I needed the bridge to more directly connect the meadow below my house with my picnic grounds - which were on the other side of the sometimes-arm-of -the-pond, sometimes-nearly-impassable-ravine.
This is the best resource I found online for building this bridge:
The bridge cost about $4000 to build and took several hundred hours of labor and thought.
Not that the thought wasn't labor, too!
I want to thank all of you that voted for me in the Woodworking Contest. I ended up 6th in the voting! Now for the judging!
Update - runner up in the Woodworking contest. I got a t-shirt - I'm happy! :)
Step 1: The Math!
An then there is the math in a suspension bridge. This is actually a fairly easy part once you use a spreadsheet and this formula:
y=(lbm/ft)/2T * x^2
which gives the sag ("y") in the catenary cable (which is not a catenary but rather a parabola) at any point along the deck ("x") as a function of the suspended weight and the tension ("t") at mid span. For my purposes, t is an input, along with the weight per linear foot of bridge (actually, half the linear weight as there are 2 cables) and the sag is what I aim for. Given the limitations of the equipment and dimensional lumber -I could not readily have raised anything longer than the 16 ft laminated posts (4 2x8x16 glued and bolted) with which I constructed the 2 towers- and minimal bury (approximately 2 ft) that meant I had at most 13.5 ft of max sag to work with. I aimed 12.75 ft of sag to allow for about 6 in of arch in the deck plus a really short suspender at mid span.
Taking all that into account, the suspended weight of the bridge is about 4000 lbs, almost all of which is the weight of the dimensional lumber used in constructing the deck. I used 2 in board for everything - 2x4x16 and 2x8x16.
With that weight, span and sag, I calculated a tension with 2000 lbs of load and treated lumber at 40 lbs/cu ft (which may be heavy, as it is more than the average weight of the pieces I weighed) at 2500 lbs. I used 2200 lbs in my calculations. From that all other loads, such as anchors, eye bolts, turnbuckles, were set. All the main load carrying material (wire rope, etc) were rated 3500 lbs or greater working load. I used 1/2 " galvanized wire rope (about 5500 lbs working load) for the catenary cable and 3/16 in galvanized aircraft cable (850 lb working load) for the suspenders. There are 37 2x8 joists on 2 ft spacing with 37 suspenders from each cable.