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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. |
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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/ |
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L_^2^, _cEI/ |
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L_^3^) are of comparable magnitude, CSI 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, 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. |