From: Grant Saviers <grants2@pacbell.net>
Subject: Re: [TowerTalk] concrete bases for freestanding towers
To: WA8JXM <wa8jxm@gmail.com>, towertalk@contesting.com
Message-ID: <4DD45E1A.5010104@pacbell.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Bases have gotten bigger. Crankup mounting plates have much heavier
steel and angles rather than plates. The catalogs show much less
concrete than the actual wet stamped drawings I have for my HDX589 UST
freestanding. An owner of an earlier 589 told me his base is much
lighter, but I haven't had a chance to measure it. The lattice pitch on
the 589 also varies, more strength where they overlap. I'm not sure
this was always so, the Tri-Ex 354 I have is constant pitch of bracing.
Why? One significant reason is the "upsizing" of the wind speed
specifications as standards were revised. Other reasons could be better
structural analysis which no longer needs a Cray supercomputer, or
liability/insurance concerns.
Grant KZ1W
### UST STILL uses the old, outdated, now defunct UBC-exposure B standard on
their tower's. This is a bit of a joke...since that implies a 70 mph wind at
the top of the tower..... but
only a 56 mph wind at 8.5' above the grnd. IMO, they should be using a
minimum of exposure C. Along comes joe ham...with his 14' of mast out the
top of the tower, overloaded
with ants....sitting in the middle of a field, or 2 x blocks from the
water/lake, or on top of a ridge / ledge/hill top etc. In which case, the
wind will be almost the same velocity at the top of
the tower VS the bottom end..... which just threw all their calcs out the
window. I would not be leaving the UST products cranked up to full height,
with a mast out the top..knowing
that the local wx forecast calls for screaming winds to come though, later
on that night. I'd play it safe....and either reduce the height to
minimum... or at least reduce the height
to 1/2.
### The base for my HDX-689 originally wanted 5' square x 10' deep. [ 9.26
yards] It's now 6' square x 9' deep [12 yards] . The original design
is flawed, and doesn't meet current eng specs.
The anchor bolts, etc, are way too close to the edge of the concrete with the
original design.
On 5/13/2011 5:18 AM, WA8JXM wrote:
> 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"???
## In the case of the Rohn BX base's...and the delhi's sold here in
canada....and also the Trylon's.... you can make the base as big as you want.
When you run the trylons through their factory software... the results
are the same, the big trlons will fold over at the 38' level [ junction of the
5th and 6th sections]. Most self support towers are designed that way.
IE: they fold over some where in the middle..and not at the base.
## Even on my HDX-689... the weak spot is the 2rd section up from the bottom.
That 3rd section folds, before the sections above and below it.
## The concrete bases for delhi's used to be 4 foot cubed..and zero re-bar.
Then they flared the very bottom. Next up, they added re-bar. Then the base
got even bigger. It's amusing actually, sincxe I have never seen one
ever fold at the base. I have seen a few that folded in the middle..which
were all grossly overloaded at the top..and were not guyed.
Jim VE7RF
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