Effective strength of concrete struts for a three-dimensional four-pile cap strut-and-tie model

For the rational analysis and safe design of reinforced concrete pile caps by using the three-dimensional (3D) strut-and-tie model method, the effective strength of concrete struts must be determined accurately. In this study, an equation of the effective strength of concrete strut for a typical and most appropriate 3D strut-and-tie model for four-pile caps is proposed. In the development of the equation, the effects of 3D stresses associated with tensile strains in reinforcing bars crossing a strut, deviation angle between strut orientation and compressive principal stress flow, the degree of confinement provided by the flexural reinforcement, and shape ratios defining the geometry of a four-pile cap, are considered. 3D strut-and-tie model analyses of 115 reinforced concrete four-pile caps were carried out to evaluate the proposed equation and to compare the proposed equation with those equations in several design specifications. The ultimate strengths predicted by using the proposed strut strength equation were most consistent and agreed reasonably well with experimental results.