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 Howe Warren Baltimore Pennsylvania "K" Pratt 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