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g. Interruptions to travel over the structure from opening the movable
span.
h. A somewhat greater obstruction of the thoroughfare in respect to
vessel traffic as well as to the passage of the water, in case of
the adoption of a swing span with its protection.
A thorough consideration of all these criteria will have to be made
before a logical conclusion can be reached as to which type of layout is
preferable.
The correct ratio of first costs for a low-level bridge and a high-level one
of the same capacity and strength at any proposed crossing must, of
course, be determined from layouts, computations of quantities of materials,
and estimates based upon current prices of the said materials and of labor.
It is sometimes necessary, however, to make a hurried economic comparison; and as an aid in so doing the author has established the following
results, based upon assumed conditions that may properly be deemed average or normal.
Let us assume a double-track-railway crossing of a river like the Missouri, requiring 1,500 feet of main spans with a vertical clearance of 50-feet
above high water for passing vessels, a variation of twenty-five feet between extreme stages of water, a horizontal bed-rock 75 feet below high-water elevation, one bank high and sloping back from high-water line at a
ratio of three horizontal to one vertical and the other bank level and very
low; and let the approach grades be one per cent. Under such conditions the deepest water will nearly always be found comparatively close
to the higher bank, and the position of channel will be permanent; consequently it would be legitimate to employ a single through span over the
second opening from the high-bank end.
With certain assumed medium unit prices for materials in place, the
minimum costs of these two structures proved to be as follows:
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