Instructables
Picture of Oklahoma Suspension Bridge
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! :)
 
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Step 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.
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That is some project. I'm sure there are several special words used to get it all together.
eugeneb43 years ago
That's freaking awesome!!
Ghost Wolf3 years ago
How big is this pond/river of yours?
Jakebutnottheone (author)  Ghost Wolf3 years ago
The pond is about 3.5 acres when full. It is only full after a stiff rain, and then only for a few weeks. Most of the year there is no water below the bridge. It's not exactly dry, either. :)
Oh cool I bet you get lots of mosquitos
Jakebutnottheone (author)  Ghost Wolf3 years ago
Not that many mosquitoes. LOTS of frogs and dragon flies, not to mention lots of fish in the pond! :)

Now gnats, that's a different story!
some people eat flys for protien
rprough3 years ago
Do you think creosoting the anchors before inserting into the ground would be a good idea?
Jakebutnottheone (author)  rprough3 years ago
Any treatment of the anchors is a good idea, but I personally am not comfortable with the idea of creosote. There are other ground line treatments available, and I may use one of those. In the long run, I will probably replace the anchors with steel screw anchors. Greater, and more certain, uplift resistance as well as a far longer life.
Forgot about the steel screw type. What comes to mind is the tiedown anchors used for a mobile to secure it. Good amount of uplift resistance.
Jakebutnottheone (author)  rprough3 years ago
The manufactured home tie downs are also often screw anchors. Using those anchors I could double up each anchor and provide the uplift I desire. I haven't yet decided how I shall do it.
hifatpeople3 years ago
what high school class teaches this? i took an "introduction to engineerig and design" class on accident and thay had nothing but BS. what class would this be? it looks like it would be really helpful in the future.
I took a class called pre-engineering and it showed us our model bridges and we took an equation to see how much pressure it could take my 1 ft long bridge could withstand about 250 lb.
care to summarise all u learned in that class?
Jakebutnottheone (author)  hifatpeople3 years ago
No high school class I know of teaches this. As for me, I thought it up, read it up, put it up.

More or less. :)
g.petinati3 years ago
This is awesome!

I'm an architect and I can say that this bridge really brought something special to the property.

Bravo!
drewgrey3 years ago
Wow, great job.
Magnificent. Congratulations!
nfarrow3 years ago
Really cool. Do you have anymore images of the bridge?
Jakebutnottheone (author)  nfarrow3 years ago
One more, which I have added to this step as the last pic in the series.
*bows to greatness*
VadimS3 years ago
Voted
rimar20003 years ago
VOTED!
Jakebutnottheone (author)  rimar20003 years ago
Thanks! There are so many great projects! I voted for several!
Astonishing.
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.
I added two additional pics to Step 2 and 1 pic to Step 6.
Did you install plates front and back of the laminated anchor?
What kind of glue did you use to laminate the anchor?
Wow.
Big washers on the other side of all 3 bolts. And I used construction adhesive to laminate the anchors (all the posts, really).

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!
I will take additional pics and add them to the instructable. The spikes are 10 in (I think) long and similar to a really, really big nail. They are pounded in about 40%. They are what the concrete bears upon, whether in up lift (anchors) or in down force (tower legs). There are 6, as I recall, in each anchor, alone with two bolt ends.
rpb3 years ago
Fascinating - many thanks for writing that up. An amazing project!
CrLz3 years ago
Amazing project- thanks for sharing the process!
RAF23 years ago

Very nice!

That is a lot of work, but the feeling of getting the job (well) done is priceless.

Greetings,
Jakebutnottheone (author)  RAF23 years ago
So priceless that two months later I am still kind of stunned!
Excellent excellent job, and good write up. This is the kind of stuff you cant find elsewhere no the internet :)
Jakebutnottheone (author)  x2percentmilk3 years ago
Well, I couldn't find it anywhere else, excepting the Pochuck bridge reference I give in the intro.
cescu3 years ago
Great job! very very nice; at university im studing bridges! are you an engineer?
Jakebutnottheone (author)  cescu3 years ago
Yes, I am an electrical engineer. I don't think that translates too well into bridge design.

Were I starting engineering school now, however, I might choose to study bridges, too!
symesy3 years ago
A little bit of grease goes a long way on thoes turnbuckles. Also if you used gal ones they can be a bit harder to work because of the galvanising on the threads, stainless ones might be more expensive but would make it a lot easier.
Jakebutnottheone (author)  symesy3 years ago
Both those things are true - but I wanted them to stay put once they were adjusted, so I put no grease on them, and they were expensive enough that for one adjustment, or very few over the life of the bridge, I was okay with the galvanized.
Fair enough. Just thought id say. Great build anyways, its really good to see people building. Good on ya.
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