This is a 76' suspension bridge across an arm of the pond on my property. It is built from treated dimensional lumber and galvanized wire rope with a small amount of plated 3/16" proof coil chain. Oh, and lots of bolts and screws!
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:
http://www.trailstobuild.com/Articles/PochuckBridgeEngineeringArticle/Pochuck%20Quagmire%20Bridge.htm
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!
Jake
Update - runner up in the Woodworking contest. I got a t-shirt - I'm happy! :)
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:
http://www.trailstobuild.com/Articles/PochuckBridgeEngineeringArticle/Pochuck%20Quagmire%20Bridge.htm
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!
Jake
Update - runner up in the Woodworking contest. I got a t-shirt - I'm happy! :)
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Signing UpStep 1: The Math!
First step in any bridge project is to measure the approximate span for the bridge - both ends of which should be at approximately the same elevation - and then decide what exactly you want to be able to cross the bridge. Those two facts set all the other dimensions. Our bridge was 76 ft across and needed to accommodate a garden tractor or a golf cart and perhaps 20 people at a time - not coincident with the garden tractor or the golf cart. A golf cart with 4 people is about 1500 lbs, 20 people are as much as 4000 lbs, while a garden tractor is only about 500 pounds. The garden tractor and the golf cart required a 5 ft wide deck.
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.
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.
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Now gnats, that's a different story!
More or less. :)
I'm an architect and I can say that this bridge really brought something special to the property.
Bravo!
Inspiring.
Intimidating.
Have you any photos of the critically important anchors and more info on how the cables attach to them?
Also, can you describe more about the spikes.
Thank you for an outstanding Instructable.
What kind of glue did you use to laminate the anchor?
Wow.
It is very difficult to line up plates on both sides of the posts. I had to do it on the towers, but it was no simple task to bore holes in the posts that aligned with the plates on both sides so that the holes pre-bored in the plates would work out.. Next time, I drill one side, bore the holes, mark the plate for the other side and then drill those holes!
Very nice!
That is a lot of work, but the feeling of getting the job (well) done is priceless.
Greetings,
Were I starting engineering school now, however, I might choose to study bridges, too!