Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Steel Design Codes
	Subarea:	AISC
	Original Author:	Bentley Technical Support Group

Why does the critical load case, along with the design forces/moments, change when I turn the slenderness check off and on?

When a code check is performed on a member, the first thing the program does is perform the slenderness check by calculating the KL/r values to ensure they do not exceed certain allowable limits. If the member fails this check, no further calculations are done for that member.  In this scenario, the design output reports the critical load case as being the first load case.  The design forces/moments correspond to that load case.  

If the slenderness check is then turned off (typically done using the MAIN parameter), the program does not check to see if the KL/r values exceed the allowable limits.  It instead proceeds to the strength checks, making sure to perform them for all load cases until determining the one that produces the highest interaction ratio.  The design output reports this load case as critical, and all the design forces/moments will correspondingly be produced by this load case. 

Consequently, should a member fail in slenderness, it is expected that the critical load case and design forces/moments will change for that member when the slenderness check is toggled on and off, going from the first load case checked in one scenario to the one deemed critical according to the strength checks in the other.