<<Another thing to think about is the coefficient of elasticity. That
may be the wrong term, but what I mean is how much does it stretch for
a given load. In a recent calculation posted on his Web site, Kurt,
K7NV, showed that Rohn towers with the bottom section sunk in concrete,
rather than pier pinned, get pretty close to the 1.0 safety factor,
below the bottom set of guys, when using Phillystran. This seems to be
related to a greater stretch per unit force. You need to know how much
this stuff stretches, and not just ultimate tensile strength, before
deciding whether it is acceptable>>
Very, very good. You've got the gist of the idea.
The term is Modulus of Elasticity, typically represented by the letter
'E'. The lower this number, the floppier and more elastic the material.
This means that it will deflect much more under a given load.
A guy material with a low modulus, or low E, will allow your tower to
deflect, or sway, much more in the wind than would a guy material with a
high E. Kurt has helped us understand that this swaying means your tower
is bending, which, when uncontrolled, will lead to buckling.
Moduli of Elasticities For reference, from prior posts:
Note: Msi stands for millions of Psi:
Fiberglass 3.5- 4.0 Msi, (epoxy/e-glass Mil Spec G-10 material)
Glasforms Fiberglass rod - 6 Msi (75% glass, from recent post)
Aluminum 10 Msi (common 6061 & 6063 alloys)
Aramid fiber 18 Msi (Kevlar 49 used for most aramid guying cable -
Steel 29 Msi (commonly used steels mild, chrome-moly, and stainless)
Steel is by faaaaaarrrr the winner here. not even close. So, by
comparison, the fiberglass is 29/6 = 4.8 times stretchier than steel.
The fiberglass is 18/6 = 3 times stretchier than the aramid.
Kurt's example strength modeling shows us that guy cables with a lower E
(more elastic) will reduce the safety factors of your tower
Fiberglass would not be a good choice if your tower is in a high wind
speed zone or heavily loaded, as many amateur towers are.
The moral of the story, it would seem, is that if you want non-resonant
guys with the best ability to secure your tower, stay with steel and
insulators or kevlar (Phillystran).
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