Oops, let me make a clarification on the elasticity comparison:
I failed to include that the relative elasticities among the materials
is for the same cross-sectional area! Sorry for the confusion.
It is possible to have equivalent stretch between steel and fiberglass
guy materials if the fiberglass rod is oversized.
Compared to a solid, 0.25" diameter steel rod, a fiberglass rod of the
properties previously mentioned would have to be about 0.25 x 2.2 = 0.55
in diameter to have the same deflection rate (spring rate)along its
The cross sectional area of the fiberglass rod in this case would then
be about 2.2 squared = 4.8 times that of the equivalent steel rod.
The breaking strength of this fiberglass rod would then be about 28,500
lb, based on the strength of 120,000 lb/sq in given before.
Based on the densities of 0.282 lb/cu in for steel and 0.073 lb/cu in
as previously reported for the fiberglass, the weight per unit foot of
the fiberglass works out to be about 1.25 times that of the steel.
Since the fiberglass is a little stiffer, yet heavier per unit length, I
would guess that the sag would be close to or only slightly less than
The point is, you *do* have to look at the 'stretchiness' of the guy
materials. The more 'stretchy' they are (low modulus), the more
oversized they must be for the equivalent stabilizing effect on your
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