Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis Solutions
	Subarea:	Miscellaneous Analysis Solutions
	Original Author:	Abhisek Mandal, Bentley Technical Support Group

 

When using I section or C section, values of MY/MZ remain the same, irrespective of the section type chosen. However for angle sections, the values of MY and MZ appear to be different. Why is it so?

Also when we are using beta as 45 degrees to align our angle section, program distributes the bending Moment about the local Y and Z axis. How to interpret this result correctly?

 

For any angle section there are 4 axes present as shown in above diagram. The U-U and V-V are the principal axis and X-X and Y-Y are the geometric axis as per the above diagram. Now in STAAD we consider the moments as per local axis orientation and by default the local axis orientation in STAAD for single angle aligns with the principal axis. So,

STAAD Local Z-Z = Principal axis V-V

STAAD Local Y-Y = Principal axis U-U

STAAD always considers this above relation and design the local moments considering the capacities or section properties in those local directions. You can validate the same from the section properties used by STAAD in design. Even if you apply beta angle these values are not changed.

Now if we see the local axis orientation of the angle section for 2 cases like with and without beta angle it shows something as below.

When there is no beta angle the local Y axis is aligned with Global Y axis and the load applied in Global Y direction (GY) will create moment in the plane of local Y axis only i.e. moment about local Z axis (MZ). Hence when there is no beta angle the MZ value is matching with other cases also and there is no My moment.

But when you are applying the beta angle as 45 degrees, the local axis orientation rotates as seen in above snap and in this case also the load in acting vertically downward. The load will cause same moment in vertical plane but as the local axis rotates STAAD tries to find the component of that moment in those local axes and designs them with the capacities along local axis. Hence the moments reported as MZ and My is M multiplied by Cosθ and Sinθ where θ is the beta angle.