On 10/23/2014 7:36 AM, dpb wrote:
....

Consider for simplicity the condition if the cables were mounted on
telephone poles on level ground 100-ft apart. Say you tension the cable
so there's 10-ft sag in the middle. Using the tabulated weight for 3/8"
cable, I estimated that it takes only about 30 lbf to achieve that sag
so we'll ignore that for the time being.

With 10 ft drop at midpoint of 100 ft run, and considering that the
applied load will essentially straighten the cable, if the load were at
the center rather than distributed the angle between the horizontal and
the cable is invtan(10/50) -- angle ~11 degrees.

Now to support that load, the vertical component of the tension has to
balance the weight of approximately 5000 lb. That vertical component is
Tv=T sin(angle) or the cable tension T is Ty/sin(angle). Substituting
numbers and noting that for small angles sin(theta)~theta, the tension
to support that 5000 lb is

T=W/sin(angle) = (W/2)/sin(11) -- 2500/0.2 -- 12,500 lb

So, you've taken up roughly half the total strength of the cable simply
by the decking. That's only a safety factor of ~2X and minimum generally
accepted is 3X while for overhead rigging and personal safety of support
5X is considered prudent.
....

ERRATUM: I forgot to divide the cable limit by the two above -- the
actual limit per cable is (optimistically) as used by your friend 14000,
not 28000.

So, the decking alone is roughly 90% of the rated breaking strength and
adding a 200 lb person is 100/0.2--500 lb.

You'll gain a little by considering the decking as a distributed instead
of point load, but that'll be only a marginal improvement and you'll
likely lose some (and potentially a lot) for the non-uniform geometry on
the downhill side as that side may be almost perfectly horizontal so the
amplification factor of 1/sin(theta) goes way up as Morgans feared
(hence his 10X estimate).

THIS IS VERY BAD...

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