Engineering Wonders of the World
Volume I

aJ'5F6f[EFSXB p B8F _ l 5 0 JLP P v/chcrzer Kollirvg Li/t Bridge^/ A short account of a very modern and ingenious type of Bridge which has done much to solve the problem of carrying land traffic across inland waterways. IN places where road and railway traffic demands facilities for passing from the one bank to the other of a river, which, on account of the many vessels passing up and down it, must not be per- manently blocked by a low-level, immov- able bridge, the engineer has to choose one of three alternatives—the tunnel, the high- level bridge, or the bridge of which part can be swung out of the way to allow a ship to pass. In cases where the two first of these alter- natives are ruled out by financial considera- tions, recourse Swing Bridges: their Dis- advantages. must be had to the movable bridge, of which there are two main types—the swing bridge, which revolves horizontally through a quarter circle on a central support set either in mid-channel or on one bank ; and the bascule bridge, of one or two leaves, which, can be raised into a more or less vertical position. The great disadvantages of the swing bridge may be summarized thus : that the central pier to support the movable portion forms a serious obstruction to navigation, dividing a waterway into two narrow channels, and rendering unusable the central and naturally most valuable part of the waterway. This greatly retards the movements of vessels. If the channel be narrow, the centre pier must be placed on the shore, and in order to obtain a balance the bridge must be much longer than the cross-water distance to be spanned. Its erection is difficult, and interferes with traffic. The opening of it leaves an unpro- tected (except by signals) chasm in the road- way ; and this fact has been responsible for several very serious accidents. Finally, if the local traffic increases to such an extent as to require the multiplication of bridges, the large amount of space occupied by a swing bridge when opened is a handicap, as two swing bridges cannot be set nearer to each other than the distance necessary between their piers to allow both to be open at the same time. Consequently where these bridges are multiplied a great breadth of right-of-way on the river banks is needed. On the other hand, no obstructing mid-