Rigid zones are used to model the physical overlap between columns/walls and slab, which prevents deformation of the slab at the column location. This also will have the effect of producing maximum design moments at face of columns/walls instead of center line of columns/walls. This in turn will produce less reinforcement and hence a more economical design, and allowed by most codes(i.e ACI-318). Deformations also will be more realistic in this case.
Should I apply rigid zones when checking punching shear?
Extended Question: I am finding large differences in the moment demands for punching shear when rigid zones are applied over columns. Is there an explanation for this behavior, and is there a recommendation for whether or not to include this option?
Answer: When the option to Include Automatic Rigid Zone Area Over Column is specified, the rigidity of the connection increases, causing deflection and slab reinforcement to decrease. Further, unbalanced moments generally increase, leading to higher punching shear ratios.
While this modeling decision is at the discretion of the engineer, we generally recommend including the rigid zones over column objects.
How do I eliminate the negative moment at the face of walls?
Answer: To decrease or eliminate negative moments, remove the rigid areas located over columns and walls, and uncheck the Wall Takes Out-of-plane Moments option. When walls are not unidirectional, or when they do not follow the direction of design strips, slab continuity will cause some negative moment which is usually small or negligible.
Results may be further improved by decreasing the maximum mesh size.