One of the most integral parts of the ship is its bottom structure. It is designed not only to give the hull the required strength to withstand the weight of the cargo but also to withstand the external hydrostatic loads that act on the bottom of the hull.
A ship’s hull is basically made up of bent plates welded together. If these plates are not stiffened, the bending moments on the plates due to the loads may exceed the value of stress that can be withstood by the material, and hence cause failure. So, the plates are stiffened (or their section modulus is increased) by adding stiffeners to them.
Now, there are two basic ways to stiffen a ship –
Transverse Stiffening
Longitudinal Stiffening
Transverse Stiffening or Transverse Framing: This is carried out in ships less than 120 meters in length. In transverse stiffening, the stiffeners run along with the breadth of the ship.
Longitudinal Stiffening or Longitudinal Framing: This type of framing employs stiffeners that run longitudinally, that is along the length of the ship, and is used in all seagoing ships having a length more than 120 meters.
Now that we have an idea of the two types of framing, we need to acquaint ourselves with another categorization of bottom structure framing in ships:
Single Bottom
Usually, all smaller ships are single-bottomed, as they do not need a double bottom to withstand the load of the cargo. In these ships, the plate floors (see the figure to understand the context of ‘Floors’ in shipbuilding) themselves act as the stiffening members of the bottom shell plating. Plate floors (as shown in the figures) constitute transversely running plates at every frame spacing.
When the hydrostatic pressure under the bottom shell exerts a bending moment in the bottom shell, the plate floor takes up the bending stress.
So, designers treat all such members taking up bending stresses as beams. Empirically, the bending moment in a beam increases with the increasing span. So, what if we could reduce the span of the plate floor to further increase its stress capacity?
This is why intercostal girders are used (see the image). The number of intercostal girders would however increase with an increasing beam of the ship since that would also result in increased length of a plate floor.
A uniform wood ceiling is provided on top of all the plate floors, to provide stowage of cargo. But that doesn’t make it a double bottom structure as the wood would not take up any stresses exerted onto the bottom structure.
Double Bottom
All seagoing ships are double-bottomed. In such a structural arrangement, a tank top is provided above the plate and bracket floors. Bracket floors are a little different from plate floors, in as much as they are not comprised of one single plate running athwartship, but only brackets at the port and starboard end, with struts that support the tank top with the bottom shell.
Bracket floors are mostly placed at each frame, and plate floors are generally placed at every three to four-frame space. The space within the double bottom (that is, between the tank top, and outer bottom shell) is used up for carrying ballast, fuel oil, dirty oil, fresh water, and other consumables.
Therefore, there're four types:
Transversely Framed, Single Bottom
Transversely Framed, Double Bottom
Longitudinally Framed, Single Bottom
Longitudinally Framed, Double Bottom
But in the fact that ships longer than 120 meters are subjected to high global longitudinal bending stresses like hogging and sagging in different load conditions, unlike smaller ships. So Longitudinally Framed Single Bottom cann't be used.
Let's learn more about the other tree structures:
1, SINGLE BOTTOM, TRANSVERSELY FRAMED
-The plate floors act as transverse stiffeners, and their spans are reduced by the use of intercostal side girders that run longitudinally.
-Most single bottom ships are provided with a bar keel that extends along the length of the ship up to a certain waterline at the stem. The bar is slightly protruded outside the outer bottom shell.
-The outer bottom shell plating just adjacent to the bar keel is called Garboard strake, and its thickness is more than the thickness of the remaining bottom shell.
-All the plate floors are flanged at their tops, so as to increase their bending strength.
-Manholes are provided on the plate floors for crew access. These holes are flange too, so as to reduce stress concentration.
2, DOUBLE BOTTOM, TRANSVERSELY FRAMED
-This is used in ships of length less than 120 meters. (See how the length factor dominates over the type of framing used)
-The bracket floors form the transverse stiffeners at every frame, and plate floors are used at every 3 to 4 frame space, or 1.8 meters intervals.
-Similar to the single bottom, to reduce the span of the plates, intercostal side girders of keelsons are used that run longitudinally. An important thing to note is that the side girders are continuous members, that is, where there is an intersection between a plate floor and a side girder, the plate floor is cut and welded on both the sides of the girder and not the other way round. Why? Remember, we needed to reduce the span of the plate floors, hence the girders will act as supporting members to the plate floors.
-Flat plate keels are used in these structures. The keel plating thickness is a very important decision-maker in the strength of the ship. This is to be calculated from the formula dedicated to this purpose, provided by the relevant classification society.
-Intercostal girders or side girders, and plate floors will have lightning holes at regular intervals to reduce the structural weight and will have manholes (flanged) to provide access.
-Drain holes will be provided on the plate floors to help drainage of liquids. Plate floors are further stiffened by flat bar stiffeners (see image below), and bracket floors, by angle struts to prevent warping.
3, DOUBLE BOTTOM, LONGITUDINALLY FRAMED
-The prime stiffening members are longitudinally running bulb sections or angle sections. The stiffeners on the bottom plating are called outer bottom longitudinals, and those that stiffen the tank top plating are called tank top longitudinals.
-The span of each longitudinal is equal to three of four frame spaces. That is, at each three or four frames, there would be a plate floor to support the longitudinal. A bracker floor is placed at almost every frame, but it does not support the longitudinals.
-Intercostal girders are used, as usual, to reduce the span of the plate floors.
If you notice carefully, the longitudinals run across plate floors through holes called scallops. So in a frame, where it is required to support the span of a longitudinal using a plate floor, the longitudinal is welded with a small plate to the plate floor, therefore rendering the scallop as a support end.
-In bracket floors, tank top and bottom shell longitudinals are supported to each other by means of angle struts.
-In plate floors, the longitudinals of the tank top and bottom shell are supported to each other by flat bar stiffeners, to restrict bending, torsion, and buckling.
-As usual, drain holes are used for fluid drainage and air holes are used for the passage of air. Note their positions in the images, to visualise the exact layout.
-Margin plates are used in some designs, to lead the flow of waste fluids (bilge) towards the bilge wells on either side of the ship.
-A continuous centre girder runs through the length of the ship, supporting the entire bottom structure, the keel plate, and the garboard strake.
Moreover, when one analyses the feasibility of a bottom structure, it is important to test for all possible modes and types of failure.
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