Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis Solutions
	Subarea:	Pdelta / Direct Analysis
	Original Author:	Sye Chakraborty, Bentley Technical Support Group

 

I am carrying out a PDELTA analysis and I get the following note

 

"** NOTE : MOMENT BALANCE DOES NOT CONSIDER SECONDARY EFFECTS OF P-DELTA OR DIRECT ANALYSIS"

 

What does the note mean and should I be concerned about this ?

 

Purpose of the note is to explain why the equilibrium check is not satisfied for moments in case of a second order analysis like PDELTA or DIRECT ANALYSIS. In a statics check or equilibrium check, the summation of applied loads are compared against summation of reactions. The applied loads are calculated by summing up all the external loads acting on the structure. In a second order analysis like PDELTA or DIRECT ANALYSIS, there are secondary moments which are generated due to the pdelta effects. However these are not considered as part of the applied load. The support reactions on the other hand include the effect of these secondary moments generated due to pdelta. Since the secondary moments can be visualized as a couple, the effect of it is not seen when it comes to summation of forces and hence equilibrium is still satisfied for forces. However for moments, this equilibrium is not satisfied as the applied load does not include the secondary moments whereas the reaction moments do. The note is intended to make the user aware of this and is a standard one. It is not generated due to any errors in the model and hence users need not be concerned about the note.