From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
if sending attachments: k1vr@gis.net
Here is a summary of the trenchant remarks made the last time the subject
of Rohn Safety factor was discussed on this reflector, in the Spring and
Summer of 1998.
From: k6ll@juno.com
To: <towertalk@contesting.com>
Date: Wed, 27 May 1998
Subject: [TowerTalk] re: Rohn safety factor
On Tue, 26 May 1998 17:36:13 EDT k1vr@juno.com (Fred Hopengarten) writes:
>I have in my head that all Rohn catalog designs include a safety factor
of 3, but at the moment I cannot find any note in the catalog about this.
>Does anyone know what safety factor Rohn uses for 25G, 45G and 55G? Can
you refer me to the drawing or page that says it?
Look at the assembly drawing for the tower section, e.g. Dwg # C630625
for Rohn 25, C630645 for Rohn 45, C630655 for Rohn 55.
In the "Nomenclature" column, items C, M, and T are specified as "with
1/3 increase in allowable stress." That's all I ever remember seeing. I
don't think it's valid to extrapolate that into a 3:1 safety factor for
an entire tower assembly, but if you choose your words precisely, you can
make some nice statements regarding the conservative nature of the
design.
The drawing also references ANSI/EIA-222-E-1991. Maybe there's something
in there regarding safety factors. I haven't looked.
Dave Hachadorian, K6LL * Yuma, AZ * K6LL@juno.com
=======================================
From: k1vr
Full-Name: Fred Hopengarten
To: k6ll@juno.com
Subject: Re: Rohn safety factor
K1VR: Dave, K6LL, may be on to something here. In the TIA/EIA-222-F
standard, to which Rohn's latest catalog specifications conform, I was
able to find two relevant statements."3.1.1.1 For structures under 700
ft. [213 m] in height, allowable stresses may be increased 1/3 for both
load combinations defined in 2.3.16."[2.3.16 refers to D +Wo, which is
deadweight + design windload on the structure , appurtenances, guys,
etc., without ice; and D + . 75Wi + I, which is deadweight + .75 design
windload on the structure, appurtenances, guys, etc., with radial ice, +
weight of ice.] and "8.2.1 For structures under 700 ft [213 m] in height,
the safety factor of guys and their connections shall be not less than
2.0."
======
From: "Steven H. Sawyers n0yvy" <sawyers@inav.net>
To: <towertalk@contesting.com>
Date: Tue, 26 May 1998
Subject: Re: [TowerTalk] Rohn Safety Factor
Sorry, but the safety factor is not 3:1. I wish it was.
The EIA-222 standard requires a 2:1 safety factor on the guy wires. I
have done comparison calculations and I come up with about the same thing
they have in their literature. Everyone should do a sanity check calc
like this to make sure they are doing it right.
The tower buckling load has a safety factor on it that varies with
height.
As an example, drawing CB70488 R1 shows a 190' tower in a 90 mph wind
zone. The base load for this tower is 9,870 lbs. The same drawing shows
the base load for the 70' tower at 3,010 lbs. Now this is the same
section of tower but the applied load is different while the guy spacing
is almost the same (31' vs. 32'). So I guess you could say that it has a
3:1 or better in this particular case.
In both cases the limiting factor is the guy strength available in the
top 3/16" guy wire.
Now if you would use 1/4" guy wire (6700#) versus 3/16" guy wire (4000#)
on a 70' tower, you won't get anywhere near the 9,870 lbs of base load so
you won't over load the tower section, but you should be able to get
about 60% increase in antenna wind load available. This is provided the
guy anchors don't pull out of the ground and you use the GA25GD guy
bracket to get the load properly distributed into the tower - especially
for the top guy. The second set can still be 3/16" and looped around a
tower leg.
Oops, I just gave away one of my tower engineering secrets.
n0yvy steve Steven H. Sawyers PE
ARRL Volunteer Consulting Engineer.
=====
From: "Lowell, Mark" <mlowell@noclant.navy.mil>
To: towertalk@contesting.com
To: <towertalk@contesting.com>
Date: Wed, 1 Jul 98 11:22:00 -0400
Subject: Re: [TowerTalk] Guy Anchor Pullout Strength
Yes, I have a copy of the Rohn catalog and have been studying it. Yes, I
have been looking at the forces. Thanks to all who have already responded
so far. Reading this reflector has got me digging into the numbers a
little (OK, a lot <grin>), rather than just copying what all the others
are doing around here (no engineering). Some of the installations make my
skin crawl after learning better from this reflector. But they stay up.
