Autodesk Robot Structural Analysis

[#1]

The Gamma Angle was in fact 90 degrees. I defined it as 0 in the "New section dialogue", not sure why it was 90 degrees in the table.  Anyway......

As your previously created user defined section had been rotated by 90 while you saved it in the user database you had to rotate it by another 90 deg to have it correctly displayed in the model. Then you replaced the section with the one that wasn't rotated while saved in the database but the 'additional ' rotation you defined remained.

Please mark the values that you found different and send me your user section database as well. Thank you.

[#2]

As requested.

[#3]

Hi Artur,

The properties that are not being displayed correctly are, Ay, Az and I think  B. 

The elastic section modulus and radius of gyration do not require definition in the user database, but i see it displaying in the detailed calculation. So i'm not sure if Robot is calculating it correctly.

Vpy and Vy are defined as the distance to the extreme fibres. Are those used to define or vet the area of shear capacity as well?  I'm just worried it will overestimate horizontal shear capacity.

[#4]

Looking at the pictures you attached I think that some values have been incorrectly defined. Assuming that the basic section is UB 914x305x289 you can see that the value of AX for your used defined section is the same as for the single UB section which seems to be wrong. The values of Ay and Az are close to the doubled areas of flanges and web but the steel design module follows the code formulas and calculates them itself. As you defined thickness of the web and flanges as for the single UB section then the values of Ay and Az in the steel design note correspond to the values for the single UB section. To change that the thickness of the web should be 2*19,5 mm and the thickness of the flange should be 2*32mm instead. Perhaps you could try another possibility that I have just thought about. It is possible to use a parametric section with additional information for the steel design module as shown on the attached picture.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[#5]

Hi Artur,

I have tried your suggestion to use a parametric section with no luck.  As you can see from the attached print screen, Robot infers the D and B of the section based on Vy+Vpy and Vz+Vpz respectively. I have tried switching Vy+Vpy and Vz+Vpz, however Robot will always infer D to be the greater of Vy+Vpy and Vz+Vpz.

Robot assumes the major axis of this particular compound strut arrangment to be parrallel to the ground, which is not applicable to this strutting system.  This assumption holds true regardless of how you draw the members, or rotate the members in the member definition dialogue. 

Therefore, assuming a user is willing to model (has already modelled)  the struts and lacing, robot can do:

1.  Lateral torsional buckling checks for an individual strut (local check between lacing) assuming you defined the LTB restraints correctly.  Robot is unable to do global lateral buckling checks about the compound struts major axis check regardless of restraint definition, because it cannot calculate the correct radius of gyration, due to the limitations I gave above.
2. Local buckling check (i.e. between lacing) for each strut about its minor axis (i.e. axis perpendicular to the ground) as long as you define the restraints correctly.  Robot is unable to do global buckling about the compound struts major axis check regardless of restraint definition, because it cannot calculate the correct radius of gyration, due to the limitations i gave above.
3. Local buckling check for each strut about its major axis (i.e. axis parrallel to the ground).  

I look forward to the version of robot that addresses this limitation.

Thanks for all your help.

[#6]

Would it be possible for you to send me the sample hand verification of LTB (with the reference to the code (I assume BS)  paragraphs) as you would like to have it in Robot please?