The outline design of a long bridge which requires several spans, involves many considerations:

(1) If the river is navigable, at least one deep and wide channel must be left for navigation. The placing of piers, the clear height of the spans above high water, and the general plans of all bridges over navigable rivers, are subject to the approval of the United States Government.

Fig. 70. Retaining Wall for Railroad Embankment.

Fig. 70. Retaining Wall for Railroad Embankment.

(2) A long bridge always requires a solution of the general question of few piers and long spans, or more piers and shorter spans. No general solution of the question is possible, since it depends on the required clear height of the spans above the water, on the required depth below the water for a suitable foundation, and on several other conditions (such as swift current, etc.) which would influence the relative cost of additional piers or longer spans. Each case must be decided according to the particular circumstances of the case.

(3) Even the general location of the line of the bridge is often determined by a careful comparison, not only of several plans for a given crossing, but even a comparison of the plans for several locations.