This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Centers. Where any portion of the outer or inner walls of a building are of brick or stone, and the openings in these walls are circular headed, the carpenter will be called upon to make wooden templets, or centers, as they are called, and set same in place for the mason to lay the stones or brick of his arch upon until the mortar has thoroughly set and rendered the arch self-sustaining.
199. In Fig. 78 is shown a wood center for a semicircular window in a 12-inch brick wall; (a) is the side view of the center and its supports, and (b) the elevation of the center in position in the wall. The faces of the center are, in this case, each composed of two pieces c of 1 1/4-inch plank sawed to the proper curve and secured at the bottom by the tie-piece b, which is securely nailed to each of the face pieces c, c, while the joint at the top is secured by the fish-plate d nailed to both pieces on the inside of the boards.
Two of these face pieces having been thus formed, they are joined together with small strips, called lagging, 11/4 inches thick, 1 1/2 to 2 inches wide, and 12 inches long, as shown at e. The bottom of the face pieces is secured by two caps i nailed to each face, under which the supports f are placed, when the center is set in the opening ready to be built upon. The lower ends of the supports f do not rest directly on the sill of the opening, but upon two slip wedges g, which are gradually driven out after the masonry has set, thus allowing the arch to gradually settle down to its own proper bearing. The braces a, a are inserted between the cleats j to maintain the supports in a vertical position against the jambs of the opening. All wood arch centers may be framed very nearly in this same manner, except in very large spans, where heavier timbers must be used.
200. Elliptical centers are formed in the same manner as those for circular arches, but usually require more pieces in the curved faces. For this reason it is advisable to lay out, on paper, a semiellipse of the required height and span according to the method described in problem 24, Geometrical Drawing, and with this paper form lay out the curved line on the several pieces of plank which have previously been arranged to occupy about their proper position in the finished center, as a, b, c, and d, Fig. 79. The curved edge is then sawed out on each piece separately, the ends are then sawed to the proper miter line, and fish-plates are nailed on the back in the same manner as d was nailed on c, in Fig. 78. When great strength is required, it is sometimes advisable to make these fish-plates as large as their face pieces a, b, c, d, and miter their ends to an even joint, as shown in Fig. 79. The ends are secured in place by the tie-pieces f and the caps g, while the center is strengthened by a strut e extending from the tie-piece f to each of the intermediate sections of the face pieces, as b and c. The small 1 1/4" x2" strips are then nailed on the curved edges in the same manner as for the semicircular center, their length being equal to the thickness of the wall in which the arch is to be built.
201. In all wood centers the joints between the different sections of the face pieces should always be in a line perpendicular to a tangent at the point where the joint line intersects the curved line of the arch; that is, in Fig. 79 the joint line n v is perpendicular to the line x y, which is tangent to the curve at v. These joint lines will all converge towards the center from which the curve is struck when the arch is semicircular, as v w, in Fig. 78, but in other curves they can be found only by first drawing the tangent, as in Fig. 79.
A characteristic difference between the trimmer-arch center and those previously described, is that the wood center is generally left in place and remains in the building-after the masonry arch is laid over it. When the mason builds the brickwork of the chimney he corbels out one course of brick about \\ inches at the level of the under side of the floorbeams, as shown at e, and the carpenter builds his center to have one end rest on the brickwork at e, and the other end notched over a 1 1/2 X2 1/2" or 3" cleat d nailed on the header beam a. The face piece of the center c is 1 1/4 inches in thickness, and its upper edge, on which the lagging f is nailed, is sawed to the curve of the arch so that the small end will be about \\ inches in thickness where it rests on the brickwork at c, and is about 4 1/2 or 5 inches less at the large end than the depth of the floorbeams. The number of pieces like c required for a trimmer-arch center depends on the length of the hearth which is to be built over it; but if the ceiling below is to be plastered, it is desirable to have one in front of the end of each tail-beam where it mortises into the header, in order to simplify the lathing.
Sometimes the lathing can be more readily accomplished by nailing a filler piece between the face pieces c, as shown by the dotted line at g, and then running the lath strips from the filler g to the nailing cleat d; but, unless the hearth is a very long one, the former method will be found most satisfactory.
203. When the roof of a building is carried up, either wholly or in part, in the form of a hemisphere, or semi-ellipsoid, it is called a dome, and the rafters supporting it must be sawed or bent to the required curvature.
204. A hemispherical dome is shown in Fig. 81, in which (b) is a half plan, and (a) is the sectional elevation.