What is the difference between FIXED and ENFORCED type supports in STAAD.Pro ?
When the FIXED or FIXED BUT type of support is specified, for those degrees of freedom which are to be treated as fully restrained, STAAD assumes the displacements to be known quantities whose value is zero. Hence, those degrees of freedom are not considered for the stiffness matrix, and their value is not calculated. (For the FIXED BUT support type, if any degrees of freedom are assigned a spring using the KFX thru KMZ terms, those are treated as unknowns and their displacements are calculated).
When the ENFORCED or ENFORCED BUT type of support is specified, for those degrees of freedom which are to be treated as restrained, STAAD assigns a spring with a very high spring stiffness - translational stiff springs for translational restraints, and rotational stiff springs for rotational restraints. Those degrees of freedom which are free to displace are assigned springs of zero stiffness. Hence, for all the 6 degrees of freedom at such supports, the displacements are calculated. The ones which have a stiff spring will end up having a negligible displacement, and those with zero-stiffness-springs will have a displacement based on the structure geometry, properties and loading.
The following are some of the ramifications of using the two support types.
1) Even though the two types follow different approaches, the end result - joint displacements, member forces and support reactions - should be the same.
2) Since the number of unknowns is less in the former method compared to the latter, the time required to compute the results will also be comparatively less using the first method. If the model is large, there may be a significant reduction in time required to perform the analysis.
3) For supports directly attached to plates or solids, if the supports are subjected to artificial displacements (called sinking support or support settlements), the effects caused by such displacements can be computed only if the ENFORCED type of support is used. In STAAD, support displacement loads cannot be applied to such models when the FIXED type of support is specified.
4) For the ENFORCED BUT type, there is no provision to specify a spring constant. The only choices, which are built into the program, are - zero stiffness or very high stiffness. So, if a user plans to assign his/her own spring constant, the FIXED BUT type is the only choice.
5) If the engineer plans to analyze a model in which support conditions change depending on the loading condition (hence he/she specifies the support command multiple times in the model), there are a few restrictions on how the supports must be assigned. These are mentioned in Section 5.38 of the Technical Reference manual where the CHANGE command is described. The ENFORCED support may offer a way to overcome some of those restrictions.