— 126 — 
In general t = V コ t" = p = G tons, and if three quarter inch rivets be 
employed, cl = 0.75", and m = 0.311 inch ton (vide Table XVIII). 
Substituting gives 
n= n’’’= and が = れ " = — 池一 nearly 
and as W varies from 13.84 tons to 24.64 tons the number of rivets passing through 
each leg of each bent connecting plate will vary from five to eight, which numbers it 
would be well to increase to seven and eleven. 
In plate girder designing care must be taken to so stagger the rows of rivet holes 
passing through the flanges that tlie latter will be weakeued as little as possible. If 
there be but one plate a single row of rivets on each side spaced about five inches 
will be sufficient ; if there bo two plates, a single row spaced three and a half inches 
will answer ; but if there be three plates, they must be wide enough to contain two 
rows of rivets on a side, with a spacing of five inches. It is bettor to use l ,r rivets 
to pass through three tliiclmossos of plate and tlie leg of a flange angle, or even to 
pass through two thicknesses and the leg, if tho plates bo as thick as half an inch. 
Beam hangers may be proportioned by tlie equation 
where A is the area of the section of one leg of a hanger, and W t tho total weight 
of a floor beam and its load not including any allowance for shock, the latter being 
provided for by the low intensity of working stress. 
Let us take, to illustrate tlie desgniug of a girder, the case of a track stringer 
for a 21’ panel. Tlie uniformly distributed live and dead load including shock is given 
in the first table of this chapter as 2750 pounds = 1.375 tous per lineal foot. The 
moment at tlie centre is 
7 5.8 foot tons. 
Let us assume the economic depth of web to be 29" and take tlie thickness as 普 ", 
making d about 26.5" = 2.21 feet, therefore 
^ = t x 221 = 8.5 8 square inches 
From tlie well known fact that a bar of wrought iron one square inch in section and 
three feet long weighs almost exactly ten pounds, we can determine the weight per 
foot of each flange angle by multiplying A by ten and dividing by six. 
This gives the weight per foot to be 14.8 pounds. Consulting Carnegie's table 
of angle irons given in Chapter II, we find that tho nearest size is a 3" x x U” 
angle, weighing 14.4 pounds per foot, which section we will adopt. 
The area of the section of the bottom flange is 8.58 十 」"， where A n is equal 
to tlie diameter of a rivet hole multiplied by twice the thickness of the leg of one of 
the lower flange angles. The thickness of the upper augle legs being U r \ we can 
assume tliat of tlie lower legs as 蚤 "， for there is to be a bottom plate. Let us use 
栽 " rivets for both flanges, because of the rather large thickness of the upper flange 
