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a pound cheaper in New York than in San Francisco, whereas concrete
and timber were about 40 per cent more expensive in New York.
Again, early in 1915 structural metal erected was let in a few cases as
low as three cents a pound, with prices of other materials correspondingly
small; while about two years later the said metal erected was for a short
time as expensive as ten cents a pound, the prices of other materials showing an increase of less than 50 per cent. In January of 1920, structural
metal erected was quoted as low as 7 cents a pound, while other prices
were much higher than in 1917. Economic comparisons in 1920 are, therefore, quite similar to those in the pre-war period, while those of 1917 were
decidedly different from those of either of the other dates.
The foregoing examples are sufficient to indicate the fact that, when
close economic comparisons are to be made, carefully selected unit prices
must be used. The statement at the beginning of this chapter to the effect
that such variations will seldom radically affect economic comparisons, is
nevertheless correct. The result of an economic study made with normally-balanced unit prices will rarely be in error by any serious amount.
This question will be discussed further in the various chapters dealing
with comparative economics.
In connection with the elaborate series of computations made by the
author in the preparation of his monograph on "Economic Span-Lengths
for Simple-Truss Bridges on Various Types of Foundations," he took occasion to figure three sets of economic curves for low-level, double-track,
steam-railroad bridges on sand foundations, one with normal unit prices
for all materials in place, another for extremely high prices, and the third
for extremely low prices. The various unit prices for each case were adjusted according to the author's best judgment, based upon an experience
in bridge estimating extending continuously over a longer period of years
than he likes to acknowledge. The results of this comparison are given in
the following table:
TABLE 4a
Economic Span-Lengths for Double-Track, Steam-Reailway Bridges on Sand
Foundations at Various Depths Below Extreme Low-Water |