[TowerTalk] How many tower sections i can install??

Thu Aug 27 15:10:58 EDT 2015

A cautionary (and perhaps encouraging) tale. In the 70's, the Department 
of State ARC had three sections of Rohn 25 on the 8th story roof of an 
office building in downtown DC, with a Mosley CL-36 on top.  All blithe 
and ignorant, we did not pierce the roof for any kind of a base at all - 
instead we just guyed the base of the tower parallel to the flat roof, 
to the same points where the guys were tied off - cast iron vent 
pipes!!  About the only thing I think we did right, in hindsight, was to 
guy the 30 feet in the middle and at the top, which probably did some 
good in mitigating both the strain on the tower and the pull on any 
single guy wire. Whatever, it stayed up for about 7 years, until a 
micro-burst with 90 MPH gusts came through, whereupon the whole works 
fell down and wound up hanging over the side of the building by the 
surviving guy wires and the feedline.  I was overseas at the time, so I 
never saw the mess, but suspect it caused some excitement for the city 
authorities. That may be why the club never put another antenna up there.

Pretty much everything I learned about guyed towers came from K7NV's web 
site <http://k7nv.com/notebook/>, particularly 
<http://k7nv.com/notebook/towerstudy/towerstudy1.html> on guyed tower 
behavior, and I recommend anyone thinking about going anything other 
than a cookbook configuration should have a look.  Douglas is an 
engineer, so I doubt this will be challenging to him.

 From a non-engineer's, common sense point of view,  I think that 
probably the downward pressure of the tower base on a concrete roof is 
less of a problem than the tension that must be handled by whichever guy 
anchor(s) happen to be upwind during a given storm.  3/16 EHS has a 
breaking strength around 4000 pounds, and all of that tension will be 
applied to the anchor point before the guy itself fails. 4-way guying 
will distribute that load more evenly, and of course two layers of 
guying would have the same benefit.

If I were doing this near the coast in Cuba, I also think think I'd 
seriously consider having a tilt-over base fabricated, and using the 
falling derrick method to lower the tower and antenna whenever storms 
approach. Depending on the roof configuration, it might be possible to 
tilt the tower over entirely, so that it lies (and is tied down) on the 
roof, with the tribander half over the side when in this position

73, Pete N4ZR
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On 8/26/2015 3:46 PM, Patrick Greenlee wrote:
> Good stuff Gerald.  Douglass may want to also consider placing the 
> base of the tower above or quite near to a load bearing interior wall 
> even though that might mess with the symmetry of a centered location 
> guy wise.  Depending on the strength of the roof in the centered 
> configuration and on the interior layout of the residence a thrust 
> member (column) located below the tower base could be used to carry 
> the vertical download through to the (assumed) concrete slab floor. 
> (Also assuming this is a single story home although in theory 
> depending on layout another thrust member on the ground floor could be 
> used as well.)
>
> If there are tower sections to spare put one under each guy to change 
> the angle of the guy and get more strength to oppose wind loading with 
> less down thrust on the tower. There is no free lunch and each of 
> these "guy helpers" will have some down thrust directed through them 
> vertically to the roof but much less than that of the tower itself.
>
> Patrick        NJ5G
>
> On 8/26/2015 10:22 AM, TexasRF--- via TowerTalk wrote:
>> Hi Douglas, you can achieve a a greater guy point distance by using four
>> guys instead of three. You would not want the guy to guy angle to 
>> exceed 120
>> degrees in any case.
>>   Your roof dimensions would allow guy to guy angles of 120, 60, 120 
>> and 60
>> degrees. With the tower mounted in the center of the roof and 
>> assuming the
>> anchors are at the edge of the roof, a guy to guy angle of 120 
>> degrees,  the
>> distance would be 20.0 ft to the anchor. The distance between anchors
>> would be 34.6 ft, 20 ft, 34.6 ft and 20 ft.
>>   So, in this case, you could guy the tower a couple of ft below the 
>> top and
>> three 10ft sections would have a anchor/height ratio of 66.66%.
>>   We typically use an 80% ratio mainly because that is the number Rohn
>> specifies for all of their tower models. It is a good compromise 
>> between load
>> capacity and space required.
>>   Considering that these towers are used up to 180ft in height or 
>> more, it is
>>   reasonable to move anchor points inward for reduced heights. The 
>> issue in
>> doing  this is that Rohn has no published data for doing this. The 
>> wind load
>> rating has  to be reduced and it takes engineering skills to properly do
>> the calculations.  It is not rocket science however.
>>   I would be concerned about possible roof damage as much as the 
>> tower  surviv
>> al. It is conceivable that using the roof edge as a turning point for 
>> the
>> guy and then a ground anchor would be stronger. Something else that 
>> would
>> benefit from an engineers blessing.
>>   73,
>> Gerald K5GW
>>           In a message dated 8/25/2015 9:14:22 P.M. Central Daylight 
>> Time,
>> co8dm at frcuba.co.cu writes:
>>
>> Hi,
>>
>> I would like read your ideas.
>>
>> I have few 10 ft  section of a tower (it is like a Rohn 25G).
>>
>> I would like install few  section over some part of my roof...The 
>> roof is
>> concrete...as you know many  house in the Caribbean has concrete roof 
>> ...it
>> is 24ft by 40ft...so, i have  calculating and the distance from the 
>> base to
>> the guy anchors is 14 ft  !!!
>>
>> I know the distance from the base to anchor points is 70 % of the  tower
>> height...so, 70% of 20 ft is 14ft, that´s mean 2 tower sections...right
>> ??...Is it possible install 3 sections adding more guy´s set ???
>>
>> Thanks,
>>
>> 73....Douglas,  CO8DM
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