— 133 — 
a t ， for a pair of steel bars pulling on a steel pin in opposite directions, or a single 
steel bar against a steel bearing, the pin in certain cases will be liable to rupture by 
8 earm g， and will therel'oro have to be porportioned to resist that stress. 
After making out the diagram of stresses, and proportioning the main members 
a ri( %e， comes the determination of the sizes of the pins, — a matter that is liable 
o occupy more time than did all the previous work. Knowing tlie sizes of all tlie 
in tlic structure, the clear width between the inner faces of tlie top chord clian- 
e 8 ( an d consequently that between post channels) can be found, after wliicli the 
9-irangoment of all the bars in tlie bridge can be decided on. Care must be taken 
ln P erfo nning the latter, that no two consecutive chord bars or ties coupled on the 
8anic l) m pull in tlie same direction， unless this arrangement reduce tlie bending- 
^onaeuts, as it can sometimes be made to do ； that tlie lighter set of bars be so placed 
as to l educe the bending- moments ; and that the diagonal ties be placed close to the 
P 0 ド s, and the beam hangers close to the ties. Especial care is needed at the panel 
n "Where tlie number of chord bars is different in tlie consecutive panels. It is 
possible to arrange the bars there, so that there will be an extremely large moment 
ズ 妳 飞 ， 01 ’ S0 that itwill be smaller tlian at any other panel point of the bottom 
1 The neglect of any of these precautions will cause an undue bending- 
on the pi u . 
Wh 孤 0 a う rau g emen t completed, the next questions to be decided are, first, under 
condition of loading will each pin take its greatest bending-moment, and, 
= り at what point on the pin will tliis bo found. In well piropoi.ticmed railroad 
c o es » tlie bottom chord pins are subjected to their greatest bending- moments when 
b 11 g e i s fuUy loaded. Under this condition, tlie stresses in the chord bars can 
a eu from tlie diagram of stresses; but those in tlie main diagonals must bo 
1 a oa for the load covering the whole bridge, and their horizontal and vertical 
c °mponents be ascertained. 
to 1 ^ ei * 】 mv i u g ] ia d some practice, one will very often be able by simple inspection 
eci d e a t wlmt place the greatest moment of flexure will exist ; blit, if not, it will 
ferei^ eSS ，y も 0 calculate tlie values of both horizontal and vertical moments at dif- 
Bur ご and find where their combined result is a maximum. As Professor 
s - ( | e 0Ws > the actual moment is represented by tlie diagonal of a rectangle whose 
to 8 le P lesen t the vertical and horizontal moments. It is usually more convenient 
of th laie ^ lG com ^ ouei1 ^ moments, add tlie results, and extract the square root 
sum, than to make out a diagram. 
10 of the stresses can be easily recorded by drawing two curved lines, 
as Bhown in the accompanying diagram, representing tlie 
directions in Tvhicli the stresses tend to bend the pin, and writ- 
ing each moment as calculated under one or other of them, 
according to whether it would produce positive or negative 
rotation. Tlxo difference between tlie sums of each column 
^ ， will give the actual horizontal or vertical moment as the case 
be. 
