3-D Pile Group
The Static Analysis of Piles(PILEGP)will analyze a three-dimensional pile group subjected to static loadings. The direct stiffness approach, in conjunction with conventional matrix methods, is used to perform the analysis. The pile cap is assumed to be rigid with the remainder of the system demonstrating elastic behavior.
PILEGP will analyze up to 200 randomly placed piles. The piles may have varying section properties, bending stiffnesses, and lengths. Battered or plumb piles are handled by PILEGP and may be fixed or pinned to the rigid cap. Friction or end bearing piles also are considered by the program. When a subgrade modulus is specified, the piles experience lateral interaction with the soil. Loads are transmitted to the piles through the origin of the coordinate axis. Factors for Group Loadings may be specified.
Program output includes shears, moments, and axial forces for each pile for multiple load conditions. Deflections at the centroid of the cap as well as arbitrarily chosen points in space also are printed. An equilibrium check of the vertical forces as well as a summary of maximum and minimum axial loads and the governing load cases are output for each pile. Diagnostic information is displayed if abnormal loading or geometric conditions are encountered.
This program is intended to accommodate the majority of problems encountered in the design of pile groups. The general direct stiffness method for three-dimensional pile foundations is utilized. This allows for a variety of features and configurations which are described in this section. Also described are the limitations and difficulties which are inherent in the general topic of pile group design and pile-soil interaction.
- Pile Group Configurations — Three configurations are offered as options within the program:
a) Standard Pile Formulation, modeled as an infinite beam on an elastic foundation;
b) Cantilever Beam Foundation, where the top portion of the pile extends through a uniform material (with a finite value of the subgrade modulus allowed) to fixity;
c) Elevated Platform Formulation (e.g.,those used offshore where the piling extends above soil level).
- Boundary Conditions — A general set of conditions may be specified for any individual pile within the group or for the group as a whole:
a) Batter may be specified for any pile to any slope or orientation (including a plumb condition);
b) The pile at the cap may be either hinged or fixed;
c) The type of pile may be either friction or bearing;
d) The interaction of the pile with the subgrade may be considered or neglected through the specification of a subgrade modulus, ks.
- Coordinate System — No restriction is placed upon the location of any pile within the group. The individual pile positions are described by X, Y, and Z coordinates which allow for stepped pile caps. Although the origin of the coordinate system can be located anywhere, the best results are obtained when it is placed as closely as possible to the centroid of the pile group.
- General Description of Method — The pile foundation consists of a group of piles, covered with a rigid cap, placed into the soil. The general direct stiffness method is related to the system through the global coordinates to obtain stiffness of the entire foundation. This method allows for the analysis of numerous pile group types. The advantages of the method are:
a) The method yields forces and moments (including torsion) in the piles which, when summated with the applied loads, are in static equilibrium (this is not the case for many popular methods used for pile group design);
b) As a direct output, the method yields the deflections and rotations of the pile cap about the origin;
c) The method allows for the determination of a pile group itself being unstable (this is not to be confused with pile buckling);
d) Interaction with the subgrade may be considered in the analysis if either the pile group is unstable or if additional group capacity is required;
e) Pile stresses at the intersection of the pile cap and the pile can be obtained considering axial and shear forces, and bending and torsional moments.