This is a simplified description of the basic components and types of bridges.

There are two categories of bridges, fixed and moveable, with several types 
being used in each of these categories.

Here are the descriptions of the basic componnents of fixed bridges.

1. Beams-The horizontal members of a bridge.
       Some simple beam bridges were logs spanning small treams.
       Later hewn beams were used and today steel girder beams are used.
2. Trestles-The supporting members of a bridge
       Trestles are used on bridges which require intermediate supports.
       Timber trestles were common in the early days of railroading,
       many being built from the trees cut on the right of ways.  
       Pile trestles came later, being made from trees which were treated
       with creosote.  These were driven into the soil to provide a etter
       bearing and then fastened together with various size timbers.
       Today trestles are built of steel from a variety of shapes such
       as "I", "WF" and "T" shaped steel.
       Another support which functions like a trestle is the Pier or column,
       usually made of concrete and shaped to allow easy flow of water.

3. Girders-Fabricated beams
       These fabricated beam types can either carry the deck on top or have
       the deck pass between the sides.  Plate steel riveted or welded to
       angle steel formed the sides of these girder type bridges.
       These sides are fastened to each other by angles and other shapes
       such as "I", "WF", "T" and channel shaped steel.

4. Arches-The circular structural member which spans the obstacle and 
       supports the bridge deck.  Early arch bridges were made from stone,
       the viaducts being examples of this type bridge.
       Some later arch bridges were made from brick in the same fashion as 
       the stone ones.

       Concrete arches are used to span small streams or drainage ditches.

6. Trusses-Structural members which form a rigid framework to support
       bridge decks.
       Trusses can be constructed to allow the deck to pass through or above.

       Here are some Truss types
		Deck Warren
		Parker or Camelback

	These types have been built with both wood and steel.

7. Suspension Type-The deck is suspended from a series of vertical cables
       connected to a main cable which is anchored at each end.
       The cable is run over two or more towers and each section is suspended
       from the main cable.

8. Cantilever Type-The deck is carried by a through truss bridge which is
       anchored at one end only.
	A. Wooden Construction
	B. Steel Construction

	All of the above types of bridges can be built either stationary
	or moveable.
	Often on longer spans several types will be combined to make up the
	entire span.

9. There are four types of movable bridges:
	A. Bascule, of which there are several types. All rotate around
	   a pinion with the bridge(leaves) being counterbalanced by a weight.
	   this configuration takes little power to move them.
	   The trunnion type rotates around the pinion with simple bearings.
	   These are often operated with hydraulic cylinders.

	B. The Scherzer type has a gear affixed to the pinion shaft.
	   As this pinion is pulled back along a fixed rack the pinion
	   rotates and opens the bridge leaf.  When the pinion is pushed
	   along the fixed rack it rotates back to the closed position.

	C. Retractile
	   This type of bridge is mounted in a skewed position on trucks or        	   carriages on one side of a body of water. To open the channel to
	   waterway traffic, the bridge is "retracted" or withdrawn to shore.
	   Bridges of this design are considered obsolete because a relatively 
	   large piece of land has to be condemned - that is, left unusable - 
	   for their operation. However, such bridges are fairly simple to 
	   operate because of the relatively small amount of electrical and
	   mechanical equipment needed to run them. 

	D. Swing Span
	   Typically, these are bridges supported on a center pier in the 
	   middle of a waterway. They are opened by rotating horizontally on 
	   wheels riding on a circular track. In this way, two channels are 
	   provided on either side of the bridge. 
	   Bridges of this type require waterways of considerable width; in 
	   addition, they are slow to operate and restrict the width of the 
	   channel. Because of this, they are rarely built today.

	E. Vertical Lift
	   A vertical lift bridge is a movable bridge which carries roadways
	   or walkways, and which can be raised and lowered like an elevator
	   in a building. Vertical lift bridges are raised and lowered using 
	   supporting end cables that are attached to rotating drums
	   in the towers on each side of the bridge.

10. Temporary Movable Spans(not used for railways)
	   Pontoon bridges fall into this category.
	   Various other Military bridges have been developed.  	

S.A. McCall