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Gothic rib-vault ceiling of theSaint-Sverinchurch in Paris
Vault(Frenchvoûte, fromItalianvolta) is anarchitecturalterm for anarchedform used to provide a space with a ceiling or roof.The parts of a vault exert lateralthrustthat requires a counterresistance. When vaults are built underground, the ground gives all the resistance required. However, when the vault is built above ground, various replacements are employed to supply the needed resistance. An example is the thicker walls used in the case ofbarrel or continuous vaultsButtressesare used to supply resistance when intersecting vaults are employed.
The simplest kind of vault is thebarrel vault(also called a wagon or tunnel vault), which is generallysemicircularin shape. The barrel vault is a continuous arch, the length being greater than its diameter. As in building an arch, a temporary support is needed while rings ofvoussoirsare constructed and the rings placed in position. Until the topmost voussoir, thekeystone, is positioned, the vault is not self-supporting. Where timber is easily obtained, this temporary support is provided bycenteringconsisting of a framedtrusswith a semicircular orsegmentalhead, which supports the voussoirs until the ring of the whole arch is completed. With a barrel vault, the centering can then be shifted on to support the next rings.
Amongst the earliest known examples of any form of vaulting is to be found in theneolithicvillage ofKhirokitiaonCyprus. Dating from ca. 6000BCE, the circular buildings supported beehive shapedcorbeldomed vaults of unfired mud-bricks and also represent the first evidence for settlements with an upper floor. SimilarBeehive tombs, calledtholoi, exist inCreteand NorthernIraq. Their construction differs from that at Khirokitia in that most appear partially buried and make provision for adromosentry.
The inclusion of domes, however, represents a wider sense of the word vault. The distinction between the two is that a vault is essentially anarchwhich is extruded into thethird dimension, whereas a dome is an arch revolved around itsvertical axis.
Pitched-brick vaults are named for their construction, the bricks are installed vertically (not radially) and are leaning (pitched) at an angle: This allows their construction to be completed without the use of centering. Examples have been found in archaeological excavations in Mesopotamia dating to the 2nd and 3rd millennium BCwhich were set ingypsum mortar.
A barrel vault is the simplest form of a vault and resembles a barrel or tunnel cut lengthwise in half. The effect is that of a structure composed of continuous semicircular or pointed sections.
The earliest known examples ofbarrel vaultswere built by theSumerians, possibly under thezigguratatNippurinBabylonia,which was built of firedbrickscemented with.
The earliest barrel vaults inancient Egyptare thought to be those in thegranariesbuilt by the 19th dynasty PharaohRamesses II, the ruins of which are behind theRamesseum, atThebes.The span was 12 feet (3.7m) and the lower part of the arch was built in horizontal courses, up to about one-third of the height, and the rings above were inclined back at a slight angle, so that the bricks of each ring, laid flatwise, adhered till the ring was completed, no centering of any kind being required; the vault thus formed was elliptic in section, arising from the method of its construction. A similar system of construction was employed for the vault over the great hall atCtesiphon, where the material employed was fired bricks or tiles of great dimensions, cemented with mortar; but the span was close upon 83 feet (25m), and the thickness of the vault was nearly 5 feet (1.5m) at the top, there being four rings ofbrickwork.
Assyrianpalaces used pitched-brick vaults, made with sun-dried mudbricks, for gates, subterranean graves and drains. During the reign of kingSennacheribthey were used to construct aqueducts, such as those atJerwan. In the provincial cityDr-Katlimmuthey were used to created vaulted platforms. The tradition of their erection, however, would seem to have been handed down to their successors inMesopotamia, viz. to theSassanians, who in their palaces inSarvestanandFirouzabadbuilt domes of similar form to those shown in the Nimrud sculptures, the chief difference being that, constructed inrubblestone and cemented with mortar, they still exist, though probably abandoned on theIslamicinvasion in the 7th century.
