The Problem of Center Construction

The above form of center is frequently employed, partic ularly in tunnels, for spans of 20 to 30 feet, precautions being taken to have the pieces break joints, to secure good bearings at the joints, and to nail or bolt the several segments firmly together. The centers for the 25-foot arch of the Musconetcong (N. J.) tunnel on the Lehigh Valley R. R. consisted of segments of 3-inch plank, 5 feet 8 inches long, 14 inches wide at the center, and 8 inches at the ends, bolted together with four #-inch and four finch bolts each, and 14- by 8 inch pieces of plate-iron over the joints. Where the center was required to support the earth also, a three-ply rib was employed; but in other positions two-ply ribs, spaced 4 to 5 feet apart, were used. Centers of this form have successfully stood in very bad ground in the Musconetcong and other tunnels;* and hence we may infer that they are at least sufficiently strong for any ordinary arch of 30 feet span.

Although not necessary for stability, it is wise to connect the feet of the rib by nailing a narrow board on each side, to prevent the end of the rib from spreading outwards and pressing against the masonry—thus interfering with the striking of the center—and also to prevent deformation in handling it.

Braced Wooden Rib.

For semicircular arches of 15 to 30 feet span, a construction similar to that shown in Fig. 226 (page 669) may be employed. The segments should consist of two thicknesses of 1- or 2-inch plank, according to span, from 8 to 12 inches wide at the middle, according to the length of the segments. The hori zontal chord and the vertical tie may each be made of two thicknesses of the plank from which the segments are made.

For greater rigidity, the rib may be further braced by any of the methods shown in outline in Figs. 207, 208, 209, or by obvious modifications of them. The form to be adopted often depends upon the passageway required under the arch. Fig. 207 is supported by a post under each end; Fig. 208 may be supported at the middle point also; and Fig. 209 may be supported at both middle points as well as at the ends.

Since the arch masonry near the springing does not press upon the center, that portion of the arch may be laid with a template before the center is set up; and hence frequently the center of a semicircular arch does not extend down to the springing line. For examples, see Figs. 222 and 226 (page 667 and 669).

Center frames are usually put together on a temporary platform or the floor of a large room, on which a full-size drawing of the rib is first drawn.

Trussed Center.

When the span is too great to employ any of the centers described above, it becomes necessary to construct trussed centers. It is not necessary here to discuss the principles of trussing, or of finding the stresses in the several pieces, or of deter mining the sections, or of joining the several pieces,—all of which are fully described in treatises on roof and bridge construction. There is a very great variety of methods of constructing such centers. Figs. 210 and 211 show two common, simple, and efficient general forms. For detailed examples, see Fig. 218 (page 663), Fig. 222 (page 667), and Fig. 226 (page 669).