Building a wooden arch bridge is not as hard as it sounds. Using two by twelve inch pressure treated boards for the main joist or beams is the trick. After selecting the site where you want the bridge to sit, measure across the water and add four feet or so to the width of the water below. You want the boards to span the stream in one piece and still bear on the ground on each side between one and two feet for support. Start off by preparing your footings for the ends of the beams to rest upon. You can use concrete or flat stones if you have them large enough but concrete will provide one level area for the full width of the bridge to sit on. If one end moves, they both move with the frost heave and so on. Dig a trench the width of the bridge, and about a foot wide and a foot deep on both ends of the bridge. Now pour the concrete and don’t forget to place a couple of pieces of rebar in the concrete for added strength.
While the concrete is drying, place your two by twelves on a good set of saw horses for cutting them to shape. Two by twelves ten, twelve, fourteen feet or longer are quite heavy and you may need some help handling them. Crown your boards so the crown is up on all the pieces. Locate the center line of each board end to end. Measure down on each end of the boards about 1 1/2 to 2 inches and make a mark. What you are going to do is create a line connecting the center line of the board to the end of the board two inches down on the ends creating an arc line. Draw one section and take a good look at it. Is it a smooth line? Does it weave up and down? If the arc looks good to your eye it probably is fine. This piece or wedge will be the template for all your other arc cuts.
Using an old skill saw that has a little play in the blade shaft is the real trick here. In a shop you could make a huge jig, use a band saw and so on but out in the field you improvise. You can carefully cut along the line you made by gently pushing sideways on the saw and it will curve around the arc. A jigsaw will do this but will take forever and the cut will not be nearly as smooth.
A helper can hold the cutoff piece to assure it does not snap at the thin end in the middle. After the cut is done use the cutoff piece to mark all the other arc cuts. You can of course make the arc steeper but do not cut away so much of the end of the board as to allow it to split under weight.
A two inch slope in six feet is quite noticeable.
The bridge assembly itself is pretty easy. Place each beam board on the footing in it’s proper location. Cut two boards for end plates the exact width you wish your bridge to be. Nail or screw these in place. The outside shape of the bridge is done. Now add all the rest of the interior joists.
I also added a couple of two by fours to the underside of the joists to keep them straight and prevent any future rollover. Using two inch by four inch Pressure Treated boards for decking provides added strength if you are crossing with a mower or other equipment. If only foot traffic is to be applied, 5/4 decking is sufficient. Screw each deck board in place leaving a small overhang on each edge as a drip edge.
The overhang will also conceal any messy cuts on the joists themselves. Handrails are an option
of course and are limitless in their designs. If this is a dangle your feet in the water bridge, you may not want handrails in the way. If this is a ten feet above the water bridge, I suggest you add some good strong handrails for safety.
Your Friendly Building Inspector