My friend (and evil Scrabble competitor) of many decades lives 52 miles to the east, two counties away from me, in the Arkansas Ozarks. Mac has a stream running past his cabin. This stream divides his cleared acreage in half. The rest of the land is forested.
Mac has a big ol’ John Deere lawn tractor and found it difficult to ford the stream for mowing. During one of our Sunday Scrabble games, he asked if I thought it possible to construct a bridge that would support the mower. Foolishly, I said that I’d do some calculations and draw up some plans for him to look at... thinking that he’d hire some responsible, skillful, knowledgeable, respected person to okay the plans and (maybe) build it for him. Silly idea.
He said, “The plans look good. When can you start.”
Serves him right if it falls down.
Step 1: Ground Work
Getting the lawn tractor to the other side of the stream was gonna' take some bridge!
Step 2: Location, Location, and Like That
The bridge had to be long enough to span the stream. On the side closest to the cabin, there was at least 3 feet taken by the width of the gabions and then still farther back to dig for solid foundations. On the far side, there was loose gravel that we had to get away from in order to find solid bearing. The minimum distance of free-span that could cross the stream and sit on solid bearing surfaces was 24 feet. And the John Deere lawn tractor had to get onto and off of the newly constructed bridge. That put us in the area of a 5-foot-wide surface.
So, the growing bridge had to [a] climb up to the existing level of the deck, [b] have clearance for the turning radius of the lawn tractor, [c] be wide enough to accommodate the width of the cutting deck of the tractor, [d] be absolutely level where it crossed the stream, and [e] slope down to the ground gently enough to allow climbing back up when the tractor was returning.
The calculations that I ran indicated that a 24-foot long, quarter-inch thick steel channel, 2 inches by 4 inches, would hold the lawn tractor, a 180-pound driver, and the bridge materials. So, using two of these steel sections would give a safety factor of 2. The lengths connecting the ramps, deck, bridge, and other spans would be made of regular 2 X 10 treated lumber.
Mac fired up his tractor and we measured the turning radius of the beast: how much space did it need to make a turn? This number would determine at what minimum angle the new decking could meet the existing deck. Then it was a matter of asking the 'clients' (Mac and his lady friend, Geri) where they wanted the bridge to cross the stream. We staked out a possible path from a ramp up to the deck, from the deck to the stream bank and over to the other side, and down to the ground via another ramp.
I used a contractor's 'dumpy' level to get the heights of the existing deck and the ground elevations at  the deck,  the near side of the stream bank,  the far side of the stream bank, and several points in between the deck and the bank. The frost line (depth of possible ice formation under the surface of the ground) in the north Arkansas Ozarks is about 18 inches. I intended to pour 18 inch square by 6 inch thick concrete footers under the supporting columns to spread the 'dead load' (weight of the structure itself) and the 'live load' (anything that might be put on the structure such as the lawn tractor, people, furniture, and the like). So the bottom of the holes for the supporting concrete columns were to be 24 inches (18 to the frost line and an extra 6 for the footer as a safety measure) below the surface of the ground level. With the depth below ground and the height needed above ground, I was able to figure the total height of each column… and the necessary amount of form material, concrete, and rebar required.
Step 3: Dig This
Once all the holes were laid out, it was time for the bar. In the Ozarks, digging a hole requires removing rocks that don't want to be removed. A 6-foot long solid steel bar with a chisel shape on one end and a dull point on the other is the implement of choice. The digger (me) lifts the bar and slams the pointy end into the ground… letting go at the last instant. You are pretty much assured of hitting a rock. Failure to let go in time often results in the 'twanging' of steel & man observed mainly in cartoons. After the rocks are loosened enough to be removed from the hole-to-be, one hunkers down and pulls them out. A shovel is an effete instrument used mostly for the removal of fragments and the little dirt that has filtered down between the ever-present rocks.Since the steel channel is rather unforgiving of errors, we dug the holes in pairs: one hole on each side of the stream for the columns which support the steel.
When the holes are deep enough, the sides of the bottom are measured and squared to 18 inches. The concrete for the footer (the weight-distributing bottom of the column) will be poured directly into the hole without additional forms… once the rebar is in place.
