On 1/12/01 5:32 PM, Dave Armbrust at ae4mr@arrl.org wrote:
>The bracket is over designed as much as I could. Where it attaches to the
>house it is 5' long and it has three bolts that go through vertical 2x4. I
>have this backed up with a horizontal 2x4. All of this including the bolts
>are stainless steel. Several locals have agreed that this is done properly
>and I believe it to be a better bracket then the one Rohn uses in their
>bracketed towers.
The important question is -- what sort of forces will be present at the
bracket? Is your bracket mounting (or for that matter, your structure)
strong enough to hold at the predicted forces?
>Unfortunately my tower does not fit any Rohn tables. There is no such thing
>as a Rohn table for 110 MPH wind.
It could be that Rohn doesn't expect a bracketed tower to stay up in such
a wind.
>Even the smallest Rohn bracketed tower
>which is only 40' calls for an bracket at 30'. I do not have a 30'
>building.
Neither do I. But some thought would find that it doesn't matter how high
the bracket is, just how much tower is above the bracket. To an engineer,
the bracket should hold the tower fixed at the bracket height. The tower
is essentially freestanding above that point.
> The best fit I could find was the 100' tower with a bracket at
>66'. This gives 34' over the top bracket.
OK, that's an important data point. 34 feet is the absolute highest I
would go over a bracket or guy point. That's as high as Rohn recommends.
Look at the freestanding tables. How much antenna can a Rohn tower
support if it were 34 feet tall? For Rohn 25, it can't be done except in
calm air. For Rohn 45, it's limited. (I don't have the table in front of
me.
> I am 39' over the top bracket
I would suggest that's too high.
Ask yourself this question -- are you willing to CLIMB the tower 39 feet
above the bracket? I would answer no. My Rohn 25 bracketed installation
is limited to 19 feet above the bracket at 26 feet. Yup, that's only 45
feet, but I'm not willing to go higher.
>but I also have concrete 15' below that bracket where as the 100' tower does
>not have any more support until 33' below the top bracket and this is only
>another bracket rather then solid concrete.
So? The tower SHOULD NOT MOVE in the bracket. If it does, you have a much
more dynamic engineering problem to solve.
> While I have pushed it by 5'
>this does not seem excessive based on my stronger design. But I am not sure
>and that is why I am writing.
The limitation isn't the bracket -- but the tower sections. Set your
sights a bit lower.
>The Rohn tables for bracketed towers and for free standing towers are not
>guyed and I assume that they have taken twist into account. These are the
>only tables I have referred to but again they do not fit.
My advise is not to exceed those examples, even if the fit is poor.
Rohn tower sections are only designed to take a limited amount of twist.
If you plan a rotatable antenna, consider mounting your rotator as low as
you can.
>Exactly. This is one of the reasons I am seeking the advise of this group.
>If I was comfortable I would not have written in the first place. I have
>already received one personal reply stating that what I just do not
>understand how strong Rohn 45 is and that what I am doing is fine. He also
>states that he has done over 300 towers all over the southeast. I do not
>want to just blindly follow his or anyone else advise. I want it done
>right.
Do you want to do it "right" or "good enough"? Your source is correct
that Rohn tower sections are actually stronger than their published
figures. The design rules are likely to be 200-300% over the rated loads.
This design factor accounts for three things: a) improper manufacture, b)
improper installation, c) deterioration of material over time.
To do it right means to never get close to the design factors.
>As the Section Manager I know of many, many, amateur towers. But I do not
>know of any that have hired a P.E. to design the tower!
That's because hams are cheap. The really, really "right" way is to hire
a PE. Then you'll have hard numbers, and you'll know if your installation
will stand.
> Prior to posting my
>message I searched the archives for information on towers, mast, etc. I had
>seen the advice where it was suggested that a PE be employed but did not see
>any examples where this was actually done. I am not trying to do a super
>tall tower. I am only 55 feet and will most likely scale it back to 45'.
>The total antenna load is 9.7 square feet including the HF beam.
Do the Rohn tables allow for a 10 square foot load on a 39 foot
freestanding Rohn 45 at 110 mph?
I don't have my catalog with me, but I don't think the numbers are
anywhere CLOSE to that.
>This is a rather small tower and load especially for Rohn 45.
Or is it?
>To suggest I hire an PE
>seems a bit excessive. I do not want to blindly follow Hank's figure of
>50,000 foot pounds without understanding how he arrived at that.
Rohn 25 is only rated at 6720 ft-lbs. I doubt Rohn 45 is 8 times that
strong. The figures are available in the Rohn catalog.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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