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.