I went to visit a ham acquaintance of mine today. He is putting up a
100-foot tower with a large HF beam on top, guyed in three places.

He is not fastening the base plate of the tower to the concrete base in
any way... the tower just rests on the top of the concrete base, and the
three sets of guys hold it in place.

He explained that the main failure mode for towers is due to twisting in
high winds with high-wind-load antennas, so by not fastening the base
plate in any way and just letting it rest on the concrete, he allows it to
rotate a few degrees if needed (by sliding on the concrete) and lessens or
prevents rotational stress on the tower.

I understand what he is saying but this is the first time in 45 years as
a ham that I've ever heard of a tower being erected in this way.

Is this a common way to install a tower, and does it make sense?

Sure does seem to me like bolting the tower base plate down to the
concrete has a lot more upside than downside, but WDIK?

Reminds me of a day back in the 1970's when a bunch of us were working on
a 120-foot tower, guyed in three places, that held our repeater antenna.
Due to events the details of which I have long since forgotten, all three
of the guys on one leg became disconnected so that there was absolutely NO
guying support on that side. One of the team was about 80 feet up the
tower at the time. The tower curved over like a banana but stayed up, and
we were able to re-install the guys before the tower came crashing down
taking our friend with it. He did have to change his pants, though. :-\

Pretty easy to imagine what would have happened if the tower base hadn't
been fastened down...

Absolutely absurd. I hope he is the one who is going up top to install his
antennas.

The other Rick, K2XT

Rick (W-A-one-R-K-T) wrote:
I went to visit a ham acquaintance of mine today. He is putting up a
100-foot tower with a large HF beam on top, guyed in three places.

He is not fastening the base plate of the tower to the concrete base in
any way... the tower just rests on the top of the concrete base, and the
three sets of guys hold it in place.

He explained that the main failure mode for towers is due to twisting in
high winds with high-wind-load antennas, so by not fastening the base
plate in any way and just letting it rest on the concrete, he allows it to
rotate a few degrees if needed (by sliding on the concrete) and lessens or
prevents rotational stress on the tower.

With the weight of the tower and the downward force of the guys aiding
the friction on the base, and the guys' resistance to the tower twisting
I doubt not fastening the base has any appreciable effect, but as you
said (farther down), WDIK?

Even so, It's very doubtful the friction will be the same over the whole
base-concrete interface, such that on a windy day his tower might just
walk off the concrete or at least far enough out from under the rest of
the tower that the force from the guys becomes unbalanced enough that it
pulls itself down, but as has been said, WDIK?

I understand what he is saying but this is the first time in 45 years as
a ham that I've ever heard of a tower being erected in this way.

Is this a common way to install a tower, and does it make sense?

If it's common it's a well kept secret.

Sure does seem to me like bolting the tower base plate down to the
concrete has a lot more upside than downside, but WDIK?

Exactly, what do we know? If he's able to get a licensed, professional
engineer (who would know) to sign off on it I'd come within a radius of
1.5 times the tower height, otherwise I'd keep my distance.

This method sounds very dangerous. I could see putting some kind of
bearing or at least a recess so the base doesn't slide out from under
the rest of the tower but just plunking it down without any method of
keeping it on the concrete, yikes!


Reminds me of a day back in the 1970's when a bunch of us were working on
a 120-foot tower, guyed in three places, that held our repeater antenna.
Due to events the details of which I have long since forgotten, all three
of the guys on one leg became disconnected so that there was absolutely NO
guying support on that side. One of the team was about 80 feet up the
tower at the time. The tower curved over like a banana but stayed up, and
we were able to re-install the guys before the tower came crashing down
taking our friend with it. He did have to change his pants, though. :-\

Pretty easy to imagine what would have happened if the tower base hadn't
been fastened down...

Death, destruction, litigation... you should advise Mr. Baseless to
have a licensed engineer look over his installation, especially if
there's any insurance companies involved in any way.

But yeah, WDIK?
W8LNA

Rick (W-A-one-R-K-T) wrote:
Is this a common way to install a tower, and does it make sense?

