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The [bridge modeler|Bridge Modeler] creates rigid [restraints|kb:Constraint] and [constraints|kb:Constraint], such as with fixed [bearings|kb:Bridge bearings], by applying large stiffness values to springs and other [link|kb:Link] 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 usersprecaution should takebe precautiontaken.

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Uncertainty exists when determining stiffness values which are large enough to simulate [rigid behavior|kb:Rigid behavior]. Values must be sufficiently large, but not so large as to cause numerical problems, such as withor [lost accuracy|kb:Lost accuracy]. {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software calculates rigid stiffness as the product of 1e4 and by multiplying the stiffness value of a unit- volume of concrete by 1e4. 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/L{_}{^}2^, _cEI/L{_}{^}3^) are on the same order of 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.

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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|kb:Coupled constraints] behavior and numerical convergence, we recommend that users {color:#ff0000}simulate rigidity by using reasonablysufficiently large stiffness values{color} on the order of *1e11 kN-m* (or kN-m/rad) forto model the rigid behavior of concrete systems.


h1. See Also

* [Rigid behavior|kb:Rigid behavior] article