I live in Gloucester County, VA. Thank you Steve, for looking up the wind
rating. We have been wondering when we will get a direct hurricane hit.
Usually, the outer banks of North Carolina take a lot of the OOOOMPH out
of the storms before they get up our way. We thought Fran was going to
get us. We got tornadoes and torrential rains in lieu of wind. Which is
worse???
I have a friend with 100 ft of Rohn 25 with guys spaced at only 35 feet.
Yikes!, now that certainly seems to cut it close. However, it has been
up for many years, through many storms. We both live in highly wooded
areas, providing shielding from maximum winds. As Steve hs pointed out,
it is possible to have less than 80% anchor spacing. I have come to the
conclusion that the Rohn spec is very conservative, as you might expect
for liability's sake.
Well, yes, I am a mechanical engineer and I do possess the skills to
analyze the forces, and so far, the numbers look ok. But it would be nice
to review the project with someone before I start, as I <humbly> could be
missing something. Rohn specs are *very* conservative, and building to
them is a very safe bet.
In the archives, someone once posted the max compressive load for Rohn 25
tower as 8430 lb per leg, 25,290 total, about what you'd expect for
steel. That's the best data I have so far. The guy preload goes up from
10% to 15% to compensate for the loss in horizontal preload, due to the
geometry. The compressive (vertical) loads do go up, but not that much
for 45% spacing. I don't own enough land for more spacing. I came up with
6518 lb total compressive load, with the upwind leg being loaded the most
at around 3500 lb.
I haven't built anything yet, and still open to suggestion, persuasion,
education, and bonk-bonks on the head! <big grin> --...MARK_N1LO...--
=====
From: Kurt Andress <ni6w@yagistress.minden.nv.us>
To: froemke@ibm.net
Cc: towertalk <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Thu, 06 Aug 1998 01:12:11 -0700
froemke@ibm.net wrote:
> Kurt, is there a model available for Rohn 25G that I could use to look
at the mechanical stresses for my next tower?
>
> Jim, K0MHC
Sorry, there are no models that you could effectively use to do whatever
it is you anticipate doing (see my response to K1VR).
I strongly recommend that you diligently follow the Rohn/ EIA specs for
the installation!
This thread was caused by my irritation about the fact that, since I
subscribed to this reflector, every comment about tower behavior
neglected the effects of the stretch behavior of the the guying lines. I
just felt compelled to say it isn't so! I figured it would cause a
firestorm, which it did, I now know why I waited so long to say it!
>From the responses, I'd say that there are two completely different
camps on the subject. Those that get it and those who don't. Well,
that's really good, because people are, at least thinking about it!
Those who want to continue in denial can do so at their leisure. Those
who want to use the information to pursue a more accurate solutions can
also do so!
I'm done with this subject! I fully stand behind my claims that wire
stretch is important and can have devastating impact on guyed structures.
Believe it or not!
Kurt
--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress
=====
From: k2av@qsl.net (Guy L. Olinger)
To: w7ni@teleport.com (Stan Griffiths)
Cc: towertalk@contesting.com
To: <towertalk@contesting.com>
Date: Wed, 19 Aug 1998 19:31:29 GMT
Subject: Re: [TowerTalk] Rohn Tower Designs
On Wed, 19 Aug 1998 03:23:36 -0400 (EDT), you wrote:
>Rohn offers a service (for a LOT of money) to analyze a NON-CATALOG
tower.
>This means there are a LOT of ways to use Rohn towers NOT shown in the
>catalog. Rohn has fully engineered and calculated the few dozen or so
>configurations you see in their catalog. There are literally thousands
of
>possible configurations not shown there. You are not resticted to using
>only the examples they show. If you choose another design, just make
sure
>it was done by a competent PE and you should be OK.
>
... snip ....
>Stan w7ni@teleport.com
Many thanks for several very forthright posts.
Along this line, take a look in your favorite Rohn catalog. Count the
number of designs which have the guy wires going to the *TOP* of the
tower. If you come up with zero, you win the prize. A lot of the 55g
designs have 15 feet free and clear.
