bridge), for eyes of diagonals. These are to ease the lateral strain of the connecting bolt or pin.

The process meeting the vertical, may be rectangu lar in horizontal section, composed of two parallel flat plates, in form as may suit the taste of the designer, united by two irregular plates formed to the profile of the parallel plates. The openings for diagonals, are, of course, through the irregular plates. These are drawn in at the bottom so as to form a square with the parallel sides, large enough to cover the flanges of the 4 segment column selected for the upright. See Fig. 41.

The inside of the square d, is filled in to form a hollow round, about an inch less in diameter than the hollow of the column, that it may have a ring or collar (represented by the inner white ring around d), project ing about 2 inches beyond the shoulder into the wrought iron column.

On the top of the joint piece may be an arrangement of oblique holes for the attachment of lateral × ties, and on the inside, facing the opposite truss, an abutting seat for the cross-stiut, which may be in the form of a 6" I beam, or such other form as may be preferred.

The foot of the post may stand upon a properly formed seat upon the connecting block of the lower chord, with an opening to receive the beam, in the same manner as described for the cast iron post. See Fig. 37.

It will be necessary for diagonals to pass through the centres of uprights, and for that purpose 10 or 12 inches. in length, as may be necessary, may be left out of two opposite segments, and the strength thus lost, restored by additional metal, in such form as may be found convenient and efficient. Or, a cast-iron middle piece may be inserted in the upright.

In the case of an upper chord of rectangular trunks, and uprights of other than a cylindrical form, the joint piece will be correspondingly modified.

The position of diagonals may be reversed, connecting by an eye with a wrought cylindrical connecting pin at the lower chord, and by screw and nut with the joint piece of the upper chord. This involves merely a question of practical economy and convenience.

Sometimes, also, the connection is made by an eye at both ends of the diagonal, depending upon accuracy, as to length, in the manufacture, for the proper adjustment of parts. It is also practicable to provide means of adjustment in the length of vertical members.

CXXIII. But, to enumerate all the changes, and peculiarities of detail admissible in the construction of the Trapezoidal Truss Bridge, even if practicable, could hardly be regarded as expedient in this place. The essential requisities are, to provide material enough of good quality in all parts, to withstand the forces to which they are respectively liable, with efficient connections of parts, by the most direct and simple means, and with such an arrangement and adjustment as may produce the most uniform, degree of strain upon all parts of each member. For instance, ecah section of the lower chord is usually composed of several bars, and it is important that each should sustain its proportionate share of the stress.

In the link chord composed of two links to each panel, if the links be properly fitted, the two sides of each must act very nearly alike, while the connecting block acts as a sort of balance beam to equalize the tension of links acting upon its two ends; and, if the two links of a pair vary slightly in length, the connect

ing block still secures equality of stress upon the two. The same is the case with regard to a chord composed of two eye bars instead of links, to each panel.

But the serious mistake is sometimes committed, of putting the two links or bars upon the same side of those in the succeeding panel, as in Fig 42; where it is obvious that the inside links (a, b, c), are exposed to more action than d, e, f.

FIG 42.

d

b

α

For, if the inside links be 3", and the outside ones 4" from centre of pin, since a and b tend to turn the pin in one direction about its centre, and d and e in the opposite direction, the forces being in equilebirio — the moments (with respect to the centre), of forces tending in one direction, must be equal to those of forces tending in the opposite direction. Hence, representing the stresses of the several links by the letters designating them respectively on the diagram, we have 3 × (a + b) = 4 × (d + e), whence, a + b = { (d + e); showing more stress upon the inside than upon the outside links.

On the contrary, if the link e, be removed to e' upon the inside of a, then d and e' act in one direction, and a and b in the other; and, assuming as before, the inside links to be 3", and the outside ones 4" from centre of pin, we have 4a + 3b = 4d + 3e. But a + d

=

be', and if the force be communicated at the ends, equally upon the two sides of the chord, giving equal stress upon a and d, for instance, the tendency is to an even balanced action throughout the length of chord.

Hence the two links of each panel should always act upon the connecting block or pin, at equal dis tances from centre of pin.

MULTIPLEX CHORDS.

CXXIV. In very long or heavy bridges, the required amount of chord section in the middle portion of the truss, is so great, that it is deemed expedient to introJuce more than two links or eye bars to the panel. This is sometimes done by alternating them upon the connessing pin, increasing the number and sizes according to the increase of stress from panel to panel toward the centre.

This mode of construction, unless the bars be arranged and proportioned with almost impracticable care and Licety, is liable to be attended by an accumulation of lateral strain upon the connecting pin, beyond what it can bear without bending, or springing so much as to materially disturb the equality of stress upon the links, or chord bars.

To illustrate this subject, let Fig. 43, represent one quarter of the chord of a 16 panel bridge. The line CC may denote the central axis of the chord running through the centres of connecting pins; D, at a distance of, say 8" from C, the line in which the diagonals act upon pins, and the other parallel lines at intervals of 3" from D, and from one another (see Figs.

on right hand of diagram), the centres of thickness of links, at which points the action of respective links is supposed to be concentrated upon the pins. Also, let a, b, c, etc., represent the panels of the chords.

Now, if 15W, or 15, represent the stress upon the chord in the two first panels, a and b, that of the succeeding panels to the centre, will be as 22, 34, 44, 52, 58 and 62 (see lower figures in diagram), and the diagonals (producing increments of action upon chord), will have a horizontal action represented by 7 in panel b, by 12 in panel c, and so on by 10, 8, 6, 4. These being added successively to 15, produce the numbers just stated for the chord in the several panels.

The first three panels, a,b and c, require only one link upon each side, as indicated by the oblique black lines. The 4th panel, d, may have 2 links on a side, and the most favorable position for them, as regards action upon connecting pins, will be as shown, diverging from the central axis, so as to bring the end toward the abutment, nearest to the main diagonal connecting with the same pin.

The first pin, connecting a and b, having two equal forces acting in opposition, but at different distances. from the centre line C, we take the moments of these forces with respect to that line, which are, for a, 15×14=210, and forb, 15×11-165. The difference (45) between these moments, equals the moment of the resultant, or the lateral stress of the pin, exerted on a leverage of.

Assuming the value of W, our unit of stress (and always understood as annexed to the figures denoting stress), to represent 5,000lbs. we have for stress of pin in this case, 45 x 5,000+L. The L, being 1" may be omitted in the expression.