— 89 — 
Although the centrifugal forco really reduces the stress on one chord, it should 
no t be assumed so to do ; for the trains do not necessarily pass over the bridge at 
tlieir maximum velocity. 
Passing to the bottom chord we may conclude at once both from the table and 
from our general knowledge of bridges, that in through and pony truss bridges the 
stresses will be greatest wlieu the bridge is loaded. When, however, tho bridge is 
empty wo liavo acting upon the windw ard chord 2°, 4° and G°, or wliat is the same 
tiling 4 C and 8°, tlie former producing C and the latter T ; and as in most cases for 
the bridges dealt with in this treatise the former exceeds tlie latter, tlie bottom 
chords should be proportioned to resist a compression of C4 — Ts, the subscripts 
denoting the horizontal line to \vliicb the utresses belong. 
When tbe bridge is loaded, 1°, 2°, 3° and perliaps 10。 act together as an oi.ili- 
llar y load while 1°, 2°, 3°, 5°, 7° and perhaps 10° act together as an extraordinary 
load. Where the latter exceeds tbe former by more than fifty per cent, tlie chords 
should be proportioned to resist tbe stresses produced by the second loading using 
an intensity of working stress of 7. 5 tous, but otherwise the stresses clue to tho first 
loading are to be taken, using an intensity of working stress of 5 tons. 
It must be noticed that T J0 affects the sizes of the chord sections on tlie con- 
vex side of tlie track only, and that., although given as either C or T, T must be 
taken, as tlie wind acts iu either direction. 
In deck bridges 11° cannot act, and as tlie effect of 5。 is small compared with 
that of 1° and 8°, the loading to be considered is that of l c , 2° and 3°. For the empty 
pony truss 
Passing to the posts we may conclude immediately that tliey tako tlieir greatest 
stresses when the bridge is partially loaded, in which case 1°, 2°, 8° give tlie or. iuary 
1 , 2°, 8°, 7 C and 12° tbe extraordinary loadings. Except iu tlie case of very 
ight posts the latter loading need not be considered ; but when it is, the post must 
be proportioned as directed in Chapter VI. 
Passing to the main diagonals we Lave 1°, 2° and 8° as the ordinary loading, 
a ud 1 , 2°, 8° and 7° as the extraordinary loading. It is self evident that the latter 
not be considered. 
Tlie same remark applies to the counters and bip verticals. 
Finally passing to the batter braces we may immediately conclude tliafc tlie 
stresses existing wlieu tlie bridge is empty need not be considered, hence tlie ordin- 
ai y loading will be for through bridges, pony truss bridges and deck bridges witli 
straight track 1°, 2° and 8®, and the extraordinary loading 1°, 2°, 3°, 7° aud 12% It 
! s Mghly improbable that the latter loading need ever bo considered. For deck 
l i<lges with curved track the ordinary loading will be 1°, 2°, 3° and 12°, the bending 
eing produced by the centrifn^al force ； and the extraordinary loading 1°, 2°, 3 C , 7° 
12 ， tlie bending being produced by wind pressure and centrifugal force acting 
^getlier. As before, tlie ordinary load is the one usually to be provided for ； if not, 
le Proportioning must be clone as directed in Chapter YI. 
*| (I ge C 4 is not quite so liable to exceed T 8 as in tho caso of through or 
ri dges， because tlie wind load carried by tlie lower lateral system is less. 
