serve as intermediate points of support for the framing on which the roadway rests in the one case, and the roof covering in the other. In bridges the roadway may lie either above the arch, or below it; in either case vertical posts, iron rods, or bridles connect the horizontal beams with the arch.

511. The greatest strain in wooden arches takes place between the crown and springing line; this part should, therefore, when practicable, be relieved of the pressure that it would directly receive from the beams above it by inclined struts, so arranged as to throw this pressure upon the lateral supports of the arch.

The pieces which compose a wooden arch may be bent into any curve. The one, however, usually adopted is an arc of a circle, as the most simple for the mechanical construction of the framing, and presenting all desirable strength.

512. Centres. The wooden frame with which the voussoirs of an arch are supported while the arch is in progress of construc

tion is termed a centre.

A centre, like the frame of a roof, consists of a number of vertical frames (Figs. 80, 81, 82, 83) termed trusses, or ribs, upon which horizontal beams, termed bolsters, are placed to receive the voussoirs of the arch.

The curved, or back pieces of a centre on which the bolsters rest consist of beams cut into suitable lengths and shaped to the proper curvature; these pieces abut end to end, the joints between them being in the direction of the radii of curvature; the joints are usually secured by short pieces, or blocks placed under the abutting ends to which the back pieces are bolted. The blocks form abutting surfaces for shores, or inclined struts seated against firm points of support below the back pieces. To prevent the shores, or the struts from bending, braces, or bridles, which are usually formed of two pieces, each with shallow notches cut into them, are added, and embrace between them the shores, or struts, the whole being firmly connected with iron bolts.

The combinations used for the frames of centres will depend upon the position of the points of support and the size of the arches.

a

Fig. 80-Represents the rib of a centre

for light arches.

a, a, rib formed as in Fig. 79.

b, b, bolster pieces which receive the

masonry.

513. For small light arches (Fig. 80) the ribs may be formed

of two or more thicknesses of short boards, firmly nailed together; the boards in each course abutting end to end by a joint in the direction of the radius of curvature of the arch, and breaking joints with those of the other course. The ribs are shaped to the form of the intrados of the arch, to receive the bolsters, which are of battens cut to suitable lengths and nailed to the ribs.

514. For heavy arches with wide spans, when firm intermediate points of support can be procured between the abutments, the back pieces (Fig. 81) may be supported by shores placed

un

Fig. 81-Represents the rib of a centre with intermediate points of support.

a, back pieces of the rib
which receive the bol-
sters f.

b, b, struts which sup-
port the back pieces.
e, e, braces.

c, solid beam resting on
the intermediate sup-
ports d, d which re-
ceive the ends of the
struts b, b.

under the blocks in the direction of the radii of curvature of the arch, or of inclined struts (Fig. 82) resting on the points of sup

A, A, intermediate points of support.

a, a, a, struts resting upon cast iron sockets on the supports A.

b, b, two courses of plank each 42 inches thick bent over the struts a, a to the form of the arch, the courses breaking joints.

c, c, folding wedges laid upon the back pieces b of each rib to receive the bolsters on which the voussoirs are laid.

port. The shores, or struts, are prevented from bending by braces suitably placed for the purpose.

515. If intermediate points of support cannot be obtained, a broad framed support must be made at each abutment to receive the extremities of the struts that sustain the back pieces. The framed support (Fig. 83) consists of a heavy beam laid either

Fig. 83-Represents a part of a rib of Waterloo Bridge over the Thames. a, a, and b, three heavy beams, forming the striking plates, which with the shores h, h form the framed support for the struts of the centre.

c, c, struts abutting against the blocks g, g placed under the joints of the back pieces ff. d, d, bridle or radial pieces in pairs which are confined at top and bottom between the horizontal ties n, n of the ribs, also in pairs.

e, e, cast iron sockets.

m, m, bolsters of the centre resting on the back pieces f.

horizontally, or inclined, and is placed at that joint of the arch, (the one which makes an angle of about 30° with the horizon,) where the voussoirs, if unsupported beneath, would slide on their beds. This beam is borne by shores which find firm points of support on the foundations of the abutment.

