A Treatise On The Principles And Practice Of Dock Engineering

SWING BRIDGE AT MARSEILLES. 449 placed side by side, each pair forming a complété span, and carrying a double raiiway track. Under normal conditions they are coupled together, but in case of repairs they can be disconnected, and each pair then acts independently of the other. Each leaf is so counterweighted that on drawing the centre and end locks, it rises to an angle of about 30°, rolling back on the abutment, and the application of power is only required to completely open the bridge or to close it. The weight of each double track is about 135 tons. The bridge can be opened or closed in thirty seconds, each leaf being operated by two horizontal struts connected to the ends of the trusses. The struts are run in and out by gearing, operated by a 25 H.P. electric motor. When the bridge is closed, each leaf acts as a cantilever, anchored by the tail end, which takes a bearing against the underside of the approach viaduct, the approach being firmly anchored to the masonry of the abutment. The end look holds the tail firmly home against its bearing. The following are the general dimensions of the bridge : — Length between ends of approaches, . . 276 feet. Span between bearings • 114 „ Width of channel, . 108 „ Headway at centre, . 35 „ Depth of truss at shore end, ■ 26 „ Depth of truss at free end, .... 6 ,, 6 inches. Width between trusses, .... • 21 „ 2 „ Total width of bridge, .... • 51 „ 10 ,, Radius of heel of truss, .... • 26 „ Weight of each double track leaf, . 135 tons. Total weight of bridge . 540 „ Counterbalanced weight on each side, • 28 „ Swing Bridge at Marseiiles.* The bridge (figs. 437, 438, and 439) over the entrance to the Marseilles repairing docks has a total length of 203 feet 5 inches and a width of 46 feet. The framework consists of three parallel trellissed girders, each 11 feet 6 inches deep, with curved upper flanges. Between one pair of girders is a single line of raiiway; between the other pair a roadway, with a footpath, 6 feet 6 inches wide, carried on brackets outside the outer girder. The width of the waterway is 91 feet 10 inches, and the swinging bridge consists of two cantilevers, 126 feet and 77 feet 5 inches long respectively. The total weight of the structure is 700 tons, of which 125 tons is due to counterpoise. The bridge is raised and turned upon a hydraulic pivot of only 22’8 inches diameter, which necessitates a pressure of over 4,000 Ibs. per square inch, obtained by means of a double-acting force pump and an accumulator. Each girder carries a roller under it near the extremity of its tail end; the three rollers are in a line parallel to the bearing girder, so that * Gaudard on “Swing Bridges,” Min. Proc. Inst. C.E., vol. xlvii. ; Price on “ Movable Bridges,” Min. Proc. Inst. C.E., vol. Ivii. ; and Barret on “The Swing Bridge at Marseilles,” Min. Proc. Inst. C.E., vol. xlii. 29