Search the forums
Tension on cable & other engineering Q's
I was at a local farm over the weekend with my family and some friends of ours. This is one of those places that, in the fall, will have hay rides, a maize maze, livestock demonstrations, a pumpkin cannon (used compressed air and launched the pumpkins about 8" in diameter 2-300 yards) etc.
They had a zip line with a little seat where you could push a toddler about 75 feet. It used something like a beam trolley device, except the casters rode on the line vs. the flanges of a beam. A ~3/8" steel cable was strung between two 6x6 posts with guy wires.
We (myself and my friend Seth) thought it was pretty cool and of course thought "I could build one of those". Our discussion evolved from there until we came upon two problems that neither of us have a definitive answer for. Opinions, yes, but no real knowledge.
Let's say you have a steel cable that is 100' long and 8' off the ground. If you place a weight (a 40 pound child) in the exact middle, we both think that the weight exerted on both ends of the cable is equal. Now, let's move the child to the one quarter point, so you're 25' away from one end and 75' from the other. The first question is- Is the force exerted on both 6x6's equal, or is the post that the child is closest to under a larger load than the far post?
A somewhat related issue is the question of cable size with respect to the length of the zip line. If one were to determine the size of cable necessary for a 100' line, would the cable size need to be increased if the line were to increase to 150'?
I know the dead load would increase, but I don't think that to be significant. I think the longer cable would need to be stronger only because you would want to increase tension on the line to counteract the amount of deflection (probably not the right term for a flexible cable, but you know what I mean) beacause of the longer span. If deflection were not an issue, my belief is that the cable size could be the same size even though you have increased the span by 50%.
So, what do you think?
Jon BlakemoreRappahannockINC.com Fredericksburg, VA