A Treatise On The Principles And Practice Of Dock Engineering

442 DOCK ENGINEERING. bridge pit is required and the system cannot be adopted in the case of low quays. From an investigation made by Mr. Findlay, it appears that for a rolling load of 10 cwts. per foot per girder, a single-leaf swing bridge is more economical than a double-leaf bridge up to 150 feet span. If the rolling load be increased to 1 ton per foot run per girder, tlie economical limit of single-leaf bridges is raised to 180 feet. Folding or Lowering Bridge at Greenock.* This forms the superstructure to a caisson closing the entrance to the Garvel Graving Dock at Greenock, and already alluded to in Chap. viii. It is only necessary to supplement the account there given of the whole structure with some particulars relating exclusively to the bridge portion, which is the design of the late Mr. W. R. Kinipple (figs. 429 and 430). The bridge roadway is carried by a series of parallel axles in pairs, placed vertical over one another, transversely to the bridge, at a distance of 30 inches, each pair being connected by four parallel rocking bars working freely on both axles. The outer bars are prolonged above the roadway level to form standards for handrailing. Two pairs of the inner bars are extended downwards into a watertight chamber of the caisson, where they are attached to boxes of ballast which act as counterweights. The raising or lowering of the bridge platform is effected by rollers fixed on each end, which work against curved plates in the abutment and the curved girder or lowering plate across the entrance to the recess. The process of hauling the caisson into its chamber brings the inner roller in contact with a convex plate, causing the handrail and platform automatically to fall to a lower level. The opening of the bridge consists of the reverse process. The outer rollers of the platform corne in contact with a concave plate which causes the platform to rise to the quay level. When the bridge is in position it is locked between the abutments so that it cannot fall, and in such a way that it does not vibrate under the heaviest traffic. The plummer blocks carrying the rocking bars are 9 feet 9 inches apart, and are supported on plated columns extending to the bottom of the caisson. Traversing Bridge at Antwerp.f The bridges at Antwerp docks are generally swing bridges, one of which is exemplified in fig. 431, but there is a traversing bridge over the entrance lock to the Kattendyk Dock, which is constructed according to the type shown in figs. 432 and 433. The structure consists of two main plate girders of a uniform height of 9 feet, one on each side of the roadway, connected at intervals of 12 feet by cross joists, between which are rivetted * Kinipple on “Greenock Harbour,” Min. Proc. Inst. C.E., vol. cxxx. ; Macalister on “ Caissons for Dock Entrances,” Min. Proc. Inst., C.E., vol. Ixv. + Vide “Anvers, Port de. Mer.”