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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