In all the instances above quoted in Sumer and Egypt the bricks, whether burnt or sun-dried, were of the description to which the termtilewould now be given; the dimensions varied from 10 inches (25cm) to 20 inches (51cm) being generally square and about 2 inches (5.1cm) to 4 inches (10cm) thick, and they were not shaped asvoussoirs, the connecting medium being thicker at the top than at the bottom. The earliest Egyptian examples of regular voussoirs in stone belong to theXXVIth Dynasty(ca.650 B.C.) in the additions made then to the temple ofMedinet Habu, and here it is probable that centering of some kind was provided, as the vaults are built in rings, so that the same centering could be shifted on after the completion of each ring. The earliest example of regularly shaped voussoirs, and of about the same date, is found in thecloacaatGraviscaeinEtruria, with a span of about 14 feet (4.3m), the voussoirs of which are from 5 to 6 feet (1.8m) long. Thecloaca maximain Rome, built byLucius Tarquinius Priscus(603 B.C.) to drain the marshy ground between thePalatineand the Capitoline Hills, was according toCommendatore Bonivaulted over in the 1st century B.C., the vault being over 800 feet (240m) long, 10 feet (3.0m) in span, with three concentric rings of voussoirs.
The enormousEyvan-e KhosroatCtesiphon(near present-dayBaghdad) was built over 1,500 years ago during theperiod as a throne room. The arch is about 37 metres (121ft) high, 26 metres (85ft) across and 50 metres (160ft) long, built entirely withoutcenteringand it was the largest vault constructed until modern times.
So far, all the vaults mentioned have been barrel vaults, which, when not built underground, required continuous walls of great thickness to resist their thrust; the earliest example of the next variety, the intersecting barrel vault, is said to be over a small hall atPergamum, inAsia Minor, but its first employment over halls of great dimensions is due to theRomans. When two semicircular barrel vaults of the same diameter cross one another their intersection (a true ellipse) is known as agroin, down which the thrust of the vault is carried to the cross walls; if a series of two or more barrel vaults intersect one another, the weight is carried on to the piers at their intersection and the thrust is transmitted to the outer cross walls; thus in the Roman reservoir atBaiae, known as thePiscina Mirabilis, a series of five aisles with semicircular barrel vaults are intersected by twelve cross aisles, the vaults being carried on 48 piers and thick external walls. The width of these aisles being only about 13 feet (4.0m) there was no great difficulty in the construction of these vaults, but in the RomanBaths of Caracallathetepidariumhad a span of 80 feet (24m), more than twice that of an Englishcathedral, so that its construction both from the statical and economical point of view was of the greatest importance.The researches of M. Choisy (LArt de bâtir chez les Romains), based on a minute examination of those portions of the vaults which still remainin situ, have shown that, on a comparatively slight centering, consisting of trusses placed about 10 feet (3.0m) apart and covered with planks laid from truss to truss, were laid – to begin with – two layers of the Roman brick (measuring nearly 2 feet (0.61m) square and 2 in. thick); on these and on the trusses transverse rings of brick were built with longitudinal ties at intervals; on the brick layers and embedding the rings and cross tiesconcretewas thrown in horizontal layers, the haunches being filled in solid, and the surface sloped on either side and covered over with a tile roof of low pitch laid direct on the concrete. The rings relieved the centering from the weight imposed, and the two layers of bricks carried the concrete till it had set.
As the walls carrying these vaults were also built in concrete with occasional bond courses of brick, the whole structure was homogeneous. One of the important ingredients of the mortar was a volcanicdepositfound near Rome, known aspozzolana, which, when the concrete had set, not only made the concrete as solid as the rock itself, but to a certain extent neutralized the thrust of the vaults, which formed shells equivalent to that of a metal lid; the Romans, however, do not seem to have recognized the value of this pozzolana mixture, for they otherwise provided amply for the counteracting of any thrust which might exist by the erection of cross walls and buttresses. In thetepidariaof the Thermae and in thebasilica of Constantine, in order to bring the thrust well within the walls, the main barrel vault of the hall was brought forward on each side and rested on detached columns, which constituted the principal architectural decoration. In cases where the cross vaults intersecting were not of the same span as those of the main vault, the arches were eitherstiltedso that their soffits might be of the same height, or they formed smaller intersections in the lower part of the vault; in both of these cases, however, the intersections or groins were twisted, for which it was very difficult to form a centering, and, moreover, they were of disagreeable effect: though every attempt was made tomaskthis in the decoration of the vault by panels and reliefs modelled instucco.
