K7NV's finite element study
<http://k7nv.com/notebook/towerstudy/towerstudy1.html> showed that
the bending stresses at the base can be the limiting factor for a
tower with the base section buried in concrete.
Kurt also showed in that study that by releasing the moment
constraints at the base and allowing the base to rotate freely about
all three axes, one increases the load limit of the tower being modeled.
A moment free connection is know as a pin joint; an example of a
three-axis pin joint would be something like a ball in a socket. The
typical Rohn pin pier installation using a BPC23/45/55G concrete
base plate is NOT a good pin joint and is not close to the
moment-free case that K7NV modeled.
A tower mounted on a Rohn concrete base plate might, emphasis on the
might, be able to rotate around the z-axis (pin pier). However, I
would not expect a BPC to tilt about the other two axes without
developing similarly high levels of stress to the base encased in concrete.
The situation that comes closer to an ideal pin joint would be the
use of a 45TG (or 55TG) tapered section at the base. That is
something that is relatively uncommon in amateur towers.
73,
Mike K1MK
At 05:58 AM 7/15/2007, David Robbins K1TTT wrote:
>Personally I would want to see the video of the steel base on the concrete
>with the thousands of pounds of pressure from the tower and guys on it
>grating past each other before I would believe that it was for relieving
>stress from torque.
>
>David Robbins K1TTT
>e-mail: mailto:k1ttt@arrl.net
>web: http://www.k1ttt.net
>AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
> > -----Original Message-----
> > From: towertalk-bounces@contesting.com [mailto:towertalk-
> > bounces@contesting.com] On Behalf Of john@kk9a.com
> > Sent: Sunday, July 15, 2007 02:39
> > To: jeremy-ca; TOWERTALK@contesting.com
> > Subject: Re: [TowerTalk] new tower
> >
> > I guess you are saying that the base will rotate before the tower breaks.
> > I
> > don't think that much torque makes it to the base on a tall tower.
> > Certainly the pier pin base will not prevent your tower from moving at the
> > top. I never saw any movement at the base of my 160' Rohn 65G tower
> > loaded
> > with large antennas, however the tower definitely twisted at the top
> > before
> > I added a 6 way star guy.
> >
> > KK9A
> >
> >
> > ----- Original Message -----
> > From: "jeremy-ca" <km1h@jeremy.mv.com>
> > To: <john@kk9a.com>; <TOWERTALK@contesting.com>
> > Sent: Saturday, July 14, 2007 10:33 AM
> > Subject: Re: [TowerTalk] new tower
> >
> >
> > The tower will still move but it is not twisting as in winding up to a
> > point
> > of possible fracture. The commercial site over on the next hill had 140'
> > of
> > 45G with the base buried in concrete and a bunch of sticks on it. After
> > about 15 years the bottom section fractured. The tower didnt collapse but
> > they replaced it with 55G on the same pad but drilled it for a pin.
> >
> > We are both in a very exposed location with negative horizons and once
> > above
> > the tree line the wind is almost always fairly strong even when it is dead
> > calm on the ground.
> >
> > Carl
> > KM1H
> >
> >
> > ----- Original Message -----
> > From: <john@kk9a.com>
> > To: <TOWERTALK@contesting.com>
> > Cc: <km1h@jeremy.mv.com>
> > Sent: Friday, July 13, 2007 10:57 PM
> > Subject: Re: [TowerTalk] new tower
> >
> >
> > >I agree that the pier pin lets the guys do the work and it is the best
> > base
> > > for large tower. I don't think the pier pin base will do anything to
> > > minimize tower twisting. The best way to stop the twisting is to use a
> > > star
> > > guy bracket.
> > >
> > > John KK9A
> > >
> > >
> > >
> > > To: "Mark Robinson" <markrob@mindspring.com>,"Paul Hemby"
> > > <phemby@hotmail.com>, <towertalk@contesting.com>
> > > Subject: Re: [TowerTalk] new tower
> > > From: "jeremy-ca" <km1h@jeremy.mv.com>
> > > Date: Fri, 13 Jul 2007 21:34:21 -0400
> > > List-post: <mailto:towertalk@contesting.com>
> > >
> > > It minimizes the tower twisting forces especially at the base.
> > Recommended
> > > by Rohn. Especially advantageous with a tall tower with a big load; lets
> > > the
> > > guys do the work. Ive been on the top of my tower with a strong wind
> > > blowing
> > > and with the 1/4" EHS it barely moves.
> > >
> > > Carl
> > > KM1H
> > >
> > >
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
>_______________________________________________
>
>
>
>_______________________________________________
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Michael Keane K1MK
k1mk@alum.mit.edu
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