HULL STRENGTH 16-1. Foreword. Cruisers, destroyers and auxiliaries have been lost by breaking in two as a result of impaired structural strength. After serious underwater explosion, considerable buckling and tearing of structure takes place (see Chapt. XV). When the main structural members are ruptured or wrinkled the vessel may break up in a seaway. The importance of these facts is obvious. Accordingly, the ensuing discussion is concerned with strength features built into a ship and how they are affected when the ship is damaged.16-2. Beam theory. An elementary knowledge of structural theory is necessary to our proposed study.If a simple beam is supported at its two ends and various vertical loads are applied over the center of the span, the beam will bend (see fig. 16-1). As the beam bends the upper section of the beam will compress and the lower part will stretch. Somewhere between the top and bottom of the beam there will be a section which will neither be in compression nor tension; that part we term the neutral axis. The greatest stresses in tension and compression occur about half way between the supports, or near the middle of the beam's length. In the case of an I-beam, the greater mass of structural material is placed in the upper and lower flanges to resist the compression and tension. Very little material is placed in the web which is near the neutral axis because the web takes little of the tension or compression stresses; however, it does take care of shearing stresses. The latter are sizeable near the supports. 16-3. Ship in seaway. A ship in a seaway can be considered similar to a beam with supports and distributed loads. The supports are buoyant forces of the water and the loads are the weight of the ship's structure and material within, such as fuel, water, ammunition, etc. The worst condition of loading and support for a ship occurs when it heads into or away from the sea, with waves approximately as long as the length of the ship. A quartering sea can also produce this condition if the ship's bow and stern either are in troughs or crests at the same time (see fig. 16-2 and fig. 16-3).16-4. Sagging stresses. The ship shown in fig. 16-2 is supported by waves, with the bow and stern riding crests and the midship region in the trough. This ship will bend with compression at the top and tension at the bottom. The ship is said to be sagging, and in this condition the weather deck tends to buckle due to compressive stress, while the bottom plating tends to stretch due to tensile stress.16-5. Hogging stresses. When the ship shown in figure 16-2 advances half a wave length, so that the crest is at midship and the bow and stern are over troughs, as in figure 16-3, the stresses are reversed. The weather deck is in tension and the bottom plating is in compression, and the ship is said to be hogging.16-6. Ship girder. In its resistance to hogging and sagging stresses, the main body of the ship can be likened to a long beam, resembling a box girder. Therefore, it is often referred to as the hull girder orFigure 16-1. Diagram to show the effect of load placed over the center of a beam, and cross section of an I-beam. 142 ship girder. Its principal strength members are at the top and bottom, where the greatest stresses occur, and these top and bottom flanges are joined together by side webs. The top flange consists of the main deck plating, especially the deck stringers, the sheerFigure 16-2. Diagram to show tension and compression when a ship is in a sagging condition.