The widest hall vaulted by the Romans was that of the throne room in the palace ofDiocletianon thePalatine Hill, and this had the enormous span of 100 feet (30m), its thrust being counteracted by other halls on either side with buttresses outside. In provincial towns and in other parts of the Roman Empire, where the material pozzolana was not procurable, the Romans had to trust to their mortar as a cementing medium, but this, though excellent of its kind, was not of sufficient cohesive strength to allow of the erection of vaults of more than about 40 feet (12m) span, which were generally built in rubblemasonry. There still exist in Asia Minor andSyriasome vaulted halls, generally attached to thermae, which are carried on walls of great thickness. There were many varieties of the Roman vault, whether continuous or intersected, such as those employed over the corridors on theColosseumand thetheatre of Marcellus, but in these cases the springing of the vault was above the summit of the arches of the main front, so that there was no intersection; on the other hand, over the corridors were either elliptical or semicircular, or over the staircases rising vaults, all of which were more difficult to construct; there were also numerous solutions of vault over circular halls, of which that of thePantheonis the most important example, having a diameter of 142 feet (43m), and over the hemicycles, which were sometimes of great size; that known asCanopusinHadrians VillaatTivolihad a diameter of 75 feet (23m), and was vaulted over with a series of ribs, between which were alternating rampant flat and semicircular webs and cells; in the same villa and in Rome were octagonal halls with various other combinations of vault. Another type of vault not yet referred to is that of thewhere theCloister vaultwas employed. Fig. 3 compared with fig. 2 will show the difference; in the former the angles of intersection are inset, and in the latter they are groins with projecting angles at the base, which die away at the summit.
The rib vault ofReims Cathedral, France.
Rib vault of church Sint-Niklaaskerk inGhent, Belgium.
Top of the rib-vaulted ceiling over the nave ofSalisbury Cathedral.
Reference has been made to the rib vault in Roman work, where the intersecting barrel vaults were not of the same diameter. Their construction must at all times have been somewhat difficult, but where the barrel vaulting was carried round over theand was intersected (as inSt Bartholomew-the-GreatinSmithfield, London) by semicones instead of cylinders, it became worse and the groins more complicated. This would seem to have led to a change of system and to the introduction of a new feature, which completely revolutionized the construction of the vault. Hitherto the intersecting features were geometrical surfaces, of which the diagonal groins were the intersections, elliptical in form, generally weak in construction and often twisting. The medieval builder reversed the process, and set up the diagonal ribs first, which were utilized as permanent centres, and on these he carried his vault or web, which henceforward took its shape from the ribs. Instead of the elliptical curve which was given by the intersection of two semicircular barrel vaults, or cylinders, he employed the semicircular arch for the diagonal ribs; this, however, raised the centre of the square bay vaulted above the level of the transverse arches and of the wall ribs, and thus gave the appearance of a dome to the vault, such as may be seen in the nave ofSantAmbrogio, Florence. To meet this, at first the transverse and wall ribs were stilted, or the upper part of their arches was raised, as in theAbbaye-aux-HommesatCaen, and theAbbey of Lessay, inNormandy. The problem was ultimately solved by the introduction of the pointed arch for the transverse and wall ribs – the pointed arch had long been known and employed, on account of its much greater strength and of the less thrust it exerted on the walls. When employed for the ribs of a vault, however narrow the span might be, by adopting a pointed arch, its summit could be made to range in height with the diagonal rib; and, moreover, when utilized for the ribs of theannular vault, as in the aisle round theapsidaltermination of the choir, it was not necessary that the half ribs on the outer side should be in the same plane as those of the inner side; for when the opposite ribs met in the centre of the annular vault, the thrust was equally transmitted from one to the other, and being already a broken arch the change of its direction was not noticeable.
