The SAP2000 bridge modeler creates rigid restraints and CSiBridge create rigid constraints (restraints), such as with fixed bearings, by applying large stiffness values to springs and other link and spring 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 precaution should take precautionbe taken.

Uncertainty exists when determining sufficiently large stiffness values which are large enough to simulate rigid behavior. Values must be sufficiently large enough to simulate fixity, but not so large as to cause numerical problems , such as with or lost accuracy. CSI Software assigns fixity stiffness as the product of 1e4 and the software calculates rigid stiffness by multiplying the stiffness of a unit - volume of concrete by 1e4. Following this computation for a typical bent cap or abutmentformulation 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^2L², cEI/L^3L³) are on the same order of comparable magnitude, CSI Software 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, rather than using the option for fixity. To avoid rigid body constraint and coupling with other links, CSI recommends reasonably problems with coupling behavior and numerical convergence, we recommend using sufficiently large stiffness values on the order of 1e11 kN-m (or kN-m) for /rad) to model the rigid behavior of concrete systems, and not true fixity. .

See Also

• Rigid behavior article