# [TowerTalk] FW: Sources for mast material

Michael Fox (K5MEF) k5mef at mefox.org
Thu Aug 7 15:54:07 EDT 2008

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>    1)  What is the meaning of the "wind speed" parameter.  The dialog box
says
"maximum wind speed expected".  In my county, basic wind speed is 70 mph
fastest mile, 85 mph 3-second gust.  So I figured the max instantaneous is
probably higher.  So I figured 90 or 100 would be a good number.  But it
really depends on what the meaning of this number is in the MARC tool
(fastest mile, 3-second gust, other).  Can you define?

We used the formulas and specs from Dave Leeson's (W6NL, ex-W6QHS) book
"Physical Design of Yagi Antennas." Maybe there's more of an explanation
there.

The over-riding spec is your county windspeed rating from the TIA-222
There are several ways of arriving at a figure.

I don't have a copy of the above book.  Perhaps someone who has a copy knows

The windspeed shown on championradio.com for my county is the fastest mile
windspeed (70 mph fastest mile).  But the current TIA-222 spec now uses the
3-second gust speed (85 mph 3sg).

So, as mentioned above, the question is, to which "wind speed" does the
WinMARC application refer:  fastest mile, 3-second gust, or "maximum" (as
the dialog box says)?  Without knowing that, the number to enter for "wind
speed" is unknown.  70 (fastest mile)?  85 (3 second gust)?  90 (more likely
"maximum")?  And, if the correct wind speed is unknown, then the resulting
tensile strength calculation is meaningless.

>  2)  Also, when the calculated total is 12,290 in-lb, the lightest
suggested
mast type shown in the list is a 40,000 in-lb yield strength.  It's seems
like it's only displaying mast types that are at least 3X higher than the
calculated total.  Maybe you could shed some light on that too?

No, that's because tubing has that as a minimum rating. As opposed to
pipe which doesn't; it's around 35 kpsi and we don't include pipe as a
generally suitable mast material.

But the behavior of the WinMARC application does not agree with that
description.  For example, if I reduce the wind speed such that the
calculated tensile strength is only 6151 in-lb, then the list of possible
mast types also changes, with the lightest version being 25,000 yield
strength  "6063-T6 Aluminum pipe" - that's a 4X difference.  A53 and A36
"Structural steel pipe" are also listed as suitable mast material.

So, again, the application seems to be listing recommended mast types that
are 3-4X higher in strength than the calculated load and the answers do
include both tubing and pipe.  The simple question is "why is it picking
those types, rather than something closer to the calculated value"?

If your design yield strength is 13 kpsi, then anything will work.
That's why I suggested Schedule 40 or 80 water pipe or conduit. You're
over-thinking this whole thing.

Yes, this situation is relatively simple.  And thanks for the straight and
simple answer on the sched. 40.  That helps a lot.

But the problem is that the application is giving answers which differ
significantly from what you're saying here.  So I'm trying to understand why
- and not getting very far.  This isn't the only antenna mast I need to
figure out.  So I'm simply trying to understand the proper inputs to, and
the meaning of the outputs from, the application so I can use it for other
configurations.

Thanks for all the helpful info.

73,

Michael - K5MEF

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