[TowerTalk] Is A Tower Weaker in Some Directions?(does it matter)

Sun Oct 5 18:57:54 EDT 2014

On 10/5/2014 6:19 PM, Stan Stockton wrote:

Regardless of normal storm path and normal wind direction, all you are 
doing is playing the odds as the wind can , and will (on occasion) come 
from any direction in gusts lasting longer than 3 seconds and exceeding 
the wind rating for your area.   The best approach as I see it is to put 
up a tower based on maximum wind speeds for your area.  Even that is no 
guarantee.  There will be occasional winds that exceed the ratings for 
your area, but to try and plan for all eventualities soon passes beyond 
the realm of expense Vs return.

As for wind resistance, I'm not an engineer, but I think you will find 
there is very little difference in tower orientation to the wind 
direction as far as wind resistance.  The tower calculates to the same 
area, or within a very small difference..

I think this is a case of over thinking the problem.

73

Roger (K8RI)

> I would say if you have to guess which way a strong wind will blow and try to figure out whether a tower will be stronger in one orientation versus another in order to sleep better the plan is marginal and a wind that is even 10 mph stronger than what you think you may experience will be a problem.  Same goes for making sure you turn the antennas in a certain direction before the big wind blows.  If that is the case, the installation is marginal.
>
> Stan, K5GO
>
> Sent from Stan's IPhone
>
>
>
>> On Oct 5, 2014, at 4:02 PM, "Gene Smar" <ersmar at verizon.net> wrote:
>>
>> 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 at contesting.com] On Behalf Of
>> Patrick Greenlee
>> Sent: Sunday, October 05, 2014 2:06 PM
>> To: towertalk-bounces at contesting.com; Cox, Norman R.;
>> towertalk at 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 at 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 at 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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-- 

73

Roger (K8RI)

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