Stone arches have long been used for spanning gaps throughout the world. Their permanence has been testified to and evidenced by the surprising number of Roman and Medieval stone arch bridges still in use. But how do these bridges work? How do you span a gap with stone?
The Slab Method
The simple and most obvious way to bridge a gap with stone is the “slab” method. Simply lay a large stone across the gap. Or lay several large stones parallel to each other to make the bridge wider. Either method will work, though technically, the latter construction is called a “lintel” bridge.
The slab method works for short spans, but there are several problems with this design. The most obvious problem is finding stones massive enough to bridge any kind of sizable gap. For a two foot gap this will be no problem, but how about for, say, a twenty foot gap? Multiple slabs on multiple piers may be an obvious solution, but this would largely congest the waterway.
Furthermore, a stone slab bridge is actually a very weak design. Stone is very strong in compression (it handles being “squashed” very well) but is incredibly weak in tension (one half of the slab of stone is being “squashed” while the other half is being “stretched”). Since a slab bridge operates in tension, the stone must be very thick and the span very small for the bridge to be sturdy under heavy loads.
Another way to span a gap with stones is corbeling. In this method, stones are stacked like they would be in a wall, but with each “course” being stepped out slightly more than the previous one. With two corbeled walls on each bank to create a bridge, the stones are stepped out until the two walls meet in the middle. This keeps most of the force in compression, and a large gap can be spanned neatly. The major limitation is that the height of the bridge will need to be rather high or the structure will become unstable. Deciding how much each stone can be stepped out over the gap is a science. While this may be suitable for narrow ravines, most streams tend to be wide and low, as opposed to narrow and deep.
So how do you keep the forces in compression while utilizing the strengths of stone without relying on its weakest points to support the bridge? That is where the arch comes in.
The arch relies on wedge-shaped stones being stacked one on the other in such a fashion as to ensure that an individual stone cannot slide down without another stone sliding up. Gravity pushes all the stones down and holds the structure together. Since each stone is wedged between its wedge-shaped neighbors, they all push on each other (compression!) and the whole structure uses gravity and friction to bridge a gap. Since the stones weigh so much, the friction between the stones is immense, and they cannot slide out. Not only that, but the more weight on the arch, the stronger it becomes because the stones press more tightly together. Thus, the strength of the stones in compression can be used to create an enduring and rugged structure.
No wonder the stone arch was so popular!