Economical Span of Bridge

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

1. Cost of Substructure
2. 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