Happy Holidays Everyone,
I think Tom's (N4KG) comments regarding the newer Rohn configurations
being inferior to the previous ones are right on target.
The only reasons for using a configuration with a cantilevered top tower
section would be to gain more antenna separation than available on the
mast alone and/or have independently rotating antennas near the top.
This approach certainly doesn't come without its cost of reduced antenna
loading.
This type of configuration, for mast mounted antennas, cannot carry as
much load as one with the top guy set connected to the tower top.
In the example presented, where the bending moment in the tower at the
top guy point was 7440 Ft-Lbs, this is probably not going to fail the
section. In the older catalogues, the 45G section is rated for 9610
Ft-Lbs.
In the newer designs, the max allowable combined stress is 50 ksi. This
would translate to 37.6 ksi allowable for any single load case. At 7440
Ft-Lbs, the stress just due to bending would be 21.5 ksi, to that we
would add stress due to shear and compression, which are quite low near
the top.
If the tower is buried in the footing, the problem is going to be at the
base. The cantilevered top configuration creates higher loads on the top
guys, resulting in higher elongation, which makes the tower lean over
more. This creates higher bending stress at the base.
This can be seen in the examples on my website.
I find the failure Tom cited very interesting (we get to to hear so
little about them!) and wanted to look into it to see what might be
learned.
We never get enough information to really make sense out of them.
Since, conjecture is the prevalent method of evaluating these
occurences, I thought it would be fun to exercise, my crystal ball, to
see what might be supposed.
>From the description, I gather that this is a 130' 45G tower, for a
basic wind speed of 90 Mph. This is because this tower has the top guys
9' below the top of tower. The Rohn 70 Mph & 110 Mph towers place the
top guys 5' below tower top.
Note:
BTW, there is a typo error on the 70 Mph 60' tower configuration. It
says to place the single guy at 45'. It should read 55'. Anyone out
there think a 60' tower with only one set of guys is the best way to
maximize the amount of antennae you can put on it?
Furthermore, I'll assume that it is configured just like the book, 1 set
of 1/4" EHS guys at 121' and 3/16" EHS guys at 78' and 35'.
Tom can correct this if my assumptions are wrong.
It is hard to say what the actual top plate load case was, as there is
not enough information. There was certainly the lateral load from the
plate supporting the mast. If the bearing was a Rohn TB type, there is
the likelyhood that the plate was also bending, because this type of
bearing does not absorb axial deflections like the industrial pillow
block bearings can. It may have been a fatigue failure.
The BPL45G top plate should be good to something like 5-6 times the
rated load in pure shear. Actually, the weak link, in simple shear,
should have been the 45G legs at about 4 times the rated load.
A guy bracket, just below the top, would not have changed the loads on
the top plate. Having the top guys connected to the ears on the top
plate would have helped. It may not have prevented the failure, but
might have kept the stuff on the tower for emergency removal.
The Rohn 90 Mph 130 Ft design says that the tower can carry 12.3 SqFt of
antennas that are made from tubing, and the load is applied exactly at
the top of the tower.
Using EIA-222-F methodology, this load is 518 Lbs. The highest lateral
load of all the 45G designs is 610 Lb on the 70 Mph 40 Ft tower. This
should be chump change for for the BPL45G top plate.
Now let's look at what was reported to be on the tower...
Tom reports that there was a Force 12 6 El 20. I looked at the Force 12
website and could only find one 6 El 20. The Magnum 620, which is
reported to have 9.1 SqFt of area. But, maybe this is a special design,
so we'll use the 8.3 SqFt cited.
The other antenna was a 2 El 40 / 2 El 80. The only 40/80 combined
antenna I found was the Magnum 2/2, listed at 14 SqFt. Maybe the actual
antenna was 80 only, and didn't have the switching gear, so we'll still
call it 13 SqFt.
So, if we take the 6 El 20 @ 8.3 SqFt and the 20/40 @ 13 SqFt we see
that there was 21.3 SqFt of antenna on the tower, right?
That's only 73% more area than the tower was rated for, right?
We need to be mindful of a few previous discussions on TT.
The EIA spec, and hence the Rohn designs, are predicated on the antenna
areas being supplied as what is commonly called the "projected area."
This also applys to crankup designs performed to UBC code.
The Force 12 figures are "effective areas", which are calculated by
multiplying the "projected area" * 2/3.
To get to the antenna areas we need to evaluate the tower, we need to
multiply the factory figures by 1.5 (1/.66).
This has been confirmed by other TT participants. Contrary to popular
belief, It's not just my own delusion.
So, in "Rohn (EIA-222) lingo", this tower actually had something like 32
SqFt of antenna on it.
That's 260% over the rating for my assumed configuration, and does not
account for the antennas being mast mounted above the tower, which will
significantly increase the tower loads.
