leading to substantially larger reinforcement ratios. The use of a strut strength factor β s = 0.60 in high-strength concrete bottle-shaped struts without web reinforcement led to strength predictions approximately 10% higher than the experimental failure loads. The limited test results suggest that, as a minimum, an effective reinforcement ratio of 0.01, calculated according to ACI Code, Section A.3.3.1, should be provided in high-strength concrete members when a strength factor β S = 0.60 is used. Additional test data, however, are required before a definite recommendation can be made in this regard.