Feedback on Rope Bridge

1. Jun 1, 2017

Sundog

I'm designing a rope bridge that will span around 40 ft over a pond. First off, I know its not recommended to build a bridge like this without having it designed by a professional. However, I plan on overbuilding it and it will not be very high. I'm also thinking about using 3 or 4 ropes to support the deck instead of the normal 2.

Using the formula H = w L2 / (8 d) I've calculated the midspan force to be around 583 lbs (estimating 350 lbs for wood, 40 foot span, and a sag of 3 feet). This is without accounting for people.

Here are my questions:
- Will the tension be distributed across the number of ropes used to support the bridge? Ex. two ropes would be 291 lbs on each rope.
- Is this the right formula to be using? Would I need to use a different formula to account for people since that weight is not even distributed?

Thank you for the help!

2. Jun 1, 2017

tech99

You need to allow for the maximum number of people in the worst position, presumably the centre, standing at one edge. I think it would be difficult to ensure that the load is shared equally by the ropes and I would be inclined to make each one able to carry the full load. I presume you will also apply a factor of safety somehow, especially as things deteriorate and you may (or may not?) have people's safety involved.

3. Jun 1, 2017