By default, beam elements transfer bending and torsional moments at their nodes. To prevent the transfer of moments at a node, an element’s fixity can be adjusted.
If an element’s fixity is changed to be a pinned connection, no rotational restraint is given to the element. One could picture this a ball joint.
A torsional fixed end condition is the same as a pinned connection except that rotation about the element axis is prevented.
Consider the simple beam layout example in Figure1:
Figure 1: Simple beam layout.
Note that all supports are translation support only (XYZ).
If the grey beam is not meant to transfer the generated moment, the end conditions need to be released. If the end condition is changed to a torsional fixed end condition (xy release), the analysis is successful and the output corresponds to what is expected (shown in Figure 2).
Figure 2: Moment diagram, torsional fixed end condition.
If, however, the end condition is changed to a pinned end condition (xyz release), the analysis is unsuccessful. The error message indicates that there is an instability in rotation about the X axis.
Figure 3: Error message: X-rotation instability
Since there is a pinned support at Node 1, and a pinned end condition is defined, there is no restraint about the element’s own axis. The torsional fixed end condition prevents the rotation about the element’s own axis.
The Frame help file gives the following suggestion:
Unless you have a try ball-joint at a certain point, it is usually beneficial for the model's stability to use torsional pins (type 'T' or 'xy') instead of full pins (type 'P' or 'xyz'). In practice, even the simplest of connection typically allow some torsional restraint, adding some justification to doing this.