Tower Basing Summary (25k)

[Peter G. Smith]

Following is a summary of comments received on the CQ-
Contest Internet reflector and on the East Coast
Megacluster on the topic of rigid versus pier-pin
basing for Rohn and other similar towers. Thanks to
N3RD, KM9P, NG0X, K9MA, K9FN, WA6SDM, N6VI/KH6, K0KR,
K7GM, N3RS, W2UP, K5ZD/1, VE3CDX, K8LX, KC7V, K1KP,
K4XU, Blck...@AOL.COM, N3JT, K1HTV, NY3M, KE3Q, K3SKE,
K1DG, W0UN and K1GW. In what follows, while I have
tried to retain the full flavor of the originals, I
have deleted liberally, chopped, merged and summarized,
so I apologize in advance if anyone feels I have
mangled his input unduly.

In this corner ... supporters of the rigid base
approach:

... we had a Rohn 25 tower with the first section
embedded in concrete. This tower was taken down and
replaced with Rohn 55. We ...decided to go with the
pier pin and Rohn baseplate, as you describe. ... The
first tower section is not rigid, and can't be climbed
on safely without temporary guys. Since our first real
guy went in at 35 feet, we had to progressively move a
set of temporary rope guys up the tower. No big deal,
just a pain in the neck and a lot of time spent.

The final 130' Rohn 55 tower does not have the "rock
solid" feel that we anticipated. We attribute that to
the fact that the tower is not rigidly anchored at the
bottom into concrete. We have every reason to expect
that this new tower will satisfactorily support the
antennas we plan for it, but the base plate and pin
approach was a new one to us, too. (see also N3RS's
comments on thee same installation, below)

- N3RD

-------

If I am breaking welds on the tower because of wind, I
have more to worry about than the tower. I've lost all
the antennas, proably a lot of trees have fallen on the
house, tower, and everything else and I am trying to
chainsaw my way back to civilization!

- KM9P

------

Remember that Rohn 25 can be used up to 180 feet.
Perhaps at that height it's really important to
minimize stress on the base. On the other hand, at
heights under 100 feet or so, there's only a fraction
of the load on the base, which may explain why so many
hams get away with embedding the first section.

By the way, Rohn sells a 3 foot section, which is
probably better to embed in the concrete than the
bottom of a 10 foot section.

- K9MA

------

I've seen very few 25/45 installations of any size that
DON'T use embedded tower sections as the base. I guess
we all think we're smarter than the factory engineers!

I have six 90' Rohn 45 towers, all with embedded
sections for bases. All have been up 16 to 18 years
with pretty big antennas on them (Telrex 8/8 on 15,
2-el. 75m, etc.) with no structural failures. There is
noticeable twist in the wind, the little torque bars
don't prevent that. All this is NOT to say ignore the
engineer's advice; they're probably right. Just wanted
to pass along what I've seen.

- N6VI/KH6

------

I have only seen the embedded section routine for Rohn
(and that is all I have ever used). I think at
"ultimate wind load" than whether your tower flexes a
bit at the bottom is the least of your concerns. The
embedded section way is easy, gives better grounding
connection with the ground, and is more likely to give
you a straight tower than any other method. In
addition, if you ever move, the short section you cut
off can be used at the next QTH as the base.

- K7GM

------

I have a Rohn 45 with tower embedded in concrete. With
a guyed tower, there really is minimal lateral force at
the base. It is all a downward force. Technically, I
think it would be easier to true up the tower section.
What I did was put 2 tower sections together and then
put it in the hole. Then with temp. guys you make it
vertical. With only 5 feet sticking out it is not easy.
With the base plate/pin, you probably will be off
vertical, but you can pull it straight with guy wires.
In theory, I can't fault either base support. I think
Rohn is pushing it a little with their explanation...

- W2UP

------

... in small installations it's much easier to plant
the bottom in concrete, true it and stack from there.
If your intended installation is so close to the design
limits that flexing of the tower member components is
even a remote possibility, you need more guys or a
bigger tower.

- K4XU

------

I like the pin idea, though getting the first sections
up could be dicey. Also, if you ever have to use the
tower in a self supporting configuration temporarily,
such as when lifting an antenna into place, you may
find the old system more convenient!!!

- N3JT

------

A section in concrete provides you a sturdy starting
point and you don't need temporary guys. It also gives
a "rock solid" feel to the tower at and near ground
level which is reassuring. The pier pin gives a
disconcerting feel of movement, even when you've added
some sections and guy wires, though that movement may
in fact add to the mechanical integrity and longevity
of the tower.

