Ship Hull Structure: Framing System

The selection of a structural framing system in any vessel must be made from a consideration of weight (TO BE MINIMIZED) , production matters (TO BE ACCESSIBLE FOR WELDING, NDT) , suitability to resist global loads and vibration. 

The shell(bottom and side)and deck plating of the ship has to be stiffened internally to prevent it from collapsing. The stiffening is generally provided by:

• Primary supporting members
• Secondary supporting members

Primary supporting members are the big members or metaphorically speaking the big bones such as web frames,girders,solid floors,etc

Secondary supporting members are the small bones of the ship such as longitudinals , hold frames,deck beams.

There are three type of Hull framing system:

• Transverse Framing System
• Longitudinal Framing System
• Combined or Mixed Framing System (Hybrids framing system)

Transverse Framing System

In a general cargo ship the transverse framing will consist of main and hold frames with brackets top and bottom, and lighter tween deck frames with brackets at the tops only. 

Scantlings of the main transverse frames are primarily dependent on their position, spacing and depth, and to some extent on the rigidity of the end connections.

Side shell with transverse framing

In way of tanks such as oil bunkers or cargo deep tanks the side frame size will be increased, except where supporting side stringers are fitted within the tank space. Frames supporting hatch end beams and those in way of deck transverses where the deck is framed longitudinally, also have increased scantlings.

The transverse framing system has been generally adopted for the structure of a fore construction so far. The side stringer supports the frames at the ends. Since the ship side inclines toward the ship centerline, the transverse frames are fitted inclined to the shell plate, not perpendicular. Therefore, the effectiveness of the shell plate as an attached plate of the frame is less.

Advantages:

• Resist Hydrostatic/local loading ( Bow & Engine room area, Cargo hold of Bulk carrier )
• Absorb the vibration (E/R, Accommodation)

Disadvantages:

• Heavier than Longitudinal framing system.
• Much deformation during block fabrication.
• The accessible for welding and NDT to be concerned during design.

Longitudinal Framing System

In this system,all the secondary supporting members are fitted in the longitudinal  direction and are known as longitudinals. Some of them are:

• Side longitudinals
• Bottom longitudinals
• Deck longitudinals

The system consist of many small,closely spaced longitudinals supporting the plating directly and being supported in turn by a few large ,widely spaced longitudinals.

Longitudinal Framing for Bottom structure

The bottom longitudinal on centreline,or centre girder, is extra large and heavy,principally to carry the loads imposed by keel blocks during dry docking ØVery deep, heavy transverse structures called transverse webs or web frames are constructed at intervals of about 3 to 5 metres to support the longitudinals ØWhere a ship’s length exceeds 120 m it is considered desirable to adopt longitudinal framing ØTankers greater than 200 m in length must be framed longitudinally

Advantages

• Longitudinally stiffened plating is more resistant to buckling especially prevalent in ship’s upper deck.
• When the deck or bottom is subjected to compressible stresses as it bends in a seaway than it would be if stiffened transversely.
• Longitudinal stiffeners supporting side shell and bulkhead plating are subjected to variable pressures from the sea or from liquid cargo. The hydrostatic pressure on each successive longitudinal and its associated plating increases with its depth below the waterline or below the surface of the liquid in the tank.
• Each longitudinal can be sized to withstand the maximum pressure associated with its depth in the ship, thus achieving an efficient use of structural material.
• Graduated size cannot be done effectively with transverse frames, which as a result are only heavy at their upper ends.

Disadvantages

• The intrusion of deep webs into prime cargo spaces for ships carrying packaged cargo is an disadvantage.
• Difficulty arises in the structural arrangements near the ends of the ship.
• As the hull narrows towards the bow and stem,however, the girth necessarily reduces and the longitudinals becomes closer together
• Difficulties in construction arise when they converge so closely that some longitudinals have to be eliminated
  
  Source: www.worldmaritimeaffairs.com