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As these last figures reverse the previously found economics of the two
types, it is evident that for bridges of high-alloy steels the span length
for equal cost is vitally dependent upon the pound price of the said alloy
steel, the lower it is the more favorable is it to the cantilever structure.
In view of the fact that at present no one has any idea of what the cost
per pound will be for high-alloy steels used in future long-span-bridge
construction, it will be well to adopt temporarily as correct the author's
before-mentioned surmise, viz., that in alloy steel bridges carrying railway
loads only, the span length for equal cost is that for which, in the cantilever bridge, there are required 4 1/2 pounds of metal to sustain 1 pound of
live load.
The author recognizes that a change in the assumed conditions would
modify somewhat all the previously found span-lengths of equal cost for
both carbon-steel and nickel-steel bridges; but he does not believe that
the variation will be material—say not to exceed 2 or 3 per cent in any
case for any one fundamental change, or 5 per cent for any probable combination of changes. For instance, if the main piers rest on piles instead of going to bed rock, this will militate a little against the suspension structure, increasing slightly the span length for equal cost. The same effect occurs if the pound price for steel cables be increased without changing the pound prices for the other metals, and vice versa.
If the unit prices for substructure be decreased, the result will be
favorable to the suspension bridge, because, while the main piers will be
affected about alike, there will be a greater saving in the anchorages of the
suspension bridge than in the anchor piers of the cantilever structure.
Let us see what effect it would have to reduce the prices of all concrete
work five dollars per cubic yard, thus bringing them close to the lowest
limits for truly-first-class construction that have existed in periods of
national depression.
In the railroad bridges of 2,700 feet span, the reduction in total cost of substructure would be $473,000 for the cantilever bridge and $928,000 for the suspension bridge, making the total costs, respectively, $14,796,000
and $14,330,000. Performing the corresponding reduction in prices of substructure for the 2,400-foot spans gives, for the total costs, respectively, $9,877,000 and $11,196,000. Plotting these points on a cross-section
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