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Revolving draw-spans are required when bridges across navigable streams are not high enough above the water to provide the proper vertical clearance for passing vessels. Before taking up the discussion of draw-spans, it will be well to consider the relative advantages and disadvantages of high and low bridges for the crossing of such streams as the Mississippi, the Missouri, and the Arkansas rivers.
As a rule, there is very little difference in the first cost of a high and of a low bridge for such a crossing, what little there is being in favor of the latter and seldom amounting to more than ten per cent. Each pier of a low bridge is cheaper than the corresponding pier of a high bridge; but this saving is offset by the cost of the pivot-pier, which is extra. The superstructure of a low bridge may be a trifle lighter than that of the corresponding high bridge, but the more expensive metal-work of the draw-span generally overbalances this. It is in the low, short trestle approaches that the low bridge costs less than the high one.
As these approaches are generally built of timber, they have to be renewed about once in every eight years, and the cost of renewal is a regular fixed charge, which lessens the annual net income from the bridge.
Herein lies the superiority of the low bridge for such crossings. Nor is this its only advantage, for, by its adoption, there is generally avoided a considerable climb at each end of the structure. On the other hand, the low bridge involves some expense for operation, which is quite an important matter when there is much river traffic, but which is of slight importance when
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