This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by Walter Hooker)
The term "pointed" is applied to vaults in which the surfaces rise to a point at the apex. They are called equilateral, drop, four-centred, etc., according to the method by which the section is set out, as explained in connection with pointed arches in Chapter VII (Arches - Plane). Pointed vaults are also termed Gothic vaults, as this form of vault is the fundamental structural form of Gothic architecture.
The simplest form of pointed vault is a continuous pointed tunnel, and ancient examples of this form exist in the Channel Islands and south-west France, but it is a form which is very rarely used in modern work, owing to the continuous thrust that such a vault exerts upon the walls. It should be noted that the primary object of Gothic vaulting is to concentrate the load upon certain points, which are made of sufficient strength to resist the thrusts; and to achieve this object the whole vault is supported upon a series of arched ribs, as shown in Fig. 122 and Plate IV. The spaces between the ribs, known as Severies, are composed of comparatively small stones, arched from rib to rib, thus bringing the centre load on the vault down to the ribs, and concentrating it upon the four corner supports. The masonry between the ribs is also called infilling or panelling, and work of this nature is often spoken of as rib and panel work. The courses of the infilling are usually, in English practice, made at right angles to the line bisecting the angle between the adjacent vault ribs at their springing. According to French practice, they are parallel with the ridge rib or joint.
It is interesting and instructive to note that in Renaissance vaulting the groins were made subsidiary to the vaulting surfaces, while in Gothic work the reverse is the case.
The English method of coursing the infilling (see Fig. 122) throws the joints out of winding at the vertices, and to make good the deficiency diagonal stones, forming serrated edges or toothing, are introduced to complete the vaulting. These engage with the courses of the panel stones and render the fabric homogeneous and stable.
The diagonal ribs (presuming the vertical arches to be formed of arcs of circles) will be elongated and flattened.
Fig. 122 shows how the simplest form of Gothic vault is set out, Abdc being the plan of a square compartment.
The ribs in this case comprise: -
1st. The wall or longitudinal ribs or ribs against the side wall of the compartment, as at BD. These govern the curvature of the vaulting in a great measure, as on them depends the height of the arch above the springing.
2nd. Transverse ribs, or ribs running transversely across the opening, as AB and CD.
3rd. The diagonal ribs AD and BC.
It is customary to set out the curves of the wall or transverse ribs first.
In this case it is also presumed that the apices of all the ribs are in a horizontal plane, so that the radii of the diagonal ribs will require to be proportionally less than that of the transverse or wall ribs in order to bring their apices at the same height and keep a similar curve; and all the centres must be upon the springing line.
In this example, the compartment being square, the curves of the wall and transverse ribs are struck with the same radius. The diagonal rib is struck from two centres, the lower arc being struck with the same radius as the wall and transverse ribs, whilst the upper arc is struck from a centre within the radius of the former and on a line at right angles with the tangent to the curves at their point of junction. The variation of curvature in the diagonal rib should occur where it becomes entirely clear of the transverse and wall ribs. This can be so arranged that the apices of the two ribs are of the same height from the springing.
The feet of the ribs on the springing line now require to be adjusted.
If there is a cap forming the base of the ribs, its spread or abacus will form the seat of the members of the ribs, and the faces of the mouldings should be so adjusted as to evenly separate as they rise above the springing line. In the present case the outer members of the ribs are adjusted to form a neat regular figure, as shown on plan.
Elevations of half the wall and half the diagonal rib are now drawn showing the nosing line and the thickness of the infilling (see Fig. 122).
The first few courses are always worked with horizontal beds, and it is not until the ribs begin to clear each other that the beds are made radiating from the centres from which the arcs are struck.
This "tas de charge," as it is called, enables the walling to be continued and the roof covering put above it, and for the vaulting to be added later under cover when settlement has ceased.
The point of clearance is found by erecting a perpendicular from the point of contact E of the outer faces of two adjacent ribs on plan until it cuts the upper outer edge of the rib in question on the elevation. See dotted lines Ea and Eb.
At these points a and b the upper beds are worked into inclined planes at right angles to the several ribs which spring therefrom, as at ac and bd.
The springers are worked from a stone of dimensions sufficient to contain the ribs at the springing line, together with sufficient material for bonding into the wall, as shown in its plan on Fig. 123.
The top and bottom joints present no difficulty, as they are horizontal and parallel. Having brought these to a surface, mark off by means of templates the moulding on the bottom bed and the spread due to the rise on the top bed. The templates for stone No. 1 and No. 3 are shown in Fig. 123. It will be seen that the top bed mould for stone No. 1 forms the bottom bed mould for stone No. 2, and the bottom bed mould for stone No. 3 forms the top bed mould for stone No. 2. It should be noted also that, save in the case of the bottom bed mould, the sections of the mouldings as shown upon the bed moulds are not the same as those shown in the section of the ribs, but the former are projections of the latter (which have their true section along radial planes cutting the curves of the ribs) upon the horizontal planes of the joints.