The back pieces of the centre (Fig. 83) may be supported by inclined struts which rest immediately upon the framed support, one of the two struts under each block resting upon one of the framed supports, the other on the one on the opposite side, the two struts being so placed as to make equal angles with the radius of curvature of the arch drawn through the middle point of the block. Bridle pieces, placed in the direction of the radius of curvature, embrace the blocks and struts in the usual manner, and prevent

the latter from sagging. This combination presents a figure of invariable form, as the strain at any one point is received by the struts and transmitted directly to the fixed points of support. It has the disadvantage of requiring beams of great length when the span of the arch is considerable, and of presenting frequent crossing of the struts where notches will be requisite, and the strength of the beams thereby diminished.

The centre of Waterloo Bridge over the Thames (Fig. 83) was framed on this principle. To avoid the inconveniences resulting from the crossing of the struts, and of building beams of sufficient length where the struts could not be procured from a single beam, the device was imagined in this work of receiving the ends of several struts at the points of crossing into a large cast-iron socket suspended by a bridle piece.

516. When the preceding combination cannot be employed, a strong truss, (Fig. 84,) consisting of two inclined struts resting

upon the framed supports, and abutting at top against a straining
beam, may
be formed to receive the ends of some of the struts
which support the back pieces. This combination, and all of a
like character, require that the arch should not be constructed
more rapidly on one side of the centre than on the other, as any
inequality of strain on the two halves of the centre would have a
tendency to change the shape of the frame, thrusting it in the
direction of the greater strain.

517. Means used for striking Centres. When the arch is completed the centres are détached from it, or struck. To effect this in large centres an arrangement of wedge blocks is used, termed the striking plates, by means of which the centre may be gradually lowered and detached from the soffit of the arch. This arrangement consists (Fig. 83) in forming steps upon the upper surface of the beam which forms the framed support to receive a wedge-shaped block, on which another beam, having its under surface also arranged with steps, rests. The struts of the rib either abut against the upper surface of the top beam, or else are inserted into cast-iron sockets, termed shoe-plates, fastened to

1

this surface. The centre is struck by driving back the wedge block.

518. When the struts rest upon intermediate supports between the abutments, double, or folding wedges may be placed under the struts, or else upon the back pieces of the ribs under each bolster. The latter arrangement presents the advantage of allowing any part of the centre to be eased from the soffit, instead of detaching the whole at once as in the other methods of striking wedges. This method was employed for the centres of Grosvenor Bridge, (Fig. 82,) over the river Dee at Chester, and was perfectly successful both in allowing a gradual settling of the arch at various points, and in the operation of striking.

519. Ties and Braces for detached Frames. When a series of frames concur to one end, as, for example, the main beams of a bridge, the trusses of a roof, ribs of a centre, &c., they require to be tied together and stiffened by other beams to prevent any displacement, and warping of the frames. For this purpose beams are placed in a horizontal position and notched upon each frame at suitable points to connect the whole together; while others are placed crossing each other, in a diagonal direction, between each pair of frames, with which they are united by suitable joints, to stiffen the frames and prevent them from yielding to any lateral effort. Both the ties and the diagonal braces may be either of single beams, or of beams in pairs, so arranged as to embrace between them the parts of the frames with which they are connected.

520. Joints. The form and arrangement of joints will depend upon the relative position of the beams joined, and the object of the joint.

Joints may be required for various purposes, either to connect the ends of beams of which the axes are in the same right line, or make an angle between them; or the end of one beam with the face of another; or where the face of one beam rests upon that of another.

In all arrangements of joints, the axes of the beams connected should lie in the same plane in which the strain upon the frame acts; and the combination should be so arranged that the parts will accurately fit when the frame is put together, and that any portion may be displaced without disconnecting the rest. The simplest forms most suitable to the object in view will usually be found to be the best, as offering the most facility in obtaining an accurate fit of the parts.

In adjusting the surfaces of the joints, an allowance should be made for any settling in the frame which may arise either from the shrinking of the timber in seasoning while in the frame, or from the fibres yielding to the action of the strain. This is done