- KE3Q

------

For most everyday tower construction, a short concrete
section sunk in 3 1/2 to 4 feet of concrete is going to
work just fine. It makes construction of the rest of
the tower a lot easier as well, since you have a firm
base to attach additional tower sections. With a pier
pin, now you need to FIRMLY support and raise enough
tower on the first lift to attach and secure the first
level of guy wires.

This is the part most people don't want to fool around
with. You really can't build the tower up from a
safety point of view. To use temporary rope guys is
asking for problems and risk injury to someone.

- K3SKE

------

The biggest problem in such an installation is getting
the first 20 feet of tower installed. You need help -
can't be done alone, compared to the anchored first
section method.

- K1DG


And now, the pier pin camp is heard from:

Before moving west, I lived in Minnesota, and we got
the occasional "wrath of God" style T-storm. Any guyed
towers that I saw in a crumpled heap seemed to have
been killed by excess torque. From that standpoint,
the pin makes sense, since the tower can rotate on the
pin rather than build up torsional stress.

- NG0X

------

In the broadcast biz (my field), I have seen lots of
guyed towers...we have four of them here and all have
single point bases that can pivot. By allowing the
tower to swivel a bit in extreme condx, you allow the
force to be shared by all the guys, rather than being
concentrated at the point the base would otherwise
enter the concrete. Our towers are Pi-Rod...a company
that uses solid steel for the legs, rather than hollow
tube...(I have one of their freestanding towers at
100') and they also recommend against securing the base
in concrete, even though their style of construction
might be able to take it.

By the way, another strong reason to not bury a section
in the concrete is that it is then much more difficult
to control moisture in a hollow-leg tower. I have seen
them rusted through so badly you would not dare climb
20' up.

- K9FN

------

As I recall from the statics classes I took, the free
pivot base is a solvable problem, the fixed base
problem is not. At least all the problems we worked
involved a pinned base. I believe the Rohn speak is "We
can't figure the loads when the base is fixed in
concrete, so don't do it."

- WA6SDM

------

Unlike that of the pier pin, alignment of the half
section is a wee bit critical and is nontrivial to
accomplish. Plus, if you flub the alignment, you have
a problem that can be solved only by demolition and
beginning again from scratch. Imagine the expense, the
effort, and the frustration. Avoid the risk. Use the
pier-pin base.

Some fellows try to correct a misalignment of the
half-section base by using differential guy-cable
tension on higher sections. Bad move. Yes, there is
some play in the leg joints that can be taken up to
correct for slight misalignment of a half-section base.
Yes, really careful use of a carpenter's level will get
one into the ballpark on alignment. But, why take the
chance that the level itself is misaligned, and so on.

Actually, I believe the majority of fellows use the
half-section base. I believe that they do so for one
or more of four bad reasons: (1) they do not know any
better, (2) they are following the crowd, (3) they
think that the half section makes it easier to get the
tower started, given that, initially, no guys are
available to stabilize the bottom several sections of
the tower, and (4) they consider the half section to be
generally more stable.

As to reason (3), I use a set of temporary guys, just
enough to stabilize the lower sections until enough of
them are stacked to reach the level at which the first
set of permanent guys is attached. This is not much
work, and certainly represents less work and risk than
is involved in "truing up" a half-section base.

As to reason (4), many fellows fail to comprehend that
the greatly predominant vector of force on the bottom
section is straight down, with essentially no shear
force being present. There is, in fact, so much down
force that, with a big tower, the base would probably
not move even if the pier pin wasn't there!

K0KR

------

I am using the pier pin approach on a brand new Rohn 55
130' tower. It doesn't seem to be as stiff as a
concrete base mounted tower, but it is surely a good
way to put up the tower. It has only been up a short
time, so I cannot say much about it beyond it is pretty
strong. [A prominent contester with large
towers/antennas] also has towers erected in this
fashion and swears by them.

- N3RS

------

I am using pier pin bases here on 2 towers (and used
them at my W3 QTH). In addition to all the good stuff
Rohn told you, they offer one big advantage to the ham
on the move. They don't cost much, and when you get
ready to move you only leave a little stub of pin
sticking out of a buried block of concrete! Buy a new
pin for the new QTH and you are back in business.

The only down side is that it makes getting the first
30 feet of tower a little harder to put up. And after
all, how often do you put up the first 30 feeet of your
tower?!

