CHAPTER XIV 
DESIGN OF STRUCTURES—PLATE GIRDERS 
191. Beams and Girders.—In Chapter VII. it has been shown that 
the straining actions at any cross section of a beam, due to any system of 
vertical loading upon it, may be resolved into two distinct effects, namely, 
‘a bending action and a shearing action. It has also been shown that, for 
economy, beams are made with a cross section shaped like the letter I, in 
which case the top and bottom flanges may, for practical purposes, be 
assumed to resist the bending moment, whilst the. web takes the shearing 
force. 
The concentration of the material into a web and flanges may be 
obtained by using a rolled steel joist or channel, and these may be 
combined with plates for stronger sections. Again, separate plates may 
be used for the web and flanges, which are united by angles. Another 
form is obtained by substituting diagonal bracing or lattice work of bars 
for the plate web. 
192. Beams of Rolled Joists or Channels and Plates.—The most 
common form of beam in use for short spans is the rolled steel joist 
(2) (0) (c) 
(d) 
(e) 
Fig. 304. 
shown at (a), Fig. 304, The standard sections for rolled steel joists are 
numerous, and range from 3 inches deep by 14 inches wide, weighing 
4 lbs. per foot of length, to 24 inches deep by 74 inches wide, weighing 
100 lbs. per foot of length. These joists can generally be obtained from — 
stock in lengths of every foot from 10 feet to 40 feet for ordinary sections. 
For convenience in rolling, the flanges are tapered in section, as shown, 
the angle between the inside of the flange and the web being 98°. Should 
the load require it, two or more of these joists may be placed side by — 
side, and they may also be strengthened by having plates riveted to their 
flanges, as shown at (4), (d), and (e), Fig. 304. A beam, formed of two 
channels connected by plates, is shown at (c), Fig. 304. . 
193. Connections between Rolled Joists.—When two or more joists 
are used side by side, without connecting flange plates, cast-iron separators, 
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