[TowerTalk] Wind loading

Tue Aug 30 22:04:45 EDT 2016

As Roger has pointed out, wind gradient is an important aspect in the design 
of any structure - tower or skyscraper.

I host a wx station here which feeds into the NOAA system. Until recently I 
had an anemometer at ground (now not in place) and one at 30 feet (the NOAA 
anemometer height recommendation). I am planning on one at the 70 foot level 
on my larger crankup.

Wind typically increases in speed with height, but obviously the terrain 
within a defined area will have a bearing on what wind speeds are. As Roger 
noted, terrain plays an important role. I often experienced a significant 
difference between the ground and 30 foot levels, sometimes as much as 25 
MPH or more. The ground anemometer was located on flat grassy ground with 
little obstruction for some 2-300 feet. The anemometer at the 30 foot level 
is 8 feet about the house roof at the front peak. Our second level deck is 
between the two as far as height. We would often see no wind at the ground 
level, be experiencing a moderate wind on the deck and see only a light 
breeze  or a significant wind speed at the roof anemometer (not the best 
location by the way!).

Roger has touched on a situation that many hams should consider doing, 
getting a fix on how the winds at their location are affected by terrain and 
by obstructions. The latter two factors will have a significant bearing on 
what happens with wind speeds.  Knowing how ones physical obstructions on 
the property affects the wind currents can help lead to better tower siting.

A little hand anemometer is a cheap investment and can be successfully used 
if you really want to know what is happening on your property and are 
willing to walk around a bit. Plotting this on a property map showing 
buildings, trees and other obstructions is easily done if you want a GENERAL 
idea. Your results may differ with certain storms of course (also a good 
time to do a walk around).

If really interested in wx at your location there is a NOAA-NWS manual 
dealing with wx instrument siting standards. I don't have it at hand but if 
anyone is interested contact me and I'll go chase it down.

Don W7WLL
MesoWest AS-531
SKY WARN LC-3H
Cranky Old Guy with Crankups

-----Original Message----- 
From: Roger (K8RI) on TT
Sent: Tuesday, August 30, 2016 5:24 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Wind loading

Wind varies with height. Even with 20 feet there can be a substantial
difference.  Much depends on the size of the rocks.  As the page the
previous link, said: close to the ground the wind is irregular.  The
rocks will make it even more so. You also have the rising air currents.
Will the bases be well above high tide?  I've flown along much of that
coastline and IIRC there was an abrupt drop off at, or just beyond the
shore.

You have increasing wind velocity as you go from the base to the top
while the antennas taper.  Going from 3/4" to 3/8ths will make for a
pretty flexible vertical.
I think the safest would to calculate the flat plate area for ea
section, and use the average. Then use the top wind speed.  This will
give you some safety factor on the higher portions of the verticals.

73,

Roger (K8RI)

On 8/30/2016 Tuesday 2:33 PM, Dick Blumenstein wrote:
> Hi Gary-
>
> I found this link from K7NV 
> <http://k7nv.com/notebook/topics/windload.html>. Hopefully, this will 
> help. I would assume you would do a force calculation for each diameter 
> piece and add the forces up of all the different pieces.
>
> Dick, K0CAT
>
> =============================
>
>
> Gary Smith wrote on 8/30/2016 2:20 PM:
>> I'm having to play games with setting up eight, short, active
>> vertical elements on an incredibly rocky area. I am not able to drive
>> in ground rods, much less the base of the antennas. I'm coming up
>> with a plan to make wooden bases for them and hold them down with
>> rocks.
>>
>> Since I live on the ocean's edge and have to contend with hurricane
>> force winds every so often, I'm trying to figure out the wind loading
>> of the verticals I'm considering. I'll buy the aluminum sections from
>> DXE and prefer to use larger sections just because of branches
>> falling at the marsh edge, as they always do. But thicker pieces will
>> have more wind loading and that is a problem.
>>
>> If I use the thinnest configuration I'll have about 22 feet tall made
>> of .375, .5, .625 & .750 sections.
>>
>> I asked what the wind loading would be and nobody I talked to was
>> able to give me a clue. Does anyone know a formula to figure the wind
>> loading of tapered elements?
>>
>> Thanks & 73,
>>
>> Gary
>> KA1J
>> _______________________________________________
>>
>>
>>
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-- 

73

Roger (K8RI)

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