My bridge project had some Lumber Jocks ask how I built it. This is the first time that I have ever tried to put something like this to paper.
My original bridge is 36” wide with a base of 96 “and a rise of 18”. The most important part of building this project is making the template. There is a math formula for finding the radius to lay out the arch. It is R= D squared plus H squared divided by 2 H. R is the radius we are looking for. D is one half of the cord or base. H is the height of the arch from the cord line. In my case the radius was 73”
I made the pattern from ½” thick mdf and used my router to cut it out. I made a long arm that I could attach the router to and located a pivot point 73” from the edge of the router bit. I used a spiral bit for this. I then drew a line on the floor (wood floor in my shop) that was longer than my radius. I marked my pivot point and then measured 55” (73” less 18”) and at this point I made a line perpendicular to my radius line. This was where I attached the mdf for the pattern. After I cut the top of the arch, I moved my pivot point the distance that I wanted for the thickness of the pattern, in my case it was 5”. Now I have my pattern.
With the pattern made you can determine what size 2” x’s you will need to make the riser from. I position the pattern and with a sharpie pen outlined the pattern. This will give you about 1/8” oversize drawing. I made my rough cuts on a band saw, but a scroll saw will work fine. Now attach the pattern to your rough cut riser with drywall screws.
I used a ½” pattern bit to cut the riser. You will not be able to cut all the way through the first time. You can use the first cut as a pattern to complete the cut. NOTE! Before you remove the pattern attach a straight edge to the riser flush to the pattern at the center end. Now you can remove the pattern. Make a Skil saw guide about 18” long and attach this to the riser flush to the straight edge you attached. Now remove the straight edge and use your Skil saw to cut the end of the riser. This gives a better cut then doing it with the router.
The bridge requires 6 risers and 4 spreaders to form the skeleton of the bridge. Four of the risers are just as you cut them, but the other two have to be modified. They will be the center risers. You need to cut off the thickness of the spreader on the end where the two halves of the bridge meet, and you need to cut a notch in the bottom part the thickness and width of the spreader. Now you need to rip the spreader for the center part the as wide as the riser is high. I made the other spreader a wide as I could. The width of the bridge skeleton should be 4” shorter then the bridge planking. This would give you a 2” over hang on each side.
In putting the sections together, you attach the outside risers to the spreaders, and then install the center riser. I used 3 ½” decking screws for this.
To install the bridge I poured a 4” thick concrete pad on each side that was level and square with each other. I position the bridge halves together with clamps and screwed the two top spreaders together. I used large concrete anchors to attach the spreaders. I made a piece to cover the point where the two halves joined. You can be fancy or plain as you want with this. I attached mine with carriage bolts and screws from the back side. The last thing is to attached the bridge planking and enjoy.
-- "When we build, let us think we build forever." John Ruskin