[TowerTalk] Mast/tower design close in guyed by shrouds and spreaders?

[Patrick Greenlee]

As a rag sailor I would mention that it is forces from wind in the sails 
that are drivers, not the motion of the base of the mast. On narrow 
beamed vessels with tall masts you have the analog to a tower with 
close-in guys.. You will see "jumper struts" and other techniques 
employed to try to keep the mast in-column or to at least control the 
bend so the mast doesn't buckle.  To a first approximation the sailboat 
analogy works but as is often the case the devil is in the details and 
in this case there is quite a lot of difference in the forces and their 
application between a mast with sails and a tower (with or without an 
antenna at the top.  The OP might mention if his desire is an active 
tower that radiates or a tower to support an antenna with significant 
wind loads.  Both differ from the sailboat example in the fine print , 
just in different ways.

In just a few days my mechanical engineering consultant/good friend and 
sailing buddy will arrive for about a 10 day stay.  We will be trying to 
dismantle a tower to refurbish, reassemble, and erect with the last step 
probably on a second visit in mid July.  If the OP will send me his 
available info and  questions then time permitting maybe I can get John 
to comment.   My friend John, helped me drop the tower and it went story 
book.  The tower is the bottom 40 ft of a 100 ft triangular tower made 
of 20 ft lengths of 4 inch ID 1/4 wall thickness steel tubing with 
inter-leg dimensions of 14 feet on centers (not 14 inches) where it 
rests on 18 inch piers  that are 7 ft into the ground. John is guiding 
me through an exercise where I have augured 9 holes 15 inches in 
diameter in three groups of three.  This to compensate for the 42 inch 
pier depth I can "drill."

73,

Patrick NJ5G  (I'm good on QRZ)



On 5/19/2014 4:21 PM, Jim Lux wrote:
> On 5/19/14, 12:19 PM, Ray, W4BYG wrote:
>> Is there any readily available design/build information for designing a
>> modest height (35 to 46 feet) ham tower or mast, that would be close 
>> guyed
>> using spreaders?  This would allow guying with close in dimensions, 
>> probably
>> anchored about the same distance from the base as the spreaders are 
>> long.
>>
>> It would be something like what is done with spreaders and shrouds on
>> sailboats.  I have studied the subject relating to sailboats but the 
>> related
>> sail mast bending forces seem to complicate the subject.
>>
>
>
> This has been discussed in the past on the list, at least in general 
> terms.
>
> Towers have bending loads just like sailboats, although you probably 
> don't have the loads due to the base moving around.
>
> The challenge is that these sorts of structures can have very high 
> compression loads on the mast (just because the angles are small). 
> However it seems that you should be able to do a design similar to a 
> braced spar (they do it for long booms on Yagis for instance).
>
> It's all about buckling loads, which in turn is all about 
> length/diameter ratios.  Generally, the compressive strength of the 
> material isn't as much a limit as the buckling.
>
>
>
> For a more conventional thing.. why not just make a bigger unguyed 
> tower.  Rather than a 1 foot diameter mast with guys/stays that come 
> down 6 feet away, make a tapered rigid structure that is 12 feet wide 
> at the bottom (like a windmill tower or a HV power line support).  The 
> design is much easier, and you can look to existing analyses for, say, 
> Rohn BX as a starting point on how to calculate the loads.
>
>
> The top part of the Eiffel tower is about 200 meters tall, and the 
> base is 40 meters on a side (about 5:1 ratio).  I think you could do a 
> bit better, since you don't need to support elevators and such.
>
> The other thing to think about is that if you design the structure to 
> flex (substantially) without failure, you might that it would be a 
> solution.
>
> There are, for instance, tensegrity structures which are very good 
> from a strength/mass/size basis, but which have unusual motions under 
> loading.
>
>> Close in guying is practical and possible with proper design.  I am 
>> aware of
>> an original 1000' TV tower in Jacksonville, FL (ch 4) that the guys (no
>> spreaders just straight guys) went out somehting less than 200'.  Ch 
>> 17's
>> original 1000' tower (in downtown Atlanta) was self supporting and as I
>> recall only had a base of about 50 maybe 75'.  So wide 2/3h and 3/4h 
>> guying
>> is not always necessary.  With good information, close in guying schemes
>> should be in some circumstances, possible and practical.
>>
>> Anyone have any insight on the subject?
>>
>> Ray, W4BYG
>>
>> "The Republic (America), can survive a fool like Barack Obama, who is 
>> after
>> all, merely a fool.   It is less likely to survive a multitude of fools,
>> such as those who made him their president." Vaclav Klaus, Former 
>> Premier
>> Czech Republic
>>
>>
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