28 July 2005
When piers and wharves need to be accessed from the waterside by small craft
such as patrol boats (which cannot berth directly), a landing float and a brow are
Flotation units may consist of foams of polystyrene and polyurethane, fiberglass-
reinforced polyester resin shells with or without foam cores, metal pontoons, metal
pipes, metal drums, and hollow concrete sections. Timber logs, the earliest form of
flotation unit and the cheapest, have a tendency to become waterlogged and their
use is not recommended. Decks of floats are variously made out of wood planks,
plywood, plywood and fiberglass-resin coatings, concrete, and nonskid metal
surfaces. Framework for floats is generally of preservative-treated timber, although
steel and aluminum are often used. All ferrous metal hardware should be
galvanized or otherwise protected from corrosion.
Anchor floats to prevent movement by wind, current, waves and impact from the
ships. Anchorage may consist of individual vertical (guide) piles, frames of batter
and vertical piles, and cables or chains. When piles are used to anchor small floats,
guides are furnished to secure the float to the anchor pile. Commonly used guides
are rigidly braced metal hoops of pipes, rollers, or traveler irons. Chains and flat bar
guides should not be used as they cause the float to hang up on the piles. See
Figure 7-1 for details. This system works well for shallow waters with a large tidal
range. In deeper water, the pile head may have to be supported by the structure or
pile driven deeper. Anchorage may also be obtained from a cable or chain system
attached to the ocean bottom or to the fixed pier or wharf structure.
Design stages for landing personnel only for a uniform live load of 50 lbs/ft2 (244
kg/m2) or a concentrated live load of 500 lbs (226.8 kg) placed at any point on the
deck surface. The float should not tilt more than 6 deg. from the horizontal when
applying the concentrated live load of 500 lbs (226.8 kg.)
Floating stages for small craft usually ride with the deck from 15 to 20 in (381 to 508
mm) above the water surface under dead load. Live loads usually lower the float
about 8 to 10 in (203 to 254 mm).
Provide fenders on all floating stages. For small craft berthing, fenders may consist
of soft, flexible rubbing strips (rubber tires, sections of hose).