[TowerTalk] Tower Questions
Joel
k2qbv@cyburban.com
Tue, 11 Apr 2000 21:15:34 -0400
Perhaps we can give you answers to some of your questions after working with the TRI EX engineering staff. Well, here goes;
Towers are designed to resist lateral wind forces. The uniform lateral wind load (pounds per foot) is computed by multiplying the projected area of a tower (square feet of area per foot of tower length) times the wind pressure (pounds per foot).
Prior to 1986, the standards used a constant pressure over the tower height. Different areas of the country were designated Zone A, B, and C with the respective constant wind pressure of 30,40 and 50 psf. If you lived in zone B, the wind pressure was considered a constant 40 psf over the entire height of the tower.
Research found that the earth creates turbulence in the wind flow. The wind would travel faster as one got further away from the affects of buildings, trees, hill, and etc. So, the code changed from a constant wind pressure to a designated Fastest Mile Wind Speed or the Basic Wind Speed designation.. Now one must find out what the basic wind speed is for the City , County, or local municipality.. In Fresno, California (next to
Visalia) the Basic wind speed that Fresno accepts is 70 mph. 70 mph is as low as the code permits. The coast of Florida is 120 mph wind area.
Under the current code, for a given wind speed, there are height coefficients. For example, Kz is the height factor, gust, exposure coefficient.
Ht Kz Press Gust Import Pressure
0-20 .87 10.87 1.16 1.00 12.63
20-40 1.06 13.25 1.16 1.00 15.40
40-60 1.19 14.88 1.16 1.00 17.29
60-80 1.29 16.16 1.16 1.00 18.77
80-100 1.37 17.22 1.16 1.00 20.01
The stanard tower analysis done by this structual engineer is for Exposure B which is suburban terrian. Some cities want Exposure C which is an open terrian where wind is unobstructed for a one mile radius. One can now see how the wind pressure increases with height. This can be equated to say the wind is faster at higher heights but the standard call for the wind speed to be measured at the 33 foot elevation. All the
anommeters around the county are to be 33 ft. off the ground. The wind speed is a 50 year old wind..
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Now to comment on the other question concerning failure of towers. The failure point of crank up towers on most towers the critical members are the braces in the lap joint. The tower and base is like a chain, it is only as strong as the weakest link.
Hope the above answers some of the questions asked on towertalk about a week ago If there are any comments, I will pass it along to the P. E. that put the above together. ..
Regards to all.
Joel, K2QBV
FIRST CALL COMMUNICATIONS,INC
(come visit us at the Visalia DX Convention this week end)
Kurt Andress wrote:
> Al Williams wrote:
> >
> > I would like to read a mini-tutorial on tower strength and wind loading > from some of our tower experts.
>
> Hi Al,
> Here's a go at answering some of your questions.
>
> > 1. Is it true that the wind speed at say 100' is greater than at 50'?
> > always?
>
> The EIA-222-F and UBC 1997 spec's both produce higher wind loads (not
> necessarily wind speeds) as the height is increased. Remember that these
> spec's use a "basic wind speed" which is for the fastest mile of wind
> measured at 33 feet. That remains constant, what increases with height
> is the windload. "Is it really true?", they think so.
>
> > 2. For a free-standing tower, is the failure mode most likely at the very
> > bottom of tower?,
>
> It all depends on what the designer did. The bending moment in a
> freestanding tower is always greatest at the base, but what the designer
> did to configure the structure to carry the loads determines where the
> first failure will occur. We might assume that a freestanding tower was
> designed to have the same stress (not bending moment) at the bottom of
> each change in section size. This would mean that all sections would
> fail simultaneously. But, we don't know that this is true for all tower
> designs.
> We'd have to analyze each one to really know what is going on.
> For these kinds of questions, ask the tower designer (usually not the
> sales guy), I'd bet he knows.
>
> > 3. Tri-ex in FirstCalls nice website states that the designer should add > up wind surface area of all of the installations on the tower and that a > tower should be selected that exceeds the total. But they go on to state > that for stacked antenna installations, the tower should be derated.
> >
>
> Ok, if that's what they say....sounds pretty vague to me.
>
> > Intuitively, it seems to me that lower beams of the stack would not cause as much stress on the tower as the higher beams and that the towers rating could be actually be increased. Or more correctly, the square footage of the lower installations could be derated. To illustrate my point, surely a 70' tower rated at 10' sq ft would withstand a much greater wind speed if the beam were placed at say the extreme 2' from
> > ground.
> >
>
> Your intuition is correct.
> Most of the freestanding tower designs I have seen are rated for the
> entire antenna load applied at the top of the tower. If the same total
> antenna area is split up into several separate loads at and below the
> tower top, then the net load at the tower base will be lower, allowing
> the total of the areas to be increased to generate the same overall
> tower loads as the original rating. What is happening along the tower
> with the new point loads is another thing, might be ok, might not,
> When the antennas are stacked on a mast extending above the tower top,
> the areas need to be reduced to create a load equivalent to the original
> one. That's probably what they meant about derating for stacked
> antennas.
>
> I've made some posts regarding this subject in the past, the stuff is
> floating about in the TT archive. Exercise the search engine and you'll
> find a lot of stuff from many sources.
>
> > Although these questions refer to free-standing towers, I would like to
> > also read a tutorial on guyed towers. I suspect that many other towertalk readers would enjoy, appreciate, and benefit from our towertalk talkers writings as I have not seen this kind of material in my 20 or so antenna books!
> >
>
> You can go to the URL below and read some stuff about guyed towers.
>
> Heights Towers has some interesting information on their website:
> www.heightstowers.com/engspecs.htm
>
> Since you raised questions about antenna loads and towers, I'll take the
> opportunity to post my bi-annual reminder that the antenna area ratings
> cited by most tower designers is not necessarily the same antenna area
> that one is likely to get from the antenna folks.
> As with most things, it is important to understand what a number really
> means!
>
> Someday, information on this topic will likely find its way into an
> amateur antenna publication. Till then, keep searching and asking the
> questions. Someone around here is likely to know the answer, if not, at
> least have an opinion about it.
>
> --
> 73, Kurt, K7NV
>
> Visit http://www.freeyellow.com/members3/yagistress/
>
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