Introduction: Oklahoma Suspension Bridge
Runner Up in the
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:
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!
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.
Step 2: Posts, Beams and Anchors
Next came the towers and anchors. Because the only way to keep the tension as low as possible, reducing the strength requirements of all connectors, is to have as much sag as possible, not exceeding approximately a 6 to 1 ratio of span to max sag. More typical designs use a range of 12 to 1 to 8 to 1 in order to minimize sideways sway. The 16 ft posts plus the 8x8 beam on top, 2 ft of bury and about 8 in above grade to the top of the joists gave me about 13.5 ft total distance for sag plus a minimal suspender length at mid span. More bury would have been better, but would have reduced the max sag and increased tension in the catenary.
I laminated 6 posts, cut 1 post in half for the top beams and 1 post in quarters for the anchors. The anchors are designed for 3.5 ft of bury with spikes on all sides, and embedded in as much concreted as is feasible - I aimed for 1500 lbs of concrete approximately 3 ft in diameter with at least 18 in of compacted dirt on top. I don't know what the total uplift resistance is, but I estimate it is in excess of 3000 lbs. This is lower than I wanted, but as much as I could do. 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, along with two bolt ends.
The anchors had 1/4 in steel plates drilled for an eye bolt for the anchor cable and additional holes for lag bolts. The towers had similar steel plates on both sides tying the beams to the posts and allowing for and eye bolt and eye nut for catenary and anchor cable connection.
I chose a fixed connection for the cables rather that a saddle as is customary in suspension bridge because I was concerned that during installation of the bridge the unbalanced loads on the towers would be too great for the minimal bury I used. With the anchor cables installed, I had no worries the towers would fall into the pond during installation!
The towers were erected with the assistance of my compact tractor as well as two human assistants. The west side tower has a tree interfering with the approach and was very difficult to raise. The east tower, with the knowledge gained on the west tower and no interference from trees went up almost easily - by comparison, at any rate.
The towers are designed for a 5 ft width between the inner faces of the posts. The eyebolt assembly is designed so that the catenary cable and suspenders will fall just outside the handrail.
Step 3: Joists, Hand Rail Sub-assembly and Suspenders
The 37 joists on a 2 ft spacing are 2x8x8 treated lumber (all the wood in the bridge is treated lumber), while the handrail assembly consists of a 2x4x4 upright with an 2x4 angled brace to the end of the joist. The spacing between the inside of the handrail upright is 5 ft - wide enough for a garden tractor or a golf cart (really, a people cart). Only every other joist has a hand rail assembly
The connection between the suspenders (3/16ths galvanized aircraft cable, 850 lb working load) and the joist is a problem. One end of the suspender must be adjustable to permit as built corrections to tension, and thereby the shape of the bridge. The ideal connection would be a eyebolt through the joist with a turnbuckle between the eyebolt and the suspender cable. The problem with that solution is expense - when you need 74 of everything, forged or even welded 8 in eyebolts are expensive, and so are the turnbuckles. Instead, I chose to drill 1 in holes through the joists approximately 2 in from the bottom and just outside the handrail upright. Through this hole I threaded 3/16ths bright proof coil chain with an 800 lb working load. I threaded a 3/16ths thimble through the end chain links, then the cable through the thimble and finally clamped the two ends of the cable with two 3/16ths galvanized clips. If I do another bridge, I might try another approach, but this worked okay, if a bit cumbersome to build and to adjust once the bridge is up.
The aircraft cable suspenders and the plated chain were cut to length with a friction cut-off saw. 74 pieces of chain, all the same length (16 links) and 74 lengths of cables, 4 of each length except only 2 for the middle suspender, (2 catenary cables, each of which with identical dimensions each way from the middle). The length of the suspenders was calculated in the spreadsheet to account for a 6 in arch in the bridge as well as sufficient cable for thimbles at both ends, the compression fitting on the top end and the two clips on the bottom end. The suspenders were installed to dimensions calculated in the spreadsheet from bottom side of the catenary cable to the top side of the joists.
Step 4: Bridge Sections and Catenary Cable Assembly, Plus Launch and Towing to Bridge Site
The catenary cable is 1/2 in galvanized wire rope with a working load of 5300 lbs at a 5 to 1 safety factor. The cable length is calculated in the spreadsheet at approximately 80 ft 7 in. I used 12 in jaw/eye turnbuckles at each end. Each suspender location, also calculated in the spreadsheet, was marked along the cable prior to installation.
The joists were assembled in four 16 ft sections and one 14 ft section (the middle section). And the trucked (tractored) over to the final assembly area. Assembly consisted of two 2x8x16 plates screwed into each joist with three 3 in deck screws, having first marked vertical on each plate at a 2 ft spacing.
At the assembly area (more on that later), the sections were dropped onto rollers and then bolted together with 2x8 plates at each joint. The two end sections were designed to hinge, while the other connections were as rigid as possible.
Once the sections were assembled, the catenary cable was affixed to the suspenders at the marked location. The connection consisted of 1/2 in clips with the closed part of the "u" down and threaded through the thimble on the tip end of the suspender.
I built 4 ft slings which were attached to the east end of the catenary cables for pulling the bridge up to height. The slings were essentially a loop of 1/2 in wire rope attached the catenary cable with two clips. The bridge will be raised into place using come-alongs attached to the slings.
One the catenary cable and slings were installed and all bolts tightened down, the bridge was pushed into the pond (I used round treated fence posts as rollers) using my compact tractor. Once in the pond, the bridge was towed to the site.
Step 5: Bridge Raising
One of the most difficult aspects of building a suspension bridge is hanging the first pieces of deck from the catenary cable. Had I not been able to assemble the bridge substructure ahead of time and float the assembly to the site, I would have had to use another process to actually raise the bridge. Most likely, I would have installed a second, and much smaller, pair of catenary cables pulled to less that half the designed sag of the main cables. Then I would have pulled the main cables across, having preinstalled the suspenders, attaching joists as the main catenary cables were pulled. I also would have installed either the outside deck boards, or the bottom hand rail boards, or perhaps both. I don't think I would have installed the 2x8 joist end boards at all had I been forced to use this raising process.
As it was, we levered up the west end of the bridge sufficiently to attach the catenary cables to the eye bolts in the top of the tower, then, again using my compact tractor, I pulled the east end up far enough to attach the 12 ft come alongs to the slings and we began to winch away, slowing raising the bridge. The turnbuckles were very hard to turn -they got HOT! But in about 4 hours from working on the catenary to calling it a day, we hung the bridge more or less level, and called it good.
Step 6: Building the Deck
Once the bridge was in the air, the garden variety work of deck building got under way. 2x8x16 lumber for decking, 2x4x16 for the hand rails, 5/4x6x16 decking for the facing boards on the handrail system.
The handrail system is an integral part of a suspension bridge - the two handrails form the trusses that provide most of the rigidity to the bridge. Without the trusses, the bridge is an elevated snake. With them, it is incredibly stable.
To make the handrails into trusses, we placed cut-to-length 2x4s from the top of one handrail support to the bottom of the next, working our way from both ends to the middle. The bridge is all but unusable without adequate trusses - witness the failure of the Tacoma Narrows bridge, due entirely to insufficient stiffness.
Entirely unexpected was the genuine beauty of the bridge. Through what can only be sheer luck, the finished bridge is simply magnificent.
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