Practical Masonry and Stone Cutting. VIII.

PRACTICAL MASONRY AND STONE-CUTTING. VIII.

by Fred T. Hodgeson.

(Copyright Applied For.)

The following, which is termed an "abridged" method of "laying out" an oblique arch, is taken from "Gwilt's Encyclopedia of Architecture." It is named "abridged" because it yields by a rapid operation the moulds of the soffits and joints within the plan of the arch.

Let A B C D, Fig. 55, be the plan. Divide A B in E into two equal parts, and draw E F parallel to A A". Then from A draw A G perpendicular to A C; prolong D B to G; divide A G into two equal parts in the point H. From H as a center describe the arc A' F G, which divide into voussoirs, and draw the joints from the center H. Image of Figure No. 55 Draw lines from each soffit parallel to E F, and below the line C D; the moulds for the soffits are comprised between the parallels of the key, and those of the joints are traced on the sides of the plan as follows: To find the moulds of the soffits through the point 2, draw 2 N parallel to G H. To find on R S the point N, through the point K draw K L, also parallel to G H. To find on 2 T the point M, and on R S the point L, draw the front line of the second soffit M N, and the front line of the first I L. The back of this sheeting soffit is formed by the same operation below the plan. The mould of the key is formed by two lines, R S, 2 T; and the front and back lines of the plan A B, C D; the two moulds of the soffits N M, T S, and L I, X V, serve to trace the two stones on each side, observing only that the lower arrisses of the soffit on the side A C, because those on top on the side B D, or that the under arris of one side may be that of the other side by reversing the mould, which will have the same effect. To find the moulds of the beds or joints, prolong N 2 to meet D G, to find the point P, and through it and the point E draw the front of the second joint P 2; prolong L M to G D to find O, through which and the point E draw the front of the point O 3. Proceed in the same manner to find the back of the other joints, which are sufficient also to trace the stones by reversing them. It is not absolutely necessary to cut out the moulds of the soffits and joints but the angles may be taken by bevels, and applied to the stones. The heads are prepared, as usual, with the moulds of the head of the straight arch. It must be observed that in this arch the face or front differs from a straight arch, being formed by different sections of a cylinder.

If we take in Fig. 56, the manner of constructing a semi-circular headed arch in a circular wall, as, for instance, a round tower, we will suppose A B C D to be a plan of that portion of the wall or tower where the arch is to be placed. Image of Figure No. 56 Bisect the arc A B, and through this point of intersection draw a line, E F, parallel to the line of the jambs G H or I K. Through any point, as L in the line E F, draw M N perpendicular to E F. Produce the lines G H and I K respectively to meet the line M N in the points O P, and the line O P will be bisected in L. From L as a center, with the radius L O or L P, describe the semi-circle O Q P. Divide this arc from L for the joint lines, and let fall perpendicular of the same to the inner face of the circular wall C D. These will be transferred to the development R S, in same manner as before described.

The most simple form of arch next to the semi-circle is the segmental, the basis of whose outline is the segment of a circle. The method by which this curve may be generated is shown at Fig. 57. Let A B represent the span, and C D the rise to soffit of key S. The tangental line B D is first bisected at the point G, and the vertical line D C produced indefinitely and at right angles to the line of springing, A B. A steel square, E, may now be placed so that one of its sides coincides with the line B G, and Image of Figure No. 57 in such a manner that a line from G may be drawn by the square in the direction G O-at right angles to B D-and produced until it cuts the line D C at O. This latter point is the center from whence the segmental curve can be described, and also that from which the beds of the arch stones must spring as shown in the sketch. Segmental arches are often used in foundations, in order to relieve pressure in some soft places, or carry the weight of walls from one point to another.

There are many forms of segmental arches, indeed, the segmental arch is in more common use than any other form. The "stilted arch" (Fig. 58), Image of Figure No. 58 so called because of its not being a true segment of a circle, was much used by French and Italian architects, but is seldom employed by either English or American builders. It has no claim to beauty and lacks constructive stability.

The elliptical arch, which is a modern innovation, and which took its rise in the building of some of our great modern bridges, is noted for its beauty of outline and its difficulty of execution, and in these points it leaves all ancient works far in the rear.

There are many methods by which an ellipse may be described, and in order that the workman may have a knowledge of some of them we will next month illustrate and describe a few of them, and will further on, describe others.

Fred T. Hodgson.

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