![]() ![]() Of course, the mortar is not being relied on to “glue” the arch together. By filling in all nooks and crannies, mortar ensures maximum friction between stones. Maximizing surface contact between stone is the primary purpose of mortar, in fact. This is achieved by precision cutting, filling in nooks and crannies thoroughly with tightly-fitting stone pieces and the use of mortar. Maximizing Frictionįriction between stones is maximized by surface contact between them. As can be seen, some of the stones do not even have a noteworthy taper, yet the arch remains stable.Ĭlearly, 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. Thus, it is possible to build a complete, stable arch in which the individual arch stones are essentially flat slabs with an insignificant slope! This small stone arch bridge uses a variety of differently sized and shaped stones. 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. For one thing, the thinner the arch stones, the less of an angle each one will have. 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. The thrust presses the stones together, causing stability through friction. A catenary is the shape taken by a downward-hanging chain that is supported at both ends. ![]() For a basic freestanding arch, this this line of thrust tends to be in the form of a catenary. The line of thrust is where the main force and pressure of the weight of the bridge is taken through the arch. The Line of ThrustĪrch stability appears to be directly related to the line of thrust in conjunction with the friction between the stones. 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. Yet, the arch stands, and has stood for centuries. It looks like, by all rights, this keystone should squeeze out the bottom, causing arch failure. The third keystone, however, is placed upside down with the tapered end up, and the thick end down. ![]() Two of these keystones are placed with the tapered end down. In Ireland, there is an old stone bridge known as Monks Bridge that features one arch that, bizarrely, has three distinct, triangular keystones. Wedge-shaped stones help keep the arch stable, yet, surprisingly, wedge-shaped stones are not critical. The friction between the stones of the arch is an important part of what keeps the arch stable. The importance of friction within a stone arch bridge is not to be underestimated. ![]()
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