zero wall thickness possible?

I'm trying to model two parallel beams with the same section.

The beams are very close to each other and take the same load cases, and will never touch each other.

Given they will perform the same, is it possible to model them as one beam, and use "box" cross section with upper/lower thickness high and SWall thick == 0?

This will result in a cross section that looks like two separate area with nothing connected in between.

Any thoughts?

Replies to This Discussion

Permalink Reply by Karamba3D on June 12, 2016 at 12:09pm

It should be possible to define hollow box cross sections with zero wall thickness at the webs.

However there are two problems in case of a direct approach:

1.) The two beams which shall be modeled as one do not touch as you stated above, so there is no shear connection between them. In such a case the cross secitons bending stiffness is E*I = E*2*b*h^3/12. Here 'b' is the width and 'h' the height of the two beams. If there is a rigid shear connection between them the term I(steiner) = b*h*h_tot^2*0.5 needs to be added to 'I'. Here h_tot is the distance between the centers of gravity of the two beams. In order to get correct results for the cross section one would have to adapt the shear stiffness 'G' appropriately.

2.) The other problem is that the torsional stiffness of the rectangular hollow cross section is proportional to the enclosed area. In Karamba that is true also in case of walls of zero thickness.

I think the best option would be to use the mechanical properties of a trapezoid cross section where the cross section height is chosen such that the resulting bending stiffnes equals that of the sum of the two beams. With the 'Modify CroSecs' you can set the mechanical properties of a hollow cross section to the values which come out of the 'Disassemble Cross Section'-component from the trapezoid.

Best,

Clemens

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