Evaluation and Proposal of Strut-and-Tie Method for the Design of Drilled Shaft Footings

Abstract

This paper presents a practical, accurate, and reasonably conservative procedure for the design and analysis of drilled shaft footings, also referred to as pile caps. A database of drilled shaft footing tests was compiled from the literature to evaluate the accuracy of an existing design guide based on the three-dimensional (3D) strut-and-tie method (STM). It was concluded that strength estimations obtained with the existing 3D STM-based design guidelines were excessively conservative, and the accuracy of the method varied with key design parameters such as strut inclination and drilled shaft size. Key enhancements to the 3D STM are proposed to resolve existing limitations and ambiguities, including the definition of tie area for bottom mat reinforcement, 3D nodal geometry, nodal strength, concrete efficiency factor, and tie anchorage checks. These recommendations are supported by experimental evidence, including data from large-scale footing tests recently conducted by the authors, and are consistent with current design code provisions. The proposed method provides more accurate (less conservative) and less scattered (more reliable) strength estimations as compared to the existing recommendation. Lastly, a complete design example of a drilled shaft footing subjected to different loading scenarios is provided in the Supplemental Materials.