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I have moved your last post to the new subject as it is a steel design topic now.
Looking at the screen capture you provided (I don't have the rtd file) I assume that you haven't assigned any member type to your bars (see the attached picture).
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Artur Kosakowski
You can get familiar with the procedure of finding the appropriate size of the cross section looking at the example 2 from the training manual available from http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=13093279.
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Artur Kosakowski
The slenderness limit for this profile has been exceeded. You should increase the size of the tube.
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Artur Kosakowski
For simplicity I would model only the 'outside' part of the element using different section and then I would create a new member to verify the capacity of the entire element as shown on the attached picture. I would not use the 'double tube' part as this is not the part where you expect the failure (double cross section). My understanding is that what you attempt to do is to find the right cross section so you should be in right to increase it when needed (or increase the thickness of the wall if there is some restriction imposed on the diameter). On the other hand you may decide that the slenderness limit of 210 is not to be applied in a particular design situation when this particular element is likely to be under tension only.
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Artur Kosakowski
Imagine a simply supported beam that is built from two different sections (one from the left hand support till the middle of the beam and then another from the middle till the right hand support). To model this situation in Robot you have to use two different bars (No.1 and No.2) but to verify the beam's capacity or deflection you still would like to 'see' it as a single element. As you know the verification in Robot is done according to the bar type assigned to the element so you need to assign appropriate bar type (that has e.g. the correct buckling length) to the entire beam. This is why I created the artificial new bar (No.4) that stands for bar 1 + bar 2. Now I can assign the appropriate bar type to bar No.4 and run its verification.
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Artur Kosakowski
The section of the new superbar will be as defined for the components which means for the imaginary beam example I used the first section till the middle of the beam and the second till its end. The super bar definition is only the information that the components should be treated as an one (continuous) element rather than as two separate (shorter) elements. As you do not create any real bar over the two existing ones there is no superposition of course.
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.
Artur Kosakowski
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