530. Iron Arches. Cast iron arches may be used for the same objects as those of timber. The frames for these purposes consist of several parallel ribs of uniform dimensions which are cast into an arch form, the ribs being connected by horizontal ties, and stiffened by diagonal braces. The weight of the superstructure is transmitted to the curved ribs in a variety of ways; most usually by an open cast iron beam, the lower part of which is so shaped as to rest upon the curved rib, and the upper part suitably formed for the object in view. These beams are also connected by ties, and stiffened by diagonal braces.

Each rib, except for narrow spans, is composed of several pieces, or segments, between each pair of which there is a joint in the direction of the radius of curvature. The forms and dimensions of the segments are uniform. The segments are usually either solid, (Fig. 94,) or open plates of uniform thickness, having

a flanch of uniform breadth and depth at each end, and on the entrados and intrados. The flanch serves both to give strength to the segment and to form the connection between the segments and the parts which rest upon the rib.

The ribs are connected by tie plates which are inserted between the joints of the segments, and are fastened to the segments by iron screw bolts which pass through the end flanches of the segments and the tie plate between them. The tie plates may be either open, or solid; the former being usually preferred on account of their superior lightness and cheapness.

The frame work of the ribs is stiffened by diagonal pieces which are connected either with the ribs, or the tie plates. The diagonal braces are cast in one piece, the arms being ribbed, or

feathered, and tapering from the centre towards the ends in a suitable manner to give lightness combined with strength.

The open beams (Fig. 94) which rest upon the curved ribs are cast in a suitable number of panels; the joint between each pair being either in the direction of the radii of the arch, or else vertical. These pieces are also cast with flanches, by which they are connected together and with the other parts of the frame. The beams, like the ribs, are tied together and stiffened by ties and diagonal braces.

Beams of suitable forms for the purposes of the structure are placed either lengthwise, or crosswise upon the open beams.

531. Curved ribs of a tubular form have, within a few years back, been tried with success, and bid fair to supersede the ordinary plate rib, as with the same amount of metal they combine nore strength than the flat rib.

The application of tubular ribs was first made in the United

Fig. 95-Represents a side view A, and a cross section and end view B through a saddle piece of the tubular arch of Major Delafield.

a, a, (Fig. A) a side view, and (Fig. B) an end view of the elliptical flanches of the end of each segment.

b, b, shoulders, or ribs to strengthen the flanches against lateral strains.

c, tie plate between the ribs.

f (Fig. B) side view of the rim of the tie-plate fitted to the interior of the tube

d, d, (Figs. A and B) saddle pieces to receive the open beams of a form similar to Fig. 94, which rest on the tubular ribs.

e, cross section of the rib through the saddle piece.

States by Major Delafield of the U. S. Corps of Engineers, in an arch for a bridge of 80 feet span. Each rib was formed of nine segments; each segment (Fig. 95) being cast in one piece, the cross section of which is an elliptical ring of uniform thickness, the transverse axis of the ellipse being in the direction of the radius of curvature of the rib. A broad elliptical flanch with ribs, or stays, is cast on each end of the segment, to connect the parts with each other; and three chairs, or saddle pieces, with grooves in them, are cast upon the entrados of each segment, and at equal intervals apart, to receive the open beam which rests on the curved rib.

The ribs are connected by an open tie plate, (Fig. 95.) Raised elliptical projections are cast on each face of the tie plate, where it is connected with the segments, which are adjusted accurately to the interior surface of each pair of segments, between which the tie plate is embraced. The segments and plate are fastened by screw bolts passed through the end flanches of the segments.

The tie plates form the only connection between the curved ribs; the broad ribbed flanches of the segments, and the raised rims of the tie plates inserted into the ends of the tubes, giving all the advantages and stiffness of diagonal pieces.

532. Tubular ribs with an elliptical cross section have been used in France for many of their bridges. They were first introduced but a few years back by M. Polonceau, after whose designs

A

a

B

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Fig. 96-Represents a side view A and a cross section and end view B through a joint of M. Polonceau's tubular arch.

a, a, top flanch, b, b bottom flanch of the semi-segments united along the vertical joint cd through the axis of the rib.

gh, side view of the joint between the flanches e, e of two semi-segments.

m, inner side of the flanches.

o, cross section of a semi-segment and top and bottom flanches.

ff, thin wedges of wrought iron placed between the end flanches of the semi-segments. to bring the parts to their proper bearing.

the greater part of these structures have been built. According to M. Polonceau's plan, each rib consists of two symmetrical parts divided lengthwise by a vertical joint. Each half of the rib is composed of a number of segments so distributed as to break joints, in order that when the segments are put together there shall be no continuous cross joint through the ribs.

The segments (Fig. 96) are cast with a top and bottom flanch and one also at each end. The halves of the rib are connected by bolts through the upper and lower flanches, and the segments by bolts through the end flanches.

For the purposes of adjusting the segments and bringing the rib to a suitable degree of tension, flat pieces of wrought iron of a wedge shape are driven into the joints between the segments, and are confined in the joints by the bolts which fasten the segments and which also pass through these wedges.

To connect the ribs with each other, iron tubular pieces are

placed between them, the ends of the tubes being suitably adjusted to the sides of the ribs. Wrought iron rods which serve as ties pass through the tubes and ribs, being arranged with screws and nuts to draw the ribs firmly against the tubular pieces. Diagonal pieces of a suitable form are placed between the ribs to give them the requisite degree of stiffness.

In the bridges constructed by Mr. Polonceau according to this plan, he supports the longitudinal beams of the roadway by cast iron rings which are fastened to the ribs and to each other, and bear a chair of a suitable form to receive the beams.

533. Iron roof Trusses. Frames of iron for roofs have been made either entirely of wrought iron, or of a combination of wrought and cast iron, or of these two last materials combined with timber. The combinations for the trusses of roofs of iron are in all respects the same as in those for timber trusses. The parts of the truss subjected to a cross strain, or to one of com

a, a, feathered struts of cast iron.

b, b, suspension bars in pairs.

m, n, tie and straining bars.

e, e and f, f, cross sections of beams resting in the cast iron sockets connected with the suspension bars.