— 90 — 
To recapitulate ； in through and pony truss bridges tlie top chords must be 
proportioned to resist the uniform live load, dead load and. engine excess stresses, 
and iu deck bridges to resist eitlicr tlie same, togetlier with curvature stresses, if 
any, or tlie combined live load, dead lodtl, engine excess, wind aiul, if necessary, cur- 
vature stresses. The bottom chords for tlirougli and pony truss bridges aro to be 
proportioned to resist a compression equal to tlie difference between tlio Avirul strefises 
and the reduced dead load stresses when the bridge is empty; also either the live 
load, dead load engine excess and, if necessary, curvature stresses, or a combination 
of tlie same witli tlie wiutl load tension. 
The bottom chords for deck bridges have to resist a compression equal to tlie 
difference between the wind stresses and the dead load stresses when tlie bridge is 
empty, anil a tension due to tlie live load, dead load and engine excess. 
Posts are to be proportioned to resist either the live load, dead load and engine 
excess stresses, or these combined with tlie transforred load and bending stresses, tlie 
latter being produced by a wind pressure of thirty ponnds per square foot. 
Main diagonals, counters and ]】ip verticals have to resist tlio live load, dead 
load and engine excess stresses ; and batter braces eitlier the three last mentioned 
for tlirougli ami pony irusB bridges, or tlie same combined with bending stresses due 
to curvature for deck bridges ; and a combination of live load, dead load, engine 
excess, transferred load anil bending stresses for eitlier tlirougli or deck bridges, tlio 
bending being duo to the wind pressure alone in tlirougli bridges or to either tlie wind 
pressure alone or tlio combined wind pressure and centrifugal force in deck bridges. 
As is customary in figuring stresses, uniformly distributed loads will be consi- 
dered as couceiitr.aled at the panel points, and the half pnncl load at each end of the 
span is not supposed to produce tiny stress in any member of the truss. 
Iu order to facilitate the calculation of stresses, tlie following nomenclature will 
be adopted. 
n = number of panels in span. 
I = length of each panel. 
d = depth of truss from centre to centre of chords. 
= perpendicular distance between central planes of trusses. 
0 = inclination of diagonal ties to tlie vertical in single intersection 
trusses. 
a = inclination of short diagonals to tlie vertical in double intersec- 
tion trusses. 
ß = inclination of long diagonals to tlie vertical in double intersec- 
tion trusses. 
O' = the angle whose tangent is 
W — live panel load on one truss (L €• one lialf the product of the 
uniform live load per lineal foot by tlie panel length). 
W x — panel dead load on one truss. 
W = tluit portion of W 1 concentrated at tlie upper panel point. 
