Engineering Wonders of the World
Volume I

46 ENGINEERING WONDERS OF THE WORLD. channel piers are necessary under any con- ditions for the support of a bascule bridge, which, moreover, does not The occupy more space laterally , when open than it does when Bridge. r shut. For this reason bascule bridges can be set very closely together, and their number increased easily as circumstances may demand. The best-known type of bascule bridge is that exemplified by the great pile rising a little to the east of the Tower of London, after which, it is named. The The Tower Bridge, opened in 1894, o , has two huge leaves or bas- cules, each 160 feet long, and projecting, when down, 100 feet over the waterway from its trunnion. The avoirdu- pois of a leaf and its counter-weight of lead and iron is 1,200 tons. This is carried by a solid steel bar trunnion, 21 inches in diameter and 48 feet long, rotating on steel roller bearings. Despite the friction-reducing pro- perties of these last, considerable friction is unavoidable where so large a weight is thrown on to comparatively small surfaces. While the Tower Bridge was in course of construction the engineers of the Metropolitan West Side Elevated Railroad of Chicago had to solve a difficult bridge problem—how to carry four tracks across the Chicago River between the Jackson Street Necessity stimulates Invention. and Van Buren Street swing bridges, which were so close together that it was impossible to build a third swing bridge between them. A bascule bridge of the Tower type was de- signed, but the plans were rejected by Mr. William Scherzer, C.E., whose opinion had been asked by the management, as including some objectionable features. Further investi- gation resulted in the evolution of an entirely new type of bridge, the Scherzer Rolling Lift Bridge. In this the fixed trunnion is abolished and The Scherzer Bridge. replaced by circular steel segments, forming part of the land end of the girder, which roll over flat surfaces as the runners of a rocking- chair move on an ordinary floor. This prin- ciple practically eliminates friction. It should be observed also that as the bascule rises it moves bodily away from the water, instead of merely turning about a fixed centre, as is the case with a trunnioned bridge. The general form of a Scherzer bridge will be gathered from our illustrations. At the rear it is usually so weighted that the whole mass is in equilibrium when the bascule is opened to an inclination of forty-five degrees with the horizontal. To lower or open the bridge fully power must be applied to overcome the equilibrium. At first sight it may not be evident at what point the power may be applied conveniently to move the bridge without shifting it as a whole on its tracks. A little consideration will show, however, that the point in question is the centre of the circle of which a rolling segment is an arc, very near which lies the centre of gravity. This point being equidistant from the ground in whatever position the bascule may be, a direct pull or push may be transmitted to it by a rod drawn back- wards or forwards horizontally Opening , . . , e the Bridge. by machinery m the rear or the bridge ; or electrically driven pinions set at the centres of the segments may be made to claw their way along horizontal racks arranged on both sides of the bridge (see p. 431). The method adopted and. the power used—whether steam, electric, hydraulic, or gasoline engine—is influenced by local cir- cumstances. As a precaution against possible slip the segments are provided along their circumference with slots which engage with teeth on the tracks. One of our illustrations shows a Scherzer bridge in course of construction. This pro-