| ECONOMICS OF CANTILEVER AND SUSPENSION BRIDGES | 103 |
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$1,032,000; while the author found for his nearest corresponding double-track, steam-railway bridge $827,000 and $161,000. While it is entirely impracticable to compare these figures, because of fundamental differences in both the loading and the foundation conditions, it is evident that Dr.
Steinman must have made some serious mistake in his calculations when he
caused the costs of his main piers, with their pneumatic foundations, and
his anchor piers, resting on bare, dry bed-rock, to be so nearly alike;
because the latter generally are insignificant affairs when compared with the
former. This same error exists in the other two cantilever bridges which he
has computed; for in his 1,000-foot span he found, respectively, $876,000
and $524,000, and in his 2,000-foot span $2,153,000 and $1,994,000. This
matter will receive additional attention later on.
In respect to his division of weights of metal in superstructure, Dr.
Steinman recorded the following:
TABLE, 13a.
Weights of Metal in Pounds
|
Main
Span
in Feet |
The
Suspended
Span |
Two
Cantilever
Arms |
Two
Anchor
Arms |
Two
Towers |
Two
Anchorages |
| 1,000 |
8,738,000 |
6,551,000 |
9,697,000 |
5,987,000 |
785,000 |
| 1,500 |
15,550,000 |
16,951,000 |
20,566,000 |
17,479,000 |
1,794,000 |
| 2,000 |
28,964,000 |
39,750,000 |
42,851,000 |
40,158,000 |
3,374,000 |
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Referring to the item of weight of the towers in both the 1,500-foot-span and the 2,000-foot-span structures it exceeds the total weight of metal in the cantilever arms, and is but little less than that in the excessively-long anchor arms. Surely this cannot be correct! Each tower consists of two braced columns, the load on each of which is composed of the vertical components of the stresses in the two upper-chord members meeting at its top, and these are not extraordinarily great. Had the upper chords been run horizontally from inner hip to inner hip, the column stresses would have been zero, barring those due to their own weight and to an insignificant wind pressure on the columns themselves only.
Such glaringly-great irregularities as these upset the entire economic
comparison and render its results worthless. Moreover, all these variations
from correctness combine to militate against the cantilever structure.
On the other hand, though, the assumption of side spans supported by the
backstays militates against the suspension structure.
In view of the preceding, the author concluded that it would be necessary to compute quantities and plot cost curves for cantilever and suspension bridges of the type and loading assumed by Dr. Steinman, adhering as closely as practicable to his general features of layout, character of metal used in the various parts, weights per foot of floor systems and lateral sys-
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