CHAPTER VIII. 
STRESSES IN TRUSSES. . 
Tlie causes of stresses in trusses are the following ; 
1* uniform live load 
2* dead load 
8® engine excess 
4° wind pressure directly 
5° wind pressure indirectly 
6* curvature of track 
The uniform 】ive load and engine excess produce tension in the bottom chords, 
main diagonals and counters, and compression in the top chords posts and batter 
braces. 
The dead load affects similarly the chords, posts and main diagonals, but does 
not affect tlie counters, or rather it tends te diminish the stresses on the counters. 
The direct stresses due to wind pressure are compression on upper and lower wind- 
ward chords, and tension on upper and lower leeward chords. 
The indirect stresses duo to wind prossure are equivalent to those produced by 
increasing the dead load on the leeward truss : they will be called “ transferred load 
stresses/ 1 Tlie wind also relieves the (lead load on the windward truss : the stresses 
due to the difference between the dead load on this truss and the reduction will be 
called “ reduced dead load stresses/* 
The curvature of the track produces a centrifugal force which acts only on either 
the lower or upper chords, according to whether the bridge be through or deck : it 
affects also the corresponding lateral system, and the vertical sway bracing of deck 
bridges. The combination of all these stresses is a little complicated, especially as 
the direct and indirect stresses due to wiud pressure may bo subdivided into tlie two 
cases, first when the bridge is empty, and second wlien it is partially or wholly 
covered by tlio moving load. Tho following table will facilitate the comprehension 
of the effects of the various loads upon the different members, T standing for tension 
and C for compression. 
