CENTRES are temporary structures of wood, with curved upper surfaces upon which arches are built, and left until they are consolidated and have taken their bearing, after which the centres are removed.
For large arches, such as those of bridges, very elaborate centres are required, with special arrangements for easing and striking them gradually; but in ordinary buildings the centres are very simple; the arches for which they are required being generally of small span and common construction.
Centres for very small and narrow arches may consist simply of a piece of wood cut to the curve of the soffit of the arch, and supported under it by props. Such centres are called " turning pieces."
For longer arches, such as those of tunnels, sewers, etc., the centre is composed of several curved frames or "ribs" RRR, supporting narrow battens nailed across them, and called "laggings"1 lll, Figs. 270, 271.
For stone arches,2 or very rough brick arches, the laggings may be of narrow battens placed an inch or more apart; but for superior brick arches they must be of close boarding smoothed off with an adze, so that the courses may be lined out on the surface.
The feet of the props rest either upon the ground or upon the footings of the walls. When the walls are very high, corbels may be introduced to support the props or struts.
Laggings should be fixed with as few nails as possible, in order to save trouble in removing the centering when done with.
When the arches exceed 3 or 4 feet span the centres are made up of pieces nailed together, as in Fig. 270.
As the span increases the form of the ribs becomes more complicated; up to spans of 20 feet, however, ribs like that shown in Fig. 271, will do very well.
More elaborate centres are never required in ordinary buildings, and it will be unnecessary further to advert to them.
In building all but the very smallest arches arrangements should be made for easing the centre, so as gradually to deprive the arch of its support.
This is done by means of pairs of greased wedges introduced between the heads of the props and the ribs (see WW, Figs. 270, 271).
After the arch has been turned, and the haunches filled in, the points of these wedges are lightly struck so as to drive them outwards from the rib under which they are placed, thus lowering the centre a very little; this causes the whole of the arch to settle slightly and uniformly and to take its bearing, the mortar being compressed in the joints.
1 Sc. cleading.
2 Frequently in arches of large stones there are only one or two pieces of lagging under each stone.
The arch is then left until the mortar has set, after which the centres are removed altogether.
Some engineers defer the easing of the centres for a day or two after the arch is built. When an arch is built of very soft stone or bricks it is better not to ease the centering, as the pressure on the edges of the voussoirs is apt to chip them.
In centres for very important stone arches, wedges or screws are frequently placed under each " lagging " separately, so that the work may be eased, course by course, and the support replaced if the settlement is too rapid.
Arrangements are sometimes adopted for easing all the wedges at the same time, so that the whole arch may settle uniformly.
These depend in practice a good deal on the rough stuff available.
For a brick arch 2' 3" thick and 20' span, a centre such as that in Fig. 271 would have the king-post, struts, and portions of ribs, cut out of 2" stuff. The laggings would be of 1 1/2" stuff for ribs 3' apart or 2" stuff for ribs 4' or 5' apart.
The laggings should be only 3 or 4 inches wide, laid close, the joints being run in with whiting and plaster of Paris. The centering should not be disturbed for a fortnight, or until the concrete has set.1
1 R. E. Aide Memoire.