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Movable bridges may be divided into the following eight types:
1. Ordinary, rotating draws.
2. Double, rotating, cantilever draws.
3. Pull-back draws.
4. Counterweighted, bascule bridges.
5. Rolling, bascule bridges.
6. Jack-knife or folding bridges.
7. Lift-bridges.
8. Floating bridges.
The ordinary rotating draws will be treated at length in the next chapter.
Very few double, rotating, cantilever draws have yet been built; in fact the author knows of but one, viz., that over the canal at Cleveland, Ohio. A number of years ago the author had occasion to figure on a large structure of this kind, but it was never built.
The principal advantages of this type of structure are a wide waterway and the retreating of the span without serious injury when struck by a vessel before it is fully opened; while its disadvantages are excessive first cost and the almost double cost of operating two independent spans; although, when electricity is used as a motive power, both spans can be operated by one man by means of a submerged cable.
This class of bridge consists of two draw-spans, differing but little from the ordinary rotating draw, each resting upon a pivot-pier and meeting at mid-channel, where they are
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