Short Note on Ship Scantlings and Framing Systems

Scantlings:

Scantling is a measurement of prescribed size, dimensions, or cross-sectional areas. In shipbuilding, the scantling refers to the collective dimensions of the framing (apart from the keel) to which planks or plates are attached to form the hull. The word is most often used in the plural to describe how much structural strength in the form of girders, I-beams, etc. is in a given section.

Framing Systems:

There are three types of framing systems that are considered in ship building.
o Transverse framing system
o Longitudinal framing system
o Combined framing system
We adopt longitudinal framing in the bottom and deck and transverse framing system in the side shells. So, we adopt combined framing system.

Functions of the Ship’s Structure

The ship is capable of bending in a longitudinal vertical plane. There must be material in the ship’s structure which will resist this bending. Any material distributed over a considerable portion of the length of the ship will contribute to the longitudinal strength. This category are the side and bottom shell plating, the inner bottom plating and any decks which there may be.

This longitudinal material forms a box girder of very large dimensions in relation to its thickness. Consequently, unless the plating is stiffened in some way, it would be incapable of withstanding compressive loads.

For this reason, it becomes necessary to fit transverse rings of material spaced from 2ft to 3ft apart throughout the length of the ship. This is the procedure which is called Transversely Framed Ship. The transverse stiffening consists of three parts:
• In the bottom between the outer and inner bottoms there are vertical plates called Floors which have lightening and access holes cut in them.
• In the sides of the ship rolled sections called Side Frames are welded to the plating.
• The decks are also supported by rolled sections welded to the plating called Beams.
The floors, side frames and beams at the various decks are connected by means of brackets so that a continuous transverse ring of material is provided.

The effect of supporting the plating in this way is to reduce that unsupported span and hence to raise the buckling strength of the plating to enable it to carry compressive loads.

Another function of these transverse rings is to prevent transverse distortion of the structure, so that the floors, side frames and beams are the main items contributing to the transverse strength of the structure of the ship.

The main force involved here is that due to water pressure and, as this will be greatest at the bottom of the ship, the bottom structure should be very heavy. This is in fact so, a very heavy girder being provided by the floor plate in conjunction with its associated inner and outer bottom plating.

The side of the ship is also subjected to water pressure of a rather lesser magnitude, and in this case adequate stiffening is provided by the girder consisting of the side frame welded to the side shell plating.

The beam with its associated deck plating forms an effective built up girder. The main factor determining the sizes of the beams is the load which they have to carry. This load may be cargo load, a load due to passengers or in the case of weather deck some weather load.

Other items of the structure which contribute to transverse strength are watertight bulkheads. Their primary object is to divide the ship into a series of watertight compartments, but since they consists of transverse sets of stiffened plating they have very considerable transverse rigidity and hence contribute greatly to the prevention of transverse deformation of the structure.

It is common practice nowadays to adopt a different form of construction in which the sides of the ship are stiffened transversely whilst the decks and bottom are stiffened by means of longitudinal.

The effect of stiffening the deck and bottom longitudinal members instead of transverse members is to increase the buckling strength of the plating.

Since these longitudinal are effectively attached to the plating, they contribute also to the general longitudinal strength of the structure. The longitudinal have to carry cargo and water pressure loads and so, in order to reduce their scantlings, they must be supported at positions other than at the bulkheads.

This is achieved by introducing deep transverse beams in the decks spaced some 6 to 12 feet apart and by having transverse plate floors in the bottom at the same spacing. These widely spaced transverse members in conjunction with the closely placed side framing then provide the transverse strength of the structure.

The longitudinal system of framing has often also been extended to the sides of the ship as well as the decks and bottom. In fact when initially developed for use in oil tankers this was the method which was adopted. This was called the Isherwood System.

At a later stage in the development of the tanker the combined system of longitudinal in the bottom and deck with transverse side framing was employed. In many of the larger oil tankers of the present day, however, the complete longitudinal framing system has been used.