Calculating horizontal Wire Cable Load

[gcheris]

We've installed a 'zip line' and are trying to figure the maximum load it can safely carry. To date, it has handled, without complaint, 210 lbs. This is fine except over time an overload will obviously weaken the cable - we don't want to do that!

The line is a 3/8th inch aircraft cable (14,400 lbs test) attached to upright 16" x 10" x 1/4" steel i-beams which are embedded in 5' deep x 4' diamenter concrete plus the tops are anchored, via the same 3/8th inch cable, apx 8' to the rear in more concrete. The attachment point on one i-beam is 11' above the other i-beam.

The trolly is a commercially mfg 14,400 lb test unit.

The deflection is about 14-20" over the 275' span (a sight guess).

Does anyone know of a formula to determine the maximum safe working load?

 

[pervect]

If you have a purely static load on the cable, by my calculations the tension on the cable will be twice the supported weight if the angle is 15 degrees and the weight is in the center.

But this doesn't really address the safety factor that you need for dynamic loads, or for fatigue.

One major concern will be the amount of energy your system can absorb. This will be given by .5*force*distance, where force is the tensile strength at which the cable starts to stretch, it's elastic limit, and distance is the amount the cable stretches when this amount of force is supplied. The distance the cable stretches depends on it's stiffness. There are various figures out there on the net, but I'm hesitant to supply any details because I don't want to get sued if I'm wrong :-).

Over and above this, you'll want to have some safety factor, at least 2:1, and probably more like 5:1.

Probably a mechanical engineer can give you a better answer than I can.

 

[Clausius2]

We've installed a 'zip line' and are trying to figure the maximum load it can safely carry. To date, it has handled, without complaint, 210 lbs. This is fine except over time an overload will obviously weaken the cable - we don't want to do that!

The line is a 3/8th inch aircraft cable (14,400 lbs test) attached to upright 16" x 10" x 1/4" steel i-beams which are embedded in 5' deep x 4' diamenter concrete plus the tops are anchored, via the same 3/8th inch cable, apx 8' to the rear in more concrete. The attachment point on one i-beam is 11' above the other i-beam.

The trolly is a commercially mfg 14,400 lb test unit.

The deflection is about 14-20" over the 275' span (a sight guess).

Does anyone know of a formula to determine the maximum safe working load?

I would prefer to see some drawing or scheme of the structure described. Do you mind drawing one?

 

[gcheris]

Thanks

Thanks to all who supplied info, it will help in solving the problem.

 

[pervect]

Good luck! I just wanted to clarify my remarks about high dynamic loads and energy a bit. Consider taking a large weight, putting it near the middle of your rig, and letting it drop a bit. If your cable system is very stiff, the weight will stop very suddenly, generating a very large force on the cable. This is what I mean by dynamic loading, and explains why I mentioend the energy absoprtion of the system as being of a concern. If you know the change in length of the cable that occurs at max rated load, you can calculate the rated energy absorption from .5*force*change-in-distance, as I mentioned. You'll have to judge for yourself how much energy the cable system might have to absorb, unless you can find some engineering guidelines somewhere. A couple of scenarios would be m*g*h, for a mass m suddenly dropped a distance h, or .5*m*v^2, for something sliding along the zipline at velocity v that stopped suddenly because a pulley "stuck"

 

[russ_watters]

...over time an overload will obviously weaken the cable...

A steel cable experiences little or no fatigue stress because its stress is virtually all tension: it will not weaken over time.