Hogging And Sagging Pdf 18
strakes of the side plating, and any continuous deck girders. The bottom flange consists of the bottom plating, including the flat keel, garboard strakes, "B" strakes, bilge strakes, etc., plus the vertical keel and any continuous longitudinal girders in way of the bottom. If an inner bottom is fitted, it also contributes to the lower flange. The side webs of the ship girder are composed of the side plating, supported to some extent by any long continuous fore-and-aft bulkheads. These side webs take up the shearing stresses which usually are greatest at the quarter-length points of the ship.The major strength members of a destroyer hull girder are indicated in figure 16-4.16-7. Transverse framing. Transverse frames and continuous transverse bulkheads contribute greatly to the strength of the hull girder by tying its various members together, stiffening them, and preventing buckling when under compression. The stanchions throughout the ship also serve to brace and stiffen the hull girder and tend to hold deck plating in position. On auxiliaries the transverses are major strength members. The longitudinal strength of a merchant-type vessel is taken up by the plating of the shell and decks, which need stiffeners of considerable size to prevent buckling.16-8. Plating in compression. Both the top and bottom flanges of the ship girder must take compressive stresses as the ship alternately sags and hogs. Unstiffened plating can take very little compression. A plate will buckle in compression at a small fraction of the load that it can withstand in tension. Therefore, the plating making up both the top and bottom flanges of the ship girder is stiffened by having shapes welded or riveted to it. The stiffening members include shapes such as I-beams, tees, channels, angles, and the like. They may run either longitudinally or transversely. The best system is to have the stiffeners run both ways to form a cellular web structure.16-9. The strength deck. The term strength deck is generally applied to the deck which acts as the top flange of the hull girder. It is the highest continuous deck, usually the main or weather deck. On a merchant-type or destroyer-type ship, where the main deck is the only continuous high deck, it is the only strength deck.16-10. Lower strength decks. If the second (or third) deck is continuously and integrally built into the vessel's structure, it will take some of the stress, although not as great a share as the main deck. These stresses are considered in the design of lower continuous decks, and the function of such decks should not be overlooked after severe structural damage. If the main deck is destroyed, the second deck becomes the strength deck, and will actually be subjected to higher unit stresses than the main deck was for a given hogging or sagging condition. (A similar situation arises if the bottom is destroyed. Intensified stresses are placed upon the next higher structure that takes the load.)16-11. Upper decks and superstructure. The decks above the main deck usually are not strength decks, and do not contribute to the strength of the hull girder. These upper decks must be interrupted at intervals down the length of the ship by expansion joints. Otherwise they will tend to take up some of the load of the strength deck and probably will fail. Cracking and buckling of deck houses and superstructure results 143 Figure 16-4. Diagram to show major strength members of a destroyer's hull girder. if this principle is neglected. The flight decks of most carriers are not strength decks. They are, therefore, interrupted with expansion joints. These upper decks and houses carry the gravity loads above them and on them down to the hull of the ship.16-12. Local strength. The structure of a ship is called upon to resist three types of local stresses, in addition to those of the ship girder. They are as follows:1. Hydrostatic pressures.2. Solid weight loads.3. Dynamic loading.16-13. Hydrostatic pressures. The pressure on a submerged body is proportional to its depth in the liquid, and acts at right angles to the surface of the object. Each square foot of shell surface is subject to a pressure of 1/35 of a ton for every foot of depth of liquid (or 64 pounds per foot of depth). The water pressure is applied to the shell and transmitted through the frames, decks and bulkheads. Although the horizontal pressures of water exerted on each side of the ship cancel each other, the force still acts upon the hull. The decks, transverse framing, and bulkheads prevent lateral crushing of the hull by the horizontal pressure of the water.16-14. Pressures due to flooding. If the shell of the ship is ruptured and flooding follows, the hydrostatic pressures formerly exerted on the shell plating are now placed upon the bulkheads of the flooded compartments. This is why bulkheads require stiffeners to prevent them from bulging, and why bulkheads that are farther below the waterline are thicker, require more stiffening, and are given higher test pressures. Flooding water will exert a considerable upward pressure against the overhead deck of a flooded compartment if the deck in question is some distance below the waterline. This pressure will be undiminished if there is an air bubble trapped above the flooding water. Therefore, some thought must be given to the problem of shoring weakened decks downward as well as upward, and to the consequences of opening a hatch, scuttle, or manhole over a flooded compartment. Hydrostatic pressures also are imposed upon bulkheads and decks by the contents of intact fuel and water tanks.16-15. Solid weight loads. The weight of every object on the ship, solid or liquid, rests at some point on a deck (or bulkhead). Included are fixed weights such as guns, barbettes, boilers, turbines, and steel of the structure itself. The crew and the consumables aboard ship also must be provided for. The load from these various items must be supported and transmitted to the shell of the ship, where it is resisted by the hydrostatic pressure.To prevent a concentration and possible excessive stress, large loads such as guns, turrets, barbettes, and handling equipment are distributed over a wide area by means of structural bulkheads and girders. 