The bridge modeler creates rigid restraints and constraints, such as with fixed bearings, by applying large stiffness values to springs and other link objects. This allows the software to calculate forces which may not be computed when links are bound by truly fixed DOF. This also avoids instability issues which may arise when constraints are coupled. SAP2000 and CSiBridge provide for coupled constraints, though users should take precaution.

Uncertainty exists when determining stiffness values large enough to simulate rigid behavior. Values must be sufficiently large, but not so large as to cause numerical problems, such as with lost accuracy.

Software calculates rigid stiffness as the product of 1e4 and the stiffness value of a unit-volume of concrete. Following this formulation for an axial stiffness component, AE/L = 1e4 * (1 * 1 * 20e6 / 1) ≈ 1e11 (kN-m). Since other stiffness components (AG/L, aEI/L, bEI/L², cEI/L³) are on the same order of magnitude,

Software uses 1e11 kN-m for rigid translational stiffness and 1e11 kN-m/rad for rigid rotational stiffness.

In the bridge modeler, rather than using the option for true fixity, users may specify link properties for bearing and foundation springs. To avoid problems with coupling behavior and numerical convergence, we recommend that users simulate rigidity by using reasonably large stiffness values on the order of 1e11 kN-m (or kN-m/rad) for concrete systems.

See Also

• Rigid behavior article