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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