The order in which these bridges may be placed as regards cost of construction, and extent of application, is as follows:—
| Number. | Span. | Description of bridge. | |
|---|---|---|---|
| 1 | 10 to | 50 feet | Cast-iron girder. |
| 2 | 50 to | 200 feet | Cast and wrought combinations. |
| 3 | 200 to | 2000 feet | Suspension. |
| 4 | 200 to | 500 feet | Cast arch. |
| 5 | 25 to | 100 feet | Boiler plate girder. |
| 6 | 100 to | 500 feet | Tubular. |
Numbers 2, 3, and 5, are the forms which are in use upon American roads. No. 1, is very liable to failure, requires much more knowledge and care in building, and is far more expensive than a wooden truss, or trussed girder. No. 4, is very expensive, and causes a greater obstruction to the water-way than any other. The enormous expense of No. 6, should, and will prevent its adoption in the United States. Let us look at the principles of construction of numbers 2, 3, and 5.
COMBINATIONS OF CAST AND WROUGHT IRON.
221. Under this head come all of the iron trussed frames used in this country.
As before observed, skill in bridge construction consists in using always that material which with the least expense is the best able to resist the particular strain to which it may be exposed. Thus wrought iron must always be used to resist tension, and cast-iron compression. Posts, braces, and upper chords should always be cast, while ties and lower chords should be made of wrought iron.
The strength of a railroad bridge must be such as to resist all extra shocks and strains, such as are produced by derailment of engines, and breakage of axles; also incidental strains arising from change of form by expansion and contraction of the metal, and from high winds and gales.
Fig. 101.
Every part of a bridge not resisting some force is worse than useless, as it adds to the weight. Lightness not only increases the economy directly, but indirectly by removing a part of the permanent load.