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A flying buttress is a specific form of buttressing most strongly associated with Gothic church architecture. The purpose of any buttress is to resist the lateral forces pushing a wall outwards (which may arise from stone vaulted ceilings or from wind-loading on roofs) by redirecting them to the ground. The defining characteristic of a flying buttress is that the buttress is not in contact with the wall like a traditional buttress; lateral forces are transmitted across an intervening space between the wall and the buttress.
Flying buttress systems have two key components - a massive vertical masonry block (the buttress) on the outside of the building and a segmental or quadrant arch bridging the gap between that buttress and the wall (the "flyer").
Although fully-fledged flying buttresses only developed in the Gothic period, they are described in the Bible in the Book of Ezekiel, chapter 41, verses 6 and 7, as part of the detailed design of a new temple, and architectural precursors can be found in Byzantine architecture and in some Romanesque buildings, such as Durham Cathedral, where quadrant arches were used to carry the lateral thrust of the stone vault over the aisles. However these arches were hidden under the gallery roof and only transmitted the forces to the massive outer walls. By the 1160s, architects in the Île-de-France were employing similar systems but with longer and finer arches running from the outer surface of the clerestory wall, over the roof of the side aisles (and hence visible from the outside) to meet a heavy vertical buttress rising above the level of the outer wall. The main advantage of such systems is that the outer walls no longer need to be heavy and massive enough to resist the lateral thrusts of the vault. Instead the wall surface could be reduced (allowing larger windows filled with stained glass), with the vertical mass concentrated into external buttresses. Early flying buttresses tended to be far heavier than is required for the static loads involved, as for example at Chartres (c. 1210) and around the apse of the Basilica of St Remi in Reims, which is thought to be among the earliest examples still surviving in its original form (dating from around 1170). Later architects progressively refined these designs and slimmed down the flyers until typically they were constructed from no more than one thickness of voussoir with a capping stone above it (see for example the cathedrals of Amiens, Le Mans and Beauvais.
Later Gothic buildings continued to use flying buttresses but often embellished them with crockets on the flyers and figural sculpture in niches or aedicules set into the buttresses. Renaissance and later architecture eschewed the flying buttress in favour of thick-wall construction. However the design was revived by Canadian architect William P. Anderson to build lighthouses at the beginning of the 20th century.
To build the flying buttress, it was first necessary to construct temporary wooden frames which are called centering. The centering would support the weight of the stones and help maintain the shape of the arch until the mortar was cured. The centering was first built on the ground by the carpenters. Once that was done, they would be hoisted into place and fastened to the piers at the end of one buttress and at the other. These acted as temporary flying buttresses until the actual stone arch was complete.
Because the majority of the load is transmitted from the ceiling through the upper part of the walls, making the buttress as a semi-arch extending far from the wall provides almost the same load-bearing capacity as a traditional buttress engaged with the wall from top to bottom, yet in a much lighter and cheaper structure. And because the flying buttress relieves the load-bearing walls with a much smaller area of contact, much larger voids are able to be built into those walls, such as for windows, than would otherwise be possible.
Often on Gothic churches, two arched 'flyers' were used one above the other. In such cases the lower flyer (positioned a little below the springing point of the vault) is designed to take the lateral force of the vault while the upper one resists the effect of wind-loading on the roof.
The vertical buttresses at the outer end of the flyers were often capped with pinnacles that provide additional vertical loading to help resist the lateral thrust transmitted by the flyer.
Another application of the flying buttress is to prop up a leaning wall in danger of collapse. An example is found at Chaddesley Corbett, where a flying buttress has been added as a more practical option instead of dismantling the leaning wall and rebuilding it. The aisle was built in the 14th century, and dismantling it would be a major work.
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