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Arched Bridge

Posted by joeabbott on April 9, 2011

notebookscanIt’s not often that I can knock a project out in a day. Maybe that comments on the size of the projects I tackle, perhaps my limited skills, or maybe a combination of the two. And yet, just this past weekend, I built a small, arched bridge for our back yard. It came out nicely, too.

The most surprising thing (to me) is how little I planned for this project.

To the right you’ll see the majority of my plans: a single page in a small, pocket-sized notebook.

I had one other abortive effort at planning, a SketchUp model that just didn’t come together, but otherwise just read a bit on the web, doodled, and started cutting!

When I decided to build the bridge, I did a search for “build arch bridge”. Bing gave me a couple promising hits but the third item in the list is what I was looking for: How to Build an Arched Footbridge – Part 1, by Rod Bird.

In his excellent walk-through, he gave me two bits of information I wasn’t able to work out without his help:

  • what sort of internal support structure do you use?
  • how wide should the arch be?
Internal Structure

Rod recommended using … none! Now, this was a bit of a surprise but considering the overall structure, makes sense. And yet I didn’t follow his recommendation, but mainly for considerations other than structural.

I ended up putting in a few stiffeners between the two arches (left and right) purely because I was building the basic “skeleton” in my garage, and then walking it a hundred yards out and up the back hill. I needed it to support and hold shape while I moved it, dug out the placement, and then put it into position.

How wide to make the arch

Rod’s article helped me with two details: use a 12” wide board and make the foot 18” long.

There wasn’t too much more in the article that I wasn’t able to work out for myself. And, when you think about how simple the job is, there isn’t too much more to divulge!

SketchUp

Now, I love SketchUp. It’s a great tool for helping me figure out a bunch of puzzles and how to approach building things. I can’t speak highly enough of it. But, I don’t understand it well enough to do everything I need it to.

I started building the arched sides and this gave me a good dimension: 7 9/64 … or, roughly, 7 1/8”. This is the width of the arch. I got this dimension from drawing an arc from the lower corners and peaking at the top, center. I then knew that, from the inside edge of the 18” foot to the point on the arc that was perpendicular. See the layout drawing below and it should be clear.

When I tried to lay the treads onto the arches, I wasn’t able to find a way to get them to sit on the curve. It just wasn’t working out. And, without that detail, building a model was useless (in my mind) so I shut down SketchUp and I pulled out my pad and pen.

Pad and Pen

The first thing I drew was the 8’x12” board (scaled down, of course!), and then sketched out the arch on it. I’d need two arches, so the first item on the cut list was two-2x12x8’ boards.

I then needed to know how many boards I needed to build the treads. I planned on using 6” boards (rather than 2x4s) so I figured out how many six-inch sections it would take to cover the 8’ span. Two details made this number a rough guess: first, I was going to put some sort of spacing between the boards; second, an arch over the 8’ span is longer than 8’. So one details was saying I’d need fewer boards (the spacing issue) and the other said I’d need more boards (the arch is longer than the straight line issue). I called it a wash and did my math.

It would take 16 six-inch treads to completely cover the 8’ span. I planned on making the treads 2.5’ (30”) long, so I knew I’d get 3 treads in every 8’ (96”) board. So I added six-2x6x8’ boards to the cut list.

Finally, I was playing around with adding handrails at one time. We were leaning away from adding them but I wanted to make sure I had the lumber … so the final item in the cut list was four-4x4x8’ posts.

The final math on the scratch pad was a bit of number crunching to determine how much edging we’d need. We had measured a path length of 51’; when you subtract the length of the bridge (8’) and divide by the length of the edging section (3’), you come up with roughly 14 edging sections to make the border on one side of the trail. Factor in the second side of the path and we needed 28-3’ border sections.

Building it

This was a simple project. I think I want to go into business building them … they’re that easy.

