This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The lintel is not intended to impart strength to the relieving arch, for should the lintel shrink or become destroyed by fire the weight over the opening must rely entirely upon the arch for support. The use of the lintel is for fixing the joinery work.
Lintels cast of coke breeze concrete in wooden moulds are often used in place of wood lintels, and they possess the advantage of being fire-resisting. They are easily formed, and they do not shrink and become loose. A wooden mould for a concrete lintel is shown in Fig. 182. The upper portion fits upon the lower, and is kept in position by projecting lugs L The trough thus formed is well oiled or smeared with soft soap on the inside to prevent the concrete from adhering to the sides, it is then filled with concrete and the top floated level by running a straight piece of wood along its upper edges.
Axed Arches are those built of bricks which have been cut with an axe, or with a bolster and club hammer (see Fig. 174), to the approximate wedge shape. The arch is set out and a template obtained as for gauged arches.
When glazed bricks are used for axed arches they are cut off to a template obtained as explained for gauged arches, but placed over the bricks so as not to cut away the glaze on the soffit.
The name "axed arch" is sometimes applied to arches whose voussoirs have been cut as in gauged work, but with less care and exactness.
Gauged Arches are arches formed of bricks which have been carefully cut and rubbed upon a stone so that the arrises are sharp, the exposed faces smooth and even, and the joints fine.
Let us suppose that we wish to build an arch 12 inches on face and 4 1/2 inches on soffit over a span of 4 feet 6 inches (Fig. 183).
A little more than half the outline of the arch must first be set out to full size on white lining paper pinned to a large board (usually from 5 to 6 feet square), the arco being struck with a radius rod - that is, a rod of wood pivoted at one end and with a pencil point fixed to the other. Now, taking a distance of 1J inch in the dividers, prick round the extrados, starting from the point B (Fig. 183), and continuing until the point C is reached. If the point of the dividers does not fall exactly upon C, then the distance taken with the dividers must be slightly increased or decreased until the extrados exactly contains an odd number of these small distances. In Fig. 183, D is the first of these pricked points after the distance has been adjusted as described above, and E is a point pricked at the same distance on the other side of the central point B, so that ED is approximately 3 inches. Join D and E to the centre A, thus indicating the joints of the key brick, and produce AD to F.
Now take a piece of board 18 by 3 1/4 by 1/2 inches (T, Fig. 183), planed on both faces and with one edge shot square and true, and place it with the shot edge coinciding with the line AE. Take a long straight edge and place it over T with one edge exactly over the line AF, and draw a line on T along the edge of the straight edge. Next cut T to this line, and shoot the edges true and square. Then T will be the template as shown in Fig. 183. The template must now be tested for accuracy, which is done by an operation called Traversing, performed in the following manner: Place the template over the drawing of the key brick as in the lower drawing of Fig. 183, adjusting it to fit exactly, and make a pencil markon the left-hand edge immediately over the intrados line. This point is called the Filling-in mark. Place the straight edge V against the edge of T, remove T and place the straight edge S against V, keeping its lower end above the intrados line. Remove V, place T against S with the filling-in mark on the intrados, and place V against T. Repeat this operation until the springing line is reached. If the edge of the template falls a little short of the springing line the filling-in mark must be shifted a little towards the wide end of the template, and if the template slightly overlaps the springing line then the filling-in mark must be shifted a little towards the narrow end of the template. It may happen that the edge of the template overlaps the springing line obliquely, in which case a few shavings must be taken off the overlapping end. Every time the shape of the template or the position of the filling-in mark are altered the template must be retraversed, care being taken that it be started from a position symmetrical to the centre line of the arch. When the template has been finally adjusted it is once more traversed, and the joints are filled in and the bonding arranged, as shown in Fig. 183, noting that in the key the lowermost brick is a stretcher. Allowance has now to be made for the joints, which should, in good work, be 1/32 inch thick. To do this the template is placed over the key with the filling-in mark over the intrados, as in Fig. 183, and the straight edge S is placed against it. The template is then slid upwards along the edge of S until its opposite edge leaves the radial line by a distance of 1/32 inch. Mark the left-hand edge of the template with a pencil immediately over the intrados. This mark is called the Cutting mark, and the edge of the template bearing the cutting mark is called the Cutting side and the opposite side is called the Bed. The template is now traversed, for four joints, using the cutting mark as previously explained for the filling-in mark, when the bed or right-hand edge of the template should come 1/8 inch away from the radial line AG (Fig. 183). If this distance exceeds 1/8 inch the cutting mark must be placed a little nearer the wider end of the template, and if less than 1/8 inch the cutting mark must be placed nearer the narrow end of the template. The bed is now placed along the radial line AD, with the cutting mark upon the soffit, and points are marked on its sides where the intrados and extrados come, and the four marks are squared across the sides and the extremities of these squared lines are joined across the face of the template, as in Fig. 184, thus securing the top and bottom bevels. The template is now complete, and it is a safe plan to write upon it its position in the building, the number of centre upon which it is to be stuck, the shape of the arch, its span and the number of the courses, as shown in Fig. 184.