Member Restraint Conditions

Member Restraints allow a member to have its restraint defined at one location along its length. They allow specification of restraint in two different ways: by use of a relatively user-friendly dialog box, or by an abbreviated “mnemonic” text syntax.

Some of the attributes of a restraint being set in combination with others imply other restraints are also set, even though the user has not specifically stated that the additional restraint axes are restrained. For example, if both the top and bottom flanges of a member are laterally restrained, then it is clear that St. Venant torsion is also achieved at this location as well. Similarly, if the top flange is restrained and full St. Venant torsion restraint is also manually restrained, then the bottom flange is assumed also to have been restrained. With regard to warping restraint, a more conservative approach is taken, and minor axis bending restraint is not assumed to imply warping restraint of the flanges, nor is warping restraint assumed to imply any overall bending restraint of the section. This is because (using an example) although an RHS section may have its “major-axis” flanges welded to a stiff end plate (thus achieving flange warping restraint), the webs could be free. This would allow significant minor axis bending flexibility in comparison to a section that had been welded all round. Inconsistent end restraints are automatically updated to ensure that sensible conditions are set.

A table summarising the available restraint axes is shown below:

Restraint Axis	Possible values	Description	Comments
Top Flange Warping	Partial None Full
Bottom Flange Warping	Partial None Full
Torsion	Partial Full Friction/ Bearing on support only None	St. Venant torsion restraint	Full restraint is assumed if both flanges are laterally restrained.
Major Axis Rotation	Partial None Full	Major (stiff) axis restraint
Minor Axis Rotation	Partial None Full	Minor (weak) axis restraint
Top Flange Lateral	None Full		Full restraint is assumed if the bottom flange is laterally restrained and the full torsion restraint is also applied.
Bottom Flange Lateral	None Full		Full restraint is assumed if the top flange is laterally restrained and the full torsion restraint is also applied.
Major Axis Shear	None Full	Restraint of shear centre against movement in the direction associated with major axis bending.
Minor Axis Shear	None Full	Restraint of shear centre against movement in the direction associated with minor axis bending.	Full restraint is assumed if both flanges are laterally restrained.