Compression is the all-important feature of a stone arch bridge that allows it to stand.
Compression and Tension
Compression pushes together a material, while tension is a force that pulls it apart. Some materials are quite strong in tension, but masonry is not one of them. For instance, take a wall made of bricks stacked without mortar. Assuming the wall is straight enough and sitting on a level enough surface as not to be wobbly, you can sit your full weight on it, easy. It is strong under compression. Now try grabbing the top bricks and pulling. The top bricks comes off, little effort required. Why? The wall has essentially no tensile strength whatsoever. If you use mortar, you can add some tensile strength, as the mortar will help “glue” the bricks together, yet even so the wall is much, much stronger under compression than tension. Even on an individual basis, you find that masonry units, whether they are unreinforced concrete blocks, bricks, or stone slabs, are far better able to handle compression than tension.
The Arch and Compression
The arch is designed as a system in which all the individual elements are held in compression. The secret is that the curved shape of the arch in a stone arch bridge tends to shunt the weight of the structure and any loads above along the line of the curve until it reaches the abutments, the abutments being the end point in which the arch locks and which takes up any and all forces, whether vertical or horizontal, and transfers it down to solid ground. Why does the arch work? Consider how all the stones are held in the arch bridge shown below.
If you place a heavy weight at the top center of the arch, the top stones are forced down, or, rather, would be, except they are locked tightly against their neighbors. Each stone pushes against its neighbor, but is locked so tightly in place that it can’t move. In this fashion, the force is carried along the gentle curve of the arch down to the ground. In fact, by adding weight to the bridge, you simply end up pressing the stones more tightly together.
It is still possible to place too much weight on an arch, enough to force the stones outwards out of the line of the arch, but the amount of weight to do this is huge, and the collapse actually occurs because of tension. After all, if sections of the arch are forced outwards, away from their original location, what is this if not a pulling apart or tension? This happens when the force on the bridge is high enough that it can no longer be contained within the line of the arch. For more details, see Improving a Stone Arch Bridge’s Serviceability by Strengthening: Part 1.
Friction and Weight
As far as collapse mechanisms of arches are concerned, one of the reasons an arch can handle so much weight is because the arch itself weighs so much. If you ever tried building an arch out of wooden blocks or other light objects, you will find it is rather easy to collapse. But if you build it out of heavy stone, you find it is much more immovable. Why? The increased weight of the stone arch presses it together (compression) more than would be true for an equivalent arch made of, say, balsa wood.
And there is one other major point: friction. Friction and compression bind the arch together into a stable system. Stones (being inherently rough) can present enormous amounts of friction. Add some weight to this, and the amount of friction increases still more, making it hard for any one stone to be dislodged. If, on the other hand, you have ever built an arch out of highly polished material, you will find it tends to be less stable than rough material.
An Arch in Inverse
One final note worth mentioning at this point is that it is possible to build a spanning system using tension. Suspension bridges act like arch bridges in inverse. Whereas the arch in an arch bridge operates in compression, the cables in a suspension bridge operate in tension. Note, however, the remarkable similarities in the curve. The basic, natural, curved lines of force are the same for the arch and suspension bridge; just the forces are compressive in an arch bridge but tensile in a suspension bridge. This natural curved line, by the way, is a catenary.