— 93 — 
depth of two inches and a half, are about as small dimensions as will be found con- 
venient. 
At each lower panel point write lightly in pencil, so that it can be after - 
wai.ds erased, the number of the panel point, beginning with zero at the riglit-liand 
end of the span. 
It is well known, and will be accepted here without proof, that the greatest 
«tresses in the chords and batter braces occur when tlio bridge is entirely covered by 
the moving load ; that the greatest stress in any diagonal exists when the live load 
extends to its foot from that end of the bridge towards which the diagonal points in a 
downward direction ; that the greatest stress in any post in through and pony truss 
bridges occurs when the maiu diagonal (or, if there bo none, when tlie heaviest 
counter) attached to its upper end (or in deck bridges when that attached to its lower 
eiul) receives its greatest stress ; and that the two diagonals of a panel cannot at the 
® ame time be subjected to the same kind of stress, excepting, of course, the initial 
tension. 
It is apparent that when the greatest stresses in all the diagonals sloping upward 
111 0ue direction, and in all tlio posts and chord panels on one side of the central 
plane, are found, the greatest stresses in tbo diagonals sloping in the opposite direc- 
'i on ， and in the posts and chord panels oq tlic other side of the central plane, can be 
immediately written. This fact is so well known, tliafc, in making a diagram of 
stresses, it is usual to write the stresses on only one-half of the members of the 
truss. 
First let us investigate the stresses due to tlie uniform live load and the dead 
1 〇 从 1， begin with a single-iutersection through or pony truss bridge. 
Tlie greatest stress in any diagonal sloping upward from right to left can be 
found by the formula 
T =~lf(n，+ l )^sec 0 + ^n f — sec の 
yliere n' is the number of the panel point at the foot of the diagonal. This formula 
ls a Pl)licable to counters as well as to main diagonals. If tlio stress should como 
0 ut negative, it shows that no counter is needed in the panel considered. It is also 
a Pplicable to the batter brace by putting (n _ I) for n f . 
Tlie stress in any post can be found by the formula. 
wlieie n f (not less than f ) is tlie number at tlie foot of the post. 
The stress iu any panel of tlie top chord is given by the formula 
' ¢7 = ど 上 tan の 
kliere n f (not greater U】 an 昼) is the number at the end of the panel nearest to the 
centre of the bridge. 
■Hie stress in any panel of the bottom chord, except tlie one at tho end of "the 
