New Structural Model for Multicomponent Pile Cross Sections under Axial Load

Piles composed of more than one material in their cross section have been used for more than 100 years. Originally this was limited to driven steel shell or pipe piles filled with portland-cement concrete. More recent developments include various types of drilled elements such as micropiles that consist of various combinations of steel shells, portland-cement grout, and steel reinforcing bars. The structural analysis or design of piles with multicomponent cross sections under axial load requires that the axial stress be apportioned to the various components. Traditionally this has been done using an approximate one-dimensional model that implies the components interact with each other only axially, not radially, and that there is no radial interaction with the ground around the pile. This note presents a new three-dimensional model that explicitly and rigorously considers not only the Poisson effects caused by axial load and the triaxial stress field that develops within and between components of a pile as a result but also how this stress field is affected by radial stresses in the adjacent ground. This new model is based on the theory of linear elasticity and yields a closed-form solution that can be either evaluated independently or incorporated within a more general analytical model for axial pile capacity. Examples of calculated results obtained using this new model are presented and suggest that Poisson effects are relatively small in magnitude so that the traditional one-dimensional model is adequate for routine use in most cases.