What is a bridge?

A bridge is a structure that spans a divide such as:

• A stream/river/ravine/valley.
• Railroad track/roadway/waterway.

Function of A Bridge

A bridge has to carry a service (which may be highway or railway traffic, a footpath, public utilities, etc.) over an obstacle (which may be another road or railway, a river, a valley, etc.) and to transfer the loads from the service to the foundations at ground level.

Classification of Bridges

• According to functions : aqueduct, viaduct, highway, pedestrian etc.
• According to materials of construction : reinforced concrete, prestressed concrete, steel, composite, timber etc.
• According to form of superstructure : slab, beam, truss, arch, suspension, cable-stayed etc.
• According to interspan relation : simple, continuous, cantilever.
• According to the position of the bridge floor relative to the superstructure : deck, through, half-through etc.
• According to method of construction : pin-connected, riveted, welded etc.
• According to road level relative to highest flood level : high-level, submersible etc.
• According to method of clearance for navigation : movable-bascule, movable-swing, transporter
• According to span : short, medium, long, right, skew, curved.
• According to degree of redundancy : determinate, indeterminate
• According to type of service and duration of use : permanent, temporary bridge, military

General Span Types

Basic Types of Bridges

• Girder/Beam Bridge
  • The most common and basic type
  • Typical spans : 10m to 200m

• Truss Bridge
  • Truss is a simple skeletal structure.
  • Typical span lengths are 40m to 500m.

• Arch Bridges
  • Arches used a curved structure which provides a high resistance to bending forces.
  • Both ends are fixed in the horizontal direction (no horizontal movement allowed in the bearings).
  • Arches can only be used where ground is solid and stable.
  • Hingeless arch is very stiff and suffers less deflection.
  • Two-hinged arch uses hinged bearings which allow rotation and most commonly used for steel arches and very economical design.
  • The three-hinged arch adds an additional hinge at the top and suffers very little movement in either foundation, but experiences more deflection. Rarely used.
  • The tied arch allows construction even if the ground is not solid enough to deal with horizontal forces.

• Cable Stayed
  • A typical cable-stayed bridge is a continuous deck with one or more towers erected above piers in the middle of the span.
  • Cables stretch down diagonally from the towers and support the deck. Typical spans 110m to 480m.