One would surely need to know if the tower is guyed or free-standing, which
was not apparent in the original message. A look at one tower
manufacturer's stress analysis for a freestanding tower shows that leg
factors of safety are calculated assuming compression of a single leg as the
limiting case. It would seem that the limiting wind loading case was
considered as broadside to one tower face.
Although mechanical engineers receive training in stress analysis, you
really would want a structural engineer to weigh in. Failure theory
considers different modes of failure, which, ultimately translate to the
principle stresses seen by individual members of the assembly. Failure
modes such as buckling and torsion do not necessarily follow intuitive ideas
of simple tension and compression loadings under a bending moment.
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Patrick Greenlee
Sent: Sunday, October 05, 2014 1:06 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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