Ecohomic Span Of Bridge

• The span of a bridge may be so determined that the total cost of the bridge is a minimum .The span to satisfy this condition is called the economic Span.The total cost of the bridge consists of the substructure and that of the Super Structure.

• The economic Span of a Bridge is the one which reduces the overall cost of a bridge to be a minimum.

• The overall cost of bridge depends upon the following factors.

1) Cost of Material and it's nature

II) Availability of skilled labour

III) Span Length

IV) Nature of Stream to be bridged

V) Climatic and other conditions

• It is not in the hands of Engineers to bring down the cost of living index or the price of the Materials like Cement,steel, timber, etc but they can help in bringing down the cost of bridges by evolving economical design .Considering only the variable items the cost of super structure increases and that of the sub structure decreases with an increase in the span length.

• Thus most economic Span Length is that which satisfies the following:-

The cost of the super structure= The cost of the sub structure

• Thus economic Span of bridge is calculated by:-

Economical

I=vp/a

Where p= Cost of one pier with its Foundation

I= Length of economic Span

a = Constant of variation