MTV wrote:
Jim Lux wrote:
MTV wrote:
Allodoxaphobia wrote:
On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote:
Phase 2 was first digging the 36' x 36" x 45" hole for the base,
then securing the tower rod base and pouring the concrete.

A 36 foot deep hole ought to hold up a freestanding 100' tower --
or more! :-)


It's actually 52" deep; the tower mfgr said min. of 45"; ARRL
handbook says 6' but only 30" sq, depending on the type of soil.

I don't know that I'd use the ARRL handbook as a source for
structural/civil engineering data. The figures in the "assembling a
station" chapter in my 1990 handbook are just an example for a TriEx
LM-470 or for a Wilson ST-77B, not a general recommendation for all
towers. In fact, the last sentence in the section is:
"Once you have that information, contact the engineering department of
your tower manufacturer or a civil engineer"

Following the mfr recommendations is probably the best bet. (and
that's if they're the *current* recommendations.. engineering and
construction standards are always evolving, what was acceptable in
1960 might not be acceptable today)

That said, there's a lot of possible tradeoffs in tower bases, even
for the same kind of tower in the same location. skinny and deep vs
wide and shallow is one. If you're not taking the manufacturer's
recommendation, then you probably need to get someone with some
engineering expertise to tell you whether what you want to do is
reasonable.

My thoughts, too, to use the mfg recommendation (45" deep) so I made it
52" deep. I can't see it not doing the job. Tower is HD aluminum and I'm
staying within all specs. Rated for 12 sq ft @ 87 mph; my wind load is
about 10 sq ft. Highest winds we had w/Hurr. Ike were between 85-90 mph.
The tower, however, will be protected by trees up to about 40', I could
lower the antenna before a storm, and I have a house brace I'll probably
use at 10'. The "Handbook" I referred to is a 2009 ARRL Antenna Book.

hmm, rating of 87 mi/hr makes it sound like it was rated under the old
EIA-222-C Zone A(30 pounds/sf) which was average 87, peak 113.

That's roughly equivalent to what the more modern standard versions call
a "basic wind speed" of 70 mi/hr. Given the more modern ways of doing
the calculation, the square foot rating is probably half of your 12 sf,
and that's at 70 mi/hr. (drag on tubes is different than flat plates, etc.)
(and ick.. why can't Glen Martin give their ratings in the more modern
TIA-222E, F or G system)

If you're in Harris county (where your call resolves to), you're in a 90
mi/hr zone, so the wind loads would be 65% higher, or, more
depressingly, your tower's probably only rated to 3.6 square feet.

Not to say that it will inevitably collapse, but....



You want to be careful about combining reinforcing methods, because the
loads may distribute in ways not intended. A tower is designed to flex
under load.. the movement is what distributes the load over the entire
structure. Adding a constraint somewhere can concentrate the load
causing it to fail. A classic example in beginning structures classes is
adding a gusset to a welded T joint that makes it weaker, because it
concentrates the loads at the end of the gusset. Another example is the
combination of 100# test piano wire and 100# test rope in parallel. The
wire is much stiffer than the rope, so as the combination is loaded, the
wire stretches very little, all the load goes on the wire, so it fails
at not much more than 100#.

definitely, you want to consult the mfrs instructions.


Today I was trying to spot the 3 guy wire supports. Have a problem with
one, so I'm thinking of using 4. Instead of all being 120 degrees, one
would be 90 degrees; other three 30 degrees apart. Problem is that one
of the 120 deg's would be right into my RV pad - not possible. An
alternative to four would be to run one onto the end of the house roof,
which I suspect would not be as sturdy as it should be?

most residential roof construction is mostly designed to use gravity to
keep the roof on and to support down forces. They don't resist upforce
well (unless you're in an area where the code encourages straps..
hurricane areas, for instance), nor do they resist side loads. The house
itself can usually take a fairly good side load (after all, there's all
that surface area exposed to the wind), but the fascia boards on the
roof can't, so you have a problem of transferring the loads from the
antenna/guy to the structure of the house.

Different areas of the country have different construction practices to
accommodate the loads that are important (e.g. my house in southern
California is designed to resist shaking loads in shear from
earthquakes, but I doubt the roof could take many feet of wet snow)


Watch out with 4 guys rather than 3.. The limiting thing on a guyed
tower is the downforce on the tower under load. If you have 4 guys, you
have more static load on the tower than with 3 (assuming you've
tensioned them the same), so you're that much closer to the failure
point. Glen Martin doesn't have a whole lot of useful engineering data
on their web site, so it's hard to even do a back of the envelope
calculation of the buckling loads on the vertical tubes.

And you gotta love the picture of a bunch of guys standing underneath
the tower they're tilting up without any sort of safety precautions.

Marv