Minimum Transverse Reinforcement for Bottle-Shaped Struts

Abstract

This paper investigates the amount of transverse reinforcement needed to resist the tension developed in a bottle-shaped strut. Bottle-shaped struts are wider at their midpoint than at either end. As the struts widen near the midpoint, tensile stresses transverse to the direction of compression are developed. Reinforcement must be placed within the strut to carry this transverse tension. An expression for the required transverse reinforcement was developed and a database of 476 test specimens was used to evaluate these equilibrium-based equations along with a series of three tests of deep beams. The strut efficiency factors presented in Appendix A of ACI 318-05 were used in this evaluation. The shortcomings of current ACI 318-05 provisions that allow the design of a deep beam without any shear reinforcement are discussed. The amount of transverse reinforcement required to maintain equilibrium in a bottle-shaped strut is proportional to the force applied to the strut and inversely proportional to the slope of the angle of dispersion. These findings suggest that additional research regarding the serviceability of structures designed using strut-and-tie provisions is needed.