- K5ZD/3

------

I have pin bases on all my towers and even my tv tower
(commercial tv) has one and its 1005ft with an 8ft face
and 5 1/2inch solid legs. If you have a choice go with
the pin.

- VE3CDX

------

Single point mounting is used on virtually all large
commercial guyed towers for the reasons Rohn stated.
However the Rohn implementation (flat plate on
concrete) negates much of the theoretical advantage. I
have been at the base of such an installation during
high winds and observed the top of the tower (130' of
45 with 2 large beams on top) being twisted 10-15
degrees, while the base was still.

- K8LX

------

I have used the pier pin method over the last 9 years
with Rohn 45 and Rohn 55 and have never experienced a
problem. We do get severe winds and thunderstorms here
during the summer and the towers have withstood all.
Currently I have 2 100 foot Rohn 55 towers up with
beams spaced up and down each tower. The same goes for
a 100 ft. Rohn 45 tower. Each uses the pier pin method.
You just have to make sure the base is flat and the
pier pin is vertical when embedded in concrete. It
sure makes life easier when putting the towers up.

- KC7V

------

I have the pier pin set up on my 70 foot Rohn 45 tower.
No problems. The pin didn't end up exactly vertical,
and the top surface of the base didn't end up exactly
level. Still, no problems.

- K1KP

------

I'm in the process right now with a 90' 45G. The pier
pin method keeps the tower from twisting. Its sorta
weird putting up the first couple of sections, though
........

- ? (I goofed)

------

I was thinking, if this twisting issue is real
important, the guys with large rotating tower setups
are in trouble, because it is common, in gusty wind
condx, for the top and and bottom antennas to get
swinging out of phase at times, thereby maxing out the
twisting torque. Doesn't matter how the base is
attached in that situation; and with your guy wires
attached to slip rings, they offer no twist resistance.

- K8LX

------

I went the route of the pier pin, with a homebrew base.
Make sure that the base surface is perfectly flat and
level. At first I was apprehensive about thinking of
what might happen if a set of guys went. But if you do
it right and use a good anchoring system and guy
hardware your fears will be reduced.

Its been about 4 years now, and the tower is really
solid as a rock.

- K1HTV

------

I was involved in erecting two 100 foot towers in one
month. The first was put up with the usual section in
concrete. I can tell you that to stand at the top of
that tower and look down is a scary proposition. It
twists and turns, because the owner was unable to have
all the guys perfectly positioned at their anchoring
points, and also because of a slight asymmetry in the
alignment at the base. A small error at the base of a
tower that size leads to a considerable misalignment at
the top. I predict that a considerable torque is
generated on that thing every time the wind blows.

For that reason, I chose to put up my own 100 foot Rohn
45 following Rohn's suggested pier pin. As soon as we
attached the lower set of guys, the bottom portion of
the tower pivoted into precise alignment. It went up
slick as a whistle. I asked a friend to bring a
surveyor's instrument over later to check out how
straight the tower was. The thing never varied more
than 3/4" from bottom to top. When I look up it, or
down, I have confidence that all forces are
appropriately balanced.

The pier pin tower is solid as a rock. I never have
detected a shift in it after the first time, when it
was equilibrating the forces from the guy wires. I
should tell you that I have it guyed with three sets of
1/4" guys, with the Rohn torque arm. I don't know
whether that made a difference, or not.

The other tower didn't shift when I was on it, either,
it was simply crooked, which would cause me to worry
when the wind blows, were it mine.

Hams in my club say that the main reason to avoid the
pier pin is that the tower might fall over complete
with a guy failure. That is, a 100 footer could damage
objects 100 feet away. A tower in concrete, they say,
will break in the middle, giving it a smller radius of
potential damage. I don't know about this. The only
tower I have any knowledge of going down was imbedded
in concrete, and it broke at its base, falling on a
house.

- NY3M
------

The local tower guru likes pier pin installations,
since they allow the tower to twist on the pin rather
than torque itself apart. Seems like a reasonable,
common sense idea.

A section in concrete provides you a sturdy starting
point and you don't need temporary guys. It also gives
a "rock solid" feel to the tower at and near ground
level which is reassuring. The pier pin gives a
disconcerting feel of movement, even when you've added
some sections and guy wires, though that movement may
in fact add to the mechanical integrity and longevity
of the tower.

- KE3Q

------

I have had my tower installed with the base pin method
for years. It has been no problem. Installation was
quick. The pin alignment (with vertical) is not very
critical. I put the first few sections together (2 or
3, can't remember) and tilted them up, guyed them and
then built the rest.

