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ECONOMICS OF BRIDGEWORK |
Chapter XXXI |
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Through the courtesy of the "Bridge or Tunnels Committee" of the
"Public Belt Railroad Commission," the author is permitted to utilize for
this memoir the results of the transbordeur investigations which he made
for the said Board. His design for that transbordeur involved multiple
spans, double track, multiple cages, and rapid transit, the speed of travel
reaching a maximum of thirty (30) miles per hour. Comparing this with
the before-mentioned existing single-span, single-track, single-cage, slow-motion structures, it is evident that the said investigation has revolutionized transbordeur designing by raising the carrying capacity per hour to something like that of the corresponding low-level bridge, when due consideration is given to the time lost by reason of passing vessels.
In the New Orleans investigation it was necessary to make estimates of
cost, based on ante-bellum unit prices, for both high-level and low-level
combined-highway-and-street-railway bridges, notwithstanding the fact that,
in all probability, the latter type would be inadmissible; and estimates
were added for four transbordeurs, two to carry cages one hundred feet
long and the others to support cages of half that length. The results of
these computations are given in the following table:
Table 32a
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| Type of Structure | Total Cost of Structure
Based on Ante-bellum
Unit-Prices |
| High-Level Bridge | $5,590,000 |
| Low-Level Bridge | 2,660,000 |
| Transbordeur with four long cages | 3,160,000 |
| Transbordeur with six long cages | 3,250,000 |
| Transbordeur with four short cages | 2,450,000 |
| Transbordeur with six short cages | 2,500,000 |
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From the preceding table may be drawn the following conclusions:
First. The high-level bridge is more than twice as expensive as the
low-level bridge. This agrees with the deductions that can be drawn from comparisons of the estimates for high-level and low-level combined-bridges, and of high-level and low-level steam-railway bridges, which were made for the New Orleans investigation.
Second. The transbordeur with short cages is a little cheaper than the
low-level bridge.
Third. The transbordeur with long cages is some 22 per cent more
expensive than the low-level bridge, but costs only 58 per cent of the price
of the high-level bridge.
The utmost capacity of the transbordeur with six short cages is 40 cages per hour in each direction; and each cage is capable of carrying a fully-loaded street-car, four automobiles, and 250 pedestrians, making 40 cars,
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