Definition of Bridge
Bridges are the structures built for carrying mostly road and railway traffic or any other moving loads over a depression or gap or obstruction such as a river, channel, canyon and valley. Classification of Bridgebased on various aspects are addressed in this article.
Types of Bridges
If a steel bridge is constructed to carry highway traffic it is called a Highway bridge. If it is constructed to carry railway traffic then it is called a Railway bridge. There is also a combined highway and railway bridges are constructed to carry both the traffic. Some bridges which are constructed exclusively for pedestrians, cycles, and animals are known as Foot bridges. Also, those bridges which are constructed to carry canals and pipelines are known as aqueduct bridges.
A bridge may be a culvert, high-level bridge or submersible bridge. A culvert is a bridge having a gross length of six meters or less between the faces of abutments or extreme vent way boundaries and measured at right angles thereto. A high-level bridge is usually constructed to carry a roadway above the highest flood level. A submersible bridge is a bridge which is designed to be in a submersible condition during the floods.
A bridge may be fixed or movable type. A fixed bridge is the one which is constructed to remain in its original position always. A movable bridge is the one which can be flexibly moved either horizontally or vertically usually to pass a channel or traffic or to allow a river.
A bridge can be classified as a deck type or through type. A deck type bridge is the one in which the roadway/railway floor rests on the top of the supporting structures, while a through bridge is the one where the roadway/railway floor rests on the bottom of the main load supporting structure. However, when the floor lies between the top and bottom of the main supporting structure, it is known as half through type bridge, semi-through bridge or pony bridge.
Classification of Steel Bridges
The steel bridges are classified according to the following criteria:
Classification of Bridge according to type of structural arrangement:
Under this criterion steel bridges are further classified as:
When the span is small, simple I-girders are used as main load carrying members. These types of sections are used to build culverts of moderate depth. But this is not sufficient for wider spans and built-up plate girders are used in such conditions to carry larges loads.
Plate girder bridge
Plate girders are most popular as railway bridges. The main advantage of them is, this plate can be transported in one piece. But the limiting depth of plate girders is only 3 to 4 m.
Truss girder bridge
When the structural requirements demand a depth more than 4m, a truss girder bridge is preferred. Truss bridges are commonly used for spans ranging from 20 m to 200 m.
For longer spans more than 200 m usually steel arch bridges or suspension bridges are provided.
Classification of Bridge according to the structural action:
They are further classified as:
Simply supported bridge
Simply supported bridges are used when the width of gap is small and only single span of bridge is needed.
Continuous span bridges
When the width of the gap is quite large, and where there is no chance of uneven settlements, a continuous bridge is used. Because of continuity in spans, moments are generally developed at pier supports resulting in the reduction of stresses at the inner spans.
In the case of a three-span continuous bridge loaded with a uniformly distributed load over all three spans, it is observed that there are two points of contraflexure in the central span. Hence to avoid this, the continuous beam/girder in the middle span is cut at the two points of contraflexure and then shear resisting joints are provided at these two points. The resulting configuration will be a cantilever bridge with a central suspended span between these two newly formed joints. This bridge consists of two simple spans, one at each end, each having an overhanging or cantilever portion along with a simple span in between the two cantilever portions.
For economic and aesthetic ground, arch bridges are generally used over deep gorges. But they usually require strong abutments to resist the thrust from the arches.
Rigid frame bridge
Rigid frame bridges are used for dry-over or under-crossing if the gaps are between 10 to 20 m. This steel framework consists of steel columns and steel girders with continuity at the knee.
Classification of Bridge according to the floor location:
Deck type bridge
For deck type bridge, the floor is commonly placed on the top of the flange in the case of a plate girder bridge and on the top chord in the case of a truss bridge. Therefore no top bracing is required.
Through type bridge
In the case of through type Bridge, the floor is placed at the level of lower chord of truss type bridge and the top chord is braced laterally.
Half through type bridge
In half through bridge, the floor lies between the top and the bottom. There are also double deck bridges which are constructed to carry the traffic on both roadways and railways at the same time.
Classification of Bridge according to the type of connections:
They are further classified as:
- Riveted bridges
- Welded bridges
- Bolted bridges
Majority of the bridges are constructed as riveted bridges in earlier days. But now the most popular method of construction is to use welded connections. Also in the past, bolted or pin-jointed bridges are made but now that construction method is abandoned, because they are less rigid and require constant maintenance.
Classification of Bridge according to movement of structural part of bridge:
Based on this criterion, bridges can be of two types:
- Fixed or permanent bridges
- Movable bridges
A fixed type bridge is the one which always remains in one position. A majority of bridges are of this type. A movable bridge is the one which can be opened either horizontally or vertically so as to allow the river or channel traffic to pass.
- Movable bridges are of the following types:
- Swing bridge
- Rolling bridge
- Floating swing bridge
- Transporting bridge
- Vertical lift bridge