Module 7 : Deck Structures Flashcards
Purpose of the primary deck
- Structural
- Provide weather-tightness and support people and equipment
The deck serves as a lid/roof to the complex box girder of the hull.
Meant to provide resistance against tension, compression, and racking forces.
Scantling size same as the topsides or could be heavier if deck is expected to carry heavier loads.
Deck will be reinforced around deck fittings and equipment bases for things like the capstan, cleats, bitts, crane base, and superstructures.
Deck strengthening ties the port and starboard sides together as well as supporting loads from above and below the deck.
Deck strength members
The primary strength members connections of the deck, sides and bottom provide the main hull girder strength to help resist transverse and longitudinal deformation. Secondary strength members support against local deformation.
Deck can be strengthened by transverse or longitudinal construction techniques.
Arrangement of primary and secondary strength members similar to that on ship’s sides.
Main deck components
Deck beams
Deck girders
Deck longitudinals
Deck plating
Other items are integral to deck strengthening : hatchway construction and additional material for heavy deck loads
Camber
Vertically higher along the centerline than at the ship’s sides, used to help shed water off the deck.
Also provides extra strength to resist longitudinal bending and better support for local deck loads.
Deck construction details
Actual size of primary and secondary deck beams laid out in classification society rules.
Depends on unsupported span, constructional details, load carried and manner in which load is applied.
Deck beams in forward area are usually heavier due to anticipated heavy loads of sea.
Continuity of strength and structure must be maintained.
Transverse construction
Primary deck strength by a fabricated T section AKA strong beam. Fitted in line with primary strength of ship’s side (web frames) to establish spacing. Either bracketed together or fitted with curved plate to appear continuous.
Strong beams are also positioned at or near the ends of hatch openings. Beams directly at the end are L shaped and form part of hatch lower coaming.
Deck girders also form part of primary strength.
Secondary strength is provided by ordinary deck beams (rolled bulb bar or angle bar). Line up with ordinary side shell frames and have beam knees fitted at corners.
Beams abreast to a hatch opening extend only to the side of the opening and are called half beams.
Deck longitudinal construction
Primary strength is from deck longitudinals and deck girders. Deck transverses are T shaped sections fitted in line with the side web frames.
Secondary strength is obtained through the use of deck longitudinals (angle or bulb bar).
In ships overs 215 m, deck beams are continuous, meaning that notches are cut in the bulkheads for passage of the beams and the notch is closed by suitable sealing plates and welding.
Where longitudinal deck beams meet at the end of a hatch opening, they are either directly welded or bracketed to the lower section of the hatch coaming.
Deck girders
Heavy longitudinal members placed under decks. Form part of the primary longitudinal strength of a ship.
Most common is built-up T shape, but could be flanged, built-up L, box shaped or rolled sections.
Girders are notched to allow tranverse deck beams to be continuous. The deck girder and transverse beam are then attached together to provide extra structural rigidity. Additional brackets are fitted depending on the shape to reduce likelihood of girder being tripped.
May be fitted in line with hatch opening or adjacent to the coaming. Girders directly at the coaming are L shaped.
Deck girders are tired to the lower section of the vessel by use of pillars. Without pillars, must substantially increase size of girder.
Deck girders are cut and bracketed at watertight bulkheads.
Some ships could incorporate external deck girder for space savings on the underside of the deck.
