At 06:39 AM 4/6/2006, Dave Tipton wrote:
>I tried to post this the other night, but it never showed up.
> I'm in the process of putting together the plans to put up a 60 foot
> Rohn 25 tower with two sets of Guys (As prescribed by the plans), with
> approximately 8 foot of mast protruding, in a 4 1/2 foot pier type
> installation, and have a few questions. (English majors, yes, I know
> that's a run on sentence)
> 1) What is the maximum allowable windload for this configuration? (I
> can't find this information on their website)
> 2) I need to use Elevated Guys. (I have small children and can't have
> anything basically at head/neck level. I'm planning to put the tops of
> my guy points at about 8 feet. (Does this change the rating?) I plan to
> also employ a house bracket at 8 feet. This is more for the initial
> installation of the tower, however, it's not bad extra insurance.
This complicates the guy anchor somewhat. Rather than a nice tension pull
on an anchor or big lump o'concrete, you've got the guy pulling on a big
lever arm. So whatever that post is planted in needs be able to withstand
the "twisting out of the ground" forces. Think of sticking a shovel into
the ground, and then pulling on the end of the handle, vs pulling at the
top of the shovel blade.
There's also the "bending the post" problem.
Two basic strategies are used:
a) plant a fairly sturdy post in a deep hole or a big hunk of concrete. The
post takes all the bending loads, and the buried part is big enough that it
won't move. Big pipe (4" or 6") or I beam (6") has been used. You'll want
to get someone to do the analysis if you're worried about structural strength.
b) plant a less sturdy post in a smaller hunk of concrete, and back guy the
post to an anchor fairly close to the post (so the post guy is pretty
vertical, and doesn't present that "low hanging clothesline" hazard). This
makes the load on the post more of a compression than a bending load, but
raises the issue of that back guy.
One thing to keep in mind (which came up in a length discussion a few
months ago on this list) is that from a "tower collapsing" standpoint, you
don't care if the post bends, as long as it doesn't actually "let
go". There's a difference between "surviving the peak wind event unscathed
and ready to operate" and "not failing during the peak wind event, but
possibly requiring repair afterwards".
So you can probably get away with a post that is strong enough to handle
the usual winds without bending, but that might bend (non catastrophically)
when the tower gets loaded by that once in 10 year peak wind. The wind
forces go up as the square, so designing for a 100 mi/hr wind requires 4
times the strength as for a 50 mi/hr wind. The post designed to not bend
in the 100 mi/hr wind is going to be a LOT bigger than the 50 mi/hr post,
and likewise, the ground anchor required is bigger. The difference is that
you'll need to plant a new 50 mi/hr rated post after the 100 mi/hr wind gust.
Say you've got a 8 foot high post 60 ft out and a 85 ft long guy cable.
That super gust comes along and bends your post so that it's in line with
the cable, and the tower now effectively has a 93 foot guy on that one
side, and will be leaning over at an angle of about 10 degrees, hardly a
disaster, in and of itself.
This might also be a challenge to explain to local building officials, who
tend to have a fairly "fail or not-fail" kind of viewpoint, at least at
first. What you save in smaller posts and anchors may be less than the
extra you spend in engineering analysis and time to convince the building
permit person at the counter.
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