Bridge Arch Support Cut From One Board

I made this project 18 years ago. I wanted to make a walk bridge for my rock garden. I had a couple of 2" x 10" x 10ft boards and was prepared to make a flat bridge when I had an idea. I thought maybe I could cut an arch out of the bottom of the board and mount in back on top. Using this method, one straight board becomes an arched board.

1ea 2" x 10" x 10 ft piece of treated wood

approximately 5 ft of 3/8 inch all thread rod

10ea of lock nuts and washers for the all thread

3/8 inch extended spade bit at least 12 to 14 inches long

1" spade bit

exterior wood glue

hack saw

wood clamps

small circular saw or jig saw

drill

sockets and wrenches to fit nuts

2 saw horses (or anything to raise the board up to make it easier to work on)

I came up with an 18 year old note that I had kept all these years. I cleaned up the note page and added some more meaningful dimensions.

I started with a nice flat and straight 2" x 10" x 10ft pressure treated board that was suitable for earth contact.

I scaled up my drawing to actual size and used that as a template.

The saw I used for cutting the board was a 5 1/2" cordless circular saw, but I could have used a hand jig saw or even an old fashion hand operated carpenter's saw.

Starting at one end of the board, I started to cut very slowly so that I wouldn't overload the saw.

Having cut the bottom out of the board, it was time to align the pieces so that they could be fastened together.

Using some 3/8 inch all thread stock, I cut five pieces each 8 3/4 inches long. I dressed up the ends after cutting the rod and prefitted the nuts to make sure they would screw on nicely. I gathered up the wrenches I would need to tighten the nuts with.

Having made the 5 through bolts, I also fashioned a simple wood clamp out of some left over all thread and 2 x 4 scraps. This jig kept the boards perpendicular. Forgive me for going off topic, but I see our young black lab in the background. She has since passed, but she was always around keeping me company. I miss her!

I used one other clamp to compress the pieces together.

In the drawing of step 1, you can see where I marked the board for drilling. Before I drilled the through holes, I used a 1" spade bit to drill a well for the locknut to set in. I drilled 10 holes in all, one for each of the top and bottom nuts.

I purchased a new extra long 3/8 inch spade bit for drilling the through holes. I believe it was 14" long. This may have been an overkill as the total depth of the pieces after cutting was about 9 inches. Using the center hole made by the 1 inch spade bit and the line drawn up the board to the top hole as a guide, I drilled the the 3/8 inch through hole in all five positions.

As I am contemplating the next couple of steps, I really wish that I had taken more pictures of all the steps. I added a drawing that hopefully details what I describe below. The holes were all drilled and the clamps were still on. Remember that the all thread pieces were cut to 8 3/4 inch lengths, thats approximately 1/4 inch shorter than the overall width of the board. I took the 5 pieces of all thread and and put 1 washer and bolt on just one end of each of the 5 pieces. I only screwed the nut on just enough so that I could tap on the nut to drive the threaded rod down into the hole. This kept me from damaging the threads on the end of the bolts. Later, I adjusted the nuts to keep the all thread centered in the piece.

Now that all bolts were driven in, I loosened all clamps and separated the pieces just far enough to run plenty on exterior wood glue between the pieces. The 3/8 inch threaded rods and the 3/8 inch hole drilled for the rods were a tight fit. To separate the pieces I cut some wedges and drove them down between the boards. After applying the glue, I removed the wedges, clamped the boards backed together and secured them with the remaining washers and nuts.

This is a picture of the completed board. It was now ready to be used as template for other supports.

The bridge is still in good shape today 18 years later, and we have a new yellow lab project manager as well!