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be called the detached, and the latter, the concrete mode of construction.

The detached plan is probably the best adapted to wrought and cast iron bridges, and also, at least, equally adapted to bridges entirely, or essentially constructed of wrought iron, when vertical thrust uprights are employed.

But it can hardly be regarded as advisable to construct iron bridges with independent members, without thrust verticals. For, although as we have seen, [XLVI,] the latter plan shows a trifle less action upon the material than the plan with verticals, the oblique thrust members in the web, are 40 or 50 per cent longer (according to inclination), as well as being in greater number, and sustaining less average action to the piece.

The 7 panel truss, Fig. 12, has 4 compression verticals, liable to an average action of 8w"; while truss Fig. 13, has not less than 6 diagonals, liable to an average compression of 4w"✓2 (when the inclination is 45°), equal to 5.65w". In the mean time, these members being over 40 per cent longer, and sustain ing only about the same aggregate amount of action, can not be so economically proportioned to perform their required labor, when acting independently, as the fewer and shorter uprights.

Still, the Trapezoid with individual members is practicable, probably with about the same economy of material without verticals as with them; and, if it be deemed expedient to adopt the former, the modes of forming and connecting the various parts may be so nearly like those already described for the latter, that particular specifications will not be given in this place.

The essential conditions to be observed, are, besides proportioning the parts to the kind and degree of strain

to which they may be exposed, to see that the forms of diagonals liable to compressive action, be made capable of withstanding such action, according to the table of negative resistances [xcII]; and, that those liable to a change of action from tension to compression, and the contrary, be formed and connected in such manner as to enable them to act in both directions.

CXXVII. In the concrete, or rivet work plan of construction, the Trapezoid without verticals may, it is thought, be generally adopted with advantage. Upon this branch of the subject, however, but little of detail will be attempted at this time, the author having had very little direct practical experience in the premises.

The first point to be attended to, of course, as in all cases of bridge construction, is, to arrange the general outline and proportions of the truss; that is, the number of panels, and depth of truss suitable for the particular case in hand. This being done, the amount and kind of force, whether thrust or tension, to which each part is liable, should be determined; for which purpose, the value of w, and of w' (the variable and constant panel load for the truss), must be assumed, or estimated according to the best data at command; when the stresses of the several parts are readily ob tained by process already explained; [XLIV, &c.].

We are then prepared to assign the requisite crosssection to each part, and to adopt a suitable form of bar, or combination of bars and plates, for each member. Thrust members will usually (if long), be formed of several parts, mostly flat plates, angle iron, T iron, and channel iron, united by riveting in such form of cross-section as may give the largest diameter practi

cable without too much attenuation of the thicknese of material, a point upon which no certain rules can be given.

Flat plates, when connected by riveting at the edges, may be of a width of 30 to 40 times the thickness perhaps, without liability to "buckle" under reasonable compression. When riveted along the centre, a width of 12 to 20 times the thickness, will be in better proportion.

UPPER CHORD.

CXXVIII. A good upper chord may be made in rectangular, or box form, of flat plates and angle iron; or, for small bridges, of channel iron, with flanges either inward or outward, upon the two vertical sides, with flat plates upon upper and under sides; the upper riveted, and the lower one either riveted, or put on with screws, tapped into the lower flanges of the channel bars.

flanges turn inward, may

The upper plate, when project half an inch, or an inch, and the lower one, come even with the sides. The channel bars should meet at the nodes, or connecting points, and a splice plate covering the joint may project below the chord far enough to form a connection with diagonals by riveting. (Fig. 44).

Diagonals acting by tension only, may be plain flat bars of width from 8 to 10 times the thickness. Those acting by thrust principally, may be of T iron with short diagonal bars riveted to the mid rib, (e Fig. 44), giving a width corresponding with that of the upper chord, or with the space between tension diagonals, so that the latter may be riveted to the cross-plate of the Tiron at the crossings, to give lateral support to

the thrust members. Angle iron may also be used instead of T iron, in these members.

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Diagonals acting by both thrust and tension, should be formed and connected with reference to the forces they are liable to.

For small bridges, small plain I bars may be used for thrust diagonals with advantage.

In all cases of tension, rivets should be so arranged when practicable, as to leave all the section available, except the diameter of a single rivet hole; that is, Dc section through two or more holes, including the one farthest from the end, should have less area than

[cxvII, Fig. 31.]

a equa e section through one hole. in Fig. 44, a, a. &c., represent tension diagonals, of plain flat bars, with cross-section proportioned to the stress in each case; b,b-thrust diagonals of T iron and short diagonal plates, as seen at e; c, c, the upper,

ana d, the lower chord; the dotted line j, shows the meeting of lower chord plate, about 4 inches toward the abutment from the poin. cf meeting of the several centres of chord and diagonals The side plates of upper chord may meet at the centre of the node, or connecting point.

The upper splice plates are of irregular form (or, they may be cut on a regular slant from upper to lower angle), but such as to cut without waste of iron. They may be clipped out upon the under side, as by the curved line, or not, as may be preferred.

The lower splice plates may be rectangular, and of such length and width as to admit of a sufficient num. ber of rivets, properly arranged, to be equal in strength to the net section of chord plate and diagonals.

It is scarcely necessary to repeat, that rivet section connecting two thicknesses of plate only, should exceed the net section of plate by as much as the direct tensile strength exceeds the shear-strength of iron.

LOWER CHORD.

CXXIX. The following plan of a flat plate bottom chord adapted to a connection of diagonals by connecting pins, is transcribed from the author's former work; and, by widening the splice plates, as in Fig. 44, is equally adapted to the concrete mode of construction; i. e., by rivet work.

The plan contemplates each half-chord as composed of two courses of plates (except near the ends), spliced alternately, one at each node so as to "break joints." The two half chords are to be placed at such distance apart as to accommodate the connections with diagonais, and with uprights, when used in connection with uprights

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