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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