Arches

image

 

I started by laying one 2×12 on a couple of saw horses and marking a point 18” from the end on each side; this was the bottom. I then found the center and marked a point 7 1/8” down from the top. I looked around my gargage and found a long piece of plastic PVC electrical conduit. Using a clever (if I say so myself) arrangement of clamps and the PVC, I was able to create a fair curve that I traced with a Sharpie. The bottom of the arch was just created.

I then used the PVC conduit to create a line from the corners, that peaked at the center of the board at the very top; the top edge was now defined.

At this point I used a jigsaw to cut out the bottom arc. I found that the blade bent quite a bit while cutting so I changed to a much coarser blade for subsequent cuts. On cutting the top edge, I found it to perform much better. To clean up the lower edge, I got out a router and a straight bit and just eyeballed a clean edge. After this a bit of 60 grit sandpaper brought it up to “good enough for landscaping work”!

Using the first arch, I laid it on the second 2×12 and traced the outline to create the second pattern to cut. On the second one, I stayed off the line just a bit and, after I’d removed the majority of material, used the router and sandpaper to get the two arches nearly identical. Again, just getting it close enough for “landscaping work”.

Treads

P1020398At this point I cut the 16 treads and found an extra board laying around that I turned into a spare tread “just in case”.

After that I pre-drilled holes in the treads to both avoid cracking a board when installing them onto the arches and to ensure all the treads looked identical (from a fastener standpoint). It just looks better when you do this.

I’d found a box of old deck screws on a shelf and planned on using them. They were 3.5” long, so very much overkill for attaching treads, but using these saved me a trip to Home Depot.

And that was the treads.

Putting it together

P1020397I planned on using spacers between the arches to ensure they were lined up and square for the installation of the treads. Once I got them in place and everything clamped up, I liked what I saw, so I drilled my pilots and stuck a couple screws in. Not only would this keep the arches in position, they’d help it hold shape when I walked it into the backyard.

At this point, I did something risky: I started putting treads on starting at both ends. This almost assured me of having to trim down a board at the center to make everything fit, and put me at risk of having the spacing look “crooked” when the two ends met in the center.

Still, I trusted to math and measuring, and felt this did the best job of creating a sturdy but light “base model” to move around while I installed it.

P1020404Two treads on each end seemed good enough.

P1020407The final detail I added was a 4×4 “skid” under each 18” foot.

I was installing this in poor soil on a steeply sloping hill, so I wanted to give the feet a sturdy base to rest on. I cut four-18” 4×4 feet and screwed them to the bottom of the arch “feet”.

Ready for installation!

Installation

This is the boring, tedious, and messy part.

Drag it out back, lay it in place, move it aside, chop/shovel/move dirt, and repeat.

Adding the skids was brilliant, and yet it cost me a lot of labor as I now had to dig down an additional 3.5-4” on all sides.

But, soon enough that was done. I then drilled 3/8” holes in the skids to allow me to sink 2’ rebar rods through the feet and into the hillside as a final bit of insurance against it moving.

Then, time to install the treads.

Oops

At this point, I realized I have yet a lot to learn.

When I was calculating the tread sizes, I used 6” … and yet dimensional lumber is a 1/2” undersized in thickness and width! My 2×6 boards were really 1.5”x5.5”. Which, over the 8’-16 tread run, put me short by over half a foot (8”, to be exact)!

So when I got to the last board on the top of the arch, I not only needed the “spare” tread that I’d cut, but I had to go back into the shop, dig up another tread, and then trim the two spares down to fit the final gap and allow for all the right spacing between all the treads.

I was pleased that this snafu took only an extra few minutes to work out and didn’t cause much problem at all. Perhaps I was still chuffed that my spacing worked out perfectly even though I had started installing treads from both ends! Regardless, with these treads going on, I was done!

And, for the record, I used 3/16” … because I happened to have two scraps of plywood in a pile that were the right size and happened to be that thick.

Coda

I highly recommend this project to anyone of just about any skill. It came together so quickly and looked good enough that I wanted to build another!

If you have questions or want to try this yourself, let me know and I’ll be happy to offer what assistance I can!

Thanks for reading.

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