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One of the most integral parts of the ship is its bottom structure. It is deed not only to give hull the required strength to withstand the weight of the cargo, but also to withstand the external hydrostatic lo that act on the bottom of the hull. If these plates are not stiffened, the bending moments on the plates due to the lo 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.
Transverse Stiffening or Transverse Framing: This is carried out in ships less than meter in length. In transverse stiffening, the stiffeners run along the breadth of the ship. We would be discussing this with interesting details a little later. 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 sea going ships having a length more than meter. The details would be touched upon in the later section of this article. Now that we have an idea of the two types of framing, we need to acquaint ourselves with another categorisation of bottom structure framing in ships:.
Usually all smaller ships are single bottomed, as they do not need a double bottom to withstand the load of the cargo. Plate floors as shown in the figures constitute transversely running plates at every frame spacing. Figure 1: Single Bottom transversely framed. Note how the upper edge of each plate is flanged to increase the bending strength of the plate floor. Now, fall back to the basics for a while. 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, deers 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 of intercostal girder would however increase with 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. All sea going 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 ends, 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 spaces. 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.
One of the most important factors in deing a double bottom of a ship, is deciding the height of the double bottom. How does a deer decide what height would be most feasible for a ship of a particular length? This is governed by the height of the keel that is required by the ship. So while estimating the scantlings of a ship, the deer first calculates using the rules specified by the authorised Classification Society the height of the centre girder, which must always be housed within the double bottom.
Hence, this factor now decides the double bottom height. Double bottom heights often increase in ways of engine rooms, as they need to take up higher stresses due to heavy machinery in those regions. In engine room region, all the frames are provided with plate floors, and no bracket floors are used.
However, there is another factor a deer must take care of, while providing an increased double bottom height in high stress regions. The height must not be increased abruptly, therefore resulting in a discontinuity, which would lead to concentration of stresses, and eventually a structural failure.
So the increase in height should be gradually tapered up and down. The taper should start a few frames forward of the engine room bulkhead, and continue up to a three or four frames aft of the engine room to allow proper stress flow or structural continuity. The intricacy in de of a bottom structure begins after one has understood the above concepts. We have seen two separate categorisations of double bottoms. First dealt with the type of stiffening used, and the second dealt with single and double bottoms.
Most budding naval architects initially find this part confusing, which is why, let us first list down the four types of possible bottom structures:. Out of these four types, three are used, and one is not. While it is easy for experienced deers to point out that one type, it may not be easy for everyone. Because there is an underlying concept to it, which we shall understand now.
Why is longitudinal framing used when we could easily have provided transverse framing in longer ships too? The answer lies in the fact that ships longer than meter are subjected to high global longitudinal bending stresses like hogging and sagging in different load conditions, unlike smaller ships. So if longer ships would be stiffened transversely, the transverse stiffeners would have no role in taking up the longitudinal bending stresses of the hull girder, and therefore lead to more chances of failure.
Hence, stiffeners are aligned longitudinally in longer ships. It should be very clear now, that since longer ships have longitudinal stiffening, and since they are also deed to carry higher amount of cargo, a double bottom is necessary. Hence, longitudinally framed single bottom structures 3 rd in the above list do not exist.
Since we are now done with the basics of a bottom structure, it would be easy to visualise each type henceforth. In modern analyses of bottom structure of ships, deers take a lot of care for various modes of failure. A disastrous mode of failure other than bending, is buckling, which a bottom structure can be often subjected to. For example, consider a ship to be hogging. The outer bottom shell undergoes a compression that le to buckling of the bottom plate, and associated structure.
Torsion can also be a mode of failure in cases of container ships. So, as a ship deer, when one analyses the feasibility of a bottom structure, it is important to test for all possible modes and types of failure. Because, for example, if a deer certifies a bottom structure only on the basis of bending stress, without taking into consideration buckling or torsion. What could happen? Hence, the principle de criteria must be decided base on all possible modes of failure, at various load cases, analysed by efficient and certified FEM tools, so as to attain a safe and economical factor of safety for the structure, from all possibilities of failures at sea.
Tags: De process naval architecture ship de ship hull de. Passionate about marine de, he believes in the importance of sharing maritime technical knowhow among industry personnel and students. I am studying marine engineering l want to now about ship structure and engine and all machinery parts. Thanks ,I really appreciate ing the forum. I would want a picture of structure after the collision bulkhead resisting panting and other parts of the vessel,I will be glad ,thanks once more.
Will be more comprehensive by adding some structural material as well. I de fictional space vehicles, and well, felt that every craft begins with a base structure. Any advice you might be able to offer is always welcome. Note their positions in the images, to visualise the exact layout. The Drain holes are not touching the bottom, thus not able to drain.
Maybe the air holes are a bit small. If the Engineer decides that de allows one or more Manholes should be a bit larger, to accommodate a bigger person especially if they are narrow and you must turn to squeeze through. How will the Fabricator make the plates with the air and drain holes quickly and for lowest cost. In which plan does the frames of the ships come?
Want to share your tips and advice? Got questions? Visit the community forum to ask questions, get answers, meet people, and share your tips! Comments I am studying marine engineering l want to now about ship structure and engine and all machinery parts. Something I noticed about the Drawing. Lightening Holes big enough to pass Tools like an Angle Grinder through are useful.Lookin for that one i can make Ship Bottom
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