SAP2000 V12 uses large stiffness for all links and springs representing rigid constraints or restraints. This avoids the instability problem that can occur when constraints become coupled. SAP2000 allows coupled constraints, and they often work fine, but not always. For the fixed links at the bearing, the use of stiff rather than rigid properties also allows the program to calculate forces that can not be calculated when using fixed links.
The downside of using large stiffnesses is (1) determining how big is big enough to act as fixed, but (2) not too big to cause numerical problems, such as lost digits of accuracy.
The BrIM of SAP2000 V12 uses the stiffness of a cubic meter of concrete, multiplied by about 1e4. This choice is possible because we have some idea as to the size of a typical bent cap or abutment, to which these stiffnesses connect. Thus in kN-m units, AE/L = 1 * 1 * 20e6 / 1 * 1e4 ≈ 1e11. Other stiffnesses such as AG/L, EI/L, EI/L3, etc, are all of about the same order in kN-m units. The value of 1e11 kN/m is used for "rigid" translational stiffnesses, and 1e11 kN-m/rad is used for all "rigid" rotational stiffnesses, converted to the current units.
For other types of structures, you have to make your own determination as to the values to use.
Note that in the bridge modeler, you can specify your own link properties for bearings and foundation springs instead of using the fixed option. Our recommendation is to use reasonably large stiffnesses instead of fixed DOF, to avoid coupling with other links or rigid body constraints used in the model.
Bridge Analysis
Constraint
Link