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)
Grant
On 10/30/2011 4:48 PM, K8RI on TT wrote:
> On 10/30/2011 2:51 PM, Jim Lux wrote:
>> On 10/30/11 10:39 AM, Grant Saviers wrote:
>>> A couple more comments, some not covered before: (all comments w/o
>>> engineering calcs  YMMV)
>>>
>>> 1. Square tube is stiffer/stronger than round pipe or wide flange beams
>>> (Ibeams) of same #/ft..
>> Hmm. I'm not sure this is true. Bending strength goes as the radius to
>> the 4th power, and for a given perimeter, a circle has the section
>> moment. (sort of like a circle has the most included area for a given
>> perimeter).
>>
>> Cross sections with right angles are easier to use in construction,
>> especially for bolted connections, and a I beam typically has more metal
>> farther from the center than a square tube (which is why they use that
>> shape).
>>
>> However, in "practical sizes commonly sold" it might be true that
>> squares are stronger than circles.
> I think you will find that per unit of weight the pipe is stronger than
> other shapes which have their maximum strength along one or more axises.
>
> Structural steel tube like pipe has a welded seam, BUT pipe is much
> softer and bends far easier than structural steel. DOM (which is also
> seamless) of a given wall thickness is even stronger and more rigid.
> IIRC DOM is also available in multiple alloys.
>
> Most towers like 25G and 45G depend on shape for most of their strength.
>
> 73
>
> Roger (K8RI)
>
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