15 August 2002
SELECTION. This chapter contains basic data for criteria to design and
construct entrance closures, including various types of gates and caissons. Advantages
and disadvantages of each type are discussed.
Requirements. Basic requirements in the choice of entrance closure are
reasonable initial costs, ease and rapidity of control, low maintenance, and feasibility of
traffic movements across the top.
Types. A review of available types reveals why floating caissons have
been adopted as standard for Navy drydocks. Other types may be suitable for smaller,
temporary, nonmilitary drydocks.
Miter Gates. Miter gates were probably the first satisfactory mechanical
gates. Each closure consists of a pair of gate leaves, hinged at the dock walls,
swinging horizontally so when closed the free ends meet in fitted contact. Gates are
moved by means of a hawser to a nearby power capstan. The sides and bottoms bear
against seats in the drydock walls and floor. This type of gate is especially suitable for
timber construction; however, they are now made of steel. These gates have the
Heavy loads on wall supports. Although loads can be relieved by roller
and track arrangements, these are not satisfactory for operational and
maintenance reasons. The same objections apply to relief of loads by
controlled buoyancy in the gates.
Required operating mechanism is costly, susceptible to breakdown, and
requires constant maintenance.
Major repairs are difficult, due to necessary removal of the gates and
inaccessibility of major features.
Recesses must be built into dock walls to maintain ship clearances with
open gates. This construction adds to the length and complexity of a
Flap Gates. A flap gate is a rigid, one-piece gate hinged at its bottom,
and swinging downward and outward. It is a compartmented structure with means for
varying its buoyancy for raising and lowering. Although this type of gate does not
impose such severe loads on the hinges as the miter gates, it has similar disadvantages
except those of recesses. Means must be provided to support a gate when down in
Set-in-Place Gates. Set-in-place gates are in various forms, and may be
built in one piece or multiple sections. They are of beam and plate construction, with