[TowerTalk] Tower Questions

[Kurt Andress]

Joel wrote:
> 
> Perhaps  we  can give you answers to some of your questions after working with the TRI EX engineering staff.  Well, here goes;
> 

SNIP discussion.

> 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. ..

Joel, 
Thanks for taking the time to collect this information, and to the P.E.. 

Wind speed gradients due to elevation shouldn't be too hard to recognize
or accept. Apparently, my comments have been misunderstood by some to
say that the wind speed doesn't change with elevation. That was not
said. The comments were directed at how the cited spec's treat the
problem by using a "constant" basic wind speed and applying varying
factors to generate varying dynamic pressures to account for the changes
of elevation.

In the response text I see components of the Uniform Building Code (UBC)
and components of EIA-222 E, & F. As pointed out subsequently by
tower2sell,the EIA spec does not use the A, B, C, exposures cited in the
response.
Actually, UBC '97 uses B, C, & D exposures (table 16-G, page 2-28). The
tabular information comes from EIA-222. The "height factor", Kz (called
"exposure coefficient" by EIA-222-F, how's that for nomenclature
confusion?)values in the table agree with what I get at 20' 40' 60' 80'
&
100'(EIA-222-F para 2.3.3, page 4), but the Gust Response Factor (Gh)
doesn't. Gh varies according to the formula in para. 2.3.4.1, which says
that it is 1.25 from ground level up to 33', then it decreases to the
1.16 value shown at 100' (continues to decrease above there).

I haven't spent enough time to make sense out of all the dynamic
pressures in the table, other than find that the 20.01 value given for
100' is what EIA-222-F produces for 70 mph basic before the drag
coefficients are applied.

Just wanted to make sure everyone understands that this was a mixed spec
response, and that no one tried to take this and go off and do
something with it. A copy of the appropriate spec is worth a thousand
posts.  

The response pointed out that crank-up tower section overlaps are
problematic as expected. One of the riginal generic questions (where is
it
gonna fail) was not answered, and I wouldn't have expected it to be.
Again, that can vary with each design. The answer can be found by
careful examination of the tower calc's.


-- 
73, Kurt, K7NV

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm