Bridges

UNIT – III

Chapter – 7

BRIDGES

 

BRIDGE TERMINOLOGY

A Bridge is a structure which provides passage over an obstacle without disturbing or closing the way beneath. The obstructions may be roads, highways, railway track, canals, rivers, valleys, etc. The required passages may be for pedestrians, road, canal, pipeline or railway track. The list of Bride Terminology is explained as follows:

1. Maximum Water Level (M.W.L.) or High Flood Level (H.F.L.): It is the level of water of .: the highest flood ever recorded in a river, for which the bridge is designed.

2. Ordinary Flood Level (O.F.L.): It is the flood level that normally occurs every year.

3. Low Water Level (L.W.L.): It is the minimum level of water in the river in the dry season.

4. Effective Span: It is the center to center distance between two adjacent supports.

5. Free Board: Free Board is the difference between maximum water level and the bottom surface of the Deck Slab. See Fig. 1.

6. Length of the Bridge: It is the overall length measured between the two end Wing Walls.

7. Water Way: Water Way is the area through which water flows under a bridge.

8. Cause Way: It is a submersible bridge which allows floods to pass over it. It is constructed on less important routes so as to reduce the construction cost.

9. Bed Level: Bed level of a river or stream is the lowest level of the river bed under bridge.

10. Afflux or Back Water: When a bridge is constructed, the structures like piers and abutments cause reduction of the natural water way. These structures are obstacles to the flow of water. Due to this obstruction, there is a rise in the level of water above the normal level on the upstream side. This rise of water level is defined as Afflux or Back Water.

11. Head Room: Head Room is the height between the lowest point of any protruding member of an under-bridge and highest point of a vehicle or truck passing under it.

12. Culvert: Culvert is a small bridge with a maximum span of 6 meters.

13. Scour Depth: Scour is the eroding of the bed material of the river due to heavy discharge caused by high velocity of water exceeding the normal velocity. Scour Depth has an adverse effect on the foundations of piers and abutments.

 

1. COMPONENTS OF A BRIDGE

See Fig. 1. A bridge can be divided into two major parts: 1. Superstructure and 2. Substructure.

1. Superstructure

Superstructure of a bridge is similar to the roof of a single storey building. It is the part of the bridge over which the traffic moves safely. It consists of a road or any other ‘flooring? over which pedestrians and vehicles can travel. These components are supported by girders, arches or suspension cables, etc. This system, in turn, rests on the Substructure. Sometimes, the 'flooring' may directly rest on the substructure.

Superstructure consists of the following components:

(i) Decking carrying the Roadway or Deck Slab

Decking consists of the Deck Slab, Girders, Trusses, Arches, etc. Deck Slab is the top horizontal slab. The surface on which traffic moves is the Deck Slab. It is laid over the Bearings of the Piers and Abutments. Deck slab is made of R.C.C. or Pre-stressed Concrete or Steel Girders.

(ii) Hand Rails

Hand Rails are provided on both sides of the bridge along its length. They prevent any vehicle or pedestrians from falling into the river by slip.

(iii) Guard Stones

Guard Stones guard the road users. They are provided on both sides of the road approaches along the edges of the footpaths. They are painted white. They indicate the presence of a bridge to the road users approaching the bridge at a distance. Hence, they prevent the vehicle users from striking the hand rails of the bridge.

(iv) Bearings

Bearings are provided above the piers and abutments. The longitudinal girders or beams of the deck slab rest on the piers and abutments by means of bearings. They are made of cement concrete. Bearings are provided for the following reasons:

(a) To distribute the load from superstructure uniformly over the piers and abutments.

(b) To allow the girders or beams to expand and contract due to thermal variations.

(c) To allow rotation of the girders at support due to the deflection of the girders.

2. Substructure

Substructure of a bridge is similar to the foundation, columns, walls, etc., which support the roof in a single storey building. The substructure consists of the following:

(i) Piers

Piers are intermediate vertical supports to the superstructure of the bridge. They are provided with foundation in the River Bed in between the two extreme supports called Abutments. They transfer the load from the superstructure to the foundation. Piers divide the length of the bridge into suitable spans and share the total load acting on the bridge.

There may be one, two or more piers depending upon the span between the piers as well as the total length of the bridge. The foundation of the piers is provided on the hard soil below the river bed. The end pier carries the load in only one direction. It is subjected to heavy unbalanced forced. Therefore, they are usually heavy.

(ii) Abutments

Abutments are the end supports to the superstructure of the bridge. They are constructed of either masonry or R.C.C. The purposes of abutments are:

(a) Abutments transfer the superstructure load (vertical load) to the foundation.

(b) Abutments laterally support and retain the earthwork of the embankment of the approaches.

(iii) Wing Walls

Wing Walls are constructed on both ends of the abutments. They retain and protect the earth filling of the approach road from the action of water.

(iv) Approaches to Bridge

Approaches to Bridge are the portions of the road constructed to reach the bridge from the general route. Approaches are provided to carry the communication route up to the Road Level of the bridge. They should be straight at least for a length of 15 meters on each side of the bridge.

(v) Foundation for the Abutments and Piers

Bridge foundations carry heavy loads. They are located at high depths from the ground. Therefore, either pile foundation or well foundation is adopted for bridge foundation. Functions of the foundation are:

(a) Foundations carry the load safely from the piers and abutments to the sub-soil.

(b) Foundations distribute the load over a larger area of the sub-soil. Therefore, no failure or uneven settlement of the sub-soil occurs.

3. Other Parts of the Bridge

(i) Revetments: These are provided for slopes at the abutments. They act as slope protection.

(ii) River Bed: River Bed is the top surface of the earth at the bottom of the river.