General node restraint

The general restraint facility allows you to apply a restraint to all otherwise unrestrained nodes.

For example, if you have a frame with two pin based supports and you want to prevent all translations in the Z direction for all of its other nodes, you could apply restraints of FFFRRR to the two support nodes and specify a general restraint of RRFRRR.

In order to input a general restraint, you simply apply the desired restraint to any unrestrained node and then select "Yes" in the General Restraint column in the Node Restraints datasheet.

Using a general restraint saves data entry time and reduces the quantity of printed output. Note that output reports only show the general restraint code on one node, even though the analysis has assumed that it applies to all unrestrained nodes.

! IMPORTANT NOTE !

The general restraint facility should be used with great care and only if you are absolutely sure of the effect it has on your model! If you apply a general restraint early in the development of your model and then forget that it exists at some later stage when it is no longer appropriate, you could be over-restraining your model. This could happen if nodes are added that shouldn’t get the general restraint. It could also happen if you initially use a general restraint to prevent all out-of-plane movements in a 2D frame for example and then extend the frame to 3D and forget to remove the general restraint. The preferred alternative to using a general restraint is the fix/release node degree of freedom tool.

$image\ebx_-1773499217.gif$ If you have applied a general restraint and require some nodes to not have a restraint at all, you can prevent them from getting the general restraint by restraining them with a code of RRRRRR.

Important note about restraining 2D frames

It is common practice amongst some engineers to restrain all out-of-plane movements in 2D frames. While this is generally appropriate for static analysis (provided there are no out-of-plane loads), it may not be appropriate for buckling and dynamic frequency analyses. This is because the frame may buckle or vibrate in an out-of-plane direction even though there are no loads in that direction. Of course, nodes that are braced in the out-of-plane direction should be restrained in that direction, however nodes that can move out-of-plane in the real structure should not be restrained in that direction in the model. Failure to do this could affect the buckling load factors, effective lengths and dynamic natural frequencies and mode shapes, and could result in unsafe designs.

For example, if a 2D frame rafter is sub-divided, the intermediate nodes should not be restrained in the out-of-plane direction unless they are braced in that direction in the real structure. Restraining them would prevent any out-of-plane buckling or vibration modes that could occur if the rafter member hadn’t been sub-divided.

Another example is a pin support for a 2D XY-plane frame column base which could be modelled with the standard 2D pin base restraint code of FFFFFR, however this would prevent rotations about the global X-axis. In reality, a column pin support would probably allow rotations about both horizontal axes and hence a restraint code of FFFRFR would be more appropriate. Restraining the rotation about the X-axis would affect the out-of-plane buckling and vibration modes of the column and could result in incorrect results.