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•will bo described presently, by only the absence of upper lateral bracing and tlio 
addition of side braces. Although the ** General Specifications n of Chapter YI 
provide for spans Laying less than five panels, the author does not recommend tlieir 
use, even if a small saving of iron be accomplished thereby. The objection to three 
and four panel trusses is the 1111 sightly appearance caused by tlie flat slope of the 
Latter braces. • 
The Pratt or single intersection quadrangular truss lias vertical intermediate 
posts, inclined tension web members, parallel chords and inclined end posts. The 
diagonal ties extend over one panel only, and the liip verticals are tension members. 
Tlie Whipple or double intersection quadrangular truss differs from the preced- 
ing only in the diagonal tics which extend over two panels and are generally lialve d 
and attached by pins to tlio mid tile of the posts, so that the assumed leugtlx of the 
latter as struts may be reduced to one half tlieir real length. 
It is better and more economical to build deck bridges with inclined end posts 
than with vertical ones, tlie track being carried ovou the piers by iron bents resting 
thereon. It is better because with vertical end posts there can bo no stress in the 
end panels of the bottom chords, an undesirable state of affairs when vibration anil 
wind pressure are considered. 
In this and in all other cases wliere tlie floor system is made continuous oyer 
tlie piers of consecutive spans, expansion of the floor system must bo provided for ; 
otherwise injurious thermal stresses would be induced. 
To all the styles ot bridge except the plate girder the following explanations will 
apply. 
Top chords are composed of two cliannels having tlieir webs vertical with a plate 
rivetted above and latticing or lacing below as shown on PI. IX Figs. 1 and 7. Where 
two cliannels and a half incli plate do not givo a sufficient sectional area to tlie cliord, 
an I beam of the same depth as the channels may be inserted between the latter as 
sliown on PI. IX Fig. 2 or, if this does not give sufficient area, channels may be built 
of plates and angles as in PI. IX, Fig. 3. What is said of top chords applies also to 
batter braces. 
Posts are composed of two channels with latticing or lacing on each side, as in 
PI. II Figs. 10 and 11. When sufficiently largo rolled cliannels are not procurable 
it is better to built thorn of plates and angles rather than to use any combination 
of I beams anti chamicls. 
Lateral and portal struts are also composed of two cliannek laced or latticed as 
shown on PI. I and in detail on Plates II and VIII. 
Intermediate struts for single track bridges are composed of sijiglo I beams with 
tlieir webs horizontal (PI. VIII, Figs. 2 and 4), and those for double track bridges of 
two channels laced or latticed. 
Bottom cliovcls consist principally of eye bars, but, as will be seen farther on, 
they may have to resist a certain amount of compression under exceptional circum- 
stances, so that in most cases of single track bridges there is a strut of two laced 
channels lying between the eye bars and symmetrically arranged ia reference to tlio 
