In a message dated 97-03-16 10:17:05 EST, you write:
> I have one pretty long mast -18' - that will support two yagis with
> a spacing of twelve feet. about 6' of the mast is in the tower with a
> thrust bearing on a flat top at the top of the tower.
Hi, Victor --
First things first. Waller County, Texas, is an 85 MPH wind zone. The
WORST county in Oklahoma is also 85 MPH and there's only 2 of them; the rest
are 70-80. You won't be making any big changes in terms of your anticipated
wind speeds.
Second, I'll take your word for it that your mast is up to the load that
you're planning and wind speed.
>
>My question is a suggested method for pulling up the mast and
>attaching the two antennas. My tower in Oklahoma was a commercial
>type - 20' very heavy sections - so I used a commercial crew and the
>help of a crane for the tower erection and subsequent mast
>extraction. The crane, however, did not have a high enough reach
>to be able to get directly over the mast, and the extraction
>procedure was very slow (and expensive).
> I have also heard briefly about other methods using a gin-pole and
> come-alongs.
>
> Since I am a very methodical planner, I would like to settle on the
> best method possible for this part of the job. Just wondered if you
> have any info you can share, or can make any suggestions?
>
There are two scenarios for mast installation.
The first is to erect the tower and bring the mast up last. While most
builders use a ginpole (usually Rohn), it is generally faster, safer and more
expensive to use a crane.
The ginpole method has some problems. First is the strength of the
ginpole. The Rohn ginpole is only rated up to 45G sections (70 pounds).
With your commercial self-supporting tower, you may want to fabricate a
ginpole of your own with hardware specifically designed for your situation.
This way, you can use robust and bigger materials so that it has the
strength to haul up a big mast. It's cheaper than a crane but please use
standard engineering and fabrication standards in building and using this
potentially dangerous piece of equipment. BTW, the disaster last year in
Texas (?) where the big commercial tower collapsed with several fatalities
was caused by ginpole and hardware failure (there were other factors
involved).
The second problem is the length of the ginpole. A 12 foot (Rohn)
ginpole is designed to pick up 10 foot tower sections and not 20-24 foot
masts. It doesn't have enough headroom to clear the top of the tower. With
an 18 foot mast, you can get away with a shorter ginpole than if you were
installing a longer mast.
So if you can build a suitable ginpole and fixture, use proper rigging
materials and techniques plus have a method for pulling the load up (a lawn
tractor, etc.), the ginpole method will work FB.
The second scenario is to erect the first 20 feet or so of the tower and
place the mast inside the tower. You'll still either need a ginpole or you
may be able to get it in by leaving some braces out while you do it. You
finish erecting the tower and then pull the mast up. All you need is a
couple of pulleys, some slings for choking (securing) the mast, a method for
pulling the haul rope and away you go. One of the nice things about this is
that you don't have a hundred or two pounds of mast flailing over your head
which is a major pucker factor when using the previous method (sword of
Damacles, etc.).
Pull the mast up through the thrust bearing and tighten up the TB bolts
when you've reached a convenient point. If you've got antennas to be
installed on the mast, you can install the first one now. After it's
secured, pull the mast up another distance and repeat for the next antenna.
The rotator goes in last and then you drop the mast onto it. Voila - all
done. Of course there are a number of nuances and details that I haven't
covered but you get the idea.
If you're interested in more information, send me a SASE and $1 and I'll
send you a reprint of the chapter from my upcoming book that describes this
operation. Send it to TOWER TECH, Box 572, Woodinville, WA, 98072.
73 and GL, Steve K7LXC
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