In a message dated 97-04-11 03:27:42 EDT, you write:
> Undoubtly steel and alloys such as chrome-molly are stronger, but they are
> also more expensive, heavier and rust alot.
>
> I have been using an aluminum mast consisting of 2-1/2" schedule 80
> 6061-T6511 with a 2" additional piece of 2" alumimun inside of it. This
> mast is supporting a full size 3 element 40 (15 sq ft.) 1' above the top
of
> the tower and a 7 element 20 (15 sq ft) on a 52' boom 10' above the 40 at
> 150' on Rohn 45. It has been in the air since about 1977 and has not
> failed. The only failure that I have had with this setup is 2 times the
> top (stock ) section of tower has bent and broke (top leg where it was
> welded to top plate broke loose) because of high winds. Section was guyed
> closer to the which is 8' down. After the tower failures I moved the guy
> attachment point closer to the top.
>
> And you thought you had a non controversial subject!! Say anything and
> somebody will disagree with you! And yes, I do sell aluminum.
>
Hi, Mike --
Got your attention, did I? Nice to hear from you.
The tensile strength of 6061-T6 is only 35,000 psi while 7075-T6 is over
70,000. One thing you have in your favor is that Marion County is only a 70
MPH wind speed zone - the lowest rating that the TIA-222 gives - so your wind
zone is not very demanding.
I ran the numbers on the installation you described in my new MARC
program and the calculated bending moment on the mast is 42,074 inch/pounds.
The mast recommended for that bending moment (3 inch assumed) should have a
39,000 pound yield strength and 0.188 inch wall which sounds pretty
reasonable for the aluminum mast you described.
Why has your installation lasted for 20 years? The main reason is that
you are using a 2.5 inch ID (3 inch OD) mast. With the insert in your mast,
it probably exceeds the 39K yield strength required. If you had used a 2
inch OD mast, the recommended yield strength should have been 87,000 psi and
0.250 inch wall. The other reason is that you don't live in a high wind
speed zone. 85 MPH winds on your system produce a bending moment of 63,069
psi and the recommended yield strength mast should be 50,000 pounds with an
0.250 wall.
Mike, you have very good tower and antenna construction skills and have
assembled a fabulous station; the fact that you put an insert in your mast
demonstrates that you are careful and thorough. The reason that I don't
recommend aluminum for masts is that 1) they don't have the tensile strength
for demanding installations and 2) aluminum is relatively soft and steel
mounting hardware (U-bolts, etc.) will gall the aluminum over time in many
cases. In other words, the constant torque forces on the hardware over time
have a good chance of gouging the aluminum and causing problems. Even steel
masts that are pinned with a bolt will gall and the hole will elongate over
time. Also, most people don't put the insert in their mast and are left with
a low tensile strength mast.
Continued good luck with your system and your contesting efforts.
73, Steve K7LXC
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