— 82 — 
Span. 
5^ and 
and 
37 f and 
39 、 40， and 
42 ，， 43， and 
45 f , 46 f and 
48 ，， 49， and 
5i ; to 
Uuif. load. 
4080 pounds. 
5927 •• 
5776 " 
3596 " 
3420 •• 
3 H 8 •• 
3080 •• 
2916 •， 
Unif. load. 
5150 pounds. 
5125 •， 
5000 " 
4875 •• 
4712 " 
4551 
45.92 " 
4255 " 
for short spans it is not at all improbable. Another reason is that as the length of 
span increases tlie ratio of (lead load to total load increases ami consequently the 
injurious effect of impact relatively (lecieases. I’liis reiluction, of course, does not 
a fleet tlie stresses on the floor system and liip vertical?, which are proportioned for 
the assumed engine loads. 
The table of equivalent uniformly distributed live loads given in the last chapter 
will be found to save much labour : aliliougli not perfectly accurate it is correct 
enough for nil practicul purposes. The table which follows it gives the percentages 
by which these equivalent loads are to be increased to resist the shock of rapidly 
passing engines. The percentages agree 11111. 】y well with the results of experiments 
upon the vibrations of bridges under passing loads. 'The two tables can be combiucd 
into the following, which is the one to be used iu calculating stresses. 
The engine excess for one truss of a single track bridge is found by subtracting 
from the total weight of engine tlie product of the length of engine by the assumed 
car load per lineal foot and dividing the remainder by two - making tlie various engine 
excesses to be used as given in the following table. 
Car Load. 
i aoo* per ft. 
115。* per ft. 
1 ioc* per ft. 
1050* per ft. 
Engine Excess. 
10.35 tons. 
10.7 tons 
11.05 tons. 
11.4 tons. 
For double track bridges the engine excesses are, of course, twice as great as the 
above. 
Wlien the panel length is eighteen feet or over Ü10 engine excesses of the two 
coupletl locomotives are assumed to bo concentrateil upon consecutive panel points, 
1 川 t fhr sliorter panels there is supposed to lie between them a panel point without 
engine excess. Of course this envision does not represent the actual distribution of 
the loading, nevertheless it is convenient and gives a small error oa the side of safety. 
I11 double intersection trusses greater web stresses are found by assuming a car to be 
placed between the two engines, thus bringing both excesses upon tlie same system 
of posts ami ilijigoimls, so this arrangemeut was adopted when calculating the web 
stresses for Table IV. 
Accurate values for the dead loads of single track bridges are given in Table I. 
The column for dead load is obtained by adding togetlier the 'veiglits of iron per 
468 1 470 c 
3 3 5 4 4 4 5 6 
sp : to 
3 0 2 46 00 0 2 
1 2 222 2 33 
d d d d Pd d 
-n -n Ln 瓜 Ln Ln ,n 
