[TowerTalk] Rohn 25 leg deformation

[Michael Tope]

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.
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. 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.

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....................................

P.S. There is a picture of the installation at

http://www.its.caltech.edu/~w6ue/Spalding/Spalding_Stack_1a.jpg

----- Original Message -----
From: <Tower2sell@aol.com>
To: <w4ef@pacbell.net>
Sent: Saturday, August 05, 2000 12:02 PM
Subject: Re: [TowerTalk] Rohn 25 leg deformation


> In a message dated 8/2/00 11:42:11 PM Central Daylight Time,
w4ef@pacbell.net
> writes:
>
> > The guy attach were the deformation occured is approximately
> >  12" below the top plate of the AS25G top section. This
> >  portion of the tower has no diaganol braces. My calculations
> >  indicate that the antenna load is consistent with the Rohn
> >  25G specification for maximum bending moment of 6750 foot
> >  pounds up to about 100 mph. These calculations assume that
> >  the overturning moment exerted at the guy point is the limiting
> >  factor in the tower's capacity, and that the 6750 foot*lb
> >  spec applies to the AS25G top section as well as the standard
> >  25G straight section.
>
> This is a bad asumption. The bending capacity of the AS25G for the portion
of
> above the diagonal braces is goverened by the bending capacity of the 3
pipes.
>
> M = 44ksi * .0682 in^3 * 3 pipes / 12 in/ft =0.75 ft k
> not  6.75 ft-k shown on C630625 R9
>
> The 25A was designed for light duty antennas like TV antennas.
>
> The drawing C870484 R4 for a 60' tower (70 mph basic) has 16.8 round or
10.1
> flat.
> This relates to about 20.2 sq ft EPA (with shape factor) Rev F
>
> The analyis of this tower is limited by the diagonal brace capacity. It
has
> 5' from the top of the tower to the top guy wire. The bending capacity of
the
> section does not control because diagonal braces are assumed.
>
> >The wind load versus force relationship
> >  that I assumed was based upon the formula given in Leeson -
> >  P = .004*v^2 (lbs/ft^2) where v is wind speed in mph. For
> >  projected area, I use KLM's specifications for wind area
> >  directly without any modification for shape (drag coefficient
> >  = 1.0). My understanding is that EIA specifications have
> >  been recently been revised downward, making these calculations
> >  somewhat conservative.
>
>
> Not correct. The basic equation is
>
> P (lbs) = 0.00256 * V^2 *Kz *Gh*EPA
>
> Kz is the height escalation factor and is a function of the height is
question
>
> Gh is the gust factor and is a function of the tower height.
>
> >
> >  Depending on wind direction, the crimped leg may be placed
> >  in tension or compression. In pure tension, I wouldn't expect
> >  the shape of the legs cross section to have much bearing
> >  on its capacity. On the other hand in a compressive situation,
> >  seems that crimped leg might be more prone to buckling in the
> >  plane of its narrow dimension. Anyway, not being a mechanical
> >  engineer, I don't know if I am fretting needlessly, or poised
> >  for disaster.
> >
> >  73 de Mike, W4EF..................................
>
> It appears to me that the crimped leg are the least of your concerns.
> Personally, I wouldn't use the 25AG et al for anything more than hanging
my
> dirty laundry from.
>
> Tower2sell@aol.com


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