Jim Lux wrote:
> Are you sure that this 2.5 times the specs isn't just a "factor of
> safety" to allow for variability in material properties and assembly
> workmanship? That is, according to the calculations, if everything
> was exactly as expected and assembled perfectly, it would have a
> failure load of 2.5*x, where x is the design load.
> One would use such a factor also when you make some simplifying
> assumptions in the analysis, with the extra margin there to cover the
> possibility that the assumption went "the wrong way".
> It's sort of like why houses are wired with AWG14 wire, even though
> the fusing current for that gauge is >100 Amps. (there's also a
> voltage drop issue, too, I admit).
35 years ago I lived in Seattle and was a member of the Western
Washington DX club. It seemed everyone in the club used R25
and many installations were over the top in terms of loading of
the R25. Seattle is a high wind area, yet I never heard of
anyone having a tower come down. I also participated in various
questionable tower raising parties. For example, W7PHO had a 90
ft Rohn 25 tower with stacked Yagi's on top. The club lifted
this tower off the ground in one piece in the vertical position
with all the antennas on top and then carried it 100 feet to a
new location, where it was lowered onto the new base. Each guy
wire had a person on it to keep the tower from falling over.
We all had to walk together. Several spotters watched the
vertical angle and yelled commands to the guy wire pullers.
Compared to this, N6TV's adventure looks tame.
Another station had a 60 ft R25 tower with a quad on the top,
and we used a crane to lift the entire thing up 20 feet, and then
add 20 ft of R25 below it. What these shenanigans illustrate to
me is that there is a rather wide gap between what Rohn, etc says is
OK, and what you can get away with. I don't have any trouble believing
a 2.5 safety factor. I personally had a 60 ft R25 tower on a
narrow lot with the guys too close in to the tower and the base
buried in dirt w/o concrete (I was renting). No problems in the
3 years I was there.
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