Matthew,
Having managed the installation of numerous towers for cellular and public
safety microwave links, I'm pretty sure you'll need to add torque arms (star
mounts) to control the twist and sway for your future microwave dishes. The
only way to be sure is to have your design and proposed antenna loads
analyzed by a structural engineer. That engineer could also specify the
replacement guy anchors required, and possibly inspect the pier pin base to
verify it can handle the vertical load.
Re. down time, is there any way to install a temporary support for those
other services while you replace the existing tower? The commercial
operators would probably insist on a hot-cut of their traffic to a temporary
antenna structure in order to maintain their service. We had to do that
many times for our tower work. It adds cost to the project, but avoids lots
of headaches when a surprise interruption would otherwise put the user
totally off the air. The crane work would take them off the air for at
least a day, if not more. With a temporary support, all their antennas can
be re-installed on your new tower and hot-cut back with minimal down time,
ie. fractions of a second.
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of
john@kk9a.com
Sent: Friday, July 19, 2019 4:14 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] tower replacement wisdom
If you need more tower strength and still maintain the 18" face, Rohn 45GSR
may be a good option. Without seeing it, I imagine that the tower pier pin
concrete base is probably OK. The guy anchors so most of the work so they
should at least examined and new ones could be added while the existing
tower is in place. With a crane and a lot of prep work you could swap in a
new tower in a day. How visible is this location? I question if it is still
a grandfathered structure if it is completely replaced.
John KK9A
[TowerTalk] tower replacement wisdom
Matthew Kaufman matthew at matthew.at
Hoping to gather some community wisdom before embarking on a tower
replacement.
I have a 2.5 acre property on a slope with what I believe to be a Rohn 55
tower that was installed as a commercial paging transmitter tower in 1971 at
150 feet, then extended sometime in the 1980s to 170 feet when they switched
from VHF to UHF and then 900 MHz paging. (The top (5th) guy level was quite
clearly added using an existing hole on the anchor and a strandvise instead
of the big grips that were used for the other guys.)
The tower has withstood at least one lightning strike, the 1989 Loma Prieta
earthquake, and numerous storms,... but the guys are getting very rusted,
the tower is rusting (despite a coating of zinc-rich paint a couple years
ago), and the guy anchors are in unknown condition (though visually "ok").
I'm not sure how long a tower should last, but it can't possibly be
"forever".
Two of the guy anchor locations are just above the tower elevation, the
third is some tens of feet lower. Each guy anchor has a metal rod that
extends 3+ feet unprotected through dirt and then into a concrete block .
The tower has a pier pin base on concrete which is below ground level, so is
in a bit of a well with retaining blocks around it (had been buried under
the earth when I acquired the property and tower).
I have the following limitations:
1. I want to have minimal downtime - the tower has a couple of heavily-used
ham repeaters, a commercial repeater, and a wireless ISP on it.
2. The county would never issue a permit for a tower like this today... the
property has a use permit for the tower, specified as "18 inch face" and a
drawing of its approximate location on the property. So I need to somehow
permit this work as "replacement in kind". A new tower would be limited to
53 feet in this zoning district, with a possible variance available to 78
feet... the trees are taller than that.
3. I have no construction drawings for the guy anchors or the tower base,
and have no way of nondestructively testing the strength of the guy anchor
rods.
4. I'd like to ensure that the tower can support the existing and future
microwave dish loads, and reduce the twist both for those and the fire
detection cameras I have up top... so maybe should go to star guying?
So... I need to choose a tower that is roughly 18" face width (possibly just
Rohn 55 again), and install it at (if I trust the foundation) or near the
existing tower location, with presumably new guy anchors that must be near
but not at the existing anchor locations (to minimize downtime), and do as
much of the removal and installation as possible with a crane (or
helicopter?). I also need to not break the bank, as this is really a hobby
tower for me, where the commercial customers are mostly to pay the utilities
and property taxes.
Thoughts? Alternatives?
Matthew Kaufman, KA6SQG
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