The corbeled so-called “arch” is a relatively uncommon design seen in stone bridges. While the corbeled design is definitely a step beyond the simple stone slab bridge, the corbeled arch is not technically a true arch. A true arch has a thrust caused by the interaction of the stones with gravity. A corbeled arch has no thrust, and relies on the cantilever principle. The corbeled arch consists of two halves, stepped out towards each other. It will thus be seen that a corbeled arch consists of two halves with but little interaction between them, while in a true arch all the stones interact creating a single strong whole.
How the Corbeled Arch Works
The corbeled pseudo-arch relies heavily on the proper balancing of stones to bridge a gap. An individual stone cannot, of course, be corbeled out more than about half its length, otherwise it becomes imbalanced and falls over. By corbeling out several stones one above another a longer span can be achieved, but eventually gravity takes over and more or less of an entire half of the corbeled “arch” will tend to fall over as a unit. Thus, it will be seen the corbeled arch is merely an exercise in equilibrium, the builder balancing the weights of the stones to ensure stability. In order to achieve greater stability it is important to ensure there is enough weight placed on top of the stones at their ends to keep them from being tipped under loads; a heavy layer of fill applied to the top of the structure as a dead load helps considerably as well.
Obviously, the more weight added to the backs of the stones of the structure, the greater loads it can carry, more concentrated weight from loads being required to unbalance the equilibrium of the “arch.” Extra fill over the corbeled arch also helps distribute the applied loads over a greater area of the structure which in turn further enhances stability.
Why Use a Corbeled Arch?
The main advantages of a corbeled arch are that it is simple to build, can span a larger gap than a single stone slab can, and also can be made stronger than a stone beam, because the weight of the load is not supported by merely one stone, but is also at least partially carried by the several stones projecting out below.
An advantage of the corbel arch as opposed to the true arch is that it has no thrust, so substantial abutments are not required. One thing to be aware of, however, is that the corbeled arch still cannot span very large gaps, and care must be taken in design to ensure stability under loads.
Stone Arch Bridges 