"Jim Lux" wrote in message
...
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.


Interesting and long discussion of Pier Pin vs Embedded Base at QTH.COM
http://www.qth.com/ka9fox/pier_pin_vs_embedded_base.txt

The Shadow who has never owned a tower but has ARRL Honor Roll -- brag
brag. Verticals do work

If the base of the tower were in a small pit then it might work, otherwise
there is a huge danger that the base will kick out and the whole lot will
come down.

73
Jeff

Some commercial towers mount similar, but the ones I've seen have a
fairly large peg sticking out of the concrete and mates with the tower
base and the guys hold it all in place.

Scott


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.

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...


--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

"Scott" wrote in message
.. .
Some commercial towers mount similar, but the ones I've seen have a fairly
large peg sticking out of the concrete and mates with the tower base and
the guys hold it all in place.

Scott

Some of these are vertical radiators that need to be insulated from ground.
I have seen pictures of such where the tower actually came to a point at the
bottom (WLW?).

Tam/WB2TT

"Rick (W-A-one-R-K-T)" wrote in message
news

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.


Somewhere on the internet was a very detailed computer simulation of this.
The base is just made so it can not kick out. You have a bending force and
a shearing force on the base of a tower. If the base can wiggle some, it
will take more stress than if the base is bolted down or placed in cement.
It also gets on the climbers nerves when the tower does its shifting.

You can think of this as placing something in the sand or dirt like a yard
stick. Place it a couple inches in sand and it will bend way over , place a
couple of inches in a crack between some bricks that can not move and start
bending it at the top, It will soon snap .