DESIGN OF STRUCTURES 223 
eeece eee ej 
_ Fia. 329. 
gives some support to the cross girder, the end of the latter is securely 
riveted to the web of the former. 
202. Weight of Plate Girders.—An estimate of the probable weight 
of a plate girder may be made by means of Unwin’s formula. 
W = total external distributed weight in tons (exclusive of girder). 
w= weight of girder itself in tons. 
7=actual span in feet. 
Jf =stress in booms in tons per square inch, 
7r=ratio of span to depth. « 
¢=coefficient varying from 1400 to 1500 for small plate girders, 
and varying from 1500 to 1800 for large plate girders. 
oe Wir 
cf —lr’ 
As a check, the following rough rule is ‘given, w= wis . 
203. Camber and Deflection.—Girders are usually given a slight 
eamber while being built, so that they just become straight when in place 
and loaded. A common allowance for camber is 3 inch to } inch per 
10 feet of span. In calculating the deflection, take E, the modulus of 
elasticity, equal to 9000 tons per square inch, for riveted structures, 
, 204. Plate Girder—Worked Example.—The method of procedure 
in designing a plate girder will be shown by working out a practical 
example. 
It is required to design a plate girder such as might be used to carry 
_ a heavy floor, the clear span being 36 feet. It is to carry twelve loads of 
_ 6 tons each, spaced 3 feet centre to centre. An inexpensive design is 
required, and it is not desirable to take up much head room. 
_ Type.—Parallel flanges. Single web plate. Punched holes. Material, 
mild steel. This will make the cheapest design. 
Actual Span.—The clear span is 36 feet. Each end reaction will be 
about 40 tons. Two square feet of bearing area at least will be required 
o 
