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Saturday, December 27, 2008

what are the Basic types Bridge

In general there are five types of bridge;
1. Girder Bridge
2. Arch Bridge
3. Cable Stayed
4. Rigid Frame Bridge
5. Suspension Bridge


1. Girder Bridge

A girder bridge is perhaps the most common and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In modern steel girder bridges, the two most common girders are I-beam girders and box-girders.

If we look at the cross section of an I-beam girder we can immediately understand why it is called an I-beam (illustration #1.) The cross section of the girder takes the shape of the capital letter I. The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. To explain why the I shape is an efficient shape for a girder is a long and difficult task so we won't attempt that here.

A box girder is much the same as an I-beam girder except that, obviously, it takes the shape of a box. The typical box girder has two webs and two flanges (illustration #2.) However, in some cases there are more than two webs, creating a multiple chamber box girder.

Other examples of simple girders include pi girders, named for their likeness to the mathematical symbol for pi, and T shaped girders. Since the majority of girder bridges these days are built with box or I-beam girders we will skip the specifics of these rarer cases.

Now that we know the basic physical differences between box girders and I-beam girders, let's look at the advantages and disadvantages of each. An I-beam is very simple to design and build and works very well in most cases. However, if the bridge contains any curves, the beams become subject to twisting forces, also known as torque. The added second web in a box girder adds stability and increases resistance to twisting forces. This makes the box girder the ideal choice for bridges with any significant curve in them.

Box girders, being more stable are also able to span greater distances and are often used for longer spans, where I-beams would not be sufficiently strong or stable. However, the design and fabrication of box girders is more difficult than that of I beams. For example, in order to weld the inside seams of a box girder, a human or welding robot must be able to operate inside the box girder.


2. Arch Bridge

After girders, arches are the second oldest bridge type and a classic structure. Unlike simple girder bridges, arches are well suited to the use of stone. Many ancient and well know examples of stone arches still stand to this day. Arches are good choices for crossing valleys and rivers since the arch doesn't require piers in the center. Arches can be one of the more beautiful bridge types. Possibly the oldest existing arch bridge is the Mycenaean Arkadiko bridge in Greece from about 1300 BC.

3. Cable Stayed

A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. A typical cable stayed bridge is a continuous girder with one or more towers erected above piers in the middle of the span. From these towers, cables stretch down diagonally (usually to both sides) and support the girder.

4. Rigid Frame Bridge

Rigid frame bridges are sometimes also known as Rahmen bridges. In a standard girder bridge, the girder and the piers are separate structures. However, a rigid frame bridge is one in which the piers and girder are one solid structure.


5. Suspension Bridge

Suspension bridge: construction that allows automobiles to travel between two points separated by an obstacle.
Side span: segment between two pylons at the ends of a bridge.
Centre span: segment between two pylons at the centre of a bridge.
Side pylon: tower-like vertical construction situated at the side, usually supporting the cables of a suspension bridge or a cable-stayed bridge.
Foundation of a pylon: very durable lower part of a tower.
Suspender: support cable.
Suspension cable: set of braided wire that supports a bridge.
Pylon: tower-like vertical support that usually supports the cables of a suspension bridge or a cable-stayed bridge.
Stiffening girder: tightener beam.

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