| 104 |
ECONOMICS OF BRIDGEWORK |
Chapter XIII |
|
tems, and cost per foot of trestle approaches; but differing with him in the following particulars:
First. Raising the grade of the structure so as to afford a vertical clearance of 150 feet above high water, and lowering the elevation of main pier
foundations to 35 feet below low water. This is more in accordance with
probable actual conditions than is indicated by the profile in Fig. 13g.
Second. Substituting steel-trestle approaches for the side spans shown
in Fig. 13h.
Third. Adopting the most economical type of substructure for each
case.
Fourth. Adopting a length of anchor arm equal to 5/16 L instead of 0.4 L.
With these premises the author computed the costs of cantilever and
suspension bridges for openings of 1,500 feet and 2,400 feet, and found that
the span of equal cost lies between these limits. Then he figured for a
2,000-foot span. This gave him three points on each curve, as shown in
Fig. 13i, besides which, he estimated in detail by proportion the costs for several other openings and plotted the results of these also. Fig. 13i shows that the span-length for equal cost is about 2,190 feet, instead of the 1,670 feet found by Dr. Steinman—a difference of over 500 feet.
It will be interesting to compare the substructure costs found by Dr.
Steinman and those found by the author for like spans and practically the
same general conditions, as recorded in the following table:
TABLE 13b
|
Main Span-length and
Character of Structure |
Cost in Dollars of the
Main Piers |
Cost in Dollars of the
Anchorages |
| Steinman |
Waddell |
Steinman |
Waddell |
| 1,000-foot Cantilever |
876,000 |
....... |
524,000 |
....... |
| 1,500-foot Cantilever |
1,262,000 |
506,000 |
1,032,000 |
290,000 |
| 1,500-foot Suspension |
1,330,000 |
440,000 |
2,428,000 |
1,222,000 |
| 2,000-foot Cantilever |
2,153,000 |
660,000 |
1,944,000 |
310,000 |
| 2,000-foot Suspension |
........ |
600,000 |
........ |
1,920,000 |
| 2,250-foot Suspension |
1,835,000 |
........ |
4,174,000 |
........ |
| 2,400-foot Cantilever |
........ |
926,000 |
........ |
357,000 |
| 2,400-foot Suspension |
........ |
880,000 |
........ |
2,400,000 |
| 3,000-foot Suspension |
3,017,000 |
........ |
6,995,000 |
........ |
|
Of the preceding nine cases there are only three which can be directly compared, viz., the 1,500-foot cantilever, the 1,500-foot suspension, and the 2,000-foot cantilever, although other comparisons might be made ap- proximately by interpolation. It will be noticed that Dr. Steinman's main piers cost two or three times as much as those of thee author, his anchor piers for
|
|