A Treatise On The Principles And Practice Of Dock Engineering

CHAPTER VIII. DOCK GATES AND CAISSONS. Définition and Relative Advantages op Gates and Caissons— METAL versus Wooden Gates—Weight, Cost, Durability, and Stkength—Single-leaf and Double-leaf Gates—Horizontal and Vertical Girdek Types—Storm Gates— Strut Gates—Stresses in Gates—Statical Forces—Methods of Finding Resultant Pressure—Zones op Equal Pressure—Rise of Gates—Analysis OE RESULTANT—Graphic REPRESENTATION—Limits of Stress—Typical Examples —Vertical Co-planar Giroers—Stress IN Panels—Exemplification of Gate Calculations —Fittings—Examples of Gates at Liverpool, Birkenhead, Manchester, Hull, Buenos Ayres, Calcutta, South Shields, and Dunkirk— Table of Dock Gates—Stresses in Caissons—Classification of Caissons Swinging, Traversing, Sliding, Rolling, Floating, and Ship Caissons —Lowering Platforms—Examples of Caissons AT Malta, Bruges, Blackwall, Cardiff, Calcutta, Belfast, Liverpool, and Greenock—Table of Dock Caissons. In localities wliere there is considérable tidal range and wliere circurn- stances render it desirable to maintain the surface of the water set apart for tlie reception of sliipping at a fairly constant level, it is evident that the entrance or entrances to a dock inust be closed in order to impound the water, and must remain closed during those portions of eacli day in which the tide falls below a certain limit. This is usually effected by ineans of (1) gates or (2) caissons, and occasionally provision may be found for both forms of closure. Graving and repairing docks are treated in like manner, but for a different purpose, the object in this case being to exclude the external water during the time of occupancy. Definitions.—The distinctive feature of a gate is that it revolves about an axis, in most cases vertical, but occasionally liorizontal, while the motion of a caisson is either rectilinear or altogether untrammelled. As with many other terms, however, employed in dock engineering, this definition is not susceptible of too rigid interpretation. There is an intermediate class of gate-caissons combining tlie hinge or axis ot the gate with the broad beani of the caisson, and exemplified at Bristol, Dundee, Havre, and other places, though, taken on the whole, the type is rare. The Relative Merits of Caissons and Gates, considered as two distinct, though compréhensive, classes based on the foregoing definitions, may be broadly gauged as follows :— 1. Gates with vertical axes need side recesses into which they may be swung when the entrance is to be opened for the passage of vessels. This