A Treatise On The Principles And Practice Of Dock Engineering

452 DOCK ENGINEERING. increase in the weight of the girders. With these modifications, it would be possible to oonstruct swing bridges, weighing about 2,500 tons, which could be safely and easily worked.” Tilting Bridge at Marseilles. At the Passage de La Joliette, 70 feet wide, at Marseiiles,* there is a large traffic of unmasted timber lighters and but few sea-going ships. As it is therefore advisable to open the passage as seldom as possible for any considérable finie, owing to the roadway traffic, a form of bridge (fig. 440) has been devised, combining the swinging principle with that of the bascule. For unmasted barges, the bridge is tilted by means of a piston pivot, and it is only rotated for large vessels. When in the tilted position the gradient of the floor is 1 in 14 and a headway of 10 feet 3 inches is afforded. The time occupied in tilting is, of course, much less than in swinging. Single Swing Bridge at Liverpool. This bridge (figs. 441, 442, and 443) constitutes a design used in three or four instances for spanning passages, 90 feet in width. The structure, which is of mild steel, consists of two main girders, each 159 feet long and 11 feet deep generally, but reduced to 6 feet in depth at the nose end. These girders are connected at intervals of 8 feet 6 inches by cross girders 2 feet deep, supporting intermediate longitudinal joists, 12 inches by 6 inches. The pivot girder consists of two box girders, each 4 feet deep at the centre and 2 feet 9 inches deep at the ends, joined by J-inch diaphragms and a covering plate, the pivot casting being bolted to and between the box girders. The main girders are 22 feet apart, centres, providing a double road- way, 17 feet wide, separated by a central cast-iron curb. A narrow space, for carters and others, adjoining each girder is also protected by a curb. The footwalks proper are two in number, each 6 feet 4 inches wide, and carried outside the main girders by brackets which are prolongations of the cross girders. Although rails have not actually been laid down upon the bridge, provision has been made for their accommodation, by spacing the longi- tudinals to suit a double railway track, and the bridge has been calculated to sustain the heaviest type of locomotive as a continuons load. * Price on “ Movable Bridges,” Min. Proc. Inst. C.E., vol. Ivii.