CENTERS FOR ARCHES 362. The centers for stone, plain concrete, and reinforced concrete arches are constructed in a similar manner. A reinforced concrete arch of the same span and designed for the same loading, will not be so heavy as a plain concrete or stone arch, and the centers need not be constructed so strong as for the other types of arches. One essential difference in the centering for stone arches and that for concrete or reinforced-concrete arches, is that centering for the latter types of arches serves as a mould for shaping the soffit of the arch ring, the face of the arch-ring, and the spandrel walls.
The successful construction of arches depends nearly as much on the centers and their supports as it does on the design of the arch. The centers should be as well constructed and the supports as un yielding as it is possible to make them. When it is necessary to use piles, they should be as well driven as permanent foundation piles, and the load should not generally be heavier than that on permanent piles.
363. Classes of Centers. There are two general classes of centers—those which act as a truss; and those in which the support, at the intersection of braces, rests on a pile or footing. Trusses are used when it is necessary to span a stream or roadway. Sometimes the length of the span for the centering is very short, or there are a series of short spans, or the span may be equal to that of the arch. The trusses must be carefully designed, so that the deflection and deformation due to the changes in the loading will be reduced to o. minimum. By placing a temporary load on the centers at the crown, the deformation during construction may be very greatly reduced. This load is removed as the weight of the arches comes on the centers. For the design of trusses, the reader is referred to instruction papers or other treatises on Bridge Engineering and Roof Trusses.
The lagging for concrete arches usually consists of 2 by 3-inch or 2 by 4-inch plank, either set on edge or laid flat, depending on the thickness of the arch and spacing of the supports. The surface
on which the concrete is laid is usually surfaced on the side on which the concrete is to he placed. The lagging is very often supported on ribs constructed of 2 by 12-inch plank, on the back of which is placed a 2-inch plank cut to a curve parallel with the intrados.
These 2 by 12-inch planks are set on the timber used to cap the piles, and are usually spaced about 2 feet apart. All the sup ports should be well braced. The centers should be constructed to give a camber to the arch about equal to the deflection of the arch when under full load. It is therefore necessary to make an allowance for the settlement of centering, for the deflection of the arch after the removal of the center ing, and for permanent camber.
The centers should be constructed so that they can be easily taken down. To facilitate the striking of centers, they are usually supported on folding wedges or sand-boxes. When the latter method is used, the sand should be fine, clean, and perfectly dry, and the boxes should be sealed around the plunger with cement mortar. • Striking forms by means of wedges is the commoner method. In Fig. 169, a shows the type of wedges generally used, although some To find the safe load for beams of hemlock from the above table. the above values must be divided by 2; for beams of short-leaf yellow pine and white oak, the values must be divided by 1.2; for white pine, spruce, eastern fir, and chestnut, the values must be divided by 1.71.
times three wedges are used, as shown by b in the same figure. They are from one to two feet long, 6 to 8 inches wide, and have a slope of from 1 to 6 to 1 to 10. The centering is lowered by driving back the wedges; and to do this slowly, it is necessary that the wedges have a very slight taper. All wedges should be driven equally when the centering is being lowered. The wedges should be made of hard wood, and are placed on top of the vertical supports or on timbers which rest on the supports. The wedges are placed at about the same elevation as the springing line of the arch.