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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 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 |
|
presented. |
|
|
. |
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Back
to Knowhere |
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Page 116
- Building stability - Box girder bridge |
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