In a message dated 96-12-15 12:30:49 EST, you write:
>> Hi to all tower and antenna types...having spent some time looking at
>> different rohn designs ..ie r45 nd r55 and r65...i am pomdering the chance
>> of "rolling my own" ...has anyone ever done this and any suggestions???
>N4SI writes:
>After I did the math some time later, I realized I had spent about
>the same for it as I would have for Rohn 25, and didn't have near the
>tower. Not only that, I had a bunch of labor in three measly
>sections. I kicked myself for a long time over it, but I now I got to
>write this story.
>
>I suggest that unless you have access to lots of materials, the specs
>for which you have available to you, and unless you have access to
>the machinery required for assembly (shop, welder) and the ability to
>make jigs, and lots of time, you may be better off cruising the bands
>or practicing using the logging software than "manufacturing" your
>own tower.
>Should you do it? Only you can answer all the questions needed for
>that response. Would I do it? Not in a zillion years. Been there, done
>that.
>
Thanks, Rod, for your real world experience. And this was only 3
sections of 25G clones!
There are some people who have the mechanical design and construction
abilities to take on a significant project such as this. If you are indeed
going to do this, call Rohn and get their calculations to compare them with
your own.
Nowhere in your post did I see anything alluding to forces, stresses or
calculations. This isn't a "hip pocket" engineering job; you need to have
done all the calculations with current standards and engineering practices
taken into consideration. One thing in your favor is that you are in a 70
MPH TIA-222 wind speed zone which is the lowest rating that they give. After
you or a structural engineer have satisfied yourselves that the materials and
construction will give you an adequate safety margin and the ability to sleep
nights in big winds, you have crossed the first major hurdle.
Now what about cost? Let's say that you are going to throw in your
labor. No problem. I'll let you worry about the cost of the materials - you
can get quotes from all kinds of different suppliers so let's assume you get
them for a reasonable cost. What about the galvanizing? A Rohn 65G ten foot
sections weighs 290 pounds. Your 250 foot tower will have 25 sections for a
total weight of 7250 pounds. Here in Seattle, hot dipped galvanizing goes
for about 45 cents per pound. Your galvanizing cost is about $3262.50. And
that's not including shipping to and from the galvanizers.
I'm not even going to delve into permits (if you're going to get one,
you'll probably need a PE stamp), insurance coverage in case of an accident
or weather-related catastrophe, your liability in any case, erection (a big
job), base and anchors, lighting and painting (now that you're over 200 feet,
you have to comply with FAA lighting and painting regulations), etc.
If you feel comfortable with all of these issues, feel free to go for it.
But why re-invent the wheel?
Earlier this fall I did an inspection of a local contester who has 3
towers up. The newest one is 130 feet of homebrewed tower and has a 2L 75M
beam on it. The tower was designed and fabricated by a local guy who was a
clever designer and had all of the equipment necessary to build it. During
the inspection I found some loose rotator bolts and some poor connector joint
weatherproofing and fixed what I found. On the 75M tower, I found a broken
girt (horizontal brace) and cracks on one of the adjoining ones. I'm sure
that the tower wouldn't have survived the winter without a major failure.
Without going into a lot of details, the owner and builder had made a design
decision that in retrospect was incorrect. A commercially designed and
constructed tower will not suffer the same problem.
Why 65G? Two-hundred and fifty feet of 55G will handle 26.8 square feet
at 70MPH which should be plenty for your 75M beam. Yes, 65G is much stouter;
250 feet of it will handle 54.5 square feet at 70 MPH!
Don't let me discourage you but I wanted to make sure that you
understood more of the implications than were apparent in your post.
73, Steve K7LXC
TOWER TECH -- professional tower supplies and services for amateurs
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