[TowerTalk] Tower concerns
Dave Armbrust
ae4mr@arrl.org
Sat, 13 Jan 2001 02:44:03 -0500
Hank,
Thanks for all your help on my tower. I really appreciate it as I am trying
to do this right. In my area amateurs are exempt from the tower ordinance
thanks to the efforts of the local hams and PRB-1. Non-amateur towers are
limited to 35 feet in residential areas and they must be wind loaded to 105
MPH, engineered plans must be submitted, the tower must be camouflaged (what
ever that means) and permits are required. But for amateurs we are total
exempt but that does not mean I do not want to do this right.
I follow you 100% on the cantilever moment of the antennas other then I was
using a higher figure of 110 MPH squared x .00256 x 1.2 (round members) or
37.17 lb per sf. You appear to be using a factor of 408 / 9.7 x .8 or 33.65
lb per sf. I will assume that your figure is more up to date but would
appreciate having the formula you are using for wind loading.
I did not understand your figure of 43,721 ft-lbs x .8 or 34,976.8 ft-lbs
for cantilever moment of the tower alone. I came up with the 18.8 sf (40' x
.47 sf per ft) x 20' (average height of tower above bracket) x 33.65 lb per
sf wind load or 12,652.4 ft-lbs cantilever moment for the tower itself.
Please explain how I should have figured the tower's cantilever moment and
the logic behind it.
I did not understand the lateral force calculation that you made either
perhaps you would be kind enough to shed some light on these for me too.
I have also come up with 3.38 sf for the mast itself and 3.31 sf for four
3/8" feed lines. Taking all of this into account I came up with a total of
32,150 ft lbs. I did not allow for the wind load of the rotor yet.
Several have suggested that I guy the tower. I really do not want to go
there as whatever method I could use would not match any guy configurations
from Rohn. In addition one of the guys would come very near the front door!
One of the reasons for me to use Rohn 45 is that I do not want to guy the
tower. I would rather take down some of the tower sections.
That brings me to the next question. Both yours and my figures show the
tower is overloaded and that I do need to take down some of the tower but
how much? Using my calculation which seems to be even less conservative
then yours it seems that I need to take down three sections! (I came up
with 15,355 cantilever moment for only 20' of tower above the bracket.)
This would leave me with only a 25 foot tower! Logic seems to state this is
going too far the other way. It seems I could do just as well with a push
up pole instead of Rohn 45g!
I would like to make a few comments off the top of my head. It seems to me
designing this tower to 110 MPH is a bit extreme. If we were to get this
kind of winds here it would most likely tear my roof off and take the
bracket and tower with it! If for some miraculous reason my roof survives
the other things like cows and my neighbor's roof flying through the air at
110 MPH will likely hit the tower and take it down anyways! Adding even
more in for gust and exposure seems even more extreme. Plus you state that
Rohn has added in a 1/3 stress increase! I am all for safety and that is
why I have asked for some guidance in the first place but it seems that each
step has added in something for safety and now it seems that the only safe
tower is an horizontal one on the ground!
It also seems that 9.7 sf (5.5 sf, 1.8 sf, 1.2 sf and 1.2 sf) is relatively
modest antenna system and to say I can only go 25 feet with Rohn 45 seems
frankly a bit ridiculous.
For me to actually go to a PE does not seem to be the answer either. He
will want to know what wind load to design it to also. If I say 110 MPH
then he will come to the same conclusions that this post does.
73--
Dave Armbrust - AE4MR
ARRL WCF Section Manager
(941)378-1701 Fax: (941)929-0040
> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of Lonberg, Hank
> Sent: Friday, January 12, 2001 8:35 PM
> To: 'Dave Armbrust '; Lonberg, Hank; 'towertalk@contesting.com '
> Subject: RE: [TowerTalk] Tower concerns
>
>
> 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
> >
> >
> > --
> > FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> > Submissions: towertalk@contesting.com
> > Administrative requests: towertalk-REQUEST@contesting.com
> > Problems: owner-towertalk@contesting.com
>
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com