- K1GW

------

And finally, an agnostic (I think) with an interesting
and relevant tale to tell:

Most of my towers are "embedded" basing. I have done
things both ways over the years on the professional
jobs I have done. Embedded is cheap and easy or quick
and dirty or whatever cliche you want to use, but can
be problematic if the first section isn't done
perfectly and if the guys aren't brought up to good
(10% tensile) tension. Pier pins allow you to not
worry about the base as much but it is always a hassle
with the temporary guying.

I guess, in summary, that the pier pin is somewhat
better, since it allows the guys to do all of the work,
both for horizontal loading (wind) and torque loading,
since the tower doesn't have any inate strength in
either the horizontal (ie it will fall down if unguyed)
or the torsional (it just twists around the pin). If
embedded and the guys are loose both the horizontal
force and the torsional force are answered (in part) by
the tower structure itself, so it is doing more work
than just responding to the vertical forces due to the
tower's weight and to the vertical component created by
the guys due both to their tension and to the wind
loading adding more vertical loading. It is just that
the tower is pretty good in its own right and if the
guys are properly tensioned and if torque stabilizers
are used the tower can handle the loads when embedded.
But if the tower's first section is not true then it
must be pulled to vertical by the guys and there are
stresses that can be unevenly distributed on the welds
and structure.

The Rohn pier pin mount for Rohn 45 and 25 isn't all
that great, since the three legs come down to a flat
plate with the pier pin in the center. If the concrete
isn't exactly flat and level, one or more of the legs
may set on the plate with no concrete underneath it.
Then the vertical load is trying to shift the legs with
respect to each other and also puts forces
differentially on the legs causing stress on the welds.
The only fool proof pier pin approach is with the
tapered base section, where all three legs taper into a
central point. So pier pins are theoretically a little
bit better, but the Rohn 25/45/55 pier pin plates
aren't really the right answer if the concrete isn't
perfect. In that case you just as well use the
embedded section and get it level. I have two levels
mounted on a piece of aluminum angle that clamps around
a leg and gives me an indication in both planes at the
same time. Then I "shoot" the tower with a transit in
both planes to confirm that it is truly vertical all of
the way up.

Maybe what I said at Dayton was my story about having a
guy slip while installing Rohn 55. I had one of my own
installations where I was using a concrete base of 36
by 36 by 48 deep on an embedded style installation. I
had 40 ft of tower up and was tensioning the 5/16
(11,000 lbs tensile) to about 1,100 pounds when one of
the Klein grips popped off. Instantly I had
a vertical tower with only two guys each with 1,100
pounds tension. The tower oscillated enough for a
second or two resulting in the concrete enlarging the
hole in the dirt enough to put the better part of an
inch gap between the concrete and the earth around it.
The concrete remained in place so that the first tower
section was still vertical but the two guys were
pulling the 40ft up point about 2 ft off vertical
resulting in a severely bowed tower. I released the
tension on the other guys and inspected the tower very
closely and saw no apparent damage. I reattached
the guys, tamped the soil down around the base to
eliminate the air space and proceeded to re-tension the
guys. As I got to 1,100 pounds again the same Klein
grip popped loose and I went through the same procedure
and the inspection revealed no damage, which I credit
to the quality of the Rohn production and to the design
of the tower and the fact that all of the forces trying
to break welds, etc were distributed over 40 ft of
tower.

Bottom line is that I have a healthy respect for Rohn
tower, I threw away the (surplus) Klein grip and bought
new ones, and the tower has been up at 200 ft for about
8 years, even though some jerk shot out a guy wire at
the 160 ft level before I had installed the 200 ft guy
wires, so once again the tower was being pulled with a
pair of 1,100 pound guys 40 ft above the last solid guy
point. But this time there was an additional 40 ft of
tower above the point where the two remaining guys were
trying to pull the tower over. Again, there was no
damage and the guy who was shooting at the Golden Eagle
that often roosts at the point where the guy attached
has not been seen since. And I really didn't have
anything to do with his disappearance, really!

I am now installing two Rohn C towers. (24 inch on a
face with 2 1/4 " dia legs and bolt on braces) Both of
these will be pier pin towers for two reasons. They
came with pier pins when I bought them surplus. And I
will make them rotating towers and mount the tapered
section on a bearing with a sprocket for chain drive.
I guess, by definition, all rotating towers are pier
pin towers at least from the bearing on up.

- W0UN

--
73, Pete
N4...@netcom.com
"Better, faster,cheaper -- choose any two"