: DEFLECTION OF BEAMS 131 
; erate than a series of separate girders covering the same spans. It 
will be noticed that the maximum shear is still at the supports when 
the te girders are converted into a continuous girder, and that the 
change in the values of the maximum shearing forces in the various 
spans is not very great. 
' The disadvantages of continuous girders are, however, serious. In 
the first place, the level of the piers is liable to changes due to unequal 
settlement or variations of temperature, and comparatively small in- 
equalities of level may cause considerable changes in the bending moment 
iagram. In the second place, travelling loads will cause the points of 
_ inflexion to change, and there will be portions of the girder in the 
vicinities of those points on which the bending moments will be alter- 
nately positive and negative. This disadvantage will obviously be greater 
the greater the moving loads are compared with the permanent load due 
to the weight of the structure. The advantage of continuity is therefore 
_ greater in long spans, where the permanent load is the most important 
one. 
} A continuous girder requires more care in construction than separate 
girders, because any want of straightness in the unloaded girder will 
_ upset the results of the designer’s calculations. 
# The sheariig force diagrams in Fig. 172 show that for a uni- 
formly distributed load the web of a continuous girder must be slightly 
heavier than the webs of a series of separate girders covering the same 
187. Cantilever Bridges.—Let the shaded diagram in Fig. 173. be 
_ the bending moment diagram for a continuous girder covering three spans. 
If the continuous girder 
6 H 
G Z 
SE 
be cut at the points of % 
WwW 
 inflexion E and F, or at . 
the points of inflexion 
 G@ and H, and joints 
Uy 
‘I iI 
be made at these points 
which are capable of 
resisting shear but not x Y E 
_ bending, the resulting fi % s A : y 
girders form what is Fig, 173. 
called acantilever bridge, 
and they will have all the advantages of the original continuous girder, 
but the bending moment diagram will not now be affected by any settle- 
ment of the piers. Moreover, the bending moment and shearing force 
diagrams for a cantilever bridge may be constructed by applying the 
simple principles of statics without any reference to the elasticity and 
deflection of the structure. The cantilever bridge. has therefore the 
advantage of being simple to design and, what is most important, there 
is the further advantage that there need be no doubt about the results of 
_ the designer’s calculations. 
& It should be noticed that when the flexible joints are made at G and 
__H, in the centre span, the cantilevers AG and DH may need to be 
_ anchored down but not fixed at A and D, as the reactions at these points 
_ may beeome negative. 
F Cantilever bridges have been used with great success for very large 
ee Rao * 
a 
> 
— . 
