A cautionary (and perhaps encouraging) tale. In the 70's, the Department
of State ARC had three sections of Rohn 25 on the 8th story roof of an
office building in downtown DC, with a Mosley CL-36 on top. All blithe
and ignorant, we did not pierce the roof for any kind of a base at all -
instead we just guyed the base of the tower parallel to the flat roof,
to the same points where the guys were tied off - cast iron vent
pipes!! About the only thing I think we did right, in hindsight, was to
guy the 30 feet in the middle and at the top, which probably did some
good in mitigating both the strain on the tower and the pull on any
single guy wire. Whatever, it stayed up for about 7 years, until a
micro-burst with 90 MPH gusts came through, whereupon the whole works
fell down and wound up hanging over the side of the building by the
surviving guy wires and the feedline. I was overseas at the time, so I
never saw the mess, but suspect it caused some excitement for the city
authorities. That may be why the club never put another antenna up there.
Pretty much everything I learned about guyed towers came from K7NV's web
site <http://k7nv.com/notebook/>, particularly
<http://k7nv.com/notebook/towerstudy/towerstudy1.html> on guyed tower
behavior, and I recommend anyone thinking about going anything other
than a cookbook configuration should have a look. Douglas is an
engineer, so I doubt this will be challenging to him.
From a non-engineer's, common sense point of view, I think that
probably the downward pressure of the tower base on a concrete roof is
less of a problem than the tension that must be handled by whichever guy
anchor(s) happen to be upwind during a given storm. 3/16 EHS has a
breaking strength around 4000 pounds, and all of that tension will be
applied to the anchor point before the guy itself fails. 4-way guying
will distribute that load more evenly, and of course two layers of
guying would have the same benefit.
If I were doing this near the coast in Cuba, I also think think I'd
seriously consider having a tilt-over base fabricated, and using the
falling derrick method to lower the tower and antenna whenever storms
approach. Depending on the roof configuration, it might be possible to
tilt the tower over entirely, so that it lies (and is tied down) on the
roof, with the tribander half over the side when in this position
73, Pete N4ZR
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On 8/26/2015 3:46 PM, Patrick Greenlee wrote:
Good stuff Gerald. Douglass may want to also consider placing the
base of the tower above or quite near to a load bearing interior wall
even though that might mess with the symmetry of a centered location
guy wise. Depending on the strength of the roof in the centered
configuration and on the interior layout of the residence a thrust
member (column) located below the tower base could be used to carry
the vertical download through to the (assumed) concrete slab floor.
(Also assuming this is a single story home although in theory
depending on layout another thrust member on the ground floor could be
used as well.)
If there are tower sections to spare put one under each guy to change
the angle of the guy and get more strength to oppose wind loading with
less down thrust on the tower. There is no free lunch and each of
these "guy helpers" will have some down thrust directed through them
vertically to the roof but much less than that of the tower itself.
Patrick NJ5G
On 8/26/2015 10:22 AM, TexasRF--- via TowerTalk wrote:
Hi Douglas, you can achieve a a greater guy point distance by using four
guys instead of three. You would not want the guy to guy angle to
exceed 120
degrees in any case.
Your roof dimensions would allow guy to guy angles of 120, 60, 120
and 60
degrees. With the tower mounted in the center of the roof and
assuming the
anchors are at the edge of the roof, a guy to guy angle of 120
degrees, the
distance would be 20.0 ft to the anchor. The distance between anchors
would be 34.6 ft, 20 ft, 34.6 ft and 20 ft.
So, in this case, you could guy the tower a couple of ft below the
top and
three 10ft sections would have a anchor/height ratio of 66.66%.
We typically use an 80% ratio mainly because that is the number Rohn
specifies for all of their tower models. It is a good compromise
between load
capacity and space required.
Considering that these towers are used up to 180ft in height or
more, it is
reasonable to move anchor points inward for reduced heights. The
issue in
doing this is that Rohn has no published data for doing this. The
wind load
rating has to be reduced and it takes engineering skills to properly do
the calculations. It is not rocket science however.
I would be concerned about possible roof damage as much as the
tower surviv
al. It is conceivable that using the roof edge as a turning point for
the
guy and then a ground anchor would be stronger. Something else that
would
benefit from an engineers blessing.
73,
Gerald K5GW
In a message dated 8/25/2015 9:14:22 P.M. Central Daylight
Time,
co8dm@frcuba.co.cu writes:
Hi,
I would like read your ideas.
I have few 10 ft section of a tower (it is like a Rohn 25G).
I would like install few section over some part of my roof...The
roof is
concrete...as you know many house in the Caribbean has concrete roof
...it
is 24ft by 40ft...so, i have calculating and the distance from the
base to
the guy anchors is 14 ft !!!
I know the distance from the base to anchor points is 70 % of the tower
height...so, 70% of 20 ft is 14ft, that´s mean 2 tower sections...right
??...Is it possible install 3 sections adding more guy´s set ???
Thanks,
73....Douglas, CO8DM
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