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.
with pieces of timber cut to the required curve, spiked to the tops of them, as shown on the right-hand side of Fig. 239, which shows a good type of centre for arches with spans of from 12 to 20 feet. The ribs are built of 12 by 2-inch boards dogged together. The tie is formed in two 7 by 1-inch timbers, notched and bolted at the ends to the lower extremities of the ribs, as shown at A. The upper ends of the braces are tenoned into the rib at the junctions of the sections and secured by dogs, as shown at B, while the lower ends are fitted and bolted between the ties.
Fig. 240 shows a very stiff type of centre, suitable for arches up to 25 feet in span, in which the load from the haunches is carried to the foot of the central or king post by means of the struts SS, while the load on the crown is carried to the ends of the tie. Should the haunches tend to rise they are prevented from doing so by means of the braces T and U, while any tendency for the crown to lift is checked partly by the brace U and the corresponding member on the other side of the centre, and partly by the bolts securing it to the head of the king post.
Fig. 241 shows a type of centre which may be used for spans up to 40 feet, when the space beneath the arch is required while the building operation is in process. Here the load on the crown is carried directly to the supports, while the haunches are tied together and each joint supported by braces.
Having designed the centres, they should be completely set out, full size, in chalk lines upon a floor or other suitable level surface. The timbers from which the sections of the ribs are to be cut are laid over the chalk diagram, and the joints are marked upon them. They are then taken up and the joints are cut, and the sections fitted and dogged together over the diagram. The curved edge is now marked upon them with a radius rod. The sections are then taken up again and cut to the curve and once more fitted into position, being kept there by driving spikes into the floor against their edges. The timbers for forming the ties and braces are then laid down in position, and marked for the necessary shoulders, notches, and bolts. These are then cut, and the whole centre fitted together to see that every member fits properly. All the joints should then be marked with some distinguishing mark to facilitate the refixing in the required position.
Small centres may be fixed together in the carpenter's shop, but large centres should always be built up in situ, as slinging the complete centres into position frequently strains the joints or distorts the curve of the ribs.
Supports for centres should be placed immediately under those members which have most of the strain - that is to say, they should be placed at the ends of the ribs, and under the junction of the braces and tie.
Centres are usually supported on upright timbers varying from 4 by 2 inches to 12 by 12 inches, according to the size of the arch. The lower ends of these timbers rest upon sleepers so as to distribute the load, and are secured by means of dogs to prevent them from being knocked out of position. When the space under an arch is not required while the building operations are in progress, economy may usually be effected by supporting each rib at frequent intervals across the span by upright supports; but should the space be required the supports of the central portions of the centre should be inclined or a truss of the kind shown in Fig. 241 should be used. It is a bad plan, however, to use inclined supports, as they are apt to work loose, and thus cause the centre to sag.
When the arch is deep, as in a barrel vault or an arch carrying a road or footpath, the ribs should be placed at intervals of from 3 to 4 feet apart, being strutted transversely if necessary, and each rib being supported by uprights. Struts are placed between the uprights to stiffen them. A horizontal timber is secured to the head of each row of uprights, and above this a second horizontal timber is placed to support the ribs of the centre, wedges being inserted between these horizontal timbers to facilitate adjustment and easing on the completion of the arch.
When a series of arches has to be constructed the supports of the centres should be carried upon the footings of the abutments, otherwise there is a tendency for the abutments to settle unevenly when the centres are removed.
In setting up the centres of deep arches the two end ribs should be set up first, and most accurately adjusted by means of the wedges, and the curve tested by means of a radius rod. The intermediate ribs are then placed in position and adjusted to a straight edge laid across the end ribs. If the curve of the intermediate ribs be untrue it can be adjusted by notching the lagging pieces over them more or less, as the case may require.
Additional stiffness can be given to the centre by means of transverse braces.
Screw-jacks are sometimes used instead of wedges for easing the centres.
When the arches are complete and the mortar set the centres should be eased gradually and evenly, so that the weight is not suddenly brought upon the abutments. The wedges should be planed and oiled or blackleaded, so that they slip easily, blocks being nailed against them to prevent them from slipping right out.
When large projecting mouldings occur at the springings the centre must be so designed so that the tie comes high enough to permit the centre to be eased and removed, the lower courses of the arch being supported upon tailing pieces, as in Fig. 240. These are rarely necessary in stone arches, as no weight is brought upon the centres until the angle of the bed joints of the voussoirs with the horizon exceeds that at which they would begin to slide - about 30 degrees, while with brick voussoirs the tail pieces form a mould for the soffit of the arch.