The first introduction of the pointed arch rib took place atCefal Cathedraland pre-dated the abbey ofSt. Denis. Whilst the pointed rib-arch is often seen as an identifier for gothic architecture, Cefal is a romanesque cathedral whose masons experimented with the possibility of gothic rib-arches before it was widely adopted by western church architecture.It was in the church atVezelay(1140) that it was extended to the square bay of theporch. Before entering into the question of the web or stone shell of the vault carried on the ribs, the earlier development of the great vaults which were thrown over the naves of a cathedral, or church, before the introduction of the pointed arch rib, shall here be noted. As has been pointed out, the aisles had already in the early Christian churches been covered over with groined vaults, the only advance made in the later developments being the introduction of transverse ribs dividing the bays into square compartments; but when in the 12th centurythe first attempts were made to vault over the naves, another difficulty presented itself, because the latter were twice the width of the aisles, so that it became necessary to include two bays of the aisles to form one square bay in the nave. This was an immense space to vault over, and moreover, it followed that every alternatepierserved no purpose, so far as the support of the nave vault was concerned, and this would seem to have suggested an alternative, viz. to provide a supplementary rib across the church and between the transverse ribs. This resulted in what is known as a sexpartite, or six-celled vault, of which one of the earliest examples is found in theAbbaye-aux-Hommes(S. Etienne) atCaen. This church, built by William the Conqueror, was originally constructed to carry a timber roof only, but nearly a century later the upper part of the nave walls were partly rebuilt, in order that it might be covered with a vault. The immense size, however, of the square vault over the nave necessitated some additional support, so that an intermediate rib was thrown across the church, dividing the square compartment into six cells, and called thesexpartite vaultthis was adopted in the cathedrals ofSens(1170),Laon(1195),Noyon(1190), Paris (122335), andBourges(1250). The intermediate rib, however, had the disadvantage of partially obscuring one side of theclerestorywindows, and it threw unequal weights on the alternate piers, so that in the cathedral ofSoissons(1205) a quadripartite or four-celled vault was introduced, the width of each bay being half the span of the nave, and corresponding therefore with the aisle piers. To this there are some exceptions, in Sant Ambrogio, Milan, and San Michele,Pavia(the original vault), and in the cathedrals ofSpeyerMainzandWorms, where the quadripartite vaults are nearly square, the intermediate piers of the aisles being of much smaller dimensions. In England sexpartite vaults exist atCanterbury(1175) (set out byWilliam of Sens),Rochester(1200),Lincoln(1215),Durham(easttransept), andSt. Faiths chapel,Westminster Abbey.
In the earlier stage of rib vaulting, the arched ribs consisted of independent or separatevoussoirsdown to the springing; the difficulty, however, of working the ribs separately led to two other important changes: (1) the lower part of the transverse diagonal and wall ribs were all worked out of one stone; and (2) the lower horizontal, constituting what is known as thetas-de-chargeor solidspringer. The tas-de-charge, or solid springer, had two advantages: (1) it enabled the stone courses to run straight through the wall, so as to bond the whole together much better; and (2) it lessened the span of the vault, which then required a centering of smaller dimensions. As soon as the ribs were completed, the web or stone shell of the vault was laid on them. In some English work each course of stone was of uniform height from one side to the other; but, as the diagonal rib was longer than either the transverse or wall rib, the courses dipped towards the former, and at theapexof the vault were cut to fit one another. In the early English Gothic period, in consequence of the great span of the vault and the very slight rise or curvature of the web, it was thought better to simplify the construction of the web by introducing intermediate ribs between the wall rib and the diagonal rib and between the diagonal and the transverse ribs; and in order to meet the thrust of these intermediate ribs a ridge rib was required, and the prolongation of this rib to the wall rib hid the junction of the web at the summit, which was not always very sightly, and constituted the ridge rib. In France, on the other hand, the web courses were always laid horizontally, and they are therefore of unequal height, increasing towards the diagonal rib. Each course also was given a slight rise in the centre, so as to increase its strength; this enabled the French masons to dispense with the intermediate rib, which was not introduced by them till the 15th century, and then more as a decorative than a constructive feature, as the domical form given to the French web rendered unnecessary the ridge rib, which, with some few exceptions, exists only in England. In both English and French vaulting centering was rarely required for the building of the web, a template (Fr.cerce) being employed to support the stones of each ring until it was complete. In Italy, Germany and Spain the French method of building the web was adopted, with horizontal courses and a domical form. Sometimes, in the case of comparatively narrow compartments, and more especially inclerestories, the wall rib was stilted, and this caused a peculiar twisting of the web, where the springing of the wall rib is at K: to these twisted surfaces the termploughshare vaultingis given.
