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The [bridge modeler|kb:Bridge modeler] creates [rigid constraints|kb:Rigid behavior] (restraints), such as with fixed [bearings|kb:Bridge bearings], by applying large stiffness values to [link|kb:Link] and [spring|tp:Line and area springs] objects. This allows the software to calculate forces which may not be computed when links are bound by truly [fixed DOF|kb:Fixed DOF in links]. This also avoids instability issues which may arise when constraints are coupled. [SAP2000|sap2000:home] and [CSiBridge|csibridge:home] provide for coupled constraints, though users should take precaution.

Uncertainty exists when determining sufficiently large stiffness values. Values must be large enough to simulate fixity, but not so large as to cause numerical problems, such as with [lost accuracy|Lost accuracy]. {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software assigns fixity stiffness as the product of 1e4 and the stiffness of a unit-volume of [concrete|kb:Concrete]. Following this computation for a typical bent cap or abutment, _AE/L_ = 1e4 * (1 * 1 * 20e6 / 1) ≈ 1e11 (kN-m). Since other stiffness components (_AG/L_, _aEI/L_, _bEI/L{_^2^}{^}2^, _cEI/L{_^3^}{^}3^) are of comparable magnitude, {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software uses *1e11 kN-m* for rigid translational stiffness and *1e11 kN-m/rad* for rigid rotational stiffness.

In the bridge modeler, users may specify link properties for *bearing and foundation springs*, rather than using the option for fixity. To avoid rigid body constraint and coupling with other links, {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} recommends reasonably large stiffness values on the order of 1e11 (kN-m) for concrete systems, and not true fixity.