Pile foundations are normally used to support bridges and are subjected to axial and lateral forces produced by dead loads, winds, traffic, waves, and ship impacts. Support configuration takes the form of a pile bent where the support members extend from the superstructure continuously below grade. Support members such as this are often termed 'pile-columns.' Resistance, within each bent, arises from pile-soil-pile interaction, and pile cap carrying capacity components. For pile foundation design, the current practice of NCDOT is to specify a long enough length to provide a point of fixity (along the length of the pile) on the basis of 1 inch lateral deflection at the top of the pile bent. The pile bent is then analyzed using frame analyses to estimate bending moment and shear forces for structural design. An equivalent length K-factor of 2.1 is assumed in the longitudinal direction (assuming free head conditions) while a K-factor of 1.2 is used in the transverse direction (assuming translation with no rotation). The shear and moment computations using frame analyses do not consider the presence of the soil around the pile from the point of fixity to the ground surface. In addition, the one-iteration moment distribution analysis does not properly take into account the impact of pile cap rigidity on the distribution of the applied load to each pile within the bent. As has been recognized by NCDOT, such limitations are excessively conservative and lead to costly specifications.
The objective of this study is to develop improved analysis criteria for design of pile bents in a cost-effective manner while maintaining adequate safety margin. Specifically, the work will be focused on the following aspects: i) Evaluating the impact of the pile cap rigidity in the transverse direction, and superstructure rigidity in the longitudinal direction to loads transferred to each pile within the bent, ii) Specifying appropriate K-factors for pile analysis under varying boundary conditions appropriate to the problem geometry and boundary connection scenarios, iii) Proposing appropriate ""point of fixity"" taking into account the presence of the soil around the pile from the ground surface down to the point of fixity, and, iv) Proposing appropriate values for maximum lateral deflection and pile shear capacity for various structure types and soil properties in order to achieve 'uniform risk' designs. The proposed work will be accomplished through a combined structural and geotechnical effort as the scope is truly an issue of soil-structure interaction. The scope will also require both detailed finite element analysis, and simplified frame analysis, for assessment of design recommendations. The analysis model will be developed using the finite element analysis software, ANSYS, which will be utilized to build model for the pile bent with the surrounding soils. The use of other programs such as FL -Pier will be explored once their robustness is confirmed. The computer programs Response 2000 and ANDRIANNA will be utilized to model the structural cross-sections and provide details required for implementing a simplified 2D plane frame analysis for estimation of pile forces, moments, and lateral deformations, as well as for aiding in the estimation of pile member limit state. While these advanced computer programs are available in the market and will be utilized for analysis of the pile bent, columns and foundation in this proposed work, it is not the intent of this research to change the current practice tools of NCDOT, but rather to explain limitations and opportunities and recommend ways for using NCDOT tools more efficiently."
