TT:
If the wind were to blow normal to one face of a tower, the opposite
single leg would be the weakest. Assuming the cross-bracing holds (does not
fail) upon exposure to high winds, the windward legs of the tower will be in
tension, i.e., the wind will attempt to elongate them when they bend away
from the wind. The remaining leg on the opposite side of the tower will be
in compression, i.e., the wind will attempt to force it downward. This sole
leg must withstand this downward, compressive force by itself. It will
deform (bend) when sufficient downward force has been placed upon it, much
as a human's knees would buckle if that human were required to support
heavier and heavier loads on his shoulders.
Now consider the case when the wind is from the single leg side of the
tower and towards the opposite face. The single windward leg will be in
tension and the two opposite legs will be in compression. Unlike the first
example with the wind applied on the face, the two leeward legs of the tower
resist the downward force from the wind with twice the counteracting force
of a single leg. To get this tower's two legs to buckle will require twice
the wind force on the opposite leg vs the single leg example above.
A properly designed and constructed guyed tower will not impose
downward force on the legs. The windward guy(s) will counteract the wind
force and keep the tower erect. The guys should not deflect sufficiently to
allow a downward force to be applied to the leeward leg(s). This is why the
foundation of a guyed tower need not be massive: it will merely resist the
dead weight of the tower and its antenna loads. There will be no
wind-caused overturning moment applied to the base as is the case with a
self-supporting tower and its massive concrete base.
This is also why one must not guy a tapered, self-supporting tower.
Guys impact unnecessary downward vertical force on the structure. If you
believe you must guy a SS structure, then re-check your design and select a
"beefier" tower. And ditch the guys.
73 de
Gene Smar AD3F
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Patrick Greenlee
Sent: Sunday, October 05, 2014 2:06 PM
To: towertalk-bounces@contesting.com; Cox, Norman R.;
towertalk@contesting.com
Subject: Re: [TowerTalk] Is A Tower Weaker in Some Directions?
Norm, Bill is right but hasn't dropped the other shoe yet. Will some
mechanical engineer type please tell us which direction of lean is strongest
in a triangular tower with X-braces or the typical zigzag bracing. Do you
want a flat side toward the wind or the side opposite.. For reference lets
assume one leg of the tower is toward the north. Then one leg is at 120
degrees and the other is 240 degrees. Which wind direction(s) would be the
most dangerous for the tower?
One would guess there are three directions of wind that hit the tower at its
least strength and three where it is the strongest. The question is in the
above example of a tower what would be the most dangerous wind ,one from the
north, 120, or 240 degrees or would worst case(s) be 60, 180, or 300
degrees?
If there is a substantial difference in max wind vs relative bearing of the
wind then those with rotating triangular towers might want to be guided by
this as regards where to leave the tower when not in use. ...and the rest
of us might wasn???t to consider this with respect to our triangular towers.
Oh woe is me... I have already committed 3 out of 4 towers to their final
positions with no regard to wind direction. I have 49 ft of Rohn 25 plus
mast to erect and have not poured the foundation for the tilt base yet (whew
just in time.)
Patrick
-----Original Message-----
From: Bill Aycock
Sent: Sunday, October 05, 2014 11:45 AM
To: Cox, Norman R. ; towertalk@contesting.com
Subject: Re: [TowerTalk] Is A Tower Weaker in Some Directions?
Norm--
Be cautious about your assumptions. The direction of the strongest winds
does not correlate closely with the direction of the storm path. I one did a
study for a Rocket test site, (over 30 years ago) that showed (for that
site) that the strongest sustained wind was almost 90 degrees from the storm
path. The strongest winds are the result of rotation, not path.
You might consult your local Weather office for help.
Bill--W4BSG
-----Original Message-----
From: Cox, Norman R.
Sent: Saturday, October 04, 2014 4:41 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Is A Tower Weaker in Some Directions?
Dear Group:
I just joined Tower Talk -- I've read through the last 5 years of
archives and have certainly learned a lot I did not know. What a resource!
I have a question:
I have just ordered a US Towers HDX-555 tower. At my QTH, I have the
option of orienting the base plate in any direction I want to. My question
is: Are these 3-legged towers weaker (or stronger) in some directions than
others? (I am not a structural engineer.) The reason I ask is that most of
the violent storms that hit here usually come from the same direction. Will
orienting the base plate in a certain direction make it any less vulnerable?
Thanks!
Norm
KE0ZT
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