One of the earliest examples of the introduction of the intermediate rib is found in the nave ofLincoln Cathedral, and there the ridge rib is not carried to the wall rib. It was soon found, however, that the construction of the web was much facilitated by additional ribs, and consequently there was a tendency to increase their number, so that in the nave ofExeterCathedral three intermediate ribs were provided between the wall rib and the diagonal rib. In order to mask the junction of the various ribs, their intersections were ornamented with richly carved bosses, and this practice increased on the introduction of another short rib, known as the lierne, a term in France given to the ridge rib.Lierne ribsare short ribs crossing between the main ribs, and were employed chiefly as decorative features, as, for instance, in theLiebfrauenkirche(1482) ofMhlacker, Germany. One of the best examples of Lierne ribs exists in the vault of theoriel windowofCrosby Hall, London. The tendency to increase the number of ribs led to singular results in some cases, as in the choir ofGloucester Cathedral, where the ordinary diagonal ribs become mere ornamentalmouldingson the surface of an intersected pointed barrel vault, and again in the cloisters, where the introduction of thefan vault, forming a concave-sidedconoid, returned to the principles of the Roman geometrical vault. This is further shown in the construction of these fan vaults, for although in the earliest examples each of the ribs above the tas-de-charge was an independent feature, eventually it was found easier to carve them and the web out of the solid stone, so that the rib and web were purely decorative and had no constructional or independent functions.
The fan vault would seem to have owed its origin to the employment of centerings of one curve for all the ribs, instead of having separate centerings for the transverse, diagonal wall and intermediate ribs; it was facilitated also by the introduction of the four-centred arch, because the lower portion of the arch formed part of the fan, or conoid, and the upper part could be extended at pleasure with a greater radius across the vault. The simplest version is that found in the cloisters of Gloucester Cathedral, where the fans meet one another at the summit, so that there are only small compartments between the fans to be filled up. In later examples, as inKings College Chapel, Cambridge, on account of the great dimensions of the vault, it was found necessary to introduce transverse ribs, which were required to give greater strength. Similar transverse ribs are found inHenry VIIs chapel and in theDivinity SchoolatOxford, where a new development presented itself. One of the defects of the fan vault at Gloucester is the appearance it gives of being half sunk in the wall; to remedy this, in the two buildings just quoted, the complete conoid is detached and treated as apendant.
The vault of theBasilica of Maxentius, completed by Constantine, was the last great work carried out in Rome before its fall, and two centuries pass before the next important development is found in theChurch of the Holy Wisdom(Hagia Sophia) atConstantinople. It is probable that the realization of the great advance in the science of vaulting shown in this church owed something to the eastern tradition ofdomevaulting seen in the Assyrian domes, which are known to us only by the representations in the bas-relief from Nimrud, because in the great water cisterns in Istanbul, known as theBasilica CisternandBin bir direk(cistern with a thousand and one columns), we find the intersecting groin vaults of the Romans already replaced by small cupolas or domes. These domes, however, are of small dimensions when compared with that projected and carried out byJustinianin theHagia Sophia. Previous to this the greatest dome was that of thePantheonat Rome, but this was carried on an immense wall 20 feet (6.1m) thick, and with the exception of small niches or recesses in the thickness of the wall could not be extended, so that Justinian apparently instructed his architect to provide an immensehemicycleorapseat the eastern end, a similar apse at the western end, and great arches on either side, the walls under which would be pierced with windows.Unlike the Pantheon dome, the upper portions of which are made of concrete, Byzantine domes were made of brick, which were lighter and thinner, but more vulnerable to the forces exerted onto them.
