Adaptable Strut-and-Tie Model for Design and Verification of Four-Pile Caps

Pile caps transfer the load from columns to a group of piles. Many pile caps support only one column, and the pile caps in turn are supported by only a few piles. Codes of practice do not provide uniform treatment for the design of these types of pile caps, which are typically short and deep with overall span-depth ratios of less than 1.5. These members have traditionally been designed as beams spanning between piles with the depth selected to avoid shear failures and the amount of longitudinal reinforcement selected to provide sufficient flexural capacity as calculated by the engineering beam theory. The strut-and-tie method also has been used for the design of pile caps in which the load path is envisaged to be a three-dimensional truss, with compressive forces being supported by concrete compressive struts between the column and piles and tensile forces being carried by reinforcing steel located between piles. However, neither of these models have provided uniform factors of safety against failure or been able to predict whether failure will occur by flexure or shear. This paper presents an analytical model based on the strut-and-tie approach. The proposed model has been calibrated using an extensive experimental database of pile caps subjected to compression and evaluated analytically for more complex loading conditions. Findings show that this model is applicable across a broad range of test data and can predict the failures modes, cracking, yielding, and failure loads of four-pile caps with reasonable accuracy. The proposed model is also demonstrated to be more safe and rational than the application of a sectional design method for the design of pile caps.