. DESIGN OF STRUCTURES 221 
a 
od is determined by the shearing forces tending to slide the boom over 
the web. Where the shearing force is small, the pitch may be large, but 
near the ends of the girder, or where the shearing force is large, closer 
riveting must be adopted. It is sometimes even necessary to adopt zig- 
zag riveting, or larger rivets at places where the shearing force becomes 
very great. It is not economical, however, to make many changes ; two 
_ different diameters or two different pitches may be regarded as the limit. 
, The riveting in the vertical joints of the web itself must be made 
_ capable of withstanding not only the shearing forces in the web, but also 
_ the stress in the web plate, due to the bending moment, for although the 
boom may be considered as carrying the bending moment, there is also 
a bending stress in the web. In fact, the stress in the outer fibres of 
_ the web is the same as that in the boom. 
Roughly, the size of the rivets may be as follows. For plates under 
# inch thick, § inch rivets. For plates from % inch to } inch thick, 
# inch rivets. For plates from 34 inch to § inch thick, 7 inch rivets. 
In each case the hole is ;4, inch larger than the rivet. These are about 
the usual proportions for punched work, and will serve as a guide. 
When many plates are to be united, larger rivets should be used. 
The pitch of the rivets should not be less than three diameters, or 
oe than sixteen times the thickness of the thinnest outside plate. 
nless it is absolutely impossible, simple pitches 3, 34, 4, 44, 5, or 
6 inches should be adopted. It is not advisable to go above 6 inches if 
the work is exposed to the weather. : 
The longitudinal pitch is easiest determined graphically. Since the 
shear stress in the web is uniformly distributed, or practically so, over 
its depth, and the shear in two directions at right angles to one another 
is the same, the shearing force per inch-run, which the longitudinal rows 
of rivets must carry, is equal at any point to the shearing force per inch 
_ of depth there. The shear per inch of depth diagram, already referred 
to (Fig. 330, p. 226), can therefore be used to determine the pitch of the 
longitudinal riveting. Let P be the safe load on a single rivet, and p 
the pitch of the row, then P/p is the shear per inch of depth or length it 
will safely carry. Set this up on the diagram as a line parallel to the 
base line for a number of different pitches. The points of intersection of 
these horizontal lines with the shear per inch of depth diagram deter- 
mine the points to which each pitch must extend. This question is 
further considered in connection with the worked example, Article 204, 
p. 223. 
200. Ends of Girders—Bearings for Girders.—The ends of girders 
are specially formed to carry the reactions. Special web stiffening is 
provided to spread the load over the depth of the web plate. Examples 
are shown in Figs. 324 and 325. When the end of a girder is carried 
on a wall, a stone templet is built into the wall to give a strong support 
for the girder. Between the stone templet and the girder a hair felt or 
sheet lead packing is placed, in order that the pressure between the girder 
and the stone may be properly distributed. It is better to limit the 
length of the bearing surface by riveting a piece of plate, called a bolster 
pale, to the under side of the bottom flange, as shown in Figs. 324 
and 325. 
The safe ‘bearing pressure between the girder and its supports will 
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