[TowerTalk] Tower concerns

Dave Armbrust ae4mr@arrl.org
Fri, 12 Jan 2001 17:32:20 -0500

> 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!

The bracket is over designed as much as I could.  Where it attaches to the
house it is 5' long and it has three bolts that go through vertical 2x4.  I
have this backed up with a horizontal 2x4.  All of this including the bolts
are stainless steel.  Several locals have agreed that this is done properly
and I believe it to be a better bracket then the one Rohn uses in their
bracketed towers.

> 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.

Unfortunately my tower does not fit any Rohn tables.  There is no such thing
as a Rohn table for 110 MPH wind.  Even the smallest Rohn bracketed tower
which is only 40' calls for an bracket at 30'.  I do not have a 30'
building.  The best fit I could find was the 100' tower with a bracket at
66'.  This gives 34' over the top bracket.  I am 39' over the top bracket
but I also have concrete 15' below that bracket where as the 100' tower does
not have any more support until 33' below the top bracket and this is only
another bracket rather then solid concrete.  While I have pushed it by 5'
this does not seem excessive based on my stronger design.  But I am not sure
and that is why I am writing.

> 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.

The Rohn tables for bracketed towers and for free standing towers are not
guyed and I assume that they have taken twist into account.  These are the
only tables I have referred to but again they do not fit.

> 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".

I do not want the tower to come down period, that is why I am writing.

> 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.

Rohn is will only stand behind their figures for only the tables they
provide.  Again I am using the tables for unguyed towers not for the Rohn
guyed tower.

> 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??.

Exactly.  This is one of the reasons I am seeking the advise of this group.
If I was comfortable I would not have written in the first place.  I have
already received one personal reply stating that what I just do not
understand how strong Rohn 45 is and that what I am doing is fine.  He also
states that he has done over 300 towers all over the southeast.   I do not
want to just blindly follow his or anyone else advise.  I want it done

> 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.

As the Section Manager I know of many, many, amateur towers.  But I do not
know of any that have hired a P.E. to design the tower!  Prior to posting my
message I searched the archives for information on towers, mast, etc. I had
seen the advice where it was suggested that a PE be employed but did not see
any examples where this was actually done.  I am not trying to do a super
tall tower.  I am only 55 feet and will most likely scale it back to 45'.
The total antenna load is 9.7 square feet including the HF beam.  This is a
rather small tower and load especially for Rohn 45.  To suggest I hire an PE
seems a bit excessive.  I do not want to blindly follow Hank's figure of
50,000 foot pounds without understanding how he arrived at that.  We are all
capable of making errors in "quick calculations" which is what Hank stated
he did.  I do not want to blindly follow Hank's quick calculations any more
then I want to blindly follow someone's advise who states he has done over
300 towers.

As I stated I am considering taking a section out.  I would rather not loose
the height but I am willing to do so for safety sake.  Most likely I will do
this but I would rather not do this by the seat of my pants and would like
to understand the process better.

> 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.
> **********************************************
> 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

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