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


beam bridge.jpg

◄ Fig. 151 - Solid timber

Ibeam bridge.jpg

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,

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Page 99 - Building stability - Beam and plate girder bridges

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