Why is that? Easy...these are designs for commercial installations.
Sidearms, microwave dishes (steel golf umbrellas if you read my earlier
post). Why such low ratings?
Well, for starters, consider 15 feet above the guys with dishes. These
things oscillate in the wind, if not *really* pinned down. A thirty foot
boom and a blizzard of elements would seem far less likely to oscillate
than a microwave dish because of the enormous damped moment on the boom.
How much less likely than that if the guys are at the top.
In terms of pure windload, with guys at the top, the load goes straight
to the guys and tower downward compression. With the commercial
15'-in-the-clear design, that top 15 is a lever with it's own windload
that gets to counterflex the tower under the top set of guys, and
requires a derating.
I could go on.
It appears that there is *not even one* ham design in the catalog. Just
a bunch of commercial designs that they copied over. It's not that they
haven't thought about it, they sell a top plate with the guy connections
right there. Just no tower designs with the plate and the wind loads to
match.
We really need ham designs that assume top guying, and dampened twisting
moments on top. And an optional derating factor for offcenter down-tower
mounting (ring-rotor, etc).
I second the call for software. We need a "Towerstress" program with
Rohn, US tower, Trylon, etc, in it.
Guy L. Olinger k2av@qsl.net Apex, NC, USA
=====
From: Kurt Andress <ni6w@yagistress.minden.nv.us>
To: <towertalk@contesting.com>
Date: Sat, 22 Aug 1998 01:54:37 -0700
Lee wrote:
> Why no torque arms? Also what do you mean by, "two top guys / anchor"?
Do you mean double the top guys?
I think Steve Sawyers is referring to a TA25 Torque Arm Stabilizer, which
allows for the attachment of 6 guys that terminate on the 3 anchors. This
is a vastly superior torque stabilizer than using the torque bars. You
can find it on the 25G accessories page.
Kurt
--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress
=====
From: "Steven H. Sawyers n0yvy" <sawyers@inav.net>
To: <towertalk@contesting.com>
Date: Sat, 22 Aug 1998 23:27:57 -0500
At 11:37 PM 8/21/98 -0400, Guy Olinger, K2AV wrote:
>
>Where can one get the formula's, etc that generate this bit of wisdom
and other such? Are they available for R45 & R55 as well?
The problem is that there is no one simple formula - it is about 50 to 60
formulas and they all relate to one another.
Generally on the shorter (less than 100 feet) you can safely increase the
guy wire size and increase the allowable wind load by the same ratio as
the guy wire strength increase.
A Rohn 25 section will handle over 16,000 lbs of compressive load if you
keep it straight. In a 90 MPH wind district the 100' tower in the Rohn
literature only has a base reaction of 4,290. The larger guys are not
going to overstress the tower IF you use the Rohn guy bracket to
distribute the load in to all three legs instead of just looping the guy
wire around the nearest leg. Off course you will have increase amount of
concrete in the ground to hold the guy wire.
Basically the same thing applies to the 45 and 55 tower sections, but
then you have to complication of Rohn including some side arm area in the
foot notes.
If you are truly interested in getting an understanding of this, then get
a copy of the EIA-222 Rev F for wind loading, get a copy of the AISC
Steel Design Handbook for allowable stresses,the Uniform Building Code
for soils loading, and a little book called Design of Cable Structures
that Tab books put out several years ago. These reference materials and
about sophomore level Structural Analysis class, or just being good in
high school physics with an understanding of vectors will get you
started.
There is nothing magic about this, it is very straight forward when you
follow a standard. If you don't follow an engineering standard, then you
can get slightly different answers, but it is always safer to follow the
engineering standards.
I am a PE and I have been doing crane and tower design for 25 years. I
have been doing enough Rohn work for the local hams to have developed a
set of basic designs. Unfortunately, no one's request ever matches my
basic design or the last one I did. So I customize what I have done
before. I have also sat with the spread sheet and done a lot
of "what if" analysis. I have successfully replicated most of the Rohn
tower designs. So I have developed a "feel for the iron" to quote one of
my first engineering supervisors.
N0YVY Steven H. Sawyers PE
ARRL Volunteer Consulting Engineer
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