Refined 3D strut-and-tie model for analysis and design of four-pile caps

Pile caps used in foundations are commonly designed for simple cases of loading and geometry using the strut-and-tie method. This approach is known to provide safe designs and rather conservative predictions of the ultimate failure load of tests. This level of conservatism is due mainly to the large simplifications made in the geometry assumed which in many cases ignore relevant parameters such as the size of the column. A three-dimensional strut-and-tie model is presented for four-pile caps in which the geometry adopted is optimized. The inclination of the direct strut from the column to the pile is obtained analytically through the maximization process of the resisting load carried by the truss assuming different modes of failure (flexural and shear). This approach is shown to provide more accurate predictions of strength of existing deep pile cap tests with lower scatter compared to design approaches in the literature and ACI 318 Code