I was asked, off-reflector, what I had done to house bracket
rohn 45G, using it as a self-supporter. It occurred to me that
perhaps it would be of interest. This is truly in the FWIW dept.
First, 45G is NOT specified for self supporting more than 30' above
a guyset or attachment point, with any kind of useful load. So,
I broke Steve's prime directive...(follow the mfr's spec), which I do
not recommend to anyone. In my case, I went 44' above the highest
house bracket, and loaded it with about 12 sq. ft. (an LPDA plus an
r7000 vertical above it).
Second, I had a flat-roofed house in VT, the roof of which was constructed
from 2x12's. The architect said it was stressed to handle 20' of wet snow.
It'd sag, but not break with that vertical load. Since he'd lived in it for
10 years, I was inclined to believe him.
Third, I constructed house brackets out of 2 x 8 and 2 x 6 material, so the
tower would lie close to the house, and would be supported both side to side
as well as fore and aft. The 2 x 8's spanned 4' of roof girder, and were
lag-screwed in (no access to the roof space)... with 12" x .500" lag screws,
which had properly drilled pilot holes, to get max thread penetration, but
not
split the roof girders. Two brackets were used...one at 6', into the main
floor
structure, and the other at 16', into the roof structure.
(I think you could probably do this off the end-wall of a conventional
attic,
if you used backing plates to distribute the load broadly.)
Why TWO brackets? I was only planning one...but when we started to dig the
hole for the foundation, we ran into a bigass rock, which we couldn't budge.
I figured if God put it there, he probably wasn't going to take it away...so
we set the tower on it. It took the vertical load, and lateral loads were
absorbed by the second bracket, around head height.
Rather than go into a lot of detail here, if you want a sketch, I'll come up
with something to email you. Suffice it to say that the tower was rigidly
held,
and stayed up for 6 years in VT, before I took it down. There was a 2x
safety
factor in the worst case loading calculations.
When I went to the foldover configuration, I lightened the structure by
replacing
one section with a steel mast, so it was 30' of tower and 5' of mast above
the fold.
This was because I didn't feel I had adequate headroom on the foldover
winch's rating.
(70lb section replaced by 30lb mast...that's 40 lbs x 30' moment arm, or
1200 lb-ft
of load I removed.)
I have used a similar house bracket design for a tubular crankup 45' tower.
In
that case, it was tied to the house at 20', and could also be tilted over
using
an eye which I installed to allow block and tackle connection.
For the record, I don't like using Rohn house brackets to hold up an unguyed
tower with any reasonable ham antenna on it. They're too elastic, and their
connection
point to the house isn't spread over enough structure nor does it have
enough attachment
points to spread the loads. If I could thru-drill, and put a backing plate
across
5 or 6 endwall studs...maybe I'd consider it. If one had manufactured roof
trusses,
rather than 2x4 or 2x6 studded walls, I would NOT consider it for an
instant.
n2ea
jimjarvis@ieee.org
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|