A Treatise On The Principles And Practice Of Dock Engineering

SWING BRIDGES. 409 increases the labour of raising the bridge, and from the nature of its office no perforations are allowable in the bridge platform. The former draw- back can be remedied to' some extent by setting back the axis from the face line of the quay, but this step considerably augments the length and cost of the bridge. It has been proposed* as an antidote to both evils that the bridge, instead of being raised, should be lowered into its vertical position and at the same time recessed within the side walls. The author is unaware of any instance in which the suggestion has been carried out. Except in the case of very high quays the project would evidently entail the submersion of a part of each leaf ; but, though this might be detrimental to the durability and appearance of the structure, from an operative point of view it would confer a benefit rather than otherwise. There are one or two obvious difficulties to be overcome, but the author of the scheme (Mr. C. J. Findlay) does not consider them insuperable. All bascules do not rotate about a fixed axis. There is a variety, known as the rolling bascule, in which the tail-end of the bridge takes the form of a circular segment, upon which the bridge rolls in a manner similar to the action of a rocking chair (fig. 435). Lifting Bridges are horizontal platforms raised vertically in such a way as to maintain a level surface throughout the process. Instances of their use are rare, and are apparently confined to rivers and canals. Indeed, their eligibility for dock work, except, conceivably, in connection with canal basins, is dubious, owing to the great height to which they would have to be raised in order to clear the masts of vessels passing beneath them. Further- more, it would be a difficult matter to secure equable movement of the platform, lifted, as it would be, from two opposite sides of the waterway, unless the action were controlled from one centre—an arrangement which is scarcely feasible in the majority of cases. The advantages attached to the system are limited to a minimum appropriation of quay space. There is a lifting bridge over a channel 100 feet wide at Chicago. Swing Bridges.—These constitute by far the most numerous and the most important class for dock work. It includes all movable bridges in which the axis of rotation is vertical. The merits of the principle are a comparatively slight expenditure of motive power, ease of movement, and less wear of the bearing surfaces, the absence of deep pits for counterbalancing purposes, and of appreciable change in level. On the other hand, two important drawbacks must not be overlooked :— 1. Swing bridges are necessarily longer than bascules or traversing bridges. Bascules may rotate about an axis as close to the edge of the coping as is considered desirable. The pivot of a swing bridge must, however, be set back a distance not less than half the width of the bridge, in order that the latter may be entirely housed within the quay line when the passage is open. Since the counterbalance must lie behind the pivot, it necessarily follows that, * Findlay on “ The Design of Movable Bridges,” Min. Proc. L.E.S., vol. il.