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of the truss. For any even number of panels, make a series of odd nunibers, 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 is of the cambre, as above stated. For, i panel truss the three even numbers 2+4+6= 12. Hence the excentricity should be is 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 f to 1 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 dekl, 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 f" 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.
The lower chords, king braces, and sway rods of the endmost panels, connect with cast iron foot pieces Fig. 36.
upon the abutments, as represented in Fig. 36. The portion of lower chord in the end panels, usually
consists of single rods, instead of B N 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.
Cast Iron Uprights are composed of two or more pieces. When of two pieces, they may be connected by flanges and bolts at the centre, where they should have a diameter of about so of the length, and a cross-section determined by the maximum stress, and the power of resistance of the material, as indicated in the table [xcii.]
The upright may taper from the centre to either end to a diameter of 5 to 6 inches, internally. The lower
end is to stand upon Fig. 37.
a properly formed seat (h Fig. 37), upon the connecting block of the lower chord, and may have an opening at the bottom, upon the innerside, where the beam
may enter and rest upon a seat (e), inside of the upright, upon the connecting block. The strength destroyed by this cutting the post should be restored by additional metal in a band or collar (c, Fig. 37), around the opening, and, if necessary, by the wing ilanges d d, extending 6 or 8 inches above the opening. To avoid too much cutting of the post, the flanges of the beams may be reduced to 3 or 31 inches in width. The post and beam seat upon the connecting block may be elevated 3 or 4 inches above the links, as may be required, so as to allow sway rods to pass through with simple screws and nuts for adjustment; thus dispensing with turnbuckles.
Holes should be cast in the central part of the pust, for diagonals to pass obliquely through. Or, what is perhaps better, the connecting bolts may be lengthened so as to permit the insertion of an open box, or frame, between the flanges, as seen at a, Fig. 37. This intermediate piece should be so constructed as to close the ends of the hollow pieces meeting it, and prevent the water from getting inside.
The top end of the upright is forked, with concaves for the connecting pin to rest in, as described in the last section, and as seen at a, Fig. 38. The cap piece of the post may be cast separate, or in connection with the upper half of the column. Both plans have been satisfactorily used. All joints, when practicable, should be accurately fitted by turning or planing.
This plan of a cast iron upright, composed of two principal parts, with or without the centre piece, is perhaps as good as any for general use; the principal disadvantage being the difficulty of giving a sufficient diameter in the middle for stiffness, without two much reducing the thickness of metal, or increasing the amount of cross-section beyond the proper theoretical proportions.
To obviate this difficulty, the device adopted in the original model of the Trapezoidal bridge, was that of using truss-rods, or stiffening rods, to secure the post against lateral deflection, after the mannner shown in Fig. 38.
In the case of using stiffening rods for the uprights, it may be recommended to form each half of the column in two pieces, somewhat in the manner above described for the whole one, without stiffeners; making the piece forming the end portion about fth shorter