A Treatise On The Principles And Practice Of Dock Engineering

BASCULE BRIDGES AT ROTTERDAM. 445 Automatic cut-offs and other precautions are adopted to prevent any tendency to accident during the movement of the bridge. The swing bridge shown in fig. 431 is notable for a lowering arm or strut designed to turn the bridge structure into an arch when in the closed position. The efficacy of the strut, however, as a compression member, is rendered dubious by the necessity for an accurate bearing, which cannot in all cases be ensured. Bascule Bridges at Rotterdam.* Between the years 1881 and 1894, a series of seven bascule bridges were constructed at Rotterdam, all upon the same principle, which is illustrated by the typical bridge (the Scheluwebrug) in fig. 434. Former bascules had been provided with a very considerable rise towards the centre of the span, in order to obtain as nearly as possible all the advantages of the arched system. This rise, however, proved very inconvenient for heavily-loaded vehicles, and in the later type the platform was made almost horizontal. In one instance only was this arrangement departed from, and in that instance the form of the bridge was parabolic at the haunches, with a very flat connecting curve in the centre of the span. In the class of bridge under notice, the two leaves of which each bridge is composed, do not abut against one another at their junction. They are only connected in the closed condition by locking bolts, for the purpose of securing uniformity of pressure and deflection. The tail ends, however, derive considerable support from their abutment, when horizontal, against an iron structure placed above the watertight pits in which the tails revolve, and strongly anchored to the foundation. Each leaf, accordingly, is capable of acting as a self-sustained cantilever. The bridges are calcu- lated to support a load of about 10 cwts. per square foot. The platforms are of oak, with a payement of blocks of “ djati,” or teak. Nearly all these bridges are moved by hydraulic power. The machines consist of oscillating cylinders receiving pressure from the town’s ordinary water main by means of a hollow trunnion. The piston actuates a crank, which is in connection with the turning axis of the bridge. An ingenious arrangement causes the withdrawal of the interlocking bolts to depend upon the closing of the two ends of the bridge to traflic by an iron grating, so that it is not possible to raise tlie bridge until this grating has been moved into position. Appended is a list of the seven bridges with the principal particulars of their design attached:— * Ysselsteyn on Le Port de Potterdam.