Stone Bridge: How to Build a Roman Arch Bridge
Introduction: Stone Bridge: How to Build a Roman Arch Bridge
Encountering a rough-cut stone bridge is always an experience of wonder to me. Every stone bridge, from an unnamed walking bridge to the 902' wonder of Pont du Gard is a testimony to the diligence and ingenuity of builders plying one of the world's most ancient trades: Masonry.
This instructable will provide a step-by-step tutorial in how to build a Stone Bridge using hand-cut stone for under $1000.
WARNING: Building a stone bridge is not an easy finish-it-over-the-weekend sort of deal. You are building a legit piece of architecture that could last for centuries. If you want to do this, it is simple enough, but not easy; you're gonna have to work for it. Some buddies, <PERSON>, <PERSON>, <PERSON>, and I built this bridge completely from hand cut stone harvested on-site. We put at least 300 hours into it, and we couldn't be more pleased with the results.
We didn't think of documenting the process until we were 2/3 of the way done, but I have drawn up some figures to help with each step.
Supplies
Much of this stuff is common household tools/supplies, but if you don't already have it, all can be purchased at your local hardware store (possible exception would be plug and feather which can be purchased on amazon.com. Assuming you have the household tools already, you're looking at $400-$800 in supplies
1) Tons of stone (Like seriously, TONS. The bridge I built spans roughly 10 ft and I probably used 5 tons of stone, harvested from the stream and the surrounding hillside. Preferably, use whatever stone you have on the property! It will look great and give you immense satisfaction to see your project built out of stone native to your land. Shot rock, [the largest size available] purchased at your local quarry will take more time to cut, but will also work great. A third option is to buy pre-cut landscaping stone from a landscaping supply company. This will be the most expensive, but you will save a ton of time cutting stone.)
2) Household Tools: A hammer (preferably 2 or 3 pound in weight with a beveled end), a shovel, a drill, a circular saw, screws.)
3) Masonry chisels: (at least one 3" wide and one 1.5" wide) ($10/ea)
4) A rotary hammer drill with 9/16 bit (for drilling holes used to split stone; I got mine at Harbor Freight for $80)
7) 3/4" Plywood, 1/4" plywood, 2x6 boards ($100-$150)
8) Plug and feathers (also called wedge and feather: by far the most specialty item, but available online. 431
Get at least 7 of these bad boys. They are your bread and butter.) ($10 for a pack )
9) Mortar ($5/ bag. and you'll need a bunch, depending on your project size.)
10) Basic concrete tools: a trowel $5, a shovel$12, and if you want, a grouting bag $5
11) Concrete for the bridge foundation ($100-$150 a ton if delivered by a truck)
Step 1: Understand the Science
Stone bridges all have arches supporting them. Stone has poor tensile and shear force resistance, but excellent compressive resistance: Like 28,000 pounds/sq. inch compression resistance. So we need to use that in our bridge. The way we do that is by creating an arch of squared off rocks that all run parallel to the radius of your arch.
To understand this, compare fig. 1 and fig 2:
Figure 1 shows a bridge that is basically just a block wall that someone cut a hole out of. This bridge will collapse because it has only the shear resistance of the blocks keeping it up over the arch.
Figure 2 shows a bridge that is supported by an arch of stones, each of which is parallel to the radius of the imaginary circle that is an extension of your arc. This is crucial because as load is applied to the top of the bridge, these stones are compressed into each other horizontally, locking them together and supporting the load of the bridge. Because of this compression with the arch of stone, these bridges age extremely well. As they settle they just become more and more locked together in compression, thus providing greater strength to the bridge. One arch bridge is thought to be about 3,300 years old!
Fig 3 shows also how the compression load of the bridge will try to "flatten out" the arc of the bridge by pressing the stones apart. 441