216 APPLIED MECHANICS 
and they are therefore of constant depth throughout their length. Hog- 
backed girders have a curved top boom, and /ish-bellied girders have a 
curved bottom boom, as shown in Fig. 310. The effect of curving one 
boom is to increase the depth of the girder towards the centre, where the 
bending moment is greatest. This permits of the cross section of the 
| rsh pei | er ae 
— Ee es | es 
=e S=_oe 
Fig. 310. 
booms being kept more nearly constant. Except under special cireum- 
stances, it is generally better and cheaper to use a parallel girder than 
one of variable depth, the cross section of the flanges being varied to 
approximately suit the bending moment. Fish-bellied girders are usually 
adopted for overhead travellers of large span. Hog-backed girders are 
frequently used for large span railway bridges. 
195. Plate Girders.— When the depth of a girder exceeds a foot, but 
is less than the limiting depth for a rolled joist, it is frequently more 
economical to build it up of plates 
and angles rather than use a rolled 
joist, and when the depth exceeds 
the limiting depth for rolled joists, 
the built up girder must be used. 
Types of built up plate girders are 
shown in Figs. 311 and 312. For 
smaller spans and lighter loads, one 
web plate and one or two flange 
plates may be sufficient, while for 
larger spans and heavier loads two, _ il | 
or even three, web plates and many IL 
flange plates may be required. When TET ge TK 
more than one web plate is used, as Fic. 311. Fra. 312. 
in Fig. 312, the girder is called a 
box girder. The box type is more suitable for large than for small 
girders, and it is better only to employ this type when there is sufficient 
room inside for the girder to be properly painted, and so protected from 
corrosion. Care must also be taken that the girder can be properly riveted 
up, a not altogether unnecessary caution. 
The depth of the girder must never be less than 1-20th of the span. 
For economy, the depth should be 1-12th to 1-10th of the span. The 
breadth varies from 1-20th to 1-50th of the span, depending on the 
amount of lateral support the girder gets. If there is no lateral support, 
the breadth should not be less than 1-20th of the span, whilst if it is 
well supported laterally, say by closely spaced cross girders, this dimen- 
sion might be diminished to 1-40th or 1-50th of the span. 
