A Treatise On The Principles And Practice Of Dock Engineering

405 CHAPTER X. DOCK BRIDGES. -Classification —Floating Bridges—Traversing Bridges—Drawbridgbs—Bascules _____Lifting Bridges — Swing Bridges — Single-leaf versus Double - leaf Bridges—Stresses in Movable Bridges—Case of the Double Cantilever— Case of the Cantilever and Beam—Case of the Arch—Case of the Con- tinuous Beam—The Theorem of Three Moments—Effect of Counterpoise — Loads imposed on Movable Bridges—Weight of Structure—Weights or Typical Locomotives—Equivalent Live Loads—Weight of Vehicles and Men — Practical Example of the Calculations for a Swing Bridge — Distinctive Features of Movable Bridges—The Pivot—Balanced Rollers and Wheels —The Counterpoise—Setting Apparatus—Interlocking Appar- atus—Notes on Design—Illustrations of Movable Bridges at Greenock, Antwerp, Rotterdam, Chicago, Marseilles, Liverpool, Leith, and Kidderpur. Narrow waterways and locks, linking together the varions parts of a dock system, are generally spanned at convenient points by bridges, in order that vehicular and foot traffic may be transmitted across them and access provided, as uninterruptedly as possible, to all quarters. On account, moreover, of the necessity of maintaining the navigation of these passages, it is essential that bridges crossing them should be of a movable nature and characterised by great rapidity of action, so as to avoid lengthy stoppages and interference with the use of either road or waterway. Such bridges are, of course, used in a variety of situations and in branches of engineering not necessarily connected with docks. Their importance, however, to the dock engineer is indisputable. Classification.—For the purpose of this treatise, movable bridges may be divided into live classes :— Floating bridges. Traversing bridges. Drawbridges. Lifting bridges. Swing bridges. Floating Bridges, as the name implies, are water-borne, either con- tinuously and wholly, or partially and during such times as they are being moved. The former variety, which are generally formed of pontoons, either singly or in combination, are rarely used otherwise than for purposes of a purely temporary nature, such as the crossing of rivers and streams Muring military operations. A striking instance of their application to