CHAPTER XX. 
幻 1 PROXIMATE METHOD OF DESIGNING A SINGLE 
TRACK BRIDGE. 
i. • or S0lne time the author has debated the expediency of writing this chapter, 
tl ia t it is poor policy to encourage the use of rule of thumb methods, but has 
^ ア concluded that its advantages will exceed its disadvantages ; for not only will 
g j^. na ^^ e an ordinary draughtsman, who lias not studied tlie theory of bridge de- 
a jj Q g’ 七 0 design any single track iron railroad bridge with sufficient exactness for 
practical purposes, but it will also serve as a check in many ways upon designs 
= by the more accurate method which this treatise explains. 
b er le a PP ro ximate method is briefly as follows ； — to proportion the main mem- 
^ trusses hy using the diagrams on Plates XIV. to XLII. inclusive, those of 
111 し ⑻ S pt em s by Table XIII. those of the floor system by Tables XI. and XII. 
in T 1 1 e ^ ai ^ s the rules and tables for same which are given in this chapter or 
^ = VIL and XX. to XXIII. inclusive. 
bers 6 of span be exactly divisible by ten, the sizes of all the main mem- 
“not ”; わ ' obtained exactly or very nearly exactly from tlie plates and tables ; but, 
direct! 81zes of the main members of trusses anil lateral systems may be either taken 
Panel】 r ° m ^ le greater span or may be interpolated from the plates : if the 
s y8tem en ° t l m 110 ^ a w hol e num ber, tlie sizes of the members of the floor 
tables ツ糾 be taken equal to those for tlie next greater panel length given in the 
of p ane | aie ^ust be exercised in interpolating sizes especially wlien tlie numbers 
u pon the m ^ W ° S P ans neares t tlie one considered are different. After deciding 
havi nff UUm ^ er panels, tlie bridge should be designed using as a guide tlie span 
, 10 satQe nil tnber of panels. The following points should be borne in mind 
Actions 巧 0 — increasing the panel length tends to increase the sizes of the 
a nd in 0 ぞ the top chords, main diagonals counters, batter braces and lateral rods, 
chord 8 eas > Ul o ^ ie depth of truss tends to diminish tlie sections of the top and bottom 
these ふ 11 ^ 111 .^ 10 ^ 0113 ^ 8 ant ^ couu törs, but to increase those of the posts ; 一 decreasing 
As produces, of course, the contrary effect. 
detail 8 ， let us take Chapter III. anil follow down tlie list there given 
u. es fo r the approximate proportioning of each member therein named, 
and the S1Zes 01 ^ s ^ a y plates for all cases are given in Tables XX [I. and XXIII” 
method of using these tables is fully explained therein. 
