Hi Gang,
Thank you for your comments. I do realize that the concrete in the ground
is nothing more than a "dead man". In addition to the weight of the concrete
alone, the resistance of the soil between it and the tower and the soil
above it and towards the tower represents a very formidable barrier to the
movement of the concrete "dead man" in the event that the tower guy could
overcome the weight of the concrete alone. Therefore there is a composite
strength of the two that is expressed by Rohn as "reaction" in ft. lbs.,both
horizontally and vertically( or lateral and uplift capacity).(This assumes
that we are dealing with "normal" soil which is defined by Rohn as soil of a
cohesive nature with a horizontal bearing capacity of 400 lbs. per square
ft. per lineal ft. of depth.)This "reaction" in foot pounds varies from 710
#(H)-780#(V)for a 70 mph, 40 ft. high, 25G tower with 18.4 sq. ft. of
windloading on top(and 1 guy wire) to 4460#(H)- 3310#(V)for a 25G tower in
90 mph wind with a 160 ft. high, 25G tower (with 5 guy wires).
My contention is that if Block 4A is suitable for both extremes then it
must be "extreme" overkill for the 40 footer and just "adaquate" for the
much taller ,5 guy tower in 20 mph stronger wind.
Being neither a "structural" engineer nor an old and experienced tower
builder, i was soliciting comment on whether others had focused on this
before and what their opinions might be like. Its been my experience that
one does not know a subject well until one knows it backwards and forwards,
inside and out. I'm attempting to learn every possible thing i can about
this subject in a very short time period. I'm really glad i stumbled across
this reflector a few days ago and i'm studying the "archive."
Now that i've become conscious of towers coupled with a personal interest
and involvement, i am seeing a lot of housebracketed towers with 1 bracket
10-20 ft. high and as many as 4 and 5 sections above the bracket with
antennas on top that would be marginal if the tower only had 2 or 3 sections
above the bracket(according to Rohn).These things are 10-15-20-25-30 yrs.
old and continue to stand year after year and defy the specs. Truly Rohn
must over engineer these things generally speaking for those who will not
adhere to "good practices."
It is not my intention to encourage anyone to ignore the specs and
standards that Rohn promulgates. Only to understand where and when i can
make intelligent choices or departures from specs.
My objection to Block 4A was based on having to hand dig the guy anchor
holes. I was told by several that a "terramite" or backhoe could not be
expected to dig square holes, 3 X 3 X 4'. I don't want the dirt in the
vertical walls to be disturbed so i elected to dig them manually. Its a lot
of work! Someone gave me the idea to use an augar to drill the dirt. I'm
going to seriously investigate that option.
Thanks guys! Hope better understanding for all of us results from all
these 2 finger typed words.:o) 73 Roy Lincoln WA4DOU
On Sat, 17 Jul 1999 18:55:12 +0100, Pete Smith wrote:
> At 01:42 PM 7/17/99 EDT, K7LXC@aol.com wrote:
> >
> >In a message dated 99-07-17 11:16:07 EDT, wa4dou@excite.com writes:
> >
> >> Anyway, how many of you have, in building or planning low towers,
noted
> >> that Rohn specifies Block no. 4A(Concrete Anchor Data)for all 25G
tower
> >> installations up thru 110 mph,
> >> and to 100 ft. with 45G in 110 mph wind before they upgrade to 4B.
> >> I realize that i've heard it said repeatedly over the years that
Rohn is
> >> very conservative in their rateings.I also realize that it can be
very
> >> dangerous to "second guess" Rohn, or to depart from their
specs.without a
> >> lot of knowledge.
> >> It is easy to look at a 50 or 60 ft 25G tower with 110 mph specs
and
> draw
> >> a conclusion that Block 3D or 3E would be totally adaquate for
anchoring
> >> purposes. My anchors are GAC25's which are only 5-1/2 feet long as
opposed
> >> to GAC30's which are 7 ft. long. Since i am digging these holes
> >> with post hole diggers and a shovel, i don't relish digging the last
1
> foot
> >> of depth between 3 and 4 ft. or even to 3-1/2 feet deep. I'd even make
a
> >> concession and pour the concrete 1-1/2 feet deep which is 50 % deeper
than
> >> called for in Block 3D and more than 3E, knowing full well that there
> would
> >> be less "earth cover".
> >> Am i barking up the wrong tree here?
> >
> > The LXC Prime Directive is to "DO what the manufacturer says". Let
me
> >put it like this - how well are you going to sleep when that big
windstorm
> >comes barreling through knowing that your hole wasn't as deep as the
> >manufacturer recommended?
>
> The weight of the concrete is far from the whole story for a buried guy
> anchor -- the weight of the earth above the anchor is the rest of it.
> The problem with burying a guy anchor more shallowly is that it will have
> just that much less ability to resist the pull of the guy wire.
>
> 73, Pete Smith N4ZR
> n4zr@contesting.com
>
> Sometimes a tower is just a tower
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