This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Although vaulted roofs are an outgrowth of masonry construction, and are almost always built of brick or stone, they are occasionally built of timber, and in any case a timber centering must be built for them. A vault may be described as the surface generated by a curved line, as the line moves through space, and in accordance with this definition there are vaults of all kinds, semicircular or barrel vaults, elliptical vaults, conical vaults, and many others.
Fig. 262. Elevation of Framing for the Niche.
In Fig. 264 is shown in outline a simple semicircular or barrel vault, known as well by the name cylindrical vault. The point A where the straight vertical wall ends and the curved surface begins is called the springing point. The point B is the crown of the vault. The distance between the springing points on opposite sides of the vault is the span, and the vertical distance between the springing point and the crown B is the rise.
It may easily be seen how a barrel vault, or a vault of any kind, may be framed with curved ribs spaced from 1 foot to 18 inches apart on centers, and following the outline of a section of the vault. If the framework is intended to be permanent and to form the body of the vault itself, then the inner edges of the ribs must lie in the surface of the vault and must be covered with lathing and plastering. If only a centering is being built, on which it is intended that a masonry vault shall be supported temporarily, then the outer edges of the ribs must conform to the vaulted surface and must be covered with rough boarding to receive the masonry.
When two vaults intersect each other, as in the case of a main vault, and the vault over a transept, the ceiling at the place where vaults come together is said to be groined. The two vaults may be of the same height or of different heights. If they are of different heights the intersection is known as a Welsh groin. Welsh groins are of common occurrence in masonry construction, but in carpentry work the vaults are almost always made equal in height and often they are of equal span as well.
Fig. 263. Section of Small Niche.
Fig. 264. Diagram of Cylindrical Vault.
The framework for each vault is composed of ribs spaced comparatively close together, and resting on the side walls at the springing line. When, however, the two vaults intersect each other, the side walls must stop at the points where they meet, and a square or rectangular area is left which has no vertical walls around it The covering for this area must be supported at the four corner points in which the side walls intersect. This is shown in plan in Fig. 265 where A B C D are the points of intersection of the walls of the vaults. The method of covering the area common to both vaults is also shown in the figure. Diagonal ribs A D and C B are put in place so as to span the distance from corner to corner and these form the basis for the rest of the framing. They must be bent to such a shape that they will coincide exactly with the line of intersection of the two vaulted surfaces. The ribs which form the framing for the groined ceiling over the area are supported on the diagonal ribs as shown in the figure. They are arranged symmetrically with respect to the center, and are bent or shaped to the form of segments of circles or ellipses.
Fig. 265. One Method of Cradling for Groined Ceiling.
Fig. 266. Another Method of Cradling for Groined Ceiling.
Fig. 265 shows one method of forming the cradling for a groined ceiling, but there is another which is also in common use. This is shown in Fig. 266. There are four curved ribs A B, B D, C D, and C A, which span the distances from corner to corner around the space to be covered. The diagonal ribs A D and C B are also employed as in the first method. Straight horizontal purlins are supported on these ribs, running parallel to the direction of the vaults, as shown in the figure. They are spaced about 16 inches apart and form the framework for the ceiling.
The only difficult problem in connection with groined ceilings is to find the shape of the diagonal rib. This rib, as has been explained above, must coincide with the line of intersection of the vaults. The problem, then, is to find the true shape of the diagonal rib.
Let us consider the two vaults shown in plan in Fig. 267. They are not of the same span, but they will be of the same height if we wish to have a common groin and not a Welsh groin; so if one is semicircular the other must be elliptical. Elevations of ribs in each vault are shown at A and B and the diagonal ribs are shown in plan at A C and B D. It is easy to find the plan of these ribs because they must pass from corner to corner diagonally. To find the elevation we must use the same principle that was employed in finding the position of the valley rafter and the shape of the curved hip rafter for an ogee roof, namely, any line drawn in the roof or ceiling surface parallel to the plate or side walls must be horizontal, and all points in it must be at the same elevation.
We start with the assumption that one of the vaults is semicircular, as shown in elevation at A, Fig. 267. Taking any line in the vaulted surface, shown in plan, as the line S P O, we produce it until it intersects the plan of the diagonal rib A C at the point 0. This point must be the plan of one point in the line of intersection of the vaulted surfaces.
The elevation of the point 0 above the springing line of the vaults is shown by the distance P S, since the line S P O is exactly horizontal throughout. This distance is laid off at E F with the line G E H representing the horizontal plane which contains the springing lines of the vaults. The point H is the point from which the diagonal rib starts. The point F, as we have seen, is another point in the curve, and we can by a similar process locate as many points as we need. This will enable us to draw the complete curve G F H of the line in which the vaults intersect, and to which the diagonal rib must conform.
Fig. 267. Diagram Showing Method of Finding Diagonal Rib for Two Intersecting Vaults.
By continuing the line from the point 0 at right angles to its former direction and parallel to the wall line, we may obtain the point K, which is a plan of one point in the surface of the elliptical vault. The elevation of this point also above the springing lines must be the same as for the point S and may be laid off, as shown at K M. By finding other points in a similar way the curve N M R of the elliptical vault may be readily determined.
LIVING ROOM OF HOUSE IN URBANA, ILL.
White & Temple, Architects, University of Illinoi
For Exterior and Plans, See Page 266.