A bridge is a structure providing passage over an obstacle without closing the way beneath.
The required passage may be for a road, a railway pedestrian or a canal of a pipeline. The obstacle to be crossed may be river, a road, a railway or a valley.
CLASSIFICATION OF BRIDGES
Bridges can be classified into various types depending upon the following factors:
Materials used for construction : Under this category bridges may be classified as:
a) Timber bridges b) Masonry bridges
c) Steel bridges d) Reinforced cement concrete bridges
e) Pre-stressed concrete bridges f) Composite bridges
Alignment : Under this category, the bridge can be classified as
a) Straight or square bridges and b) Skew bridges
Straight or square are the bridges which are at right angles to the axis of the river.
Skew bridges are not at right angles to the axis of the river.
The Relative position of bridge floor: Under this category, the bridge and classified as
a) Deck bridge b) Semi through bridge and c) Through bridge
Deck bridges are the bridges whose floorings are supported at the top of the super structure. Through bridges are the bridges whose floorings are supported at the bottom of the super structure. Semi-through bridges are the bridges whose floorings are supported at some intermediate level of the super structure.
Function of Purpose : Under this, the bridge can be classified as
a) Highway bridge b) Railway bridge c) Foot bridge
d) Viaduct and e) Aqueduct etc.
Position of High floor level : Under this, the bridges may be classified as
a) Submersible bridge and b) Non-submersible bridge
Submersible Bridges are the bridges whose floor levels are below the high flood level. During flood seasons, it allows the water to pass over the bridge submerging the communication route. In economic point of view, these bridges are constructed.
Non-submergible bridges are the bridges whose floor levels are above the high flood level.
Life : Under this, the bridges may be classified as
a) Permanent bridges b) Temporary bridges
Type of Superstructure : Under this, the bridges may be classified as
a) Arch bridges b) Truss bridges
c) Portal frame bridges d) Balanced cantilever bridges
e) Suspension bridges etc.,
Span length : Under this category, the bridges can be classified as
a) Culverts (span less than 6m)
b) Minor bridges (span between 6 to 30m)
c) Major bridges (span above 30m
d) Long span bridges (span above 120m)
Loading : Road bridges and culverts have been classified by Indian raod congress into
a) Class AA bridges b) Class A bridges
c) Class B bridges according to the loadings they are designed to carry.
COMPONENT PARTS OF A BRIDGE
Broadly, a Bridge can be divided into two major parts.
1. Sub structure 2. Super structure
1. SUB STRUCTURE
The function of the sub structure is similar to that of foundations, columns and walls of a buildings, becaus e it supports the super structure of the bridges and transmits the load safely to the ground.
The substructure consists of the following:
a. Abutments b. Piers c. Wi ng walls d. Approaches
e. Foundations for the piers and abutments
a. Abutments: The end of su perstructure of a bridge is called abutments.
Its main functions are
1. To laterally support t he earth work of the embankment of the approac hes.
2. To transmit the load f rom the bridge superstructure.
3. To give final formati on level to the bridge.
Bridge abutments can be made of brick masonry, stone masonry, plain concrete or reinforced concrete
b. Piers : Piers are the intermediate supports for the superstructure. Piers transmit the loads from the superstructur e of the bridge to the foundations. A pier essent ially consists of a column or shat and a fo undation. They may have different configurati ons as shown in figure. These piers may be constructed with stone masonry or concrete.
c. Wing walls : These are th e walls provided at both ends of the abutments to retain the earth filling of the approach road. They are constructed of the same materi al as those of the main abutment.
d. Approaches: The portio n of the road constructed to reach the bridg e from their general route or height is known as approach of the bridge. The alignment and the level of the approaches mainly dep end on the design and layout of the bridge.
e. Foundations for the Piers and Abutments : The foundation of a bridge structure distributes the load from the piers and abutments over the larger area of sub soil. It prevents the tilting and ov er-turning of the piers and abutments and also unequal settlement of the sub soil.
The different types of funct ions adopted for bridges are:
i. Spread foundation ii. Raft foundation iii. Pile foundation
iv. Caisson foundation v. Well foundation
2. Super structure : The super structure is that part of the bridge over which the traffic moves with safely.
It consists of:
a. Decking b. Parapet or hand rails, guard stones etc. c. Bearing
a. Decking : It is provided to allow the road surface to be built in over it. It may consist of a slab, trusses, arches etc.
b. Parapet or Hand rails, guard stones: These are the protective works provided on both sides of the deck along the roadway in order to safe guard the moving vehicles and the passengers on a bridge. Foot paths are also provided for pedestrians to walk along the bridge. In order to prevent a vehicle from striking the parapet wall of the hand rails, guard stones painted white are provided at the ends of the road surfaces.
c. Bearing: It is part of the bearing structure provided to distribute the load coming from the superstructure and also to allow for longitudinal and angular movements.