[TowerTalk] Rohn 25 leg deformation

[Michael Tope]

Hi Tower2sell,

I see what you mean now by the "portal" analogy. In
terms of shear load, the top guy must balance the
horizontal wind load on the antennas. I was so
preoccupied with overturning moment (balance of torques)
that I neglected to consider the balance of lateral
forces. The tendency here is as you suggest for the
portal formed by the tower legs just above the top
guy attach points, and the top plate to deform from
a rectangle into a parallegram. Ignoring any
discrepancies in my windload calculations for the
moment, at 50 lbs/ft^2 (~100mph) the portal would see
around 1000 lbs of horizontal shear force. Given that
the vertical height of portal is around 6", the bending
moment on the unreinforced leg span would be around
0.5kft*lb. Still within spec, but close enough for me
to sit up a take notice. More reason perhaps to move
the guy attach to the top plate as this would move the
shear up load off the unreinforced leg spans.

Thanks for the input!


Mike, W4EF.................

----- Original Message -----
From: <Tower2sell@aol.com>
To: <w4ef@pacbell.net>
Sent: Sunday, August 06, 2000 12:06 PM
Subject: Re: [TowerTalk] Rohn 25 leg deformation


> In a message dated Sun, 6 Aug 2000 12:06:55 PM Eastern Daylight Time,
Michael Tope <w4ef@pacbell.net> writes:
>
> << Hi Tower2sell,
>
> >Regarding the bending capacity of the AS25G, although
> >I must admit that I made a leap of faith when assuming
> >that the AS25G has the same bending capacity as the 25G
> >straight section, I do not agree with your calculation
> >of its maximum bending moment capacity. It appears
> >that you simply added the section moduli of each leg
> >algebraically with no attention given to the >geometrical
> >arrangement of the legs. This would suggest that >bundling
> >the legs together side by side would provide the same
> >structure as having the three legs in a triangluar
> >arrangment 12" on a side with a steel plate welded to
> >the top, and a steel gusset welded in the middle. >Perhaps
> >you are confusing the AS25G with the 25AG2. With the
> >25AG2, the three pipes do come together at a single >point.
>
> This may be the case. When you said there wer no diagonal braces I assumed
a standard ROHN catalog part top section such as 25AG 1 to 5. The AS25G is
not listed in my catalog so I may be shooting blind. The principals are
still the same. If you do not have diagonal braces the shear load does not
distribute the same. This means that without braces a portal action develops
and puts the individual legs in bending. The effective section modulas
reverts back to the sum of the three legs. The bending moment in the legs is
then the shear load times the distance from the top to the bottom of the
portal. The tension and compression truss loads are then combined with the
leg bending stresses.
>
> This discription is almost too complicated to discuss in an e-mail.
>
>
> >In this case, the bending capacity would be limited as
> you suggest.
>
> >As far as the intended purpose of 25G goes, 20 sqft of
> >EPA is a pretty big TV antenna.
>
> What I ment was the 25AG series top sections were developed for TV antenna
use. Since seams to be a moot point.
>
> >Admittedly though, given
> the choice, I would have gone with Rohn 45 for this
> installation. Unfortunately, I had to use what was already
> >there.
>
> >The equation I used for wind pressure vs speed is from
> Leeson, and is as you point out a simplification. This
> is explained in the book. The difference between the
> leading coefficients (.004 vs .0256) is the drag
> coefficient for round members. When I get a chance, I
> will rerun the calculations with the correct gust factor
> >and height factor. Thanks for pointing this out.
>
>
> This looks like an old Rev C type formula. Which used a uniform wind and
no 1/3 increase.
>
>
> >I may redo the upper guy attach to this tower using the
> holes in the AS25G top plate. This should distribute the
> shear load of the guys across the top plate, instead
> of right into the legs (our current installation puts
> the wimpy little welds to the rotor shelf in tension).
> Also, its unclear to me that the top section takes the
> full brunt of the antennas overturning moment as we have
> 10 feet of 105KSI chrome moly in the tower. The cross
> section of the mast just above the thrust bearing
> obviously carries the full overturning moment of the
> antenna load, but this get transferred to the tower in
> a rather complex way since the transition from mast to
> tower isn't abrupt (the top tower sections with mast
> inside has some composite bending moment capacity).
>
> Anyway, I can think of cheaper ways to build a clothesline.
>
> >73 de Mike, W4EF....................................
>
>
> Mike, I was just trying to make the point that sometimes other elements of
the tower control the design. The 25G design in the ROHN catalog is very
simplistic and safe design. The sheets have made some very simplistic
assumtions, one that you will be using 25G sections. They do not tell you
that the shear load in the top of the tower controls the design.
>
> A picture is worth 1000 words. You do not have a standard ROHN catalog
installation and the values shown in the catalog are not appropriate for
your application. You seam to be guyed at or near the top of the tower which
significatly reduces the moment and shear load on the section. (this is
good)
>
> Back to your original question. Does chrushing the the leg reduce the
capacity. The answer is somtimes and somewhat. Meaning that if you have
braces and the truss action controls - I don't think you could measure any
difference.
>
> If you do not have braces then the failure mode may be in bending of the
pipe which would lead to a local buckling type failure (step on a coke can
and you'll know what I mean) Now, the deformation provides a discountinuity
in the  pipe wall that could enhance local buckling. Now the good news is
that the U-bolt should stiffen the section so that the actual local buckling

bending should be right above the U-bolt. Even if the pipe wasn't originally
deformed.
>
> I don't think that using a few 25G tower setions for a clothes lines would
be that unreasonable. I did build my own ladder once. It was 11' long, cost
$8 and was indistructable. Only problem was it took two people to carry it.
(minor inconvience)
>
> Tower2sell@aol.com
>
>
>
>



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