DESIGN OF STRUCTURES 219 
many of a type are required. It is often more economical to have the 
same web thickness throughout, especially in small spans, where the web 
plate can be obtained in one piece, than to use plates of different thickness 
and special joints. 
. 198. Web Stiffeners.—To prevent buckling and twisting it is neces- 
sary to give web plates lateral support. This is done by riveting to 
them at intervals angle- or tee-section bars placed vertically. Fig. 322 
Fig. 322. 
shows examples of stiffeners applied to single web girders. When the 
stiffeners are straight and not set in to meet the web plate, intermediate 
packing pieces are introduced. Stiffeners 
formed of plates and angles are shown on 
the main girders in Figs. 328 and 329, 
p. 223. 
Box girders should have diaphragm 
plates fitted between the webs at intervals, 
as shown in Fig. 323. This ensures that 
the cross section of the girder remains 
rectangular, and that all the parts bend 
together. Manholes must be provided so 
that the space enclosed may be got at. 
The distance apart of the stiffeners is 
determined by the shearing force upon the 
web. It is necessary, however, to place 
a stiffener wherever a local load occurs 
upon the girder. ; 
No satisfactory theory for the spacing 
of the web stiffeners has yet been formu- 
lated, and the rules which will be given 
wen are almost entirely empirical. In 
pter IX. it was shown that a shear 
stress in one direction must be accompanied 
by another of the same intensity, but in a 
direction at right angles to that of the first. 
Also, it was shown that these two shear stresses are equivalent to tensile and 
compressive stresses of the same intensity as the shear stresses and at right 
angles to one another, but in directions making angles of 45° with the 
directions of the shear stresses. A square panel of the web plate is therefore 
Fig. 323. 
e 
