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[TowerTalk] centermast

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Subject: [TowerTalk] centermast
From: (Kurt Andress)
Date: Sun, 15 Aug 1999 11:20:54 -0700 writes:
> > I have a 100 Ft. SSV tower sitting in 16 Yards of concrete.  I have a (I
> >  believe been awhile) 2 in. ISD gas pipe for a center mast with I think 3/8
> >  wall thickness. 


> >  Should I replace the mast with a larger one or can I just insert another
> >  mast inside the present one and bolt through ?
> > wrote:

>      First of all, Chippewa County is an 80 MPH windspeed zone.
>      Second, unless you know the strength of the existing gas pipe, it
> probably isn't up to the job. Pipe is not rated for strength - just liquids -
> so by itself it's probably doomed.
>       My MARC (Mast, Antenna and Rotator Calculator) Program shows a bending
> moment of over 28K in-lbs and recommends a mast with 55kpsi yield strength
> and a 1/4 inch wall thickness. (MARC is available from
>       You need either a carbon alloy steel tube or seriously reinforce your
> existing one. I don't know what would be suitable - maybe we'll get some real
> engineering input.


Well, I'm sure I don't know the answers to the questions posed, but
here's some input.

First, I suspect the areas of some of the antennas, used to produce
Steve's numbers may be incorrect or misleading for this exercise.
And I expect the required information is not readily available. 

Without the projected areas and the knowledge of what the areas
represent, it's hard to approach the analysis. That's when I usually put
my pencil down.

Once the information is known, the azimuth of worst case load from the
combined antennas can be found (either 0 or 90 deg) and the projected
areas at that azimuth are used in the formulas from the specification of
choice to determine the loads on the mast.

If the choice is EIA-222-F:
Take the projected area and multiply it by 1.2, the force coefficient
for long cylindrical members, to get an effective area.
Then determine the wind pressure based on the basic wind speed and
antenna height, including gust and exposure factors.
Pressure x effective area = the load applied to mast.
This is a simplified version of the path presented in the spec.

If the choice is some other spec:
Start with the projected areas and follow the methods. They all start
with the same projected area requirement. Apply varying drag, gust, and
exposure coefficients. Some pressure formulas differ.

So, I'm not convinced anyone knows yet what the loads are on the mast,
or that it is clear which spec we want to apply to the problem.

But, let's say that the mast does have a 28 kin-lb moment at the tower
top, and the mast is 2" sched 160 pipe (that's nominal 2.375 OD x .343
wall). Had to make a guess, cuz we're still not sure what size it is.

Then, the stress in the mast is nominally 28.6 ksi.

Now, someone gets to pick a material:

ASTM A53 electric welded Grade A        yield = 30 ksi
ASTM A53 electric welded Grade B        yield = 35 ksi
ASTM A53 furnace welded                 yield = 25 ksi
ASTM A106 Grade A                       yield = 30 ksi
ASTM A106 Grade B                       yield = 35 ksi
API 5L  Grade A                         yield = 30 ksi
API 5L  Grade B                         yield = 35 ksi
API 5L  Grade A25                       yield = 25 ksi
Add any other options here!

The thing might be marginal, might not.

With the correct info, the mast stress may be higher than
we're discussing. May be less....

As far as what to do if the truth were known....

A new mast, that can handle whatever the loads are, is straight forward. 

Getting a tube insert to work properly requires a fairly good fit.
It might be a bit hard to find a tube that fits well inside the existing
The tolerance stackup on the actual mast and insert would determine what
might work.
Not out of the question, but certainly a challenge to sort out.

Best of luck with the project!

73, Kurt, K7NV 

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