One cuts four 20-inch lengths of iron rebar. These four are pounded about half their length into the bottom of the hole in a square pattern about 8 inches on a side. Now one cuts four 16-inch lengths of rebar. Using iron "tie-wire", the 16-inch pieces of rebar are wired to the upright rebar in the ground but only 3 or 4 inches from the bottom. It will look like a tic-tac-toe board, only made of iron. Then the concrete is mixed by hand in a wheelbarrow and poured over the iron until there is at least six inches of concrete in the bottom. The 4 upright pieces of rebar still exposed will be used to anchor the column to the footer.
We want the footer to bond to the column that we're going to pour on top. After 24 hours, the footer should be solid enough to work on. Cut 4 pieces of rebar long enough for the column, keeping in mind that the top of each column will be slightly different (more on that later). Use iron tie-wire to fasten each of the uprights to the stubs of rebar sticking out of the footer. Either bend a 4-sided loop or cut straight pieces (much easier) to connect the 4 upright iron rebar pieces on at least 2 levels, say 12 inches up from the bottom and 12 inches down from the top. Wire all together. Lift the form over the top of the rebar cage and slide it down to rest atop the concrete footer.
Now comes the "more on that later" part. Mac and Geri and I decided to "sink" the steel channel and the treated lumber into the concrete columns. This would prevent any twisting of the material. However, It also meant that some columns would have just two 2 X 10 timbers embedded in the concrete and some would have 2 X 10s and steel embedded. To make it a trifle more complicated, Mac decided that it would be nice if we could use one of the hollow steel channels to carry a water line and an electrical line to the other side of the stream. Inserts, two-by sections wrapped in plastic bags, had to fastened inside the basic forms to provide for all these intrusions.
Once the forms are plumbed and leveled, concrete is mixed in the wheelbarrow and dye is added to yield the color desired by Geri and Mac. As the concrete is poured from the wheelbarrow or shoveled into the form, it is a good idea to 'rod' the still liquid mass. This 'rodding' consists of shoving a section of rebar into the liquid concrete and removing it again. Repeat. This removes air pockets, settles the concrete into otherwise unfilled areas, and mixes the material of multi-wheelbarrow batches. Rapping the sides of the forms with your hammer will ensure that there are no voids when the forms are stripped.
Step 4: Cool Railings
Mac had picked up some antique cast iron railings and he wanted them incorporated into the bridge. We sunk one long leg into the concrete column at each end and bolted the other legs through the 2 X 6 covering.
When the first span and railings were set, we went on to do the second... level with and parallel to the first.
Step 5: Connecting Wood & Concrete
After the concrete had set for 3 days, the wood joists were temporarily removed and the forms were stripped off the columns. Before any of the wood was permanently slid into place, black tar water-proofing was smeared over the wood that would be in direct contact with the concrete.
When the outside joists were in place, cross members were cut and nailed to provide support for the center joist pieces that run the whole length of the decking.
Step 6: A "seating Area" Is Added
We used rim joists to tie the joist ends of the octagon into a rigid whole. As extra support, we lag-screwed 3 10-foot lengths of angle iron under the octagonal area; the angle iron braces cross under the joists and are securely fastened to all five wooden timbers. Standing on the outside edge of the octagon creates minimal deflection because the entire structure would have to warp to have any substansive movement.
Step 7: All Hands on Deck
Step 8: Finishing Up
Mac and Geri picked out posts, finials (the decorative top pieces), and the balusters (the upright pieces between the heavy posts). The octagonal section between the iron railings was fairly straight-forward carpentry. Measure, cut, and drill the posts before bolting them to the rim joists along the bottom. The balusters are pre-drilled and screwed to the joists between the posts.
The sloping section between the original deck at the cabin and the bridge section across the water needed a bit more care. Each post had to be cut for length and then cut to fit the sloping angle. Finally, the last cuts were to make sure that the post would be plumb: the level had to show that it was perfectly upright from both sides.
The bridge, all 65 feet of it, was done... for my part.
Mac added cedar posts at the midway points of both ramps, constructed handrails of two-by lumber, and protected the entire structure with exterior grade solid stain.
Step 9: We're Happy Campers!
That winter, the Ozarks had an unusually heavy snow. Mac and Geri grabbed this picture.
The spring brought very heavy rains and the spring branch carried off the water from a several-square-mile watershed. (Remember the flooding and the washing out of the stream bank?) A large tree root & trunk about the size of a Volkswagon slammed into the bridge but did no damage.
And that John Deere lawn tractor? Makes it across the bridge with no trouble.
I reckon that Mac and Geri and I did all right.