The symmetrical arrangement of the cables extending down from 

both sides of each tower enables the deck load to be carried by 
the cables in a balanced fashion. The tower acts like the apex
of 
a
triangle whose legs are the cables and whose base is the deck. 
Like the suspension bridge, the internal tensile stresses of the 
cables suspending the deck are transferred to the tower, which 
then displaces them to the ground as compressive stresses. 
Fig. 191 Stress diagram 

. 
Box
girder bridge 
. 
Another
way of moving the mass of a beam outwards from the center of its
cross section, in 
order to
increase its moment of inertia, is to build it like a hollow box.
This design is able 
. 



Fig. 192  Box girder bridge 
designs 
(static demonstration models) 
click image to enlarge 
typical crosssections 
. 

. 
to
resist twisting forces better than the
Ιbeam
girder shown previously. Therefore it is 
capable
of spanning longer distances. This also makes box girders a logical
choice for use 
in
bridges that have a curve to them, such as freeway overpasses and clover
leafs. 
. 



crosssection of model's girder 
. 
Fig. 193  Box girder freeway overpass
(scale visualization model)
click image to enlarge 

. 
Box girders are also called orthotropic beams because they can
resist 

multiple stresses at once. The inherent stability of the box
girder is 
due to the fact that each face, or plate, of the box acts as a shear
plate 
to
resist bending induced shear stresses. The stability of such
"plate 
action" structures differs from the stability characteristics of the

triangulated structures discussed so far.
Plate action will be
covered 
in
depth later in this lesson when the structural stability of
buildings is 
presented. 

. 
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to Knowhere 

Page 116
 Building stability  Box girder bridge 

