My dad and I did that to a rotating pipe mast in the late 1950s. 60 feet
of 4" galvanized pipe with a bearing and guy wires at about 565 feet. It
stood (except for the times we took it down to work on antennas) until
about 2004 when a contractor that coveted the land cut a guy wire.
Unfortunately my dad had moved his licensed address away from home so I
couldn't sic the FBI onto the contractor and I was wondering how I was
going to take it down anyway.
The bending strength a tube is proportional to the diameter to the 4th
power. So a little loss in height costs a big loss in bending strength.
A 10% loss in diameter results in a bending strength down to about 63%.
So in a tube its vital to maintain full diameter. Many times a wooden
dowel is fitted where the bending stress is highest, not nearly so much
to add the stiffness from the dowel but just to keep the tubing from
collapsing. A tube collapsed to half height has 6% of its original
bending strength and soon will have collapsed to two metal thickness
were it bends (and sometimes breaks). Column load is more complex but
bending is a part of failure if it doesn't buckle. Wood inserts help
slow buckling too. I'm not so much a fan of foam, I don't think its
strong enough in the tube to keep the tube round and full diameter,
though foam faced with very thin metal shows a lot of strength, I think
that structural foam is a lot more dense than foam in place out of a can.
Stay wires may contribute to the endwise crushing of the tubing to lead
to a buckling failure, but when I've had stay wired masts fail, its been
because the stay wire opposite the wire antenna load failed in tension.
73, Jerry, K0CQ
On 1/8/2011 5:59 PM, Richards wrote:
Cool. That is a new one on me. The photo really helps me visualize it.
A local ham suggested I fill the tubing with that expanding foam
insulation stuff that comes in a aerosol can... thinking it would
expand, and become stiff and dampen any vibration from flexing. I had
not thought of that... and am not too sure it will do what we intend...
but it also is a different approach.
DX-Engineering agrees with Rick... I need heavier, larger sized tubing.
(But that is 1) too easy, and 2) fails to use what I have on hand.
(I understand that scrounging and using parts on hand is a venerable old
ham tradition!)
Thanks for the idea. Kind of a "pull-yourself-up-by-the-bootstraps"
solution... but one that works.
================ James -K8JHR =========================
On 1/7/2011 10:33 PM, Dr. Gerald N. Johnson wrote:
You can solve your willowy tubing with stay wire braces, probably want
to use a poly rope instead of wire. Stay wire braces have been used on
slim masts, elevator legs (for grain transfer at an angle), crane booms,
and vintage aircraft wings.
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