Hello Dave,
I've been watching this thread from a distance. I am not a PE, and do
not have the formulas that you guys are using. I do have some
reasonable real world experience based on several freestanding towers
of my own and helping with several others. My "gut" reaction is as
follows:
A) You have too much load on that tower for the 110 MPH loads you
expect to see in FL., and the antenna's you're going to mount on it.
B) There are many variables that you have not taken into account (at
least in your logic as posted - perhaps you have privately). For
instance, how will the tower be bracketed to the house? Will it be
screwed to the gutter board? Will it be lag bolted to a wall that is
reinforced? What do the brackets look like. I have seen several
towers mounted with brackets at the 1/4 or 1/3 mark and no guy
wires.....come down in relatively low wind conditions -- not because
the tower failed, but because the bracket failed. Those towers were
NOT used as they were intended/designed, and they failed!
C) It's dangerous to do your own calculations on such a project
unless you really are experienced in such calculations. What safety
number have you used. You admitted you didn't take the tower itself
into the calculation because you didn't have data. Additionally, I
didn't see any wind sail numbers for a rotor for the HF beams -- do
you plan to have them in a fixed position? As you stated,
manufacturers use a safety factor to cover a great many variables
*they* haven't considered even with all they know about tower design.
You have even less knowledge and have NO safety factor.
D) Have you taken twist into consideration?? A guyed tower doesn't
react the same way relative to twist that a freestanding tower does.
With no proper guys on the tower, you will have significant twist on a
tower that isn't designed to take twisting loads -- and that may spell
the doom to your installation rather than bending moments.
E) How close are your neighbors? You say you can't guy the tower
properly because you are ten feet from the property line -- are you
willing to gamble on damage or personal safety on your neighbors
property? It's one thing if the tower fails and you do considerable
damage to your own home. It's kinda "shame on you" if that happens
<G>. If the tower fell on your neighbors property and did damage, or
worse yet, did bodily harm to your neighbor, would you stand up in
court and say "I would have guyed it properly, Judge, but I didn't
have room so I just put it up based on my own caculations".
F) The comparisons you're trying to make against a Rohn tower are not
valid at all! The Rohn tower is a properly guyed tower. The dynamics
of a properly guyed tower are completely different than a free
standing tower. The base requirements are even different. Think of
it like this. Do you think Rohn would stand behind their numbers if
you decided to take the upper guys off their installation?? Are you
aware that if you guy a freestanding tower, you destroy it's ability
to function??? The two types of design are not comparable.
Additionally it troubles me a great deal that you are a "ARRL WCF
Section Manager". You should be a person who sets an example for
others regarding the proper installation and operation of a good
Amateur Station -- are you??.
I think Hank gave you some really good advice when he said "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.". The few hundred
dollars you spend will be well worth the investment. I hope this
response doesn't sound personal attack. I don't mean it that way at
all, but I am really concerned that you are heading for some real
trouble.
73 from.......
Bob Otto
N8NGA@one.net
Cincinnati, Ohio
**********************************************
DXCC 10M ** DX is !! ** WAS 10M
There is a very fine line between
"HOBBY" and "MENTAL ILLNESS"
**********************************************
When trouble arises and things look really bad,
there is always one individual who perceives a
solution and is willing to take command.
VERY OFTEN THAT PERSON IS CRAZY!
**********************************************
Friday, January 12, 2001, 3:36:57 PM, you wrote:
DA> Hank,
DA> I am not quite following your calculation of 50,000 lb-ft of moment.
DA> Perhaps you can explain.
DA> What I come up with is .00256 x 110 MPH squared x 1.2 (round members) x 9.7
DA> SF x (60'-15') (above bracket) comes to 16,225 lb-ft. (Equation is out of
DA> the ARRL antenna book chapter 22). This does not take into account the
DA> windload of the 40' of 45G sections as I do not have those figures.
DA> 73--
DA> Dave Armbrust - AE4MR
DA> ARRL WCF Section Manager
DA> (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
DA> --
DA> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
DA> Submissions: towertalk@contesting.com
DA> Administrative requests: towertalk-REQUEST@contesting.com
DA> 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
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