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h2. General approach

* It may be best to begin with a simple model, then gradually build upon its sophistication.

h2. Release only one end

* Ensure that releases are not assigned to both ends of [frame|kb:Frame] objects. Releasing both ends may keep load from being able to transfer, leading to [numerical problems|kb:Numerical problems]. Information on numerical problems is available using the Standard Solver, available through Analyze > Set Analysis Options > Solver Options. Check for instability warnings in the log file by selecting File > Show Input Log Files.

h2. Avoid full fixity and full rigidity

* Rather than applying full fixity to DOF, and full rigidity to structural objects, restrained and [rigid|kb:Rigid behavior] conditions should be modeled using sufficiently large stiffness values, perhaps on the order of 1e11. Stiffness values may be assigned using spring constants, [property modifiers|kb:Property modifiers], or section properties. Full fixity or full rigidity may cause numerical sensitivity in linear solutions, and singularities in [nonlinear|kb:Nonlinear] numerical formulation, leading to convergence problems or instabilities.

h2. Assign mass and inertia 

* During dynamic analysis, ensure that [mass|kb:Mass], inertia, and perhaps rotational inertia are distributed throughout the structure. Mass and inertia couple with dynamic behavior as an integral part of mathematical formulation. When [links|kb:Link] are located at support points, inertia either may be assigned using the link property, or it may be added directly at the [joint|kb:Joint]. Rotational inertia is added by assigning a mass value between ^1^/~10~ _md_ ^2^ and ^1^/~100~ _md_ ^2^, where _m_ is a characteristic mass located a characteristic distance _d_ from the support.


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* *General approach*. It may be best to begin with a simple model, then gradually build upon its sophistication.

* *Release only one end*. Ensure that releases are not assigned to both ends of [frame|kb:Frame] objects. Releasing both ends may keep load from being able to transfer, leading to [numerical problems|kb:Numerical problems]. Information on numerical problems is available using the Standard Solver, available through Analyze > Set Analysis Options > Solver Options. Check for instability warnings in the log file by selecting File > Show Input Log Files.

* *Avoid full fixity and full rigidity*. Rather than applying full fixity to DOF, and full rigidity to structural objects, restrained and [rigid|kb:Rigid behavior] conditions should be modeled using sufficiently large stiffness values, perhaps on the order of 1e11. Stiffness values may be assigned using spring constants, [property modifiers|kb:Property modifiers], or section properties. Full fixity or full rigidity may cause numerical sensitivity in linear solutions, and singularities in [nonlinear|kb:Nonlinear] numerical formulation, leading to convergence problems or instabilities.

* *Assign mass and inertia*. During dynamic analysis, ensure that [mass|kb:Mass], inertia, and perhaps rotational inertia are distributed throughout the structure. Mass and inertia couple with dynamic behavior as an integral part of mathematical formulation. When [links|kb:Link] are located at support points, inertia either may be assigned using the link property, or it may be added directly at the [joint|kb:Joint]. Rotational inertia is added by assigning a mass value between ^1^/~10~ _md_ ^2^ and ^1^/~100~ _md_ ^2^, where _m_ is a characteristic mass located a characteristic distance _d_ from the support.
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