4. Trusses. A truss is a series of members taking stress in the direction of their length, placed together so as to form a triangle or system of triangles, which, when placed upon supports a certain distance apart, will, in addition to their own weight, sustain certain loads applied at the points where the members intersect. These points are called panel points.
5. Bridge Trusses. A bridge truss is one in which the members that carry the superimposed loads are in the same plane. Usually this plane is vertical.
6. Truss Bridges. A truss bridge is a structure consisting of two or more—usually two—bridge trusses connected by a system of beams called the floor system, which transfer to panel points the load for which the trusses are designed.
7. Girders. These are beams consisting of a wide, thin plate, called a web plate, with shapes, usually angles and narrow, thin plates called flanges, at the top and bottom edges. All are firmly riveted together. (See Part IV, "Steel 8. Girder Bridges. These consist of usually two, sometimes three, girders connected as in the case of truss bridges by a system of beams.
9. Deck Bridges. In cases where the floor system connects the trusses at their tops, the bridge is called a deck bridge, since the traffic moves on a deck, as it were (see Fig. 4).
12. Pony-Truss Bridges. When the height of the trusses of a through bridge is less than the height of the loads that go over them, they are called pony trusses, and the bridge a pony-truss bridge.
13. Lateral Bracing. In all deck bridges, and in all through bridges except pony-truss bridges, the chords which are not con nected by the floor system are connected by a horizontal truss system called the lateral bracing. In all bridges the chords which are con
nected by the floor system are connected by a horizontal truss system, also called the lateral bracing. One of these systems is called the top lateral system, as it connects the top chords; and the other is the bottom lateral system, as it connects the bottom chords (see Fig. 7).
14. Portals. In through bridges, the end-posts of the pair cf trusses are connected by a system of braces in order to preserve the rectangular cross-section of the bridge. This is called the portal bracing (see Fig. 7).
15. Sway Bracing and These serve the same purpose as the portal braces, and are either small struts or systems of cross-bracing placed at the intermediate posts. " The former are called knee-braces, and the latter sway bracing.
16. Floor Systems. In both highway and railway bridges, there are beams running from the intermediate posts or hip ver ticals across to the like members opposite. These are called floor beams. In highway bridges, there are smaller beams running parallel to the trusses and resting at their ends upon the floor-beams. These are called floor-joists, and the plank or other floor rests directly upon them. In railway bridges, two beams or girders per track run parallel to the trusses and are connected at their ends to the floor-beams. These are called track stringers (or simply stringers). The ties rest directly upon them. The various members of the floor system of a railway bridge are shown in Fig. 7. The diagonals connecting the top chords, and those connecting the bottom chords, are the top and bottom laterals respectively.
17. Names. Trusses may be classified according to their names, the character of their chords, and the system of webbing. Table I gives the classification of the more important of these accord- . ing to name.
Of the types of trusses listed in Table I, the Warren, Howe, Pratt, Bowstring, and Baltimore are now built; and of these construc tions probably 90 per cent are Pratt trusses. The Baltimore truss is used for long spans only.