I have set some fixity at the base of a column. Why have the deflections not reduced due to this increased base fixity?

Environment

Not environment-specific

Question

I have a small simple goalpost frame that I’m designing. I was trying to utilize the software's function to set the bases to be nominally pinned and assign a 20% rotational stiffness to limit the frame deflection. However, when I run the model its giving me the same deflection values as when I don’t assign the stiffness. Would you be able to explain to me why this isn’t working for me?

Answer

The most likely explanation is that pinned end releases have been applied to the bottom of the columns. This means the support fixity does not play any part, since the connection of the column to it is pinned.

For more about member releases, we recommend reviewing the article "(N-1) rule for the number of pins to apply at a node where N members meet." .

Solver View

If you switch to the Solver View you can turn On the Scene Content, 1D Elements > Releases to review where releases have been applied in the model.

To resolve you need to alter the fixity at the bottom of the columns to remove the release, by setting them as Fixed.

Comparing Deflections

Pinned Bases (fixed column releases)

Nominally Pinned Bases 20% (fixed column releases)

(N-1) rule for the number of pins to apply at a node where N members meet.