[TowerTalk] Subject: Wind survival + load ratings... vs,

[Kurt Andress]

Jim Thompson, Jim Lux et al......
Your comments on this subject are appropriate and appreciated, but there 
is more that needs to be explained....this is not a simple matter and is 
most likely why few have ever gotten any of it anywhere close to right......

Both Jim's are right that the members of a yagi antenna deflect when 
exposed to the wind! This causes them to experience lower loads as they 
become inclined to the wind as explained by Dick Weber, K5IU in his 
papers from ~20 years ago as he explained his "crosswind principle". 
There is absolutely no way to easily account for this principle in 
analyses without spending exorbitant amounts of time to manually 
calculate the deflections of each of the portions of each of the 
sections in a member at each wind speed increment! This is a non-linear 
phenomenon, and no one has created software to be able to automatically 
do it! So, linear Static Analyses will be conservative by the amount of 
deflections the members see. It is important to note that properly 
designed ~100 mph 20M thru 10M antennas do not experience huge 
deflections, like 40M & 80M antennas do, so the intrinsic safety margins 
they will get from becoming unloaded by deflections will be much lower 
than for the larger antennas. I think we should accept static linear 
analyses for them as they are, with their unknown safety margins that 
come from their deflections. The relative comparisons of the safe wind 
speeds of the different antennas are always valid, as long as they are 
analyzed with the same methods. TIA/EIA 222 has yet to require the 
complexities of appurtenance member (I.E. antennas) deflections into the 
standard, towers on the other hand are analyzed with non-linear methods 
that are accurate for structures with relatively low deflections. I.E. 
nobody designs towers with huge deflections.....my website 
K7NV.com\notebook shows what happens with that stuff....

Jim Thompson, I think your evaluation of the Optibeam 80 is lacking some 
important knowledge. I learned more than I ever want to know about that, 
by being the mechanical design engineer, and lead for all tower work for 
the W7RN station. We tried twice to redesign, reinforce, and do 
everything possible to make two of those antennas survive up there, with 
no success. After a few years of effort, and constant search & 
inquiries, I learned that many EU antenna builders do not have ready 
access to high strength alloy tubes, like we do here. They can get ~40 
ksi equivalent tubes in the small tube sizes (maybe up to around 1" 
diameters), but not in the larger ones.....the larger tubes are rolled 
and welded, with visible seams, not drawn, with a yield strength of 
about 1/2 of what we can get here with 6063-T832 drawn tubing. I heard 
some of the tubes were made in Turkey..... So, plug 23,200 psi yield 
into those analyses and see what it does. In the UK, they can get real 
6061 alloy tubes, but they are 1/16th wall and don't telescope, so they 
have to grind tubes to get them to fit together.
So, to solve the W7RN problem, I designed from scratch, new elements for 
one antenna with 6063-T832 American sourced tubing. The static YS 
analyses say they are safe @ 130 mph. My Linear Finite Element analyses 
show that the tips of the elements are deflected far enough to be 
parallel with the wind at that speed, so they are capable of well over 
that wind speed. Emperical experience says those elements survived the 
worst winter in over 10 years up on the Comstock, with some significant 
icing....they are the only elements that have survived up there for 
longer than a few weeks ;-) YMMV......etc,...... back to net......