I didn't attempt an analysis of the actual situation. That effort would
have required far too much assumption on my part.
I swore off talking about this anymore, but this is just such an
excellent example, I couldn't keep my mouth shut.
This is still a gigantic problem, that just goes to sleep every time I
bring it up. People are taking published antenna area figures, that are
incorrect (or at least are not known to be correct) for tower
evaluation, and applying them to their towers. I expect we will have
some more of these discussions, when anyone has the moxy to step forth
to report their failures.
Tom's cited tower failure might be a good example of how far not to go
when overloading a tower.
It is certainly a good data point for the bold experimenter, but we
still don't know what the wind exposure and history was at this site.
Everyone that plans to overload their towers, please install
anemometers, so we can have more meaningful information! Anemometer
builders, please provide fastest mile wind speed calculations, so we can
directly use the information.
I still agree with Tom that the current Rohn designs are far from the
best solution for most of our types of applications. In my mind, that is
an entirely different matter, than the cited failure.
I'm just not so sure we can pin the blame for this failure on Rohn.
Remember, when we step outside the box, we take matters into our own
hands!
I wonder how much this experiment cost, and whether it wouldn't have
been more cost effective to have someone design a tower configuration to
fit the antennae?
Y'all have fun with it!
--
73, Kurt, K7NV
Father Physics & Mother Nature control our world, the rest of us are
either passengers or students!
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
---Original Text---
n4kg@juno.com wrote:
>
> The old Rohn catalogs only gave load ratings based on the
> full rated height of each tower type (200 ft for R25, 300 ft for R45).
> No deratings were called out for lower heights. All guying
> illustrations showed guys equally spaced along the full height
> of the tower (i.e. top guy attached to the top of the tower).
>
> The new Rohn catalog (mine is dated 1994) shows increasing
> loads for lower heights. This is a welcome clarification but the
> examples show the top guys for R25 placed 5 ft down from the
> top of the tower and the top guys for R45 placed 9 ft down from
> the top of the tower.
>
> The ratings are appropriate only if ALL of the antenna load is
> placed along the open tower above the top guy. If you extend
> a mast above the top of the tower, the bending moment applied
> to the tower can become excessive.
>
> To minimize the bending moment applied to the tower, the
> largest antenna should be placed as close to the guys as
> possible. With the guys 9 ft down from the top, the only way
> minimize the bending moment and rotate the antenna is to
> use an expensive ring rotor or side mount just above the
> guy wires. If the large antenna is placed one foot above
> the top of the tower the bending moment will be 10 (ft) X
> the antenna windload vs 1 or 2 (ft) times the windload if
> the guys are placed at the top of the tower.
>
> The bending moment for an antenna at the top of a 12 ft mast
> will be (12 + 9) X the antenna windload vs. (9 + 1 or 2) X the
> antenna windload if the guys are placed at the top of the
> tower.
>
> Example: Consider two antennas with 8 sq ft of windload
> placed 1 ft above the top of the tower and 12 ft above the tower
> top with a wind force of 30 psi (87 mph). The windload forces
> will be 240 lbs at each antenna.
>
> With the top guy 9 ft below the top of the tower, the bending
> moment is (9+1) X 240 + (9 + 12) X 240 = 7440 ft lbs.
>
> With the top guy 1 ft below the top of the tower, the bending
> moment is (1+1) X 240 + (1 + 12) X 240 = 3600 ft lbs.
>
> The bending moment contribution of the mast and tower
> was not included in these calculations.
>
> Notice that the guying configuration shown in the new Rohn
> catalog results in more than twice the bending moment
> applied to the tower for these moderate sized antennas
> (similar to TH6 for example). Full size 3L40's or 2L80's
> will be much worse.
>
> When using a rotating mast to support more than one
> antenna above the top of a tower the top guys should
> be placed as close to the top of the tower as possible.
>
> This is the ONLY way to safely realize the full capacity
> of a guyed tower with multiple antennas. The amateur
> community needs to encourage Rohn to provide load
> ratings for their guyed R25 and R45 towers with the
> guys placed at the top plate or first step down from
> the top.
>
> You may ask, "Is the bending moment at the top of
> a guyed tower a real consideration?"
>
> I'm glad you asked. The answer is a resounding YES!
>
> One of locals had a Force 12 6L20 (8.3 sq ft) mounted
> above a Force 12 2L40 / 2L80 (13 sq. ft.) on 130 ft of
> Rohn 45 with the top guy 9 ft below the top of the tower.
> The rotor was approximately 5 ft down from the top of the
> tower.
>
> A storm ripped the top plate off the top section causing
> the mast and antennas to plummet to the ground. He
> suspects a cold weld may have contributed to the
> accident but the above calculations show that the load
> born by the tower is considerably higher when the guys
> are placed below the top of the tower.
>
> de Tom N4KG
--
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