In the following discussion, the "segment length" is the distance between two adjacent cross sections that are restrained or between a cross section that is restrained and the end of the design group. A cross section is assumed to be restrained when a full or partial restraint is applied to either flange or when a lateral restraint is applied to its critical flange.
Member design segments
The design group in the diagram above consists of three analysis members of different lengths. The group has full or partial restraints at the ends and three equally spaced lateral restraints on the top flange. For the bending moment diagram shown, the first top flange lateral restraint is ineffective because the bottom flange is the critical flange at that point. Thus, the first segment continues past the first top flange restraint to midspan where the top flange has become the critical one.
When determining the effective restraint at a cross section, SPACE GASS looks at the restraint applied to the critical flange, however it also looks at the other flange to see if a restraint has been applied to it and, if so, whether or not it affects the cross section restraint. Thus, the effective restraint for the cross section can be dependent on the restraint applied to both flanges.
In the following table, the "critical flange" is as per the critical flange definition, the "other flange" is the non-critical flange and the "effective restraint" is the cross section restraint that SPACE GASS uses in the code check.
For restraint type definitions see also Flange restraints. A C (continuous) flange restraint is assumed to be equivalent to a series of L (lateral) flange restraints spaced at increments of 1mm for the entire length of the continuous restraint.
|
Restraint on Critical Flange |
Restraint on Other Flange |
Effective |
|
None or U |
None or U |
None |
|
L |
None or U |
L |
|
P or F |
None or U |
F |
|
S or R |
None or U |
R |
|
None or U |
L |
None |
|
None or U |
P or F |
P |
|
None or U |
S or R |
S |
|
L, P or F |
L, P, F, S or R |
F |
|
S or R |
L, P, F, S or R |
R |
For single angle sections, it is unclear whether or not lateral restraints applied to either leg are effective in providing any restraint to the section. Consequently, you should be very careful when applying lateral restraints to single angle sections and you should use them only if you are sure they are effective in restraining the section. SPACE GASS will apply them if you specify them and so the decision about whether or not they should be used is entirely up to you.