Steve:
Your calculations are a little off. Please note the following:
Antenna Ht Area Pw M (ft-#) M(in-#)
TH7 .5 9.4 25 117.5 1410
WARC 5 3.1 25 387.5 4650
Discl 10 2.7 25 675 8100
214FM 12 1.7 25 510 6120
Mast: 300.24 3603
Total 1990.24 ft-# 23,883 in-#
Mast is uniformly loaded there for it sees 2/12x25=4.17 #/ft
the mast moment is the uniform load x length x length/2
4.17x12x6=300.24 ft-#
The formula for bending stress Fb=M/S where S= section modulus of
tube
Then bending stress in 2" dia x 0.25 wall tube
Fb=23883/S S=.5369 inches cubed
Fb=23883/0.5369 = 44,483 psi
The ultimate strength of 6061-T6 Tube is Fu=38,000 psi
The yield strength of 6061-T6 Tube is Fy=35,000 psi
Using an allowable Fb of ).6Fy then the allowable Fb= 21,000 psi
It is clear to me that the 2" dia x 0.25 wall 6061-T6 tube is
overstressed and with the above load cominbation actually has failed.
I hope this will help in the reflectors discussions
73
Hank Lonberg P.E.
KR7X
______________________________ Reply Separator _________________________________
Subject: Re: [TowerTalk] The last aluminum mast comment
Author: k5ed@dzn.com (Ed Jensen) at ~INETMAIL
To: <towertalk@contesting.com>
Date: 10/9/98 7:54 AM
Hi Steve,
You wrote:
> I think that you forgot to multiply by the wind pressure. At 70.7 MPH,
it's 20 psf, 80 > MPH is about 25 psf. Your analysis doesn't allow for the
load
> distribution up the mast. (Sorry, I'm not an engineer.)
I wrote:
>> Antenna area (sq ft) load (#) (@ 80mph
using 25#/sq ft)
>> TH7 9.4 235
>> Disc1 (40m 1 el) 2.7 68
>> 214FM (2m 14el) 1.7 43
>> WARC 2/2 (12/17m yagi) 3.1 78
>> mast .1/ft 2.5/ft
As you can see Steve, I did multiply by the wind pressure (25 psf).
My distribution again was:
> Antenna area (sq ft) Hgt above tower(ft) bending moment
(ft#)
> TH7 9.4 .5
118
> WARC 2/2 3.1 5 390
> Disc1 2.7 10
270
> 214FM 1.7 12
516
> mast 1.2 6 (avg)
180
Note that bending moment is calculated from areaXwind pressureXHgt above
tower. So I did allow for load distribution up the mast.
The load WILL be supported. Try the MARC program (I don't have it) with the
antenna distribution I posted and let me know the results. Placing the high
area antennas close to the tower makes a big difference.
73, Ed
Ed Jensen, K5ED, El Paso, TX, k5ed@dzn.com
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>Subject: Re: [TowerTalk] The last aluminum mast comment
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