Selection of bridge type

SELECTION OF BRIDGE TYPE

The selection or choice of an appropriate type of bridge and planning of its basic features constitute a crucial decision to be taken by the bridge engineer. The designer must consider all the preliminary data available to him from the detailed investigation before arriving at a solution. The completed structure should be the most suitable selection.

Numerous factors influence the selection of a bridge. These factors are:

1. Desired traffic

2. Purpose of the bridge (river, road way, railway, etc.)

3. Type of foundation soil

4. Span of the bridge and number of spans

5. Loads including incident loads like wind force, earthquake force, etc., on the bridge [Refer Section 1 below.]

6. Head room to be provided below the bridge

7. Life of the bridge (permanent or temporary)

8. Type of superstructure to be provided (R.C.C., Steel Truss, Arch, Cantilever, etc.)

9. Materials of construction of the bridge (R.C.C., Steel, etc.)

10. Position of floor of the bridge (submersible or non-submersible type)

11. Relative position of floor of the bridge (deck or through or semi-through type)

12. Nature of alignment (straight or skew type)

13. Economy in construction and Cost factor

14. Aesthetically pleasing appearance

1. TYPES OF LOADS ON BRIDGES

The loads and forces to be considered in designing bridges and culverts are listed below:

1. Dead Load

Dead Load is the self-weight of the bridge and the weight of the permanent fixed structures like water line, etc.

2. Live Loads

Live Loads are loads of temporary and varying nature. Live loads on the bridges include traffic on the road and trains. Class AA Loading, Class A Loading and Class B Loading are the live loads classified by Indian Road Congress (I.R.C.).

Class AA Bridges

These are the bridges designed for Indian Road Congress (I.R.C.) Class AA loading and checked for Class A loading. They are provided within certain municipal limits, in certain existing or contemplated industrial area and along certain specified national highways.

Class A Bridges

Class A Bridges are the permanent bridges designed for I.R.C.  Class A Loading. The I.R.C. Class A Loading is based on the heaviest type of commercial vehicle which is considered likely to run on Indian roads. Therefore, this type of loading is to be normally adopted on all the roads on which permanent bridges as well as culverts are constructed.

Class A type of loading consists of one driving unit and two trailer units.

Class B Bridges

Class B Bridges are the temporary-designed bridges for I.R.C. Class B Loading. I.R.C. Class B Loading is the same as I.R.C. Class A Loading. The only difference is that the wheel loads should be 40% less than Class A Loading. This type of loading is adopted for temporary bridges and in specified areas.

Class B type of loading consists of one driving unit and two trailer units.

3. Impact Effect of the Live Load

Impact is caused due to the fast moving heavy vehicles over rough or uneven surfaces. Moving loads on a bridge has a jumping effect due to the uneven surface of the road way. The jumping causes vibration of the bridge structure. It is called Impact Effect of the live Load.

4. Wind Load

Wind exerts a horizontal force, called Wind Load, on the bridge.

5. Longitudinal Force

It is produced on a bridge due to the application of brake of vehicles.

6. Force due to Water Currents

The pressure of water flowing under bridge on the supports is called Force due to Water Currents.

7. Centrifugal Forces

Centrifugal Force occurs due to the moving of vehicles on a curve. That is, if a bridge is located on a curve, its structure is acted upon by the centrifugal action of the moving vehicles. Hence, the bridge members should be designed to carry safely the stress induced by the centrifugal force.

8. Earthquake Forces

If a bridge is situated in a region subjected to earthquakes, allowances should be made in the design for the Seismic Force. les bridge is situated for resic