Art. 120. 



CONSTRUCTION. 



215 



they could carry compression, one could not change in length 

 without affecting the other, and the stresses would be statically 

 indeterminate (42). 



Fig. 154 shows a simple truss supported at both ends. The 

 top chord is in compression just as the top flange of a girder, 



Top Chord 



a C ' 

 Bottom Chord 



6 pane/5 (a) p - L 



PRATT TRUSS 



Fig. 154. 



similarly supported, is in compression. In like manner does the 

 bottom chord compare with the bottom flange of a girder. The 

 web members resist the shear by direct tension and compression, 

 the diagonal members being all tension members and the vertical 

 members being in compression. The chord members can resist 

 no part of the vertical shear since they have no vertical compo- 

 nents. If one or both chords were inclined, they would carry a 

 part of the shear. The chords resist all of the bending moment. 



Panels are the longitudinal divisions of a truss, formed by 

 the web members. 



Panel points are the centers of the joints where the web 

 members are attached to the chords. 



A panel length is the distance between panel points usually 

 measured horizontally. 



Chord Members are the upper and lower members of a truss. 



A main tie is a web member in tension under full load. For 

 a certain position of the live load its stress may reduce to zero. 



A counter tie is a diagonal web member in tension under cer- 

 tain partial loads only. Counter ties are not necessary in trusses 

 carrying dead loads only. If, in Fig. 154, a continuous live load 

 moves across the bridge from right to left, it tends to distort the 

 rectangle CDdc so as to produce tension in the main tie, and 

 there will be no stress in the counter tie. But if the live load 

 moves from the left to the right, the distortion will be in the 

 opposite direction until the head of the live load passes a certain 

 point in the bridge. This distortion and that due to the dead 



