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.
Michael - K5MEF
We are what we repeatedly do. Excellence, then, is not a single act, but a
habit. -- Aristotle
From: K7LXC@aol.com [mailto:K7LXC@aol.com]
Sent: Tuesday, August 05, 2008 3:59 PM
To: email@example.com; firstname.lastname@example.org
Subject: Re: [TowerTalk] Sources for mast material
In a message dated 8/5/2008 6:23:13 A.M. Pacific Daylight Time,
> I used Champion Radio's WinMARC tool to calculate the recommended masts
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
What was the bending moment figure you're working with? 40,000 psi or
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.
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