Centres are framed structures for temporary purposes, used in throwing arches over apertures, etc. They have their top shaped to the required curve of the arch, are made specially for the dimensions of the opening, and act as a support to the constituent parts of the arch during construction, and until each member has taken its proper bearing, and the whole has become one solid mass.
For small arches, and only 4 1/2 inches deep on soffit, a wood turning-piece is used - i.e., a strong piece of wood about 3 inches wide is cut to the required curve of the arch, and fixed just below the springing, generally by means of bricks held up by nails driven into the joints of the reveals, as Fig. 899.
For openings of a larger size the centres are formed of two outside pieces, cut to curve, and with small pieces called laggings nailed at right angles to their faces across the top, as Fig. 900, while the bottom is secured in a similar manner, the whole being frequently fixed by bricks in precisely the same manner as last explained, though when of larger sizes they are horsed up by upright supports strutting up, as it were, from the sills or other flat horizontal surfaces below, as shown on the illustration.
The centre itself is raised into its exact position by means of two sedges driven together and placed on the horizontal piece across the support, or on the supports themselves, or bricks at each springing, as at XX on Fig. 901. For rough arches these laggings, of about 3/4 inch x1/2 inch in size, are spaced at about their own width apart; while centres for faced work, and especially for rubbed and gauged arches, must be dose-lagged - i.e., their lagging must butt up to each other and the arrises eased off, to give the line a true unbroken curve.
When the opening is so wide and the rise so high that the curved ribs (to which the laggings are nailed) cannot be cut out of ordinary-sized stuff, the centres have to be built up by lapping and nailing two thicknesses together to make the one rib, as Fig. 901.
Fig. 902 shows the centre for a still larger arch, built up and framed, the radii and cross pieces being halved or dovetailed together, and to the ribs, as shown.
Fig. 903 is a sketch of a framed centre, as seen from a point of view above it.
Foundations, as a rule, should be 3 feet below the general surface of the ground - i.e., the trench excavated to receive the bottom of the concrete, or brick footing, should be 3 feet deep, especially in clayey ground, which is liable to be affected by both heat and rain, the former in particular causing it to crack to a depth even exceeding 3 feet. Of course it will be understood that the nature of the ground varies enormously in different parts of the country, sometimes in different parts of the same town, and even, in some cases, in different streets or gardens, two soils differing in nature often being closely adjacent.
A good foundation should be in solid as opposed to made-up ground; it should be incompressible, and of the same nature throughout, the strati being at right angles to the weight they have to carry - i.e., horizontal, instead of dipping or sloping. When punned with a rammer it should sound "solid," a hollow sound indicating an inferior nature below; and it should resist the pick or crossbar well when driven in, without disintegration of the surrounding mass.
All foundations, of whatever kind, should be made perfectly level (except where they are stepped) before they receive the concrete or footings, ground of a stony or rocky nature being levelled with the pick, and the softer kinds well rammed. The solid ground should always be reached; and if there are great variations, etc., arches should be thrown over weak places, when at a great depth, from piers built on the solid. If a solid stratum be reached, with an inferior one below, it is unadvisable to go any deeper, if that solid stratum is sufficiently strong to carry the weight required.
Where the nature of the ground will not allow of the building being built on it with safety, concrete, of specified and adequate depth and strength, is employed in the trench to assist the natural ground by distributing the weight.