It has generally been maintained that the wall ribs were integral and important members of a true ribbed vault and that they actually aided in the support of the masonry panels. There are, however, a number of reasons for believing that this is not entirely so but that these ribs were comparatively unimportant as far as their relation to the vaults was concerned and were of much more importance, in the first place as cover joints, in the second as window heads, and in the third as relieving arches in the clerestory wall. Two important facts lend strength to the theory that the wall rib was not as a rule a supporting member. The first of these lies in the fact that it was quite frequently omitted even from vaults of the true Gothic form, and the second, in the fact that, when present, there are perhaps as many cases in which the curve of the vault fails to follow that of the rib as there are of the reverse condition. In fact, it is a question whether in the majority of cases the vault panel actually rests upon or even cuts into the face of the formeret. Take, for example, a number of chevet vaults[365] and examine them in this respect. At Saint Germer [(Fig. 63)] the wall rib is largely a relieving arch in the clerestory wall which is made much thinner beneath it; and while the curve of the chevet cells follows in general that of the arch, it does not exactly correspond with it. In the large chevet vault of Saint Remi at Reims [(Fig. 64)], and in many other vaults not over the apse, especially in the English churches and those in which a group of clerestory windows is found in each bay, no wall rib is used, showing that such a rib was not at all necessary as far as the construction and support of the vault was concerned. Moreover, in many of the churches in which a wall rib is used along the exact line of the vault surface, it is too small to act as a supporting member and would seem to be merely a cover-joint to hide the intersection of the vault surface with the clerestory wall.[366] Finally and most important of all are the cases in which this rib is used primarily as a window head. In some of these, as for example in the apse of La Madeleine at Vézelay, and those of the cathedrals of Soissons [(Fig. 67)] and Chartres [(Fig. 68)], the curve of the vault corresponds with this window-head arch, but in many other apses such as those of Bourges cathedral [(Fig. 76)], of Saint Étienne at Caen [(Fig. 70)], and of the Sainte Chapelle at Saint Germer, the builders without hesitation curved their vault surface away from the line of the window-head which would otherwise be the natural wall rib.[367] Although from the preceding facts, it would seem evident that the wall rib was not an essential structural member of the Gothic vaulting system it may have been of advantage in many instances for holding a temporary wooden centering during the construction of the vault panels.
Fig. 64.—Reims, Saint Remi.
Radiating-Ribbed Chevets continued
Returning again to the radiating-ribbed chevet, especially that of Saint Germer-de-Fly [(Fig. 63)], it is important to note the one great weakness which this vault possesses. It lies in the position of the radiating ribs which abut the apsidal arch at its crown, in other words at a point not at all suited to meet the pressures which are thus brought to bear against it. A rather heavy arch between the apse and the remaining bay of the choir, though no heavier than those in the vaulted bays of the nave, aids in resisting the pressure but nevertheless such a vault is not strictly logical from a structural standpoint. It is not as well buttressed, for example, as the ribbed half dome of Saint Georges at Boscherville [(Fig. 61)], or the transept chevet at Tournai [(Fig. 53)], in which a tunnel vaulted bay precedes the arch against whose crown the radiants are brought to bear.
It is not surprising that this vault was but little used in subsequent Gothic architecture. It is possible, however, to cite a few examples, among them the cathedrals of Séez (Orne) (end of the thirteenth century), Cambrai (Nord) (cir. 1250), and Dinan (Côtes-du-Nord) (end of the thirteenth century), the cathedral of Saint Sauveur at Bruges (Belgium) (probably thirteenth century), and the abbey church of Moissac (Tarn) (probably fourteenth century). There is also a peculiar form in which the ribs are narrowed toward the crown, in Santa Maria sopra Minerva at Rome (after 1285). Two other slight variants of the type, one in the church of Saint Pierre-le-Guillard at Bourges and the other in the cathedral of Moulins are later discussed.