Rotation of Angle Members

Question

I have a couple of questions concerning the rotation of singe angle members. Which command should I use to rotate these angle members? "Beta Angle" or "Beta 45" (or 135 or 225 or whatever your required angle is)? I pose this question because I would have thought the results would have been the same but they are not. The results for "Beta Angle" actually match those of an angle member modeled along its principal axis with no rotation or "Beta 0". I also would have thought the properties for the weld design, namely Sz, Sy, CH & CV, would have changed with this "Beta Angle" command since the member end forces are no longer being applied in the same manner to the weld. Any thoughts?

Sye · Answer

Your understanding of the beta angle is correct. However for analysis, you do not need to find out Iy or Iz about global axis. All you need to do is specify the loads along the global axes and NOT along member local axes. The software would automatically account for the correct stiffness in each global direction based on the beta angle specification.

Sye · Answer

You can get member forces printed in terms of global directions by using PRINT MEMBER FORCES GLOBAL LIST ... in the command file. 
 The same can also be set using the GUI as shown next

Kris Sathia · Answer

The OpenSTAAD functions that extract the member forces and moments provide them only along the principal axes. Knowing the Alpha value which is 45 for an equal leg angle, you need to transform them to the geometric axes of the angle.

Kris Sathia · Answer

By the way, for the AISC 9th edition, using the parameters AXIS and FLX, you can state that the angle should be designed on the basis of its geometric axes. The program will transform the principal axes moments to the geometric axes before performing the design.

Sye · Answer

I will need to know more details to troubleshoot. What is the service request or service ticket number ? 
 
On a general note, single angles are oriented such that the principal axes are oriented parallel to the global planes ( beta 0 orientation ). You can assign beta angles to the angles such that the legs are oriented parallel to the global axes. Loads can be applied in global directions. Anyway send the specific details with the model and we will assist you.

Sye · Answer

Local axes are always aligned with the principal axes of cross sections. There is no way to orient the local axes for angles so that it is aligned with the legs.

Sye · Answer

Although you cannot change local axes for angles and align it with the geometric axes but you can always orient angles so that the legs are aligned with the global axes. This can be done using the beta angles. Loads can also be applied along the global directions for the scenarios where your angle legs are aligned with the global axes. For angles which are inclined to the global planes you would need to resolve the load in the global directions and apply it that way. As far as design of single angles is concerned, design as per geometric axes is only allowed for certain limited cases whereas you can always design single angles based on principal axes as that is the more generic case. In addition, our design code implementations are equipped to automatically check if the member can be designed as per geometric axis or not and if so, design as per both principal and geometric axes is carried out. I am specifically referring to the AISC 360-16 specifications here but I believe other international codes also impose conditions on design of angles as per geometric axes as well.

Sye · Answer

Since for angle sections the principal axes ( and hence local axes) are not parallel to the legs, you do not need to match the alignment of the local axes for angles with the global axes because if you do so, the angle legs would not be aligned with the global axes. Rather you would want to align the legs with the global planes which is where the beta angle assignment comes into the picture. You can rotate the angle by any rotation angle about it's local x axis using a beta angle which would give you the ability to align angles as you want it to be. 
 If you need to apply wind loads at certain angle to the global direction ( horizontal), there is no direct way of doing that in STAAD.Pro. The wind load needs to be resolved into two components along the two global directions and applied that way. 
 Also I would like to add that the wind load generation ( Open Structure option ) can take into account the orientation of angles when it calculates the member load. Based on the orientation of the angle section, the software would figure out the area of obstruction and calculate the udl on the member accordingly.