queen posts, between them,-of a single top beam, termed the plate of the side frame, which rests upon the uprights, with which it is

connected by a mortise and tenon joint,—and of diagonal braces and other smaller braces, termed check braces, placed between the uprights. The curved-built beam, termed the arch-timbers, is bolted upon the timbers of the open-built beam. The bridgeframe may consist of two or more ribs, which are connected and stiffened by cross ties and diagonal braces. The roadway-flooring (Fig. 135) is laid upon cross pieces, termed the floor girders, which may either rest upon the chords, or else be attached at any intermediate point between them and the top beam. The roadway and footpaths may be placed in any position between the

several ribs.

There is great similarity between the combination adopted by Burr and those of the two bridge-frames just described. The main difference consists in the application by Burr of what he terms the arch-timbers, to strengthen and stiffen an open-built beam. It may be remarked from the Figs. 134, 135, that the framing of the open-built beam is faulty, in that the top beam, or plate, is not only of less dimensions than the bottom beam, or chord, but is weakened by mortises, and moreover affords no other support to the queen posts, or uprights, which act as suspension pieces for the chord, than that of the pin which confines the tenon in the mortise. From the manner in which the archtimbers are formed and connected with the parts of the open-built

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beam, they add but little if any more strength and stiffness than would be given by straight timbers reaching from the springing point of the arch timbers to their crown; and they are certainly

B

Fig. 135-Represents the half of a cross section of Fig. 134 through the crown of the arch timbers, in which the letters designate the same parts as in the preceding Fig. g, rafters of roof truss.

h, h, diagonal braces of bridge frame. B, side view of the pier.

less efficacious in subserving their end than would be inclined struts, occupying a like position at bottom, and abutting against a straining beam, placed either under the centre part of the chord, where the locality would permit it, or under the centre portion of the plate. In localities where fine timber is less abundant than in those in which the most of Burr's bridges have been built, a judicious regard to economy would undoubtedly have suggested a selection of forms for the secondary parts of the frame, which would have prevented these parts from being as much cut to waste as the Figs. show they must have been in the example taken to illustrate this system.

(I) This structure, constructed under the superintendence of Moncure Robinson, Esq., is upon Town's plan. The width of the bays varies from 130 to 153 feet. It consists of two ribs, each of which is formed of a double lattice, with two chords at bottom and one at top. The roadway, for rails, rests on the top girders. The ribs are braced by vertical diagonal braces, and by horizontal diagonal braces between each pair of the top and bottom girders. The piers are of rustic work; they are 40 feet above the low-water level, and 4 feet thick at top. The example here selected for illustration (Fig. 136) is taken from another bridge, of nearly the same width of bay, erected subsequently to the Richmond bridge, by the same engineer, in which the top

chord also is doubled, as the former bridge was found to be rather weak.

(K) This bridge is constructed on Howe's plan. It consists (Fig. 137) of two ribs which are connected at top and bottom, in the usual manner, with cross ties and diagonal braces. The roadway-flooring rests upon the cross girders at bottom. The bridge is not roofed, as is usually the case, the ribs being covered in on the sides and at top by a sheathing of boards, and the flooring-boards by a metallic covering.

The bridges constructed according to Colonel Long's plan have been mostly applied to medium spans. In the printed description of the different improvements of this system patented by Colonel Long, he very judiciously introduces struts, which he terms arch braces, either below the top or the bottom string, as the locality may demand, for the purpose of preventing sagging, which must necessarily take place in time in all open-built beams of considerable span, if not strengthened in this way.

Fig. 137-Represents a side view of the truss and an end view of the pier, M; a cross section of the truss and side view of the pier, N; and a top view O, of the pier of the railroad bridge at Springfield.

A, inclined plane of the ice-breaker of the up-stream starling.

a, a, iron side-stays of the ribs anchored into the piers at top.

606. Bridges of cast iron admit of even greater boldness of design than those of timber, owing to the superiority, both in strength and durability, of the former over the latter material; and they may therefore be resorted to under circumstances very nearly the same in which a wooden structure would be suitable. 607. The abutments and piers of cast-iron bridges should be built of stone, as the corrosive action of salt water, or even of fresh water when impure, would in time render iron supports of this character insecure; and timber, when exposed to the same destructive agents, is still less durable than cast iron.

The forms and dimensions of the stone abutments and piers are regulated on the same principles as the like parts in wooden bridges with curved frames. The piers may be either built up high enough to receive the roadway-bearers, or else they may terminated just above the springing plates of the bridge-frame,

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and form supports for cast-iron standards upon which the roadwaybearers may be laid.

608. The curved ribs of cast-iron bridge-frames have undergone various modifications and improvements. In the earlier bridges, they were formed of several concentric arcs, or curved beams, placed at some distance asunder, and united by radial pieces; the spandrels being filled either by contiguous rings, or by vertical pieces of cast iron upon which the roadway-bearers were laid.

In the next stage of progress towards improvement, the curved ribs were made less deep, and were each formed of several segments, or panels cast separately in one piece, each panel consisting of three concentric arcs connected by radial pieces, and having flanches, with other suitable arrangements, for connecting them firmly by wrought-iron keys, screw-bolts, &c.; the entire rib thus presenting the appearance of three concentric arcs connected by radial pieces. The spandrels were filled either with panels formed like those of the curved ribs, with iron rings, or with a lozenge-shaped reticulated combination. The ribs were connected by cast-iron plates and wrought-iron diagonal ties.

In the more recent structures, the ribs have been composed of voussoir-shaped panels, each formed of a solid thin plate with flanches around the edges; or else of a curved tubular rib, formed like those of Polonceau, or of Delafield, described under the head of Framing. The spandrel-filling is either a reticulated combination, or one of contiguous iron rings. The ribs are usually united by cast-iron tie-plates, and braced by diagonal ties of cast and wrought iron.

609. The roadway-bearers and flooring may be formed either of timber, or of cast iron. In the more recent structures in England, they have been made of the latter material; the roadwaybearers being cast of a suitable form for strength, and for their connection with the ribs; and the flooring-plates being of cast iron.

The roadway and footpaths, formed in the usual manner, rest upon the flooring-plates.

The parapet consists, in most cases, of a light combination of cast or wrought iron, in keeping with the general style of the

structure.

610. The English engineers have taken the lead in this branch of architecture, and, in their more recent structures, have carried it to a high degree of mechanical perfection and architectural elegance. Among the more celebrated cast-iron bridges in England, that of Coalbrookdale belongs to the first epoch above mentioned; those of Staines and Sunderland to the second; and to the third, the bridge of Southwark at London; that of Tewkes