Bridges
Bridges
Structures built for crossing on obstruction are called bridges. The obstruction is called bridges. The obstruction is in the form of rivers, channel, railways lines or roads. Bridges have space below for the movement of water or vehicles.
Classification of Bridges
(a) According to the usage
(i)Road/Highway Bridge
(ii) Railways Bridge
(iii) Foot bridge
(iv) Aqueduct bridge.
(v) Viaduct Bridge
(b) According to the materials
(i) R.C.C Bridge
(ii) Pre-stressed and composite bridge.
(iii) Steel bridge
(iv) Concrete bridge
(v) Masonry bridge
(vi) Timber bridge
(c) According to the deck slab position
(i) Thorough bridge
(ii) Semi-through bridge
(iii) Deck bridge
(d) According to the structural form
(i) Arch bridge
(ii) Slab bridge
(iii) Beam bridge
(iv) Suspension Bridges
Components parts of bridges
(a) Sub structure
(b) Super structure
(a) Sub structure
It supports the super structure of the bridge and transmits the load safely to the ground.
It consists of
(i) Abutments
(ii) Piers
(iii) Wing –Walls
(iv) Approaches
(v) Foundation for piers and abutments
(b) Super structure
It is a part of a bridge over which the traffic moves with safety. It consists of
(i) Deck slab
(ii) Parapet or hand rails, guard stones
(iii) Bearing
Components of a Bridge
A bridge consist of
(a)Deck with rod surface
(b) Piers
(c) Abutments
(d) Foundations
(e) Bank connections
(f) Road Approach
(g) Hand rails
(h) Guard stones
(a) Deck with road surface:
Deck is the top horizontal portion of the bridge. It may be made of R.C.C or pre-stresses concrete or steel girders and joist.
(b) Piers
The intermediate supports that transmit the load from the deck to the foundation. It may be constructed to masonry, concrete or R.C.C. There may be one or more piers depending upon the length of the bridge and span between the piers. The height of the abutments and piers are equal.
(c) Abutments
Two end supports of a bridge are called abutments. Piers, abutments carry the vertical loads. Bank connection like depends up on the nature of the subsoil, velocity of water etc. Well foundation is the most commonly adopted bridge foundation.
(e) Bank connections
Bank connection like wing walls and return walls provide a connection between the bridges abutments and road approach.
(f) Road approach
Road approach is in the form of embankment properly protected on the slopes by revetment.
(g) Handrails
It is provided on both sides of bridges to prevent users, vehicles from falling in to the stream. An is erected along the length of the bridge.
(h) Guard stones
It indicates the presence of a bridge to the road users approaching the bridges at a distance. It is available on both sides of the bridges. They guide the vehicles for a safety entry on to bridges
Factors influencing the selection of bridges
>Span of the bridges and no. of spans.
>Purpose of bridges (road, foot, bridge etc).
>Intensity of loading on the bridge.
>Type of super structure adopted (truss, framed, arch, cantilever, or suspension bridge).
>Headroom to be provided below the bridge (bow string girders for high head room)
>Type of foundation soil, Available.
>Nature of material used.
>Nature of alignment (Straight or skew bridges)
Relative position of floor of the bridge (through semi-through or Deck Bridge)
>Prevailing wind force and earth –quake possibilities.
>Life expectancy of the bridges (temporary or permanent bridges)
Types of bridges
(a)Arch bridge
Arch bridge can be used up to spans of about 200 meters. Arches may be filled spandrel (or) open spandrel type. They require very stable abutments to withstand inclined end reactions. There will bw no bending anywhere in the arch. The Vibration due to impact forces is minimum.
(b) Bow string Girder Bridge
Bow string girder bridges are economical when sufficient head room is needed under a bridge. The roadways are supported by cross girders that rest on main beams. The main beams are hung from the arch by hangers. This bridge is adopted for spans varying from 30 m to 35 m. the load carrying structure consisting of the arch, hangers and tie known as bow girders.
(c) Cantilever bridge
One cantilever extends from each support to cover one span. Central beam is provided between the free ends of the cantilever. The cantilever is counter balanced by counter weights.
(d) Girder and Sub Bridge (or) T-beam Bridge
Longitudinal beams and deck slab form a shape like T. cross beams are connecting T-beams. It transmits the load from the slab to the piers and abutments. Piers and abutments may be R.C.C or masonry. Kerb of R.C.C. is provided on either side of the roadways. T-beam Bridge may be used up to a span of about 20 m. it is cheaper than steel bridge. Maintenance cost is less than that of steel bridges. It is durable. Compact and presents a neat appearance.
(e) Suspension bridges
This type is much larger than any other bridge. Tall towers are erected on piers and abutments form the end supports. Two sets of cables are hung in the shape of catenary from tower to tower. Cables are the most important parts of high tensile steel. The roadways are suspended from the cables by means of suspenders.
(f) Plate Girder Bridge
The deck slab of plate Girder Bridge is made of R.C.C or steel plate. Longitudinal ribs support the deck slab/plate. These ribs are carried by transverse floor beams that rest on the plate girders. Laterals bracings are provided both eat the top and bottom of the girders to take the wind pressure.
(g) Through type steel bridge
The track (railways bridge or the deck slab (highway bridge) is carried by stringers running longitudinally. The cross girders are connected to the main girders. Sway bracings and knee bracings are provided at the top of the bridge.
(h) Slab bridge (R.C.C slab Bridge)
It is a single span bridge and is suitable for spans not more than 10 m. it consists of reinforced cement concrete slab of uniform thickness. The ends of reinforced cement concrete slab of uniform thickness. The ends of the slab rest on abutments of bricks masonry , stone masonry or R.C.C. over the slab is laid a wearing coat of cement concrete of thickness 75 mm. Kerb of brick masonry or R.C.C are provided on either side. Parapet wall is also provided on both sides of the bridges. It is the Simplest form of bridge and easiest to construct.
Important terms
(a) Culvert
It is a small bridge constructed to allow water flow across, a railway track or road embankment. It may be slab culvert, arch culvert, box culvert or a big pipe.
(b) Causeway
It is a low level submersible bridge. It allows water to flow over it. It is used only where the traffic is light.
(c) Submersible bridge
It is a high level cause way
(d) Bed level
Bad level of a river or stream at the site of the bridge refers to the lowest level of the bridge (i.e) river bed.
(e) High flood level (HFL)
HFL is the highest level recorded at a point of stream or river.
(f) Afflux (or) back water
The rise in water on heaping up of water on the upstream side of the bridge is known as AFFLUX.
(g) Water way
The area through which the water flows under a superstructure is known as water – way of bridges.
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