OK, this happened to a close friend of mine about 10 years ago. The names are
withheld to protect the guilty. Or ignorant, in this case. :-)
He had 53 feet of Rohn 45, house bracketed at 10'. He had a 3'x3'x3' concrete
base, and two C3's and a shorty fourty on top. No guys. A microburst wind
took it all down, which actually upturned the concrete base along with it.
Now I don't think I would categorize this as a base failure, as a single set
(or two) of guys would likely have prevented this.
Chris
KF7P
On May 13, 2011, at 19:56 , Gary Joe Mayfield wrote:
I was referring to the cylinder (dirt) bases that are for the BX
freestanding towers. The newer catalogs say they are not recommended and
are provided for convenience. Again I think the only thing to change is the
liability. I really like the question someone else asked. Have you ever
heard of a tower base failing? I would think that once the base is strong
enough the tower always fails first, there would be no point in having the
base any stronger....
73,
Joe kk0sd
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of K8RI on TT
Sent: Friday, May 13, 2011 4:27 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] concrete bases for freestanding towers
On 5/13/2011 11:02 AM, Dick Green WC1M wrote:
> I would bet that Rohn's specs for its dirt base required a certain soil
> consistency and limited the windload. You might be able to get away with
> using a base like that in some locations, but not others, such as places
> with loose, sandy soil.
They work quite well in most any soil except mud/swamp. However I'd have
any soil evaluated where the use of one is contemplated. They are for
*guyed towers only* where the base serves only to keep the tower from
sinking or the base from sliding sideways out from under it. Depending
on the design of the base, *most* sand works quite well for this. OTOH
I'd certainly not use one on a sand dune. <:-)) Another point for a dirt
base in sand, the tower needs to be guyed very well ( rigidly ) as any
movement can cause the base to *work* its way down into the sand. This
*typically* is not a problem with well shorter, guyed towers.
In the past I've used them under towers holding 5L 20 and 6L 15 meter beams.
I would not use them under a heavy tower, or one that will support a
heavy load. I'd not use on any tower larger than a 25G or a 25G taller
than 60 feet. That is based on the vertical load that base must support.
I'd rephrase Joe's statement to, "The old installations were based on
what worked (was sufficient) while today's are based on a large safety
factor due to liability".
The typical guyed ham tower installation (not the larger ones most of us
on TT use) would work just fine with a dirt base. Yes, I put a dirt base
under a 50' 25G on the West end of my shop. But all it supports is a
C19-XR and a 7L 6-meter yagi for the time being. The plans are to
replace it with a 60 or 70 foot crank up and that will definitely not be
on a dirt base. <:-))
73
Roger (K8RI)
> 73, Dick WC1M
>
> -----Original Message-----
> From: Gary "Joe" Mayfield [mailto:gary_mayfield@hotmail.com]
> Sent: Friday, May 13, 2011 8:58 AM
> To: 'WA8JXM'; towertalk@contesting.com
> Subject: Re: [TowerTalk] concrete bases for freestanding towers
>
> Ken,
>
> I just put in a base for a HDBX-48 on Wednesday. Several people
> here on the list thought the 5 yards of concrete specified by Rohn was not
> enough and the rebar specified by Rohn was completely in adequate. Most
of
> the local old timers thought I was putting in crazy too much concrete, as
> their towers had been put in with at most 3 yards and had been up for
years.
> One local in particular whom I know has put up many towers (all of which
are
> still up) advised me all that was really needed a little under 2 yards of
> concrete. I respect this guy because I know for a fact he has been there
> and done that. It was kind of difficult to go against years of local
> experience and wisdom, but in the end I chose to follow the prime
directive.
> At $93 per cubic yard is was not an easy decision to make on my budget. I
> was able to save some $ by hand digging the hole myself the day before,
> which was a lot of work for a fat aging man.
>
> The strange thing is Rohn sells or at least used to sell a cylinder
> "dirt" base for these towers that has/had a very small volume. I suspect
> the true answer is a little of both. The old numbers were used to sell
> towers - look how easy this is, and you only need to a 2 yards of
concrete!
> The new numbers are driven by liability.
>
> 73,
> Joe kk0sd
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of WA8JXM
> Sent: Friday, May 13, 2011 7:18 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] concrete bases for freestanding towers
>
> Many years ago I had a 64' freestanding tower with a 3el triband beam
> (TA-33) on it. The base was only 2 cu yards of concrete. As far as I
> remember, that was all the manufacturer (Heights) recommended at the time.
>
> Now when I look at anyone's recommendations, the base requirements are
much
> larger. Rohn (and others) recommends 3 cu yards even for a 40' BX tower.
> I had used only 1 cu yard for a freestanding 40' tower.
>
> Is my memory faulty, or have the recommended bases grown over the years?
> Were the old recommendations inadequate, or has everyone grown super
> conservative over the years? "If one yard is adequate, three will be
> better, so let's use five yards"???
>
> As one ham commented on the air yesterday, the only failures he has ever
> heard of were in the tower itself, not the base falling over.
>
> Ken
> _______________________________________________
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