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Bridges |
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A bridge
is basically a beam that is designed to carry a live load such as
vehicles, people, |
and
animals across its span. That is, it carries dynamic loads in
addition to the dead loads |
of its
own weight and stationary objects. Therefore the stresses imposed on
a bridge are |
more
complex than on a structural beam member. Nevertheless the same type
of static |
analysis
can be used to see how a bridge reacts to loads in order to shed light on
the |
dynamics
of its behavior. In this section you will explore the structure and
behavior of |
beam, truss,
cantilever, suspension, arch, and box girder bridges. |
. |
Beam
bridge - typical span 10 to
200 m ( 30 to 600 ft.) |
. |
Probably
the oldest type of bridge used by humans or animals is simply a log fallen
across |
a stream
or gully, that is, a simply supported beam. Improvements were made
eventually |
to the
log to shape it into a more useable form. Light duty pedestrian or
short span road |
. |
 |
◄ Fig. 151 - Solid timber |
 |
beam bridge |
click image to enlarge |
Fig. 152 - Steel
Ι-beam |
girder bridge ► |
(scale visualization models) |
|
. |
bridges
can be constructed from solid, heavy timber beams. Modern highway beam
bridges |
are
typically constructed from rows of steel or reinforced concrete
I-beam
girders which |
carry
the roadbed, or deck. Since the maximum length a beam bridge can
efficiently and |
safely
span is limited by the bending moment exerted by a load mid-span,
Ι-beam
bridges |
are only
suitable for short spans of thirty to six hundred feet. They are
used extensively for |
highway overpasses, elevated walkways, and causeways, which use many short
bridge |
spans
joined together in series to cross wide stretches of ground. |
. |
Plate
girder bridge |
. |
An
extreme example of increasing the height of a beam to increase its
stiffness is the plate |
girder
bridge design in which the girder is many times taller in cross-section
than it is wide. |
The
thinness of the girder makes it susceptible to twisting, or torque,
stresses induced by a |
load, and
sideways wind loads. In a deck plate girder bridge, which carries
its load on top, |
Back to
Knowhere |
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Page 99 -
Building stability - Beam and plate girder bridges |
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