when the greatest movable load is less than four times the weight of structure, as is usually the case. But instead of dispensing with that member, and other counters on the left, they may be made in two pieces each, of §" or " iron, connecting with the upright at the crossing by screws and nuts, in the manner above described; thus preventing the uprights from deflecting lengthwise of the truss, where the greatest weights act upon them, and where otherwise, they would require to be heavier.

GENERAL TRANSVERSE SUPPORT.

CXXI. The system of cross-struts and diagonal ties serves to preserve the upper chords in line, but does not prevent the whole structure from swaying bodily to the right or left; a result which would be fatal to the structure.

In the arch truss Fig. 27, the width of base at the bearings upon abutments, resulting from the peculiar form of the arch, affords the required stability in this respect.

In case of the trapezoidal truss, when high, various devices have been resorted to for producing the same results. For deck bridges, cross tying between king braces at the ends, is an easy and efficient means of accomplic..ing the object. For through bridges, guys from the cornecting bolt at the elbow of the obtuse angle, anchored in the abutment, may be employed. But this requires extra length of abutments and piers, and the ects of change of temperature, are, to tighten and slacken the guys, so as to impair their efficiency.

To obviate the latter objection, double acting guys (acting by thrust and tension), applied at one side only

of the bridge, have been employed: the effect of temperature being only to very slightly sway the bridge laterally, but not so as to be detrimental to stability. This also, rectires 5 or 6 feet more length of pier, than what ir sary to bear the vertical pres

sure.

Again, the king races have been made with two branches diverging from the elbow to a base of 2 or 3 feet in width, according to height of truss. This plan has been used in a large number of bridges, with satisfactory results. But it contracts to a small degree, the available width of bridge; not, however, so as to produce material inconvenience.

Another device is, the introduction of two or more long beams, extending 5 or 6 feet outside of the trusses, say at the first thrust uprights from the ends (as over Figs. 31, 31, Fig. 18), with guys extending from the connecting bolt at the upper chord, to the ends of said long beams (see g Fig. 38).

Arches may also be introduced at the ends of the bridge, attached to the king braces, say a quarter of the way down from the top, and with the connecting bolt at the elbow. These may be made with a full, or an open-work web, and flanges of 24 or 2 inch angle irou upon both sides of the web, at the top, and around the arch, and either angle iron or plain flat bars, along the sides next the king braces.

A web of" plates placed edge to edge, and battened upon both sides with plates of the same about 4" wide, riveted alternately on each side of the seam, with angle iron, etc., as above, riveted once in 6", forms a stiff and substantial arch for the purpose under consideration, such as have been used effectively in a bridge of 160ft. span.

Moreover, simple arch braces extending from the king brace to a stiff and substantial cross beam from

elbow to elbow

(see Fig. 40), will effect nearly the same result as the arch. In both cases, a considerable degree of lateral stress is liable to be thrown upon the king braces, which accordingly should be strong, or supported by truss rods, and struts

opposite the feet of the arch or braces.

Whether the truss rods be used or not, it is advisable that the connection with the king brace be made by means of a bolt running through the whole diameter of the king brace, with nut or shoulder bearing externally and internally upon both sides, to counteract any tendency to collapse.

Fig. 40 presents an end view of a bridge, showing arch braces, with truss rods to sustain the thrust of arch braces against king braces. The internal figure gives an enlarged view of the connection at the elbow. A strap a (about "x5"), bent twice at right angles, is riveted or bolted to the flanges of an I beam (about 9" deep), leaving a space of about 4 inches from the end of the I beam, for eyes of two sway rods and a nut upon the large connecting bolt. This bolt in large bridges being from 3 to 4 inches in diameter through the elbow, reduced to 2 or 24 inches in the part pro

iecting through the strap above mentioned, and the eyes of sway rods.

The truss rods may not be necessary (with substantial king braces), for spans not exceeding 150 feet. But they will add to the security, in all cases of railroad bridges having cast iron king braces. These members being over twice the length of the cylinders in the upper chord, are usually cast in two pieces, and connected by bolts and flanges in the middle, where they have a diameter of about of the length of brace, and taper to the size of the upper chord at the er.ds

CXXII. WROUGHT IRON THRUST MEMBERS.

The trapezoidal bridge, as described in detail in the preceding section, and as originally intended, is a wrought and cast iron bridge. But it will readily be seen that with slight modification of detail, it is easily adapted to the use of wrought iron upper chord, vertical posts, and main end braces; which latter, for convenience, have been designated in this work, as king braces.

All of these members may be in the form of the patent wrought iron column of the Phoenix Iron Co. of Pennsylvania, formed of flanged segments, united by riveting; or of rectangular wrought iron trunks, as well as various other forms of section.

For the Phoenix column, a cast iron connecting piece may be inserted at the joints of the upper chord, with ends formed to enter the squared ends of the chord cylinders, and receive them against a shoulder of the connecting piece. This piece may have an opening in the under side to receive the diagonale and uprights, where they are secured by a transverse

connecting bolt, in the same manner as at the joint of the cast iron chord cylinders, as before described. In this case the upright may have a cast iron top piece, formed as seen in Figs. 35 and 38 upon the top of cast iron uprights. A separate top piece has sometimes been used with cast iron verticals.

The connecting piece may also be formed as indicated in Fig. 41, with a downward branch like process to meet and receive the squared end of the vertical in the same manner as the horizontal part connects with chord cylinders. In this case the connecting piece must have openings as at bb Fig. 41, for the eyes diagonals to enter.

Fig. 41, shows an inside view of the joint piece, as it would appear it cut vertically and longitudinally, and the near half removed. The horizontal part consists of a cylindrical shell a little thicker than the wrought iron chord cylinder, with ribs upon the outside corresponding with those of the wrought cylinders, and as shown in end view c. Upon the inside, the ring and flanges a a, project inward, leaving usually a space of about 5 inches (according to dimensions of