Fixing length of typical span in design of bridges across river/elevated road/metro project is very important structural design decision. The cost of building one typical span can be broadly divided into following two components

- Cost of Substructure
- Cost of Superstructure

The cost of substructure is covers cost of bridge bearing, pier cap/pier head, pier and foundation (Open/Pile/Cassion). To take decision on most economical length of typical span, initial design and cost estimate is done for four to five different span lengths.

A graph is plotted with span length as abscissa and cost as ordinate. The graph is completed using cost of superstructure and substructure for all different spans. Figure 1 shows typical graph of Span versus cost. Point A on this graph correspondence to the economical span for the bridge project.

Figure 1 Span Versus Cost for Superstructure/Substructure

the graph will be informative if you can add the total cost vs span as well

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