Rotation of Angle Members

I'm no expert with STAAD (I assume this is a STAAD question), but for asymmetrical members such as single angles there are two relevant axis systems. The principal axes (e.g. major and minor axes) which are skewed to the angle of the angle legs, and the axis defined by the two legs.

The STAAD help clarifies as follows:

Using BETA ANGLE and RANGLE
Single angle sections are oriented according to their principal axes by default. If it is necessary to orient them such that their legs are parallel to the global axes, the BETA specification must be used. STAAD offers the following additional specifications for this purpose:

• BETA ANGLE
• BETA RANGLE


Both of the above options will result in an orientation with the legs parallel to the global axis. The ANGLE option rotates the section through the angle (90° - α) (where α = angle between the principal axis system and the geometric axis system of the angle). The RANGLE option rotates the section through an angle equal to (180° - α). For unequal angles, the right option must be used based on the required orientation.

Certainly, the rotation of the angle relative to the direction of the load will impact the capacity, interaction ratio, etc.

If you have a case where the physical rotation of the angle using the two methods is the same, but the capacity is different (for the welds or whatever) then you might want to post a copy of the test file here for the experts.

 

What I've come to realize is this: The animated model shown by STAAD is not necessarily the same model STAAD analyzes.  I say this because if you were to interchange "BETA 45" in your input file with "BETA ANGLE", the picture would be the same, leading one to believe STAAD recognizes that command and will analyze it as shown.  But that it does not.  That rotation is never taken into account in the analysis and that is the reason for difference in results.  I've come across this problem with a few other items so I think it is a programming issue that should be addressed by STAAD in future versions.  So as far as how to use BETA ANGLE and BETA RANGLE, apparently choosing them from the pull down window options is the only acceptable way of using them.

What I've come to realize is this: The animated model shown by STAAD is not necessarily the same model STAAD analyzes.  I say this because if you were to interchange "BETA 45" in your input file with "BETA ANGLE", the picture would be the same, leading one to believe STAAD recognizes that command and will analyze it as shown.  But that it does not.  That rotation is never taken into account in the analysis and that is the reason for difference in results.  I've come across this problem with a few other items so I think it is a programming issue that should be addressed by STAAD in future versions.  So as far as how to use BETA ANGLE and BETA RANGLE, apparently choosing them from the pull down window options is the only acceptable way of using them.

Let us try to understand the feature with the help of a simple example file which is attached.

The file contains 3 members. All of these have been assigned an angle section L8x4x1 ( STAAD name : L80X40X16 ). From AISC 13^th edition table on page 1-40, for an L8X4X1 section,  the tana = 0.247 which means a = 13.874 degree (approx).

By definition, ANGLE option rotates the section by 90 - a. That means an ANGLE assignment should be the equivalent of assigning a section, a beta value of 90 - 13.874 = 76.126 degree    

Here are the details of the beta assignments for the members :

Member # 2 : Manually a beta angle value of 76.126 degree has been applied to the member

Member # 3 : "ANGLE" specification is applied to the member. Please follow the steps listed below to assign "ANGLE" specification correctly. STAAD automatically calculates the beta angle 76.1133 degree and writes the information in the input editor

Member # 4 : No beta angles assigned. So by default it has a beta angle value of zero.

Steps to assign the "ANGLE" specification

From within the Properties Whole Structure box, go to the Beta Angle tab

Click on "Create Beta Angle".

Choose the radio button "Angle" and click on OK.

You will see a new item listed in the window called "Beta Angle".

Select it.

Select the member to which you want to assign this specification.

Once the member is selected, the Assignment method would automatically get set to "Assigned To Selected Beams".

Click on Assign.

Note that the actual value of the beta angle corresponding to the Angle assignment now shows up in the window. If you open the input editor, you will see the angle assignment getting written down in terms of the actual beta angle value

 

Finally same load is applied and same supports defined for all members. When the member forces are examined after analysis, the member end forces/member section forces are found to match exactly for members 2 and 3. This is because the ANGLE specification is identical to the beta 76.126 . However the forces for member 4 are different and rightly so because the orientation of member 4 is not identical to the other two members.    

I have a question about loading on single angle (beta is applied). when i assign a point load to member in local axis then load is shown in principal axis. Applied load is not parallel to any other global axis. then how can i applied wind load as point load ?

thank you for help.......