A Treatise On The Principles And Practice Of Dock Engineering

TRAVERSING BRIDGE AT ANTWERP. 443 two series of longitudinal joists. A footwalk, 4 feet G inches wide, is carried on brackets outside the main girders. The width of the passage opening is 90 feet, and the total length of the bridge is 158 feet 6 inches. The roadway is paved with blocks of creosoted pine, laid upon a ^-inch bed of asphalt, which in its turn covers a floor of jointed oak. At the tail end of the bridge there is a counterbalance of 106 tons. The total weight of the movable platform is 370 tons. To open the passage, it is necessary to lift the bridge to such a height as will enable the tail rollers to run back on the level of the roadway by which the bridge is approached, and this is effected by placing under each girder a hydraulic press with a large roller fixed on the head of the ram. The ram is 2 feet 7 inches in diameter and the roller 3 feet 7 inches; the latter is mounted on a 9-inch axle. The amount of lift is 3 feet. When the water enters the presses the bridge is lifted, but the tail end, which preponderates, does not begin to rise until the horn, or projection, at the nose end of the bridge cornes in contact with a small inverted roller just below the surface of the coping. The tail end then ascends until the bridge becomes hori- zontal at its full elevation, as shown on the diagram by the dotted lines. Fig. 431.—Swing Bridge at Antwerp. It is then drawn back upon the press rollers and the tail rollers by the action of a horizontal cylinder and ram, with chains and multiplying sheaves, situated beneath the bridge. The ram is 20 inches diameter with a stroke of 12 feet, and there are four sheaves at each end, multiplying the power eightfold. The chain is 1| inches diameter. An iron pathway, to bear upon each press roller, is fixed to the underside of each girder. The process of closing is the same as that just described, but in the inverse order. The bridge was constructed by Messrs. Sir W. G. Armstrong & Co., of Newcastle, to the late senior partner of which firm the engineering profession is mainly indebted for the many present valuable applications of hydraulic power. Several bridges have been designed upon the same principle, and are reported to work very well, notwithstanding the excessive weight which is necessarily carried on the main rollers. In one case, where the arrange- ment is a little different from that described above, the load on each roller amounted to nearly 100 tons, and yet it was at work for more than 20 years without any important renewals of the working apparatus. In this instance both the rollers and the roller paths were of cast iron, 9 inches broad, the diameter of the roller being 3 feet.