Applies To
	Product(s):	STAAD.Pro
	Version(s):	CONNECT Edition
	Environment:	N/A
	Area:	Steel Design
	Subarea:	EN 1993-1-1:2005
	Original Author:	Modestas Turulis, Bentley Technical Support Group

 

 

The calculated buckling moment resistance in STAAD as per EN 1993-1-1:2005 is less than the actual calculated buckling resistance compared with Blue book capacity. Why?

 

The buckling moment resistance (Mb) in the lateral torsional buckling check is calculated by following the section 6.3.2.1 of EN 1993-1-1:2005. As can be seen in the equation 6.56, buckling moment (Mb) depends on the Elastic Critical Moment (Mcr):

EN 1993-1-1:2005 does not, however, specify a method to evaluate Mcr, so STAAD will use the NCCI document SN-003a-EN-EU for doubly symmetric sections. This is a general equation from this document to calculate Mcr:

One of the most common reasons on why the Buckling Moment resistance (Mb) or Elastic Critical Moment (Mcr) calculated by STAAD.Pro differs from other applications or hand calculations is the use of the Zg value, which is described as follows:

This value in STAAD.Pro is controlled by the design parameter ZG, which, by default, is set to a value of D/2 (D - depth of the section) in STAAD.Pro:

This is a more conservative approach with the load applied at the positive value of half the depth from the shear center, which is a destabilizing load condition:

SCI publications and some other applications usually use this value as 0, meaning that the load is applied at the shear center (stabilizing condition). So in order to get same results between STAAD.Pro and manual calculations or some other applications, assign the design parameter ZG 0 to the required member or make sure that the same Zg value is used in all instances.