connection with the beam, not less than of the distance of upper chord from the beam.

(D

Fr. 35.

Fig. 35 will serve to illustrate the modes of connection for most of the members of a bridge of the kind under consideration. That part of the upright between a and b, is contracted in length. Otherwise, the parts are represented in nearly correct proportions. At c, is represented the connec tion of the upright with the end of the beam, by means of a double eye and bolt, as shown at h. This receives the web of the beam, to which it is secured by the transverse bolt, which should be long enough to receive the eye of a sway rod under both head and nut. The stem of this fixture extends through the upright at its widest part (whence it may taper in both directions), and is secured by a nut upon a screw of about 1" in diameter. The beam should rest with its lower flange upon a small projection cast upon the upright, and not hang upon the connecting fixture.

If so preferred, the sway rods may be connected by a screw and nut cast in the end of the connecting block, as seen at d. This plan has been used, but the connection by the bolt at c is deemed preferable.

The outer and inner flanges of the upright at the top, being increased to nearly an inch in thickness, according to size of bridge, and extending 3 or 4 inches above the web, terminate in semicircular concaves to receive the pin connecting the diagonals with the upper chord. A full view of the flange at the top of

the upright, with the piu resting in the concave, 18

shown at e.

A heavy cross-bar from flange to flange at a, and light cross-bars at intervals of 16 to 18 inches from a to b, serve to support the flauges, and stiffen the piece.

The diagonals are formed with eyes to receive the connecting pin at the upper end, and screws and nuts to connect with the block at the lower chord, in the same manner as in the arch truss.

The main diagonals, those inclining outward from the centre of the truss, should be in pairs, and in size, proportioned to the stress they are liable to, as determined by the process fully described in sections XXXIX, &c.

The links acting in conjunction, horizontally, with the main diagonals, should go on next the end of the connecting block, as that arrangement obviously produces less stress upon the block.

The upper chord, usually formed of hollow cylinders, has openings in the underside at the joints, for uprights and diagonals to enter, where they connect by means of the transverse pin already mentioned. The cylinders should have an extra thickness for 3 or 4 inches from the ends, and a strong collar arcund the opening, to restore the loss of strength occasioned by the opening; and the ends should be squared in a lathe, to secure a perfect joint and a straight chord.

If it be required to give a cambre to the truss, the ends of cylinders should be slighly beveled at the ends, making the under side a trifle shorter. This is easily effected by throwing the end opposite the one being turned, out of centre more or less, according to the cambre required. An 8 panel truss requires an excentricity equal to of the requiredrise in the centre

of the truss. For any even number of panels, make a series of odd numbers, 1, 3, 5, &c., to a number of terms equal to half the number of panels; add the terms of the series, and divide the required cambre by the sum, and the quotient equals the required excentricity to give the proper bevel.

For an odd number of panels, take as many even numbers 2, 4, 6, &c., as equal half the greatest even number of panels; add the terms and divide as before for the excentricity. For illustration, for 8 panels, the four odd numbers 1+3+5+7=16, whence the excentricity should be of the cambre, as above stated. For panel truss the three even numbers 2+4+6=12. Hence the excentricity should be of the cambre.

The reason for this rule will be obvious without more particular demonstration.

At the obtuse angles of the truss, a hollow elbow is inserted (g, Fig. 35), reaching about 10 inches each way from the angular point, at the centre of the connecting pin, with an opening in the under side for upright and diagonals to enter, where they are fastened by a pin or bolt, as at the intermediate joints; the cylinders meeting the elbow, being shortened by as much as the elbow extends from the angle, either way.

The vertical member connecting with the elbow, is exposed to tension only, sustaining a weight equal to the gross panel load of the truss. It may be composed of two wrought iron suspension rods, united in a single eye at the top, and diverging downward to a connection with the beam and connecting block; or, it may be of cast iron, like the intermediates, with wrought iron eye plates, in place of the cast iron flanges with concaves as seen at e. These should be fastened by efficient means to the cast iron part of the upright; which lat

ter should have a cross-section nowhere less than one square inch to each 2,000lbs. of the gross panel load. A complete wrought iron connection from beam to elbow, however, is to be preferred.

The thickness of web and flanges of the uprights, should be from to inch, and the cross-section of upper chord cylinders should be about 20 per C. greater than that of the portion of bottom chord forming the opposite side of the oblique parallelogram included between consecutive main diagonals and included sections of chords; as de kl, Fig. 12.

The upright should be so formed as to bring the centres of upper and lower chords in the same vertical plane.

Sway rods in this class of bridges, should be about " in diameter, with a turn buckle near one end for adjustment, and an eye at each end, for connection with the bolt at c. The screw working in the turn buckle is cut upon the short piece, which should be " larger in diameter than the long piece which has no screw upon it.

FIG. 36.

The lower chords, king braces, and sway rods of the endmost panels, connect with cast iron foot pieces upon the abutments, as represented in Fig. 36. The portion of lower chord in the end panels, usually consists of single rods, instead of links, with an oblong eye at one end to receive the connecting block, and a screw and nut for connection with the foot piece (Fig. 36), at the other end.

This plan of construction will generally yield precedence to the Arch Truss plan, for short spans, except for deck bridges upon rail roads, in which case the

structure will be secured laterally, by x ties, or sway rods between beams, and between king braces at the ends; no x bracing being required between lower chords.

Low trusses constructed in the manner above described, have been used satisfactorily for supporting the outside of wide side walks; answering the purposes of a protection railing at the same time. For this purpose, the uprights are only 5 or 6 feet long, so as to bring the upper chord about 4 feet above the flooring. The first instance of this kind was in the case of the canal bridge on Genesee street in Utica, built 18 or 20 years ago, and repeatedly copied since.

CXX. Bridges from 80 to 100 feet for common roads may be constructed with single canceled trusses, 13 to 14 feet high; in which case the panels will require to be wide (horizontally) in order to avoid an inclination of diagonals too steep for good economy.

But for railroad purposes, the trusses require a depth of about 20 feet to afford sufficient head room under the top connections, unless the beams be suspended below the bottom chords. Hence, the

Double Cancelated Truss

should be adopted for "through bridges" of spans exceeding 70 or 80 feet.

Figures 18 and 20 exhibit in outline, the general character of the double cancelated trapezoidal truss bridge; and, it is only necessary in this place, to de. scribe feasible modes of forming and connecting the various members; which may be done essentially as described in the preceding section, with such modifi cations as follow.