Where the spans are not controlled by features crossed—such as roads, railroads, streams, or existing buildings—and there is freedom to locate piers, the lengths of spans will be controlled by aesthetic, economic, and structural requirements. Generally, from an aesthetic standpoint, spans should have a length at least 3 or 4 times the pier height.
The profile of the site crossed will influence the span proportions. On the uphill end of a crossed hillside, the end spans will be shorter than at the bottom of the valley. The type of bridge will also affect the selection of span ratios, from both aesthetic and structural standpoints. Where spans are continuous, the end span should not be made too short, because uplift may occur under live load, and loss of positive reaction at the abutment will occur sooner if the abutment settles.
The most economical bridge will generally not be either the one with the most economical superstructure or the one with the most economical substructure, but the one with the least combined cost. That determination is made by performing a cost study wherein a number of different span lengths are investigated, along with the cost of their substructures. To be meaningful, the superstructure and substructure designs should be fairly detailed. The superstructure and substructure costs are then plotted. The optimum span length will be at the low point of the combined cost curve. A typical cost study curve is shown in Fig. 4.11.
