The pier cap is defined by dimensions, i.e., size, start and ending elevation, skew angle, and factor of reduced moment of inertia. There are five types of pier caps: Hammerhead, Straight, Tapered, Variable and Integral. Regardless of the type of cap selected, the program will define sections that are symmetrical about the centerline of the cap in the Z-direction. For hammerhead or multiple column pier, the bearing points sit on the top of the cap. The eccentricity of the bearing points can not be more than half of the cap width. For integral pier, girders are monolithic with the pier cap and their connection is considered to be at the centerline of the cap.

You can add columns to the pier structure, except for hammerhead structures. The shape of the column can be rectangular, round, tapered, rectangular chamfered, hexagonal and octagonal. Columns could also comprise of several segments with linear or parabolic variation for each segment. You can also add a drilled shaft to the column. The minimum and maximum column sizes are 12 in. and 1000 in., respectively, for U.S. units, and 300 mm and 25400 mm for metric units.

There are two types of drilled shaft available: circular and rectangular. The section of a drilled shaft is defined by the width and depth or diameters. To determine the location of a drilled shaft, the program uses the total height of the drilled shaft and the partial height of the drilled shaft below fixity point.

A drilled shaft is divided into two portions: above fixity point and below fixity point. The drilled shaft is fixed (no displacements and rotations) at fixity point, in the frame analysis. The specifications for the structural design of the drilled shaft are in accordance with AASHTO LFD Art. 4.6.6 and AASHTO LRFD Art. 10.8.3.

Substructure takes the portions of the drilled shaft below fixity point as a member supported laterally. In this case, only the axial load is considered. For the portion of the drilled shaft above fixity point, Substructure assumes there are no lateral supports. Therefore, it is designed exactly the same as a reinforced column, taking into consideration both axial load and moments.

There are four footing types available that you can design for each column or combination of columns: spread, pile/shaft cap, combined, and strap. The forces and moments at the bottom of the column (or top of the footing) are used in the footing design. To calculate the bearing pressure or pile reactions, the assumption of infinite rigid footing is assumed.

By default columns are fixed at bottom. However, program allows users to specify a spring matrix at the bottom of every column if partial fixity is desired. Program provides two options for this. First option allows to define diagonal entries in the spring matrix. This is useful when uncoupled springs are to be defined. User may specify any or all of the KxKx, KyKy, KzKz, RxRx, RyRy or Rzz spring values. First three correspond to transitional DOF and the last three to rotational DOF. If full restraint is required for any DOF, corresponding K value should be set equal to zero. To model no restraint, specify a value greater than zero but very small value.

For example to model a 3D pin condition at column bottom, specify KxKx, KyKy, KzKz equal to zero and RxRx, RyRy, RzRz equal to 0.01. When data from versions prior to 4.2 is read in with a single Ky spring, program reads that in as a diagonal matrix with KyKy value. All other values are set equal to zero.

The second option available for user is to specify the full 6x6 stiffness matrix. This can be used to model fully coupled springs as would be the case of a pile cap resting on a group of piles. If such matrix is available, once specified, program will use the stiffness in analysis.

In the spring stiffness matrix, negative values are not allowed for leading diagonal entries. However, off diagonal terms in full stiffness matrix may be negative.

When Spring value greater than zero is specified for a DOF, program computes the displacement at that node and reports on analysis screen.