144 16-16. Bottom framing. The bottom framing, in which floors and keel are integrated, forms a rigid cellular construction. It is to this bottom framing that loads of great magnitude are brought, by stanchions, or in the case of temporary loads, by shoring. It is sometimes necessary to shore all the way down from the main deck in cases of unusual topside cargo. Other heavy loads, like the ship's main propulsion machinery, are bolted to foundations which are built directly on top of the bottom framing.16-17. Dynamic loading. In addition to local stresses due to the above loads which are static in nature, the various members of a ship's structure may be subjected to dynamic loads of unpredictable intensity and duration. Pitching and pounding, wind pressures, collisions, the recoil of gunfire, turning forces, inertia due to changes of motion, and blast effect from the bursting of enemy shells, bombs, or torpedoes, all impose dynamic stresses of varying magnitude and time on the inner and outer portions of the ship. If plates are turned out from a torpedo hit, the resulting scoop action may build up a greater pressure in the flooded compartment than would be caused by the hydrostatic head alone. It is difficult to estimate the size of structure necessary to withstand some of these impulses, and they are allowed for on the basis of experience, plus liberal safety actors in designing for static loads plus some dynamic toad. For this reason there is a considerable excess of strength built into Naval ships.16-18. Panning. Panting is the term applied to the action of a section of plating when it pulsates in and out under the influence of waves or dynamic impulses. To overcome this tendency additional members are provided in the region of dynamic loads. The panting frames or breast plates in the fore peak are an example. A tendency of plating to pant can be overcome temporarily by shoring.16-19. Underwater explosion damage-large ships. Underwater explosion damage impairs the strength of a ship in two ways. Structural strength members are damaged by being ruptured or buckled. Flooding of compartments also takes place, thus increasing the loading on the previously damaged ship girder (see Chapt. XV).A given underwater explosion frequently opens a bigger hole in the shell of a large vessel. The large ship, on the other hand, is better able than a small one to withstand the destruction and loss of structure, inasmuch as only a small proportion of the main structural members will be damaged. These main structural shapes and plates are also considerably heavier and are more numerous than in the case of a small ship. The chances of serious structural failure on a large ship due to a given underwater explosion or bomb hit are small.16-20. Underwater explosion damage-small ships. In the case of auxiliaries, cruisers, destroyers and other small ships, underwater explosions in the midship region have ruptured a large proportion of the principal strength members. Following extensive damage to the major strength members ships of this type may break up, unless the strength of fractured members can be replaced before the vessel is subjected to the action of heavy seas.If the structural damage is severe, partial restoration of the main strength members may be necessary before proceeding far (and then only at reduced speeds) even with a calm sea and when destination is not too far away. Such repairs if not practicable underway can frequently be accomplished at advanced bases.16-21. Explosions at bow or stern of small ships. Underwater explosions at either the bow or stern of a slender, small ship, such as a destroyer or destroyer escort, usually cause local destruction which is intense, but neither widespread nor particularly serious. However, the effect of such an explosion is to shake this type of vessel like a whip. Waves of flexural vibration pass down the length of the hull, producing stresses like those in hogging and sagging but of shorter duration and of greater intensity. The result, although not usually obvious, can be serious. It consists of compression failures in the midship region, evident in wrinkled deck plating, wrinkled shell plating, buckled longitudinal girders, and either buckling, laying over of flanges, wrinkling, or other failure of any of the members in the waist of the ship that contribute to her longitudinal strength. Such failures may be hidden from sight below the waterline, under boilers, behind stores and equipment, or beneath the surface of subsequent leakage water. It is a feature of compression members that once buckled they can never again develop even a fraction of their original strength. Attempted straightening is of no avail; the only effective measures are replacement or duplication.16-22. Flooding and the ship girder. The load on the ship girder is increased by the entrance of damage water. The increase in stresses due to this augmented load depends both on the amount and on the location of the flooding. 145 Damage and consequent flooding in the middle length increases sagging stresses. This means increased tension at the bottom and compression at the top. Measures to correct trim caused by damage in the middle length should be of a nature to reduce sagging stresses. This may be accomplished by any or all of three methods, as follows: 2ff7e9595c
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