Introduction

• A bridge is a structure built to span physical obstacle without closing the way underneath such as a body of water, valley, or road, for the purpose of providing passage over the obstacle.
• If we stand at the middle of the plank, it sags even more. So, we must provide for the weight of whatever our bridge is designed to carry the ‘live load’.
• There are many different designs that such serve a particular purpose and apply to different situations. Designs of bridges vary depending on the function of the bridge.
• Designs of bridges will vary depending on the function of the bridge and the nature of the area where the bridge is to be constructed.

Types of Bridges

1. Beam Bridges
2. Cantilever Bridges
3. Arch Bridges
4. Suspension Bridges
5. Cable-Stayed Bridges
6. Truss Bridges
7. Floating Bridge

Beam Bridges

• Consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers.
• A beam or ‘girder’ bridge is the simplest and most inexpensive kind of bridge.
• In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers.
• The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.
• The beam itself must be strong so that it doesn’t bend under its own weight and the added weight of crossing traffic.

• 

Cantilever Bridges

• A cantilever bridge is a bridge built using cantilevers structures that project horizontally into space, supported on only one end.
• A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be crossed, meeting at the center.
• The cantilevers may be simple beams however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.

• 

Arch Bridges

• The arch has great natural strength. Thousands of years ago, Roman built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet.
• Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end.
• These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out.

• 

Suspension Bridges

• This kind of bridges can span 2,000 to 7,000 feet- way further than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
• They also tend to be the most expensive to build.
• These cables rest on top of high towers and are secured at each one end by anchorages.
• The towers enable the main cables to be draped over long distances.
• Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

• 

Cable-Stayed Bridges

• The cable stayed bridge is newer than the other type of bridge. Large upright steel supports are used to transmit the load into the ground.
• Two bridges support the load of the roadway in very different ways.
• The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end.
• In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.

• 

Truss Bridge

• All beams in a truss bridge are straight. Trusses are comprised of many small beams that together can support a large amount of weight and span great distances.
• Truss is a structure made up of three or more members which are normally considered to be pinned or hinged at the joints.
• Load applied to the truss is transmitted to joint so that each individual members are in either pure tension or compression.

• 

Floating Bridge

• Permanent floating bridges are useful for traversing features lacking strong bedrock for traditional piers.
• Such bridges can require a section that is elevated, or can be raised or removed, to allow ships to pass.
• Pontoon bridges are usually temporary structures, some are used for long periods of time.
• Pontoon bridges are supported by floating pontoons with sufficient buoyancy to support the bridge and dynamic loads.

•