on 9/3/02 11:57 PM, K7LXC@aol.com at K7LXC@aol.com wrote:
> In a message dated 9/3/02 9:23:51 PM Pacific Daylight Time,
> na9d@speakeasy.net writes:
>
>> I am shortly going to be putting up my first real tower - 5 sections of Rohn
>> 25.
>>
>> I plan on guying the tower with two sets of guys per Rohn specs. My
>> question is about anchoring them. I have purchased Rohn GA5604 screw
>> anchors. The specs on these say that they have 2500 pounds worth of
> holding
>> power. I was actually surprised when I received them at how small the
> screw
>> portion was.
>>
>> My question is this: Are these sufficient for my tower installation or do
> I
>> need to think about digging a hole and putting these anchors in concrete?
> I
>> was originally planning on doing this but now I am wondering if I need to
> go
>> through all that extra work.
>
> No, they're not sufficient. Yes, do the extra work.
>
> Fifty-feet of 25G has 2 sets of guys; the maximum guywire strength at the
> anchor before failure is 8,000 pounds. This is SIGNIFICANTLY more than the
> capacity of the screw-in anchors. Even if you used one screw-in per guy,
> you'd still have a mismatch. I'll sure someone will mention screw-in creep
> due to wet soil conditions.
>
I just looked at the Rohn website and saw a couple of things:
1.) I answered my question of guy tension: 400 pounds
2.) For 110 MPH winds on a 50 foot tower, you have forces on the anchor of
1480 pounds vertical and 1680 pounds horizontal. The rod is at a 41.3
degree angle. So the amount of force along the direction of the anchor rod
is the vector sum of the two forces. Applying the formulas of trigonometry
to this, the force along the rod is approximately 2240 pounds which is less
than the 2500 pounds rated holding power.
3.) For 90 MPH winds (what most people rate for), the forces are 1350
Horizontal and 1180 vertical.
So looking at that, I have to conclude that an anchor with a 2500 pound
holding capacity will work quite well.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Life Member: ARRL, NRA
Member: AMSAT, DXCC
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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