Absolutely right, Steve.
In my county, I am required to get a building permit for almost any
structure. For building any tower, I am required to have engineering
drawings signed off by a PE! Unless you have documentation for every piece
of hardware in the system, as well
as soil testing, you really dont know how much holding power your guys will
have.
To be safe and secure without soil samples, you need to use concrete
anchors. That's why the Rohn book only shows these types of anchors in
their handbook for all but the smallest towers. These rely only on the
weight of the concrete to hold the tower in position. However, this is
about the most expensive way to go, especially for larger towers.
The other alternative which may require you to cut the soil testing corner,
is to use some sort of ground anchor. These include screw-ins, but there
are several other types and methods of installation. Without going into a
lot of detail, they all have holding strengths directly related to the size
of the anchor plate, the type of soil, installation method, and depth. I
would guess that Rohn wont even risk their insurance with data on these,
since their performance is directly related to the installation
particulars, not just the anchor itself. However, AB Chance sells many
different types of ground anchors at very reasonable prices and with data
sheets on most combinations of anchor and anchor rod. Grainger is a
distributor for Chance, and where I ordered my rods and anchors. As an
example of cost savings, I paid about $25 each for an expansion anchor set
that is good for over 10,000 lbs in all but the worst soils. Compare that
with a concrete anchor system from Rohn that will run you at least a couple
hundred by
the time you are done.
Keep in mind that I am not an engineer and you must do your own homework.
I just give you the seeds for you to figure things out for yourself. If
you dont feel comfortable with something, do it the Rohn way and you almost
cant go wrong.
However, I would never use any undocumented structural hardware.
If you dont do soil testing, but can narrow down your soil to a certain
range and then select a ground anchor system that will hold way more than
you estimate for your ultimate load in your worst case soil estimate, you
should be safe and can still save a pile of dough over custom concrete
anchors.
73, Tyler K3MM
To: nw9g@netusa1.net, towertalk@contesting.com
cc: (bcc: Tyler G Stewart/BENN/CEC)
Subject: Re: [TowerTalk] Guy anchor help please
In a message dated 97-04-16 21:42:23 EDT, nw9g@netusa1.net (Steve Narducci)
writes:
> Today the weather was good enough for me to get out and
> start my 100 foot of rohn 25g tower. I went to the farm
> implement store and purchased three 42" screw augers with
> a 6" screw at the bottom which has a 1/2" solid shank with
> a welded loop at the top. I used my post hole digger to
> get down about 8 to 10 inches to get the auger started.
> It was tough but I managed to get the augers down to the
> top of their necks on the ground.
> My question. Is this auger safe for 100 foot of
> tower? How much pull would it take to yank these out of
> the ground? Finally, should I put a bag of cement on the
> top 10 inches of the auger that is in the cavity below the
> ground level? Or would that make any difference at all?
> I am somewhat scared to climb this monster and I just
> want it to be safe. Thanks so much and 73's....
Are these screw-in anchors adequate? Probably not.
The tower construction Prime Directive is to "do what the manufacturer
says". Nowhere in the Rohn literature does it give a spec to USE screw-in
anchors for anything. The one they offer in their catalog (which is bigger
than the one that you mentioned) states that it has 2,500 pounds holding
power.
The A.B.Chance Company publishes an "Encyclopedia of Anchoring" for
screw-in applications. They list nine different classifications of soils,
each with a different holding capacity. Your first problem is that you
don't know what your soil classification is so you can't give it any kind
of rating.
The second thing is the screw-in anchors that you purchased are too
small. If the Rohn one has 2,500 pounds holding power and 3/16 inch EHS
has an ultimate breaking strength of 3,990 pounds, the anchor will pull at
about 62.5% of the breaking strength of the guy wire making the screw
anchors the weak link in the chain.
You didn't mention what county you are in or what your wind speed zone
is. That plays a key part in your installation planning.
Cutting to the chase, don't use the screw-in anchors. Install anchors
per the Rohn catalog (available from TOWER TECH for five bucks) and you'll
be able to sleep nights when that big storm comes rolling through.
Congratulations for thinking about the soundness of your decision and
posting your concern on TowerTalk. It was in your best interest.
73, Steve K7LXC
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