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.
In constructing arches the voussoirs are headed one upon another, commencing with the springers and building up towards the crown on either side, until the arch is finally completed by the insertion of the keystone. As the crown of the arch is approached the bed joints of the voussoirs become less obliquely inclined to the horizon, or, in other words, become more nearly vertical, while the tendency of the voussoirs to slide upon one another gradually increases. It becomes necessary, therefore, to support them until the arch is completed, the nature of the support depending upon the size of the arch and the weight of the voussoirs.
When small arches of only a few inches rise are to be constructed the voussoirs are supported during the course of construction upon solid pieces of timber, with the upper surface cut to the curve of the intrados of the arch, as shown in Fig. 236.
When arches of considerable span and rise have to be constructed the voussoirs are supported during the course of construction upon frameworks of timber called Centres. These centres vary considerably in their construction, according to size of the arch and the magnitude of the loads to be borne.
Centres may be classified as -
1. Centres with Solid Ribs.
2. Centres with Built-up Ribs.
1. Centres with Solid Ribs are used for small arches. They are composed of two ribs of 1-inch boards with the upper edges cut to a curve. The lower corners of the ribs are nailed to Stretchers, also out of 1-inch boards; the length of these is a little less than the thickness of the wall in which the arch occurs. To the upper edges of the ribs 1 1/2 by 3/4-inch Lagging Pieces are nailed, as shown in Fig. 237. The centre thus formed is supported upon uprights, the lower ends of which rest upon the upper surface of the sill below, and a piece of wood slightly larger than the width of the opening is fixed between the uprights to keep them in position. Wedges are inserted between the head of the upright and the stretchers, which enable the centre to be eased when the arch is completed.
Centres with Built-up Ribs are of two kinds -
(a) Centres with Laminated Ribs.
(b) Centres with Ribs built up of Solid Timber. Before designing a centre for any particular arch the principles upon which all centres are constructed should be understood.
During the construction of an arch ring the load increases as voussoirs are added on either side, and the increasing load creates varying stresses in the ribs of the centre. Thus when the arch ring has been built up to about half-way between the springing and the crown there is a tendency for the load to push in the lower portions of the ribs, and in doing so to raise the upper portions, while when the arch ring is complete the tendencies are reversed; that is to say, the upper portions of the ribs tend to sag, while the lower portions tend to bulge outwards.
It is clear from the above facts that the ribs must be supported at intervals by means of braces which are not only capable of resisting these varying stresses, but must be so placed that the two tendencies to sag or rise counteract one another.
The centres, being of a temporary nature, should be so designed that their timbers may be cut or spoilt as little as possible, so that they may be used again for other purposes.
The joints should also be designed to resist the varying stresses, tenoned joints being useless for the purpose, save where the tenon is used to prevent lateral motion in the timbers.
(a) Centres with Laminated Ribs, as their name implies, have their ribs built up in sections, out of thin boards in two or more thicknesses, nailed together so as to breakjoint, as in Fig. 238. The length of the sections of the ribs is determined by the width of the board from which they are cut, boards 9 by 1 inch being generally used; and the sections should never be so long as to cause their end to be less than 4 1/2 inches wide. One of the lowest sections on each side of each rib is stopped short of the springing so as to form a rebate for a Tie, which varies from 6 by 1 inch to 9 by 2 inches, according to the span of the arch, this tie being nailed at each end to the rib. To prevent the haunches from sinking and the crown from rising, or vice-versd, during the construction of the arch, the rib is strengthened by three braces, varying from 6 by 1 inch to 9 by 1 inch, according to the span. Each brace is cut at the upper end to fit into the angle formed by two sections of one lamination, and is nailed to the overlapping section of the other lamination, while the lower ends are securely nailed to the middle of the tie.
For a wall arch two such ribs are connected together by nailing two stretchers, one at either end of the ties. Lagging pieces are then nailed across the curved edges of the ribs. If the arch is to be rough, 2 by 1 inch lagging pieces are used, spaced openly as shown on the left-hand side of Fig. 238; but if the arch is in gauged brickwork the lagging pieces are placed close together, and their edges are trimmed with a plane to the desired curve, or else the lagging is formed of two thicknesses of 11 by 3/8-inch pine bent to the curve of the ribs. When a small arch is constructed of stone the voussoirs are supported by a single lagging piece occurring at each joint, as shown on the right-hand side of Fig. 238. This type of centre is used for spans of from 5 to 12 feet. (b) Centres with Ribs built up of Solid Timber. - In this kind of centre the ribs are built up out of solid pieces of timber, the length of the sections being regulated, as in laminated ribs, by the width of the timber from which they are cut. When timbers are not at hand of sufficient width to enable sections of convenient length to be cut from them, the sections may be formed out of narrow timbers firred up, i.e.