For thin wall tubing of a given wall thickness
and material, it is approximately true that strength
is proportional to the square of the outer diameter,
and stiffness is proportional to the cube of the outer diameter.
Meanwhile, weight is approximately proportional to outer
diameter, and wind force (for a given MPH) is proportional
to outer diameter.
Thus, doubling the diameter (all other things being equal)
doubles the survivable wind force per square foot,
which increases the MPH wind survival by the square root
of 2 = 1.414, since wind force is proportional to the square of MPH.
In terms of erection, larger diameter is an even bigger win.
OTOH, the amount of sag for a given diameter is the same
no matter what the wall thickness. The increased weight
exactly cancels out the increased stiffness.
BTW, you don't need software to figure the above. Any
mechanical engineering handbook has all these facts.
If you do use the fence rail, realize that the zinc
plating is too thin to keep RF from impinging on the
steel, a magnetic material. Hard to say how lossy
it is at RF, except that the DC resistivity is 6X that
of copper.
The fence rail is likely even stronger relative to EMT
than would be predicted by the diameter. EMT is specifically
designed to be bendable. Not what you want. OTOH, fence
rail, or course, is designed NOT to bend.
I hope that all makes sense.
Rick N6RK
On 1/1/2020 9:54 AM, Gedas wrote:
Hi Rick es HNY !
But let me ask my question again, is it better to go with a larger dia
pipe (even if it has a greater wind load) or better to go with a smaller
dia tubing with less wind load? Assuming the wall thickness is the same
for all the members.
Gedas, W8BYA
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