Design of Pile Caps With Strut-and-Tie Models

Abstract

The present ACI Building Code (ACI 318-99) procedure for shear design of pile caps is the same approach used for 2-way slabs. The procedure involves determining a section thickness such that the concrete shear stress is greater than the applied shear stress on the critical section. For footings supported on piles, the ACI Code recognizes that these general provisions are not applicable as the depth of the footing increases and some of the pile loads fall within the critical section. In deep pile caps, the critical section may even be located outside the footing, making it impossible to investigate shear at d or d/2. For these situations, the ACI Commentary states that the designer should examine shear strength at the face of the column and refers to procedures outlined in the CRSI Handbook (1996). ACI has recently published proposed revisions for the ACI 318-02 Code that promote the use of strut-and-tie models as an alternative to the existing ACI and CRSI procedures. The design methodology involves limiting the concrete stresses in the compression struts and nodal zones to ensure that the tension tie yields before significant diagonal cracking develops in the compression struts or crushing in the nodal zones. This paper details current ACI and CRSI procedures and the proposed ACI provisions for strut-and-tie design.