Thediagramshows the outlines of the solution of the problem. If a hemispherical dome is cut by four vertical planes, the intersection gives four semicircular arches; if cut in addition by a horizontal plane tangent to the top of these arches, it describes a circle; that portion of the sphere which is below this circle and between the arches, forming a sphericalspandril, is thependentive, and itsradiusis equal to thediagonalof the square on which the four arches rest. Having obtained a circle for the base of the dome, it is not necessary that the upper portion of the dome should spring from the same level as the arches, or that its domical surface should be a continuation of that of the pendentive. The first and second dome of the Hagia Sophia apparently fell down, so that Justinian determined to raise it, possibly to give greater lightness to the structure, but mainly in order to obtain increased light for the interior of the church. This was effected by piercing it with forty windows – the effect of which, as the light streaming through these windows, gave the dome the appearance of being suspended in the air. The pendentive which carried the dome rested on four great arches, the thrust of those crossing the church being counteracted by immense buttresses which traversed the aisles, and the other two partly by smaller arches in the apse, the thrust being carried to the outer walls, and to a certain extent by the side walls which were built under the arches. From the description given byProcopiuswe gather that the centering employed for the great arches consisted of a wall erected to support them during their erection. The construction of the pendentives is not known, but it is surmised that to the top of the pendentives they were built in horizontal courses of brick, projecting one over the other, the projecting angles being cut off afterwards and covered with stucco in which the mosaics were embedded; this was the method employed in the erection of the Prigordian domes, to which we shall return; these, however, were of less diameter than those of the Hagia Sophia, being only about 40 to 60 feet (18m) instead of 107 feet (33m) The apotheosis ofByzantine architecture, in fact, was reached in Hagia Sophia, for although it formed the model on which all subsequent Byzantine churches were based, so far as their plan was concerned, no domes approaching the former in dimensions were even attempted. The principal difference in some later examples is that which took place in the form of the pendentive on which the dome was carried. Instead of the spherical spandril of Hagia Sophia, large niches were formed in the angles, as in theMosque of Damascus, which was built by Byzantine workmen for theAl-Walid Iin A.D. 705; these gave an octagonal base on which the hemispherical dome rested; or again, as in the Sassanian palaces of Sarvestan and Firouzabad of the 4th and 5th century, when a series of concentric arch rings, projecting one in front of the other, were built, giving also an octagonal base; each of these pendentives is known as asquinch.
There is one other remarkable vault, also built by Justinian, in theChurch of the Saints Sergius and Bacchusin Constantinople. The central area of this church was octagonal on plan, and the dome is divided into sixteen compartments; of these eight consist of broad flat bands rising from the centre of each of the walls, and the alternate eight are concave cells over the angles of the octagon, which externally and internally give to the roof the appearance of an umbrella.
Nave ofLisbon Cathedralwith a barrel vaultedsoffit. Note the absence ofclerestory windows, all of the light being provided by theRose windowat one end of the vault.
Although the dome constitutes the principal characteristic of the Byzantine church, throughout Asia Minor are numerous examples in which the naves are vaulted with the semicircular barrel vault, and this is the type of vault found throughout the south of France in the 11th and 12th centuries, the only change being the occasional substitution of the pointed barrel vault, adopted not only on account of its exerting a less thrust, but because, as pointed out by Fergusson (vol. ii. p.46), the roofing tiles were laid directly on the vault and a less amount of filling in at the top was required.
The continuous thrust of the barrel vault in these cases was met either by semicircular or pointed barrel vaults on the aisles, which had only half the span of thenave; of this there is an interesting example in theChapel of Saint Johnin theTower of London- and sometimes by half-barrel vaults. The great thickness of the walls, however, required in such constructions would seem to have led to another solution of the problem