Axial, bending and Torsional Stiffess

Axial stiffness I'm not sure unless you mean Euler buckling??

Bending stiffness is the resistance to bending and torsional stiffness is the resistance to torsion.

It's not really related to yield stress directly, but you work out all the stress on the pipe and for multiple ones use Von Mises stress and if you get to 90% of yield that's normally the maximum combined stress value.

You can calculate it directly from shape, thickness and material strength.

I know you're a junior engineer here, but have you done any study of stress and forces?

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