Basically, my problem is small part of a bigger assignment, but I'm completely stuck at this point. This is really a math problem, but I figured I'd post it here anyway considering I might have misunderstood the physics. 1. The problem statement, all variables and given/known data A vertical pipe is subjected to a axial force of 1089N and an Mb of 600Nm. The maximum stress in the pipe is not to be larger than 150MPa. I am to find the relation between di and do (di/do), where di is the inner- and do is the outer diameter of the pipe (which is part of a bigger construction.) 2. Relevant equations von Mises theorem, but there are no sheer stresses in the pipe. Also, sigma=F/A+Mb/Wb+Mv/Wv. However Mv is zero in this case. 3. The attempt at a solution sigma=F/A+Mb/Wb -> 150e6=1089/[(pi/4)*(dy^2-di^2)+600/(pi/32dy)*(dy^4-di^4) I can't seem to find a method to isolate di/dy from this equation, mainly because of the single dy in the Wb for circular pipes. Any help would be appreciated. If something is unclear don't hesitate to ask.