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Centering for Arches

arch, center, wedges, centers and mortar


No arch becomes self-supporting until keyed up, that is, until the crown or keystone course is laid. Until that time the arch ring, which should be built up simultaneously from both abutments, has to be supported by frames called centers. These consist of a series of ribs placed from 3 to 6 or more feet apart, supported from below. The upper surface of these ribs is cut to the form of the arch, and over these a series of planks called laggings are placed, upon which the arch stones directly rest. The ribs may be of timber or iron. They should be strong and stiff. Any deformation that occurs in the rib will distort the arch, and may even result in its collapse.

Striking the Center. The ends of the ribs or center frames usually rest upon a timber lying parallel to, and near, the springing line of the arch. This tim ber is supported by wedges, preferably of hardwood, rest ing upon a second stick, which is in turn supported by wooden posts, usually one under each end of each rib. The wedges between the two timbers, as above, are used in removing the center after the is completed, and are known as striking wedges. They consist of a pair of folding wedges, 1 to 2 feet long, 6 inches wide, and having a slope of from 1 to 5 to 1 to 10, placed under each end of each rib. It is necessary to remove the cen ters slowly, particularly for large arches; and hence the striking wedges should have a very slight taper, the larger the span the smaller the taper.

The center is lowered by driving back the wedges. To lower the center uniformly the wedges must be driven back uniformly. This is most easily accomplished by making a mark on the side of each pair of wedges before commencing to drive, and then moving each the same amount.

The inclined surfaces of the wedges should be lubricated when the center is set up, so as to facilitate the striking, • Screws may be used instead of wedges for lowering centers.

Sand is also employed for the same purpose. The method fol lowed is to support the center frames by wooden pistons or plungers resting on sand confined in plate-iron cylinders. Near the bottom of each cylinder there is a plug which can be withdrawn and replaced at pleasure, thus regulating the outflow of the sand and the descent of the center.

There is great difference of opinion as to the proper time for striking centers. Some hold that the center should be struck as soon as the arch is completed and the spandrel filling is in place; while others contend that the mortar should be given time to harden. It is probably best to slacken the centers as soon as the keystone course is in place, so as to biing all the joints under pressure. The length of time which should elapse before the centers are finally removed should vary with the kind of mortar employed and also with its amount. In brick and rubble arches a large proportion of the arch ring consists of mortar, and if the center is removed too soon the compression of this mortar might cause a serious or even dangerous deformation- of the arch. Hence the centers of such arches should remain until the mortar has not only set, but has attained a con siderable part of its ultimate strength.

Frequently the centers of bridge arches are not removed for three or four months after the arch is completed, but usually the centers for the arches of tunnels, sewers, and culverts are removed as soon as the arch is turned and, say, half of the spandrel filling is in place.