On 10/29/11 9:48 AM, Cqtestk4xs@aol.com wrote:
> My GUESS would be that 4" galvanized pipe would work. I would sink it
> about four feet in a base about 3 x 3 x 1.5 feet of concrete. That would be
> in a hole 3 x 3 x 4 so you would only fill the hole up 18 inches with
> concrete and back fill the rest with the dirt from the hole. if it was in my
> backyard, I would sleep well at night with the set up. You might be able to
> get by with less, but do you really want to try. Towers make a big boom
> when they come down.
>
There was quite a lot of discussion on this list about loads, etc. on
elevated anchors a few years back (1-10 years.. I can't remember)
http://lists.contesting.com/_towertalk/2006-01/msg00188.html is at
least one of the cases.
A couple observations..
A lever arm is a great way to lever the block of concrete out of the
ground. Think of it as a mini free-standing tower with enormous loads
on the top of the tower. Bill's idea of casting the concrete block and
then backfilling with dirt isn't a bad one to help with this (since it
means you have to lift a whole cubic yard out of the ground: a ton or
more of concrete and dirt. It's sort of the same idea as the screw in
anchor or a deadman.
you've got a short tower (40 ft), so the loads won't be all that high.
Your guy tension is, say, 300 pounds, and at a 45 degree angle (for
example), so there's 214 lbs pulling sideways on the top of your 4 foot
pipe. If a 2000 block of stuff 3 feet across were sitting in the
driveway with that pipe sticking out, you'd not be able to tip it (i.e.
4ft * 200lb <<1.5 ft *2000 lb) However, if you got up close to half the
breaking load of the guy (1500lb) you're talking serious trouble..
you've got almost the weight of the block pulling sideways on a 4 ft
lever arm.
However, shifting the block in the ground is probably as far as it gets.
Will it uproot the block and drag it across the yard? Maybe, but 1500
lb of concrete and dirt will probably take a thousand pounds to drag.
It kind of depends on what the maximum expected load on the guy wire is.
One thing to think about is how it will fail... this scheme has several
"fail soft" aspects, which might be a good thing... the pipe might bend
so that it forms a straight line with the guy (assuming you've attached
the guy in a way that changing the direction of pull doesn't have a
lower failure load. Worst case, this is like slacking off the guy by 4
ft.
So, why don't you like the screw in anchors? Is it to get more "walk
under" clearance?
What about a combination of a screw in anchor and a vertical post
(loaded in compression). Think of guy leading to the anchor as a sort
of "back guy" holding the post upright.
This has the advantage of having better ultimate failure
characteristics.. the anchor stays put, and maybe the post fails, but
your tower just leans... it doesn't fall over.
> The three anchor bases would need a total of 1.5 yards and your base for
> the Rohn 25 will need another yard or so. If your concrete company has a 5
> yard minimum (many do) simply pour the rest into the three anchor holes.
> They will never go anywhere...for sure.
>
> As an extra you might want to dump concrete down the pipe...stops the pipe
> from filling with water and adds some strength. If you do I would order
> the mud with small agregate for easier pouring.
Not a lot of additional strength from filling the pipe with concrete,
but the filling it to prevent water is useful.
In any case, this is a bit more complex that it seems at first glance.
You want to either copy a proven engineered design, or get some
engineering assistance.
Start by looking at K7NV's handy collection:
http://k7nv.com/notebook/topics/TowerTips.pdf. Page 24 is where the
discussion on anchors starts, with how to calculate pullout loads, and such
You could also proof test your anchors after installation. get some
cable and a comealong and try to uproot them.
>
> Bill KH7XS/K4XS
>
>
> In a message dated 10/29/2011 4:09:54 P.M. Greenwich Standard Time,
> yvadir@gmail.com writes:
>
> I live on a small city lot and am in the process of installing a 40 ft
> Rohn 25G tower in my back yard with a pier pin base. I'd really like to
> avoid using screw in anchors in such close quarters and would prefer to
> use an elevated guy anchor like pipe set in cement. I originally thought
> that 11 ft would be great to clear my roof line but have been told that
> would require a pipe larger than what I can obtain around here. Revising
> my plans a 4 ft high pipe in the corners of the back yard would still be
> better than going to ground level. Can anyone give me an approximate
> idea as to what size pipe and what size base might be needed?
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|