[TowerTalk] Mast inquiries

[Jim Thomson]

Date: Wed, 13 May 2015 07:54:46 -0700
From: "Earl Morse" <kz8e at wt.net>
To: <towertalk at contesting.com>
Subject: Re: [TowerTalk] Mast inquiries
Message-ID: <20150513075446.2B5CCEA8 at m0005309.ppops.net>
Content-Type: text/plain; charset="UTF-8"

Run the calculations.

This article might help:

http://www.w9smc.com/BH/2007/k9cc%20article.pdf

You will need this table to take your wind load to PSI.

http://bristolite.com/interfaces/psi_wind.aspx

See how much the force that a beam of X windload places on the mast at the point where the mast exits the top of the tower if it is mounted Y feet above the tower.

Basically it is the windload of the antenna (at max desired windspeed) multiplied by the moment arm (length of mast between top of tower and antenna) minding the units.  That will tell you if you have good enough pipe or need to go to CM.  I did these calcs 10 years ago and have them written down for a XM240 mounted 1 foot above the tower and a KT34XA mounted 10 feet above the tower on the same mast.  I ended up using CM but can't remember off the top of my head what the numbers were.  They are in a notebook somewhere and I could dig them up but a quick recalc shows that it would be almost 30000 in lb at the point where the mast exits the top of the tower (neglecting the load of the mast itself). 

Here's the calcs:

XA 9 sq ft windload mounted 10 feet up on mast
XM240 5.5 sq ft wind load mounted 1 ft up on mast

Antenna Loads
XA 9 sq ft X 25.6 lb/sq ft = 230.4 lb
XM240 5.5 sq ft X 25.6 lb/sq ft = 140.8 lb

Load translated back down to the top of tower
XA 230.4 lb x 10 ft = 2304 ft lb
XM240 140.8 lb x 1 ft = 140.8 ft lb

Both antennas together are 2444.8 ft lb

Converted to in-lb
29337.6 in lb

That doesn't count the load of the mast so you have to add that in there too.

Now look and see if your mast is capable of that load with some safety factor.

This is the kind of stuff where if you are really serious it might be worth getting a PE involved to take in account all the factors, but the calcs aren't too difficult and will give you a better warm and fuzzy feeling.

Earl
N8SS

Disclaimer:  I'm a EE not a ME/CE.

##  For the real deal....use the free  ARRL software for the mast calculations. Its  dead on , and uses the latest 222- rev G specs.   BTW.... you will get a real eye opener when you start comparing
stuff like a 2.0 inch mast with a .25 inch thick wall   vs  a 2.0 inch od mast with a .375 inch wall.    Then compare that to a 3 inch od mast with a .25 inch thick wall.    The 3 inch mast blows em all away hands down.

##  The kicker is.... both the 3 inch mast x .25 wall   and the 2.0 OD mast x   .375 wall  both weight the same  per ft.   But the 3 inch mast is one helluva lot stronger than the super thick wall  2 x .375 mast. 

##  also remember that 4130  CM material is only 70 ksi..in its basic un-heat treated form.   Heat treated 4130  CM  will range from 107 ksi..all the way up to 120 ksi.   UN heat treated 4130 is not expensive at any 
steel yard..and comes UN galvanized as well.    Array solutions tells me the heat treating process is done at 1300 deg F.    The hot dipped galvanizing process is done after the fact..and is done at just 900 deg F.  
The galvanizing process will  NOT screw up the heat treating process.   BTW, you can also get 4130 CM  in 2.5 inch OD  x .375 wall.   You can also get  3.0 x .275 in CM. 

##  For any mast it boils down to section modulus  X  yield strength =  max  bending moment.   A few mins with the free  arrl mast software  will give u more than an eye opener.   DOM is typ 75 ksi..and up to 89 ksi. 
  DOM  is stronger than  UN heat treated CM.     Section modulus is just a function of OD and ID and wall thickness.  Large diam,  thin wall masts have a greater  section modulus  vs  a   smaller diam, thick wall mast.  

##  Don’t go cheap on a mast.... unless you install just one yagi within 0-2 ft above the top of the tower.   Too many folks install junk......then way down the road want to add a 40m yagi 8-10 ft above the top of the tower
with disastrous results.   I believe I have seen well over 3 doz pix of bent mast over the years.    Most are extremely dangerous to remove.  When u see a pix of a 3 x high stack of yagis on a 15 mast...bent at a right angle..it
makes you cringe.   In a lot of cases, a crane is not an option to safely remove the mess... you cant always get a crane in there. 

## if you buy an expensive mast..like DOM, or CM..make damn sure it comes with paper work depicting the yield strength.    Yield strength X section modulus = bending moment.   If you dont know what the yield strength
is, get it checked out asap....or dont install it.   Dont guess. 

Jim  VE7RF