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