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


(static demonstration models)

click image to enlarge

typical cross-sections



to resist twisting forces better than the Ι-beam 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.





cross-section 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



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Page 116 - Building stability - Box girder bridge

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