CHAPTER III. 

 STEEL HIGHWAY BRIDGES. 



Definition. A truss is a framework composed of individual members so fastened together 

 that loads applied at the joints produce only direct tension or compression. The triangle is the 

 only geometrical figure in which the form is changed only by changing the lengths of the sides. 

 In its simplest form every truss is a triangle or a combination of triangles. The members of the 

 iss are either fastened together with pins, pin-connected, or with plates and rivets, riveted. 



Types of Truss Bridges. The bridge in Fig. I consists of two vertical trusses which carry 

 the floor and the load; of two horizontal trusses in the planes of the top and bottom chords, re- 

 spectively, which carry the horizontal wind load along the bridge, and of cross-bracing in the planes 

 of the end-posts, called portals, and in the planes of the intermediate posts, called sway bracing. 



ftrtal 



*X, U 10 



FIG. i. DIAGRAMMATIC SKETCH OF A THROUGH PRATT TRUSS HIGHWAY BRIDGE. 



The floor is carried on joists or stringers placed parallel to the length of the bridge, and which are 

 supported in turn by the floorbeams. The names of the different parts of the bridge are shown 

 in Fig. i. The main ties, hip verticals, counters and intermediate posts are together called 

 "webs." The bridge shown in Fig. i, is a through pin-connected highway bridge of the Pratt 

 type, the traffic passing through the bridge. In a deck bridge the roadway floor is carried on top 

 of the main trusses. The bridge shown has square abutments; if the abutments are not at right 



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