Suspension bridges

I’ve built a bunch of “monkey bridges” in my spruce forest for the kids to play on.

The materials are much low weight for a particular span than any rigid bridge would be.  And if trees or existing rocks can be used for the foundation on each end, relatively little construction materials would be flown or carried in.  Over a 10-foot span, you just use logs or lumber.  At 20-30 feet, you can still do lumber, but you’d really like an intermediate support (maybe a wire cage filled with rocks) to reduce the size of lumber needed and/or avoid have to do some engineered truss design.

At 50 feet, a rigid span would be huge, mostly to hold up its own weight.  While a suspension bridge for a few people at a time can be two 3/4″ steel cables  with LOTS of safety margin.  I’ve done 100-foot spans with just 3/8″ steel cables, knowing it only be a few kids on it and no group of 15 people swinging it back and forth, or trying to ride a four wheeler or a horse over it.

If you make the support cable what holds the treads, it will steep at the ends (think Indiana Jones) – the support cable needs to hang in a catenary curve.  In something like the Golden Gate bridge, it’s the two cables above that holds it all up, the road bed just hangs from it.