Dave:
My anaylsis basis comes from the 1997 UBC Building Code Wind Design
criteria. For a design wind of 110mph 9.7 sf of antenna at 60 feet has
a wind load of 408 pounds. The cantilever moment due to the antennas is
approximately 408 x (60-15) = 18,360 ft-lbs. Using an .8 reduction for
round members this is 14,688 ft-lbs which is still higher than Rohn's
allowable value of 11,400 ft-lbs with a 1/3 stress increase. Just based on
the antenna area your tower will be overstressed based on allowable load.
The tower itself has an area of approximately 0.47 sf/ft. The wind load on
the tower is a function of the wind pressure times the area. The code
applies some additional factors for gust and exposure plus it increases the
wind pressure as you elevate from the ground. For 55 feet plus of Rohn 45g
this accounts for 1,859 pounds of lateral load and 43,721 ft-lbs. of moment
at the bracket level. The total moment at the bracket is 18,360 plus 43,721
which equals 62,081 ft.-lbs. I reduced this value to .8(62,081)= 49,665
ft-lbs. which I rounded to 50,000 ft-lbs. The lateral force at the bracket
from the tower above it is 408 + 1859 = 2267 lbs. 0.8(2267)= 1814 lbs.
My original value is higher due to the fact I added in an rotator at 50 foot
level also.
I do not mean to critize your idea in any way. If you have to get a building
permit, many jurisdictions don't require one, and you need an set of
calculations by a licensed engineer then you just may have some problems
with this particular arrangement. If you don't then more the better, and you
can do what you want reqardless of any advice you receive pro or con.
It is your decision as to how you will proceed and at this point I can not
provide any more input regarding this matter. I have given you some free
engineering to allow you to make a reasoned decision on this matter. I do
not wish to be further involved as my professional license and means of
income could be at stake. In my professional opinion; if you wish to proceed
along your original idea you should seek professional engineering assistance
if required to get a permit. If no permit is necessary, then do what you
will and deal with the consequences.
73 and good luck
H.S.Lonberg Jr. P.E. / KR7X
-----Original Message-----
From: Dave Armbrust
To: Lonberg, Hank; towertalk@contesting.com
Sent: 1/12/01 12:13 PM
Subject: RE: [TowerTalk] Tower concerns
Hank,
I am not quite following your calculation of 50,000 lb-ft of moment.
Perhaps you can explain.
What I come up with is .00256 x 110 MPH squared x 1.2 (round members) x
9.7
SF x (60'-15') (above bracket) comes to 16,225 lb-ft. (Equation is out
of
the ARRL antenna book chapter 22). This does not take into account the
windload of the 40' of 45G sections as I do not have those figures.
73--
Dave Armbrust - AE4MR
ARRL WCF Section Manager
(941)378-1701 Fax: (941)929-0040
> -----Original Message-----
> From: Lonberg, Hank [mailto:Hank.Lonberg@Harrisgrp.com]
> Sent: Friday, January 12, 2001 2:42 PM
> To: 'ae4mr@arrl.org'; towertalk@contesting.com
> Subject: RE: [TowerTalk] Tower concerns
>
>
> Dave:
>
> Doing some quick calculations based on 9.7 sf of antenna lumped at 60
feet
> and 45g tower section
> with 55-15=40 foot of cantilever and 110mph design wind. This
> generates over
> 50,000 lb-ft of moment.
> This is at the 15 foot bracket point. Rohn 45g allowable moment
> on the tower
> section is 11,400 lb-ft.
> Also developed at the 15 foot level is 2100+ pounds of force due to
the
> lateral wind loads.
>
> You should reconsider what you are planning, and are currently doing
if
> 110mph is indeed the design
> wind load for your area. If you insist on going ahead, get a local
P.E. to
> help you with this one.
> The ARRL has a list of volunteer consulting engineers.
>
> Hope this gives you an idea of what you are up against.
>
> Take care,
>
> Hank Lonberg P.E. / KR7X
>
>
>
> -----Original Message-----
> From: Dave Armbrust [mailto:ae4mr@arrl.org]
> Sent: Friday, January 12, 2001 10:20 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Tower concerns
>
> I am in the process of putting up a tower and I have some concerns
that I
> hope the group can help me with.
>
> The tower is currently 55' of Rohn 45. 5 straight sections and 1 top
> section. The bottom section in buried in 4' of concrete as per
> Rohn's specs
> for a bracketed tower. It is bracket to house at 15 feet and will be
> unguyed. I can not do so as the tower is less then 10' from the
property
> line.
>
> For the mast I have 21' of Schedule 40 1 1/2 galvanized steel
> pipe. I plan
> on having 8' of this in the tower and 13' above the top of the tower.
>
> Antennas will consist of the following from the top down:
>
> Diamond X510NA (1.2 SF) (17.2 feet long)
> Cushcraft 719B (1.2 SF)
> Cushcraft 13B2 (1.8 SF)
> One of the following HF beams:
> MA5B (3.22 SF)
> A3S (4.36 SF)
> A4S (5.5 SF)
>
> The wind rating for my county (Sarasota, FL) is 110 MPH.
>
> I am getting a little concerned about the 55' height and am
considering
> taking one section out leaving me at 45'. I would really rather
> not do this
> but will consider it for safety sake. I have been told they have
> gone much
> higher then this with 25G and no guides. Rohn does not have any
> charts that
> fit my configuration. The closest I can come is to look at their 100'
> bracketed tower (5.5 SF at 70 MPH, 2.0 SF at 80). It has two brackets
one
> at 33 feet and one at 66 feet. Table may be found at:
> http://www.rohn.net/CommPro/Towers/Bracketed/Bracketed.htm. In this
case
> the tower extends 34 feet above the top bracket. I am pushing it
> a bit with
> 40' above the top bracket but the cement is only 15' below the top
bracket
> instead of only another bracket 33' below the top bracket. My
> installation
> should be stronger then the 100' bracket tower example Rohn uses.
>
> If I look at the self supporting Rohn 45 towers
> http://www.rohn.net/CommPro/Towers/Bracketed/SStowers.htm it
> shows 5.1 SF at
> 70 MPH and 1.4 SF at 80 MPH for a 40' self supporting tower and
> 2.3 SF at 70
> MPH for a 45' tower.
>
> I know I am pushing the Rohn figures a bit but I also understand that
they
> have some room for error in their figures. With lawsuits today who
can
> blame them. As I stated earlier my county has a 110 MPH wind rating
but
> none of Rohn's towers examples have charts for this. Commercial
towers in
> this county must be rated to 105 MPH per zoning ordinances. My total
wind
> load is going to be somewhere between 7.42 SF and 9.7 SF. I am only
10'
> from my property line making it impossible to do any sort of
reasonable
> guides.
>
> I am also concerned a little about the mast. The Diamond X510NA
> (1.2 SF) is
> 17.2 feet long and rated for 90 MPH. The wind load at 90 MPH is 29.87
> pounds at 21.6 feet or 645 foot pounds. Assuming a 25kps rating for
the
> schedule 40 it should hold up and logic tells you the 17.2' antenna
would
> give way before the 13' of 2" steel pipe will. The small beam at
> 5.5 SF is
> only a load of 137 foot pounds.
>
> Do I really need to take some of the tower down or do I need to reduce
my
> already low antenna loads?
>
> 73--
> Dave Armbrust - AE4MR
> ARRL WCF Section Manager
> (941)378-1701 Fax: (941)929-0040
>
>
> --
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Submissions: towertalk@contesting.com
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Problems: owner-towertalk@contesting.com
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