[TowerTalk] Sources for mast material

[Michael Fox (K5MEF)]

Hi Steve,

 

We'll you're definitely the guy to know.  

 

To answer your question, the total wind load calculated in WinMARC is:

 

Wind Speed      Total

70 mph             6,151 in-lb

80 mph             8,197 in-lb

90 mph             10,243 in-lb

100 mph            12,290 in-lb

 

My central question is the meaning of the "wind speed".  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?

 

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 almost like
it's looking for something that's at least 3X higher than the calculated
total.  Maybe you could shed some light on that too?

 

Thanks in advance.

 

73,

Michael - K5MEF

 

--

We are what we repeatedly do.  Excellence, then, is not a single act, but a
habit. -- Aristotle

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From: K7LXC at aol.com [mailto:K7LXC at aol.com] 
Sent: Tuesday, August 05, 2008 3:59 PM
To: towertalk at contesting.com; k5mef at mefox.org
Subject: Re: [TowerTalk] Sources for mast material

 

In a message dated 8/5/2008 6:23:13 A.M. Pacific Daylight Time,
towertalk-request at contesting.com writes:

>  I used Champion Radio's WinMARC tool to calculate the recommended masts
for
my planned VHF/UHF stack.  If I enter an extra safe wind speed of 100 mph
(my county is 70 mph basic speed), the tool suggests the following masts:

Wall  Yield Strength    Description

----  --------------    ------------------------------------------

0.120       40,000      6063-T835         Drawn aluminum tube
0.120       42,000      2024-T3           Drawn aluminum tube
0.120       46,000      A 500 Grade B     Structural steel pipe
0.120       50,000      1010              Carbon steel CDBW 


Howdy --

 

    What was the bending moment figure you're working with? 40,000 psi or
what? 

 

    This is much simpler IMO. Your 100 MPH safety factor has skewed the
whole thing. That's 2-3 times the wind pressure as your REAL windspeed
rating. I'll never discourage anyone from over-engineering something but you
should back up a step or two. 

 

    Your vhf/uhf stack is pretty small. If you lower the windspeed to 90 or
even 80 - it'll change what's required - resulting in you saving a bunch of
money and hassle. (I don't discourage anyone from spending money either.)

 

    All you really need is - dare I say it - some galvanized pipe; e.g.
Schedule 40 water pipe,  conduit, EMT, etc. It's rated around 35,000 psi
yield strength which should be fine for your proposed installation. And you
can drive down to your big box hardware store and get what you need. Cheap.
Maybe you have some in your boneyard.

 

    While the galvanized pipe is 1.9" OD, your yagis won't know the
difference. That's what I'd do. 

 

Cheers,

Steve     K7LXC

TOWER TECH 

 





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