[TowerTalk] Booms

[Roger (K8RI) on TT]

How does the square boom handle torsional forces compared to a round boom?

73, Roger (K8RI)

On 3/11/2017 11:16 AM, Jim Thomson wrote:
> 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
>
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-- 

73

Roger (K8RI)


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