The importance of friction within a stone arch bridge is not to be underestimated. The friction between the stones of the arch is an important part of what keeps the arch stable. Wedge-shaped stones help keep the arch stable, yet, surprisingly, wedge-shaped stones are not critical. In Ireland, there is an old stone bridge known as Monks Bridge that features one arch that, bizarrely, has three distinct, triangular keystones. Two of these keystones are placed with the tapered end down. The third keystone, however, is placed upside down with the tapered end up, and the thick end down. It looks like, by all rights, this keystone should squeeze out the bottom, causing arch failure. Yet, the arch stands, and has stood for centuries. Clearly, there is more to arch stability than properly placed wedge-shaped stones, though wedge-shaped stones do help prevent vibration from working things loose.
The Line of Thrust
Arch stability appears to be directly related to the line of thrust in conjunction with the friction between the stones. The line of thrust is where the main force and pressure of the weight of the bridge is taken through the arch. For a basic freestanding arch, this this line of thrust tends to be in the form of a catenary. A catenary is the shape taken by a downward-hanging chain that is supported at both ends. The thrust presses the stones together, causing stability through friction.
When Wedge-Shaped Stones Are Not
One curious fact about arch stability is the fact that even with properly shaped and properly placed stones, it is possible for the stones not to really be distinct wedges at all. For one thing, the thinner the arch stones, the less of an angle each one will have. Also, for a given number of arch stones, the flatter the arc and the longer the span the less of an angle each arch stone will have. Thus, it is possible to build a complete, stable arch in which the individual arch stones are essentially flat slabs with an insignificant slope!
Clearly, in these scenarios, it isn’t the wedge shape holding the stones in place; it is the pressure of the arch, and the resultant friction.
Friction between stones is maximized by surface contact between them. This is achieved by precision cutting, filling in nooks and crannies thoroughly with tightly-fitting stone pieces and the use of mortar. Maximizing surface contact between stone is the primary purpose of mortar, in fact. By filling in all nooks and crannies, mortar ensures maximum friction between stones. Of course, the mortar is not being relied on to “glue” the arch together. Furthermore, as mortar does deteriorate, it is advisable to treat a mortared arch like a mortarless arch during construction. This is done by ensuring maximum contact between stones and extensive filling in of gaps with stone fragments. This takes more time, but can and does materially reduce the amount of mortar needed and strengthens the arch as well.