Cathedral walls serve a number of purposes. They must be pillared at their base to allow for an open gathering space for worship. Their upper tiers must be windowed to allow air and light into the interior, and they must hold up a roof over a large, open area. Once these requirements are understood, the design principles of Cathedral walls, including their height, layout, roofing techiques, and use of columns and 'arches' can be better understood. Even the purpose of the peculiar 'flying buttresses', which surround most Gothic Cathedrals can be appreciated once the difficulties of holding a large, heavy roof in place are understood.
Many of the building techniques used in Cathedral architecture were first developed and used on a large scale by the Romans during the imperial age. The most important of these were the 'Arch' and 'Vaulted' Ceilings. The arch was known as an important architectural technique since ancient times, but the Romans were the first to sytematically use the arch to build massive structures on a large scale. Roman aqueducts, bridges, public buildings, coloseums, baths, monuments, and temples all used arches as a means of supporting elevated structures.
Arches were invented in order to span an area, or support weight between two pillars. Vaults are cross-supported arches that are used in forming a roof, and domes a specialized form of vaulting that produces a continuous semi-circular roofline. Although they are used for widely varying purposes the architectural principles behind all are the same. The purpose of all these structures it direct the weight of a roof or of a load-bearing span towards the pillars supporing the vault or arch. In other words, arches are used to transfer the weight applied above it to its supporting structure. The basic principle is simple, and it can be used in many ways to create a variety of structures.
Arches were used on a regular basis by ancient architects for a number of purposes, but most often to 1) To create open spaces for doors or windows, 2) to support heavy structures on pillars or piers. The basic components of an arch are:
The diagram above shows a simple arch, but there are many variations, such as a pointed arch, a horseshow arch, or an ogee, that work on the same principles. Most pointed arches are symetric and have a joint, rather than a keystone at their apex.
The fact that the force on a springer stone pushes it outward, away from the arch means that architects need to figure out a way to counter that force. There are several ways to do this. One is by building arches that counter-balance each other. Another is by buttressing the piers or columns that support load-bearing arches or vaulted ceilings. Clever techniques of using rows of interdependent arches and buttresses are a critically important part of Cathedral architecture.
Arcade—An arcade is succession of arches, each counter-supporting the columns or piers of each other. The outward force imposed on the pier supporting one arch is perfectly offset by the couter-force of the opposing arch, so arcades are extremely stable structures. They are typically found in churches beween the Nave area and the bay aisles. Arcades also form enclosed walkways in cloisters, and interior passageways in churches.
In addition to their use in Cathedral architecture, a great many structures use some form of arcade. Aqueducts, and bridges, for example, are weight bearing arcades.
Buttress—A Buttress is structure that pushes back against a pier or pillar that is supporting an arch. The design of a buttress depends on how much force is being exerted on a pier and on what direction the pressure is coming from. A wall or pier that is supporting a heavy ceiling, must either rely on a buttress to support it, or it must be extremely thick. Cathedrals use buttresses to support their columns so that the walls of the cathedral can be be thin enough for light to enter the building through windows and arches.
Flying Buttress—A flying Buttress is a massive structure built that is built away from the pier or pillar that it is intended to support. An arch or arches are then built between the buttress mass and the pier. Flying buttresses allow for tall piers, large windows areas, and a lighter interior, all characteristic of the Gothic cathedrals of the 12th century.
Once one understands the importance of arches, arcades, and buttresses, the plan for wall elevations of Cathedrals makes more sense. A row of tall columns or piers defines the main interior of the cathedral, and between these piers are three levels of arches. The ground level arches, separate the Nave or Choir from the covered aisles or ambulatories to the north and south. The upper level arches form the clerestory, or main window area of the chruch, and the middle row of arches, the triforium, is required for structural support.
