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Buckling of Hollow Tubes (counter intuitive results?)

Buckling of Hollow Tubes (counter intuitive results?)

Buckling of Hollow Tubes (counter intuitive results?)

(OP)
Hi,

I have a space frame made from beam elements in Abaqus and am trying to find the buckling load. When all the beam profiles are solid there is no problem and i find an eigenvalue and also a LPF from Riks analysis, I have checked this with a simple single beam also to check my method.

However I then wish to do a sort of optimization. So I make the section hollow, and in a number of tests keep the external diameter constant each time and vary the internal diameter. This seems to cause a problem in Abaqus as with a constant external diameter the structure becomes stiffer when increasing the internal diameter, i.e. having less material. I'd understand if the external diameter was also larger as the second moment of area would be increasing but here I am clueless?

Can any one offer any advice, it would be much appreciated.

Thanks
Tom

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