A Treatise On The Principles And Practice Of Dock Engineering

4o6 DOCK ENGINEERING. more permanent ends is afforded by the Liverpool and Birkenhead landing stages on the River Mersey, which, themselves constructed on the same principle, are connected with the shore by floating bridges, consisting of a series of pontoons, flexibly linked together so that they are able to adapt themselves to the fluctuations of tidal level. The length of the Liverpool bridge is 550 feet and its width 35 feet. The Birkenhead bridge is 678 feet in length by 30 feet in width. Neither of these bridges is, however, a movable bridge in the sense intended in this section. There is a pontoon bridge, which is movable in the true sense of the word, over the Kaiser William Canal at Holtenau. It consists of two main or turning pontoons, meeting at the centre of the canal, united to two bearing pontoons at their shore ends. The bridge, which carries a 15-foot roadway and two 2 feet 6-inch footpaths, is opened by turning the pontoons round their shoreward ends, and this is accomplished by having a chain, one end of which is attached to a mushroom anchor in the bed of the canal, and the other to a bollard on the bank, wound round the barrel of a winch,, which is on a small pontoon alongside of, and fixed to the main pontoon. The second kind of floating bridge is represented by caissons, which, however, only act incidentally as bridges, their primary function being that of closing a waterway. It has already been noted that one of the advantages appertaining to a caisson, in comparison with a pair of gates,, is this capacity to discharge dual duties, whereby the additional expenditure for a bridge is avoided. Caissons as a class have already been dealt with in Chap. viii., so that there is no need to pursue this branch of the subject further. Traversing Bridges are supported by the quay at or about the coping level and are projected forward or withdrawn in a straight line—in other words, their motion is rectilinear and approximately horizontal, or with just sufficient inclination to enable them to clear the edge of the roadway abutting on their recesses; for, except in the case of footbridges, which may be provided with approach steps at each end of the bridge, forming part of the moving structure, the wheel track of a traversing bridge must lie somewhat below the quay level in order that its floor may form a continuous horizontal plane with the roadways. Consequently, for the purpose of removal, the tail or inner end of the bridge must be raised to the height of the roadway before it can be drawn backwards. Several arrangements have been devised for the working of traversing bridges, of which the following are a few typical instances : —• (a) The nose or forward end of the bridge rests upon rollers driven in .between the bridge girders and the wall-bearing plate. In order to open the passage these rollers are withdrawn, and, at the same time, the tail end is lifted. The bridge tilts about intermediate wheels, fixed at the quay edge, and upon these and the tail-end wheels the structure is supported during withdrawal. (b) The same effect of tilting the bridge is obtained by making the tail