Some towers have a swivel to allow for twisting
but the base under the swivel is firmly embedded
in concrete.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote in news:Oq6Mi.56304$YL5.4223
@newssvr29.news.prodigy.net:

Rick (W-A-one-R-K-T) wrote:
Is this a common way to install a tower, and does it make sense?

Some towers have a swivel to allow for twisting
but the base under the swivel is firmly embedded
in concrete.

Hi All:

I've seen profesonal towers put up that way, sorta. They had a single pin
that a hole in the flat tower base set over. The pressure of the guys keeps
the base on the concret pad and the pin keeps it from kicking out to the
side. With out the pin, it's going to come done sometime.

John Passaneau W3JXP

On Mon, 01 Oct 2007 13:38:14 +0000, John Passaneau wrote:

I've seen profesonal towers put up that way, sorta. They had a single pin
that a hole in the flat tower base set over.

Hmmm... Now that you mention it, I recall that the base plate had what
looked like a capped tube rising from its center. Maybe that tube goes
down over a pin of some kind that was embedded in the concrete.

I'll have to ask him about that.

"Rick (W-A-one-R-K-T)" wrote in
news
On Mon, 01 Oct 2007 13:38:14 +0000, John Passaneau wrote:

I've seen profesonal towers put up that way, sorta. They had a single
pin that a hole in the flat tower base set over.

Hmmm... Now that you mention it, I recall that the base plate had what
looked like a capped tube rising from its center. Maybe that tube
goes down over a pin of some kind that was embedded in the concrete.

I'll have to ask him about that.


HI again:

That's what it looked like.

John Passaneau W3JXP

Rick (W-A-one-R-K-T) wrote:
On Mon, 01 Oct 2007 13:38:14 +0000, John Passaneau wrote:


I've seen profesonal towers put up that way, sorta. They had a single pin
that a hole in the flat tower base set over.


Hmmm... Now that you mention it, I recall that the base plate had what
looked like a capped tube rising from its center. Maybe that tube goes
down over a pin of some kind that was embedded in the concrete.

I'll have to ask him about that.


that would be what's known as a "pier pin" base, and, as noted, it
greatly reduces the torsional load on the tower. Whether the force
causing the tower to fail is torsional, I'm not sure. Guyed towers fail
by buckling from the compressional force exerted by the guys.
Obviously, putting another force on the tower in addition to the
compressive force is going to increase the load on at least some member
of the tower, and if failure of that member causes enough asymmetry to
get the buckle going, then it is an issue.

On Mon, 01 Oct 2007 13:48:40 -0700, Jim Lux wrote:

that would be what's known as a "pier pin" base

Yup. Fairly common in commercial tower installations as I've seen
several. Seems to be unique in ham installations, though.

73, de Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

Jim Lux wrote in
:

....
that would be what's known as a "pier pin" base, and, as noted, it
greatly reduces the torsional load on the tower. Whether the force
causing the tower to fail is torsional, I'm not sure. Guyed towers
fail by buckling from the compressional force exerted by the guys.

The vertical elements of a tower framework should take their load in line
with the element. They are slender elements which are tied in by bracing
to prevent buckling.

It seems to me that in a typical construction where the end of that
vertical element in each section is not free to hinge, that twisting of
the section deforms the vertical element and would assist buckling if the
downwards force in the element is very large... as it is on very large
structures.

(If you read the USCG manual on towers, they are obsessed with ensuring
that not only is the tower vertical, but that each vertical element is
vertical (ie twist within specified limits.)

I wonder about the benefit in shorter structures, but can see that
dynamic forces caused by rotator brakes trying to instantly stop a
rotating beam might be better accommodated by the pier pin base.

For hints on amateur applications of towers, look at these pics:

(note the extension of the winch handle)

http://www.users.bigpond.com/vk3bjm/...s/image016.jpg

and the counterweight in more detail:
http://www.users.bigpond.com/vk3bjm/...s/image018.jpg

Owen