CHAPTER XV. 
DÜÜ13LE TRACK BRIDGES. 
For reasons given in Chapter I, special attention has hitherto been given to 
single track bridges, Inifc ns the Japanese engineer may sometimo be called upon to 
design a double track bridge, there will be given in this chapter, although they may 
have been previously mentioned, the principal differences between bridges foi- sin- 
gle track and those for double track roads. 
In the first place, of course, double track bridges are wider and their live loads 
twice as great as for corresponding single track bridges. This causes the weight 
of the track stringers and floor system proper to bo doubled, and a largo increase 
on the size and weight of the floor beams. Tfee live and dead load stresses on 
the trusses arc about doubled, thus necessitating in many cases the abandonment 
of rolled channels for the top chords and batter braces. 
The total wind pressure per lineal foot is increased because the area of the ver- 
tical projection of one truss is greater, fincl as the trusses are farther apart the 
lengths of all the members of the lateral systems and sway bracing are increased, 
consequently the weights of these portions of the structure are doubly augmented. 
For reasons given in Chapter IX, the stresses in the vertical sway bracing are 
greatly iucreasecl. The wind pressure ueetl not be considered to affect the stresses 
in the chords or posts, for in the first place ceteris paribus tlie wind stresses both 
direct and iudirect are reduced by the increase of width of bridge, and in the second 
place they are relatively less important by reason of the doubling of the live and 
dead load stresses. For the >samo reason stiffened bottom chords are not required 
in double track bridges. 
In these low particulars only does the designing of double tvack bridges clitfer 
li-om that of single track bridges, and the author is confident that anyone who has 
thoroughly studied the latter and perfected himself therein will have no trouble 
whatsoever mill the former. 
