Design of Pile Caps With Strut-and-Tie Models


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Title: Design of Pile Caps With Strut-and-Tie Models

Author(s): J. Dimig, E. Rogalla, and J. Duntemann

Publication: Special Publication

Volume: 209


Appears on pages(s): 269-278

Keywords: foundation; pile cap; reinrorced concrete; strut-and-tie model

Date: 9/26/2002

The current ACI Building Code (ACI 318-99) procedure for the shear design of pile caps is the same approach used for two-way slabs. The procedure involves determining a section thickness such that the concrete shear stress (@,) is greater than the applied shear stress (vu) 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 dn. For these situations, the ACI Commentary states that the designer should examine shear strength at the face of the column and it refers to procedures outlined in the CRSI Handbook (1996). ACI has recently published proposed revisions for the ACI 318-02 Code, which promotes 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 insure that the tension tie (longitudinal reinforcement) yields before significant diagonal cracking develops in the compression struts or crushing in the nodal zones. This paper explains the existing ACI and CRSI procedures and the proposed ACI provisions for strut-and-tie design.