Translation:Dictionary of French Architecture from the 11th to 16th Century/Volume 1/Discharging arch
It is the arch which is set in constructions above the lintels of the doors, above the openings in general, and the weak parts of lower constructions to defer the weight of higher constructions on points of support which stability is assured. The archivolts of the gates and doors are true relieving arches (see: ARCHIVOLTS, variety of the Arch); however one hardly gives the name of relieving arches that to the arcs whose facing levels plain walls, which are distinguished from horizontal bases only by their apparatus, and sometimes however by a weak projection. In high Roman constructions in small materials and blockings, one often meets relieving arches out of bricks and hardcores set in full wall, in order to defer gravities on points of the foundations and bases established more firmly than the remainder of the masonry. This tradition is still preserved for the Romance period. But at that time constructions in blocking were not any more of use, and one finds only très-rarement arcs intended to divide gravities in a blank wall. Moreover in the Romance buildings construction becomes almost always a reason for decoration, and when while building one needed relieving arches one sought to show them, that is to say by a projection, and even sometimes by a net decorated or profiled with the suction face. Such are the relieving arches which are seen along the wall on the low sides of the Church St-Étienne of Nevers (end of the 11th century) (74).
Here these arcs are especially intended to charge the piles on the low sides which receive the pushes of the vaults; the walls not being armed with buttresses, this addition of load gives to the principal points of support a great stability. It is a system which makes it possible to brick up thin walls between the piles intended to receive the weight of constructions, it presents consequently a saving in materials; one sees it applied in many churches of Poitou, of Anjou, of Auvergne and Saintonge for the Romance period. Useless to add that these relieving arches are always extradossés; since their essential function is to defer the higher loads on their diagrids, they must tend to make slip masonries on their kidneys.
The pinion of the southern transsept of the church of Our-injury-of-Port in Clermont-Ferrand is thus related to two relieving arches outside, resting on a column (75). Often in the civil architecture of the XI E and XII E centuries one meets doors of which the lintels are relieved by relieving arches coming to support their diagrids on ' ' a portée' ' spared at the two ends of the lintels (76), sometimes also above
lintels one sees a key posed in the base which surmounts them and which forms an installed plat band thus deferring the weight of the walls on the two jambs (77).
A vacuum is left then between the under-surface of the key and the lintel to avoid the load of this key in the event of movement in the construction industries. Relieving arches are posed above interior splayings of the doors and the windows in almost all the civil buildings of the Middle Ages.
These arcs are semicircular arch (78) (castle of Polignac, Haute-Loire, XI E century), seldom in tierce point, and generally convex only to take less height under the floors (see: WINDOW). For the ogival period, the manufacturers have to cross great empty spaces, they unceasingly seek to decrease at ground floor the points of support, in order to leave the most possible place with crowd, not to obstruct the sight; this principle leads them to establish a part of higher constructions cantilever; if in through naves they establish propping up above on the low sides, to defer the push of the large vaults outside, it is necessary, in the direction length, that they avoid making weigh the walls of the galleries cantilever on the vaults on these low sides, too light to carry the load of a wall so thin that it is. Consequently, to avoid the annoying effect of this weight on vaults, relieving arches were spared in the thickness of the head walls of the galleries on the first floor. These archs defer the load of these walls on the diagrids of the transverse ribs on the low sides (see: CONSTRUCTION, TRIFORIUM, GALLERY). One finds relieving arches in tierce point, in the high galleries of Our-injury of Paris, in the triforium of the naves of the cathedrals of Amiens (79), of Rheims, of Nevers. But to Amiens, the higher windows being posed on the interior open-type screen of the triforium, these archhes relieving carry only the weight of a thin wall, which rises only to the support of fenestration.
In the buildings of Burgundy, and a part of Champagne, the windows, instead of being posed on the interior blind arcade, are in retirement on the walls external of the triforium. In this case, the relieving arch is all the more necessary as this external wall carries with fenestration the rocker of the cornices of crowning, it is sometimes posed immediately above the suction face of the archivolts, in order to avoid even the load of the filling, which as in Rheims, Paris and Amiens, furnishes the lower part with the arch in tierce point, or even, the relieving arch is only one convex arch, set in the thickness of the wall, a little above ground-level of the gallery, as one can notice it in the church of Saint-Father-under-Vézelay (80).
One meets relieving arches, at the base of the central towers of the churches resting on the four transverse ribs of the transsepts, as with the cathedral of Laon. Under the belfries of the bell-towers, as with Our-injury of Paris. There are also some above the vaults, to defer the weight of the sideboards and the frames on the piles, and to relieve the mullions of the windows holding place of formerets, as in the Ste Chapelle of Paris, as in Amiens, with the cathedral of Troyes (81). To the XV E century, the relieving arches were extremely of use to carry massive constructions, resting seemingly on up to date constructions; to relieve the clotheshangers of the large pinks of the weight of the pinions of face.
It is not necessary to say, that the arches play a great part in the construction of the buildings of the Middle Ages, the architects had arrived, as of the XIII E century, to acquire a perfect knowledge of their force of resistance, and their effects on the piles and the walls, they put a particular care in the choice of the materials which were to compose them, in their apparatus, and the way of their joints. Roman architecture did nothing but open the way in the application of the archs to art to build; the architecture of the Middle Ages also traversed far it was possible to do it, at the point to misuse even this principle at the end of the XV E century, by a too absolute employment perhaps, and the refinements pushed with excess. The essential quality of the arch, it is elasticity. The wider it is, the broader the space which it must cross is, and the more necessary it is that it is flexible. The manufacturers of the Middle Ages followed this principle perfectly by multiplying the joints in their archs, by composing them of equal archstones, always extradossés carefully. It is only to the XVI E century, whereas art to build, itself, subjected the use of materials to forms which were appropriate neither for their qualities, nor with their dimensions, that the arch was not applied any more because of its true function. The logical principle which had made it admit, ceased directing the manufacturers. While imitating or believing to imitate the forms of Roman antiquity, the architects of the rebirth deviated more of the principle of ancient construction than the architects of the XII E and XIII E centuries; or rather, they did not hold no account of it. If in their massive constructions, inébranlables, the Romans had included/understood the need for leaving with the archs a certain elasticity by extradossant them, and by forming them of concentric rows of archstones, when they needed to give them a great resistance, with stronger reason in the masonries of the Middle Ages, where all is balance, and movement consequently, was one not to lose sight of the fact the principle which must direct the architects in the construction of the arcs. Day when one ceased extradosser the archs, where one of unequal archstones like dimension, and weight consequently, installing them with ' ' crossettes' ', and connecting them to horizontal bases, by means of right joints with the tail, one wanted to compose them did not include/understand more the true function of the arch (see: CONSTRUCTION, VAULT).