DESIGN OF STRUCTURES 225 
Uxtra stiffening to make a } inch plate suitable is not advisable for three 
-reasons—the necessary stiffeners would be awkward to get in, the stress 
square inch would approach very near to the limit, and, what is even 
_ more important, the web riveting in the 3 inch plate would be very difficult 
todesign. On the other hand, it need hardly be pointed out that the extra 
stiffeners required by the ,', inch plate will be much cheaper than if a 4 
‘inch plate were used with no extra stiffening. A +% inch web plate will 
4 Bharefore be used, stiffened as shown. 
_ Longitudinal Riveting in Web and Flanges. —? inch rivets in }# inch 
holes will be adopted. The value of a rivet in single shear at 5 tons per 
square inch shearing stress is 2°59 tons. A rivet in double shear bearing 
in a 4%, inch plate will carry 3°55 tons, the bearing stress being limited to 
10 tons per square inch. Dealing first with the single row in double 
shear through the web plate, a 3 inch pitch represents a shear per inch of 
depth (or length) of 1°18 tons, a 4 inch pitch 0°89 tons, and a 6 inch pitch 
0°59 tons. A 5 inch pitch will not work in between the stiffeners, and 
need not be further considered. The above values are set up in the 
diagram as thin full lines. It is now seen that a 3 inch pitch must 
_ extend from the end of the girder to A, a 4 inch pitch from A to B, and 
a 6 inch pitch from B to the centre. Since too many changes are not 
desirable, a 3 iach pitch will be adopted extending to B, and a 6 inch 
pitch from B to the centre. Over the last two panels, near the end, a 3 
ch pitch with the same size of rivets is inadequate. A closer pitch 
_ means zig-zagging the rivets and a deeper flange angle, and thickening 
the web plate is not desirable, as has already been seen. The third 
alternative is to use larger rivets. There are only a few larger rivets 
required, and probably the cheapest way out of the difficulty wiil 
be to punch all the holes alike and then reamer the few holes at 
the ends out to % inch diameter. The load upon one of these { inch 
rivets at 3 inches pitch is 3x 136=4:1 tons. Its bearing area is 
0-383 square inch, and the bearing stress will therefore be 10°7 tons per 
square inch, but since in these reamered holes the rivets will be in 
‘much better condition than in the ordinary punched holes, this may be 
allowed. 
The riveting in the flanges joining the flange plates and angles is 
_ determined in exactly the same manner as for the web, except that there 
_are two rows of rivets in single shear, instead of one row in double shear. 
The possible pitches are shown on the same diagram as dotted lines. It 
| must be remembered when choosing the pitch that the rivets should 
wig-zag with those in the web. 3 inch and 6 inch pitches only are 
admissible. The 3 inch pitch will extend from the end to C, and the 6 
inch from C to the centre. This will necessitate one odd 44 inch pitch, 
shown in the plan of the girder. No larger rivets are necessary, the 
3 inch pitch giving ample strength. 
Boom Section.—The distance between the centres of gravity of the 
flanges may be taken roughly as that over the backs of the angles. 
Tf angles 4 inches x 4 inches x 4 inch be used, punched 2} inches from 
‘the back, this will be 24+ 4} = 284 inches. Using this figure, draw the 
diagram (Fig. 331), showing the force in the boom everywhere (got by 
dividing the ordinates of the bending moment diagram by 28} inches). 
_ The force in a boom at the centre is roughly 160 tons. The stress being 
P 
