[TowerTalk] Booms

[Jim Thomson]

Date: Fri, 10 Mar 2017 21:12:43 -0600
From: Ed Karl <edk0kl at centurytel.net>
To: towertalk at contesting.com
Subject: [TowerTalk] Booms

Hey Troops-

I've been around for a while, always wondered about this. Looking at the 
discussion
of square vs round booms. How come the elements are on top of the round 
boom?
Seems like less inclination to rotate out of alignment if they were 
already on the bottom ...

ed K0KL.

##  Nobody mounts els on top of the round booms, they all mount below.
Since the els sag at their tips, I see no advantage to mounting els  below
the round or square boom.  With eles  mounted below a boom,   you now have
the entire weight of the ele, plus any ice loading, sitting on the mounting bolts,
like those used on stauff clamp assy..which are only the .25 – 20 variety. 
If els mounted on top, you have zero weight on the bolts, or u bolts.  
Thats not that big a deal, except for maybe 40m eles, in
which case you would have at least 2 x ss  u bolts on each side..or a min of 2 x
stauff clamps per side.  

##  I would still like to know if I have the maths correct  as far as a 4 inch round boom
vs  a 3 inch square boom. 

## A 3 inch square boom has a section modulus of   1.323 
A 4 inch round boom has a section modulus of 1.45 

## A 3 inch rectangular boom that is say has 40 ft long has a projected area of exactly 10 sq ft.
A 4 inch round boom that is the same 40 ft long has a projected area of exactly  13.332 sq ft. 

##  Both latest eia- tia  and also ubc-97 D specs say to multiply any flat surface  by 2.0 
Round tubes are multiplied by just 1.2 

##  so the effective area of the 3 inch sq boom ends up being  10 x 2 =  20 sq ft. 
The effective area of the 4 inch round boom ends up being  13.332 x 1.2  =  15.998 sq ft
20  / 15.998    =  25%  more effective surface area for the 3 inch sq boom. 

## Bending moment of a cylinder or tube is just section modulus X yield strength.   This is how they
calculate  the max stress where your mast enters the top of the tower, or at any point along the length 
of a yagi ele.   Of course, with greater effective surface areas and or longer eles or a longer tower mast, the
stress increases fast like. 
IF the same format Is used for  square  tubing, then the round boom reigns supreme..hands down.  Round boom
has a bigger section modulus... by 9.6 %.   1.45 / 1.323 = 1.09599  = 9.6%     The sq boom has 25 %  more effective surface area.

##  mounting els  to a round or sq boom is a no brainer, several methods will work on each type.   Thats a moot point.
If Im correct, I see no advantage to the square boom.   Its a helluva lot weaker vs a round boom, assuming the circumference
and yield strength is identical in both cases.....and both booms are the  same length.  

Jim  VE7RF