You are correct that a larger tube wins for same amount of steel. My
comparison was same size/same moment and showed the extra steel needed
for round in a 4" shape. The #/ft measure was a bad choice on my part.
It should have included "for a given size" since otherwise a very large
diameter thin tube has a higher moment but is useless structurally
(unless it is pressurized as in liquid fuel rockets or Coors cans).
On 10/31/2011 6:11 AM, Jim Lux wrote:
> On 10/30/11 7:59 PM, Grant Saviers wrote:
>> Like your logic, but squares have more steel "further out" from the
>> neutral axis so they win (or at least moment calculators say so)
>> see http://www.engineersedge.com/section_properties_menu.shtml
>> Compare a 4" od x 3" id pipe with a 4" sq od x 3.5" id sq tube. Many
>> shapes are there to play with.
>> About the same moments and the pipe is much heavier. 5.5 sq in cross
>> section of steel for the pipe and 3.75 sq in for sq tube, a 50% increase
>> in weight per foot for tube.
>> And there is a diagonal moment calculator for squares (squares win in
>> all cases).
>> What am I missing? (of course there is a lot more to structures than
>> this one number)
> You're comparing a 1/2" wall round tube with a 1/4" wall square tube.
> Since the moment is Router^4-Rinner^4 the added wall thickness adds
> weight without much strength (tube is better than rod, as it were).
> If you make the weight/ft (cross sectional area) the same (I did it by
> making the tube bigger), you see the expected advantage for round.
> Try 4" square tube, 3" inside (1/2" walls)
> area (weight per length) = 7
> moment = 14.6
> Compare to
> 5" ID round tube, 4.01" id (1/2" walls)
> area = 7.01
> moment = 18.0
> If your constraint were "what's the strongest that will fit in a 4x4"
> square, then square tube will win, since even a solid 4" diameter round
> bar won't be as strong in bending: that r^4 term out to the corners.
> Large diameter thin walls is the strongest in bending and torsion, etc. BUT
> the practical problem is that as it gets thinner, small defects become a
> bigger problem. If you column load it, it's even worse (the standing on
> an empty aluminum can thing).. a tiny asymmetry in loading or the metal
> will lead to buckling.
> Yes, if you know the direction of the load, and it's always the same,
> then some other shape will be better (a long thin sheet, for instance)
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