Fig 86 is a perspective view of a part of one panel of the Howe truss, and shows quite clearly how the parts are arranged. For an analysis of these structures, see Wood's Treatise on Bridges and Roofs.

bridge in this country. The bridge-frame (Fig. 155) consisted

B

Fig. 155-Represents a side view of a portion of the open-curved rib of the

bridge over the Schuylkill at Philadelphia.

A, lower curved built beam.

B, top beam.

a, a, posts

c, c, diagonal braces,

o, o, iron diagonal ties.

m, m, iron stays anchored in the abutment C.

of five ribs. Each rib is an open-built beam formed of a bottom curved solid-built beam and of a single top beam, which are connected by radial pieces, diagonal braces, and inclined iron stays. The bottom curved beam is composed of three concentric solid-built beams, slightly separated from each other, each of which has seven courses of curved scantling in it, each course 6 inches thick by 13 inches in breadth; the courses, as well as the concentric beams, being firmly united by iron bolts, &c. A roadway that rests upon the bottom curved ribs is left on each side of the centre rib, and a footpath between each of the two exterior ribs. The bridge was covered in by a roof and a sheathing on the sides.

611. Burr's Truss.-Burr's plan, which (Fig. 156) consists

in forming each rib of an open-built beam of straight timber, and connecting with it a curved solid-built. beam formed of two or more thicknesses of scantling, between which the

framework of the open-built beam is clamped. The openbuilt beam consists of a horizontal bottom beam of two thicknesses of scantling, termed the chords, between which are secured the uprights, termed the queen posts,-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.

Fig. 157.

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The curved-built beam, termed the arch-timbers, is bolted upon the timbers of the open-built beam. The bridge-frame

may consist of two or more ribs, which are connected and stiffened by cross ties and diagonal braces. The roadwayflooring 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.

612. Pratt's Truss. This truss (Fig. 157) has the same general form as Howe's, but differs in its details. The verticals here are wooden posts instead of iron rods, and the diagonals are iron ties instead of wooden braces.

613. McCallum's Truss. This truss (Fig. 158) is a modi

Fig. 158.

fication of Howe's, the essential difference of which consists in a curved upper chord instead of a horizontal one. The long braces at the end-called arch braces,—are not essential to this system. This system is stiffer than similar ones having horizontal chords.

614. A simple but effective structure, shown in Fig. 159,

Fig. 159.

has been in use for some time on the N. Y. State canals for common road bridges, and for crossings on farms. There are no counter-braces, which, as may readily be shown, are unnecessary for short spans. (See Wood's Treatise on Bridges and Roofs, pp. 120 and 121.) The lower timber may be spliced, or in any other manner made continuous throughout. Another timber, which is placed on this, extends over two or four of the central bays. The, verticals, which are iron rods, are made divergent, as shown in Fig. 159a.

615. Wooden Arches. A wooden arch may be formed by bending a single beam (Fig. 160) and confining its extremi

ties to prevent it from resuming its original shape. A beam in this state presents greater resistance to a cross strain than when straight, and may be used with advantage where great stiffness is required, provided the points of support are of sufficient strength to resist the lateral thrust of the beam. This method can be resorted to only in narrow bearings.

For wide arches a curved-built beam must be adopted; and for this purpose a solid (Figs. 161 and 162) or an open-built beam may be used, depending on the bearing to be spanned by the arch. In either case the curved beams are built in

the same manner as straight beams, the pieces of which they are formed being suitably bent to conform to the curvature of the arch, which may be done either by steaming the pieces, by mechanical power, or by the usual method of softening the woody fibres by keeping the pieces wet while subjected to the heat of a light blaze.

Fig, 161.

g

Fig. 161-Represents a wooden arch A, formed of a solid-built beam of three
courses, which support the beams c, c by the posts g, g, which are formed
of pieces in pairs.

b, b, inclined struts to strengthen the arch by relieving it of a part of the
load on the beams c, c.

Fig. 162-Represents a wooden arch of a solid-built beam A, which supports
the horizontal beam B by means of the posts a, a. The arch is let into
the beam B, which acts as a tie to confine its extremities.

616. The number of ribs in the bridge-frame will depend on the general strength required by the object of the structure, and upon the class of frame adopted. In the first class, in which the roadway is usually above the frames, any requisite number of ribs may be used, and they may be placed at equal intervals apart, or else be so placed as to give the best support to the loads which pass over the bridge. In the second class, as the frame usually lies entirely, or projects partly above the roadway, &c., if more than two ribs are required, they are so arranged that one or two, as circumstances may demand, form each head of the bridge, and one or two more are placed midway between the heads, so as to leave a sufficient width of roadway between the centre and adjacent ribs. The footpaths are usually, in this case, either placed