1. 1. Introduction

2. 1.1 Ship structure
A ship structure usually consists of a network of plates and supporting structure.
The supporting structure consists of large members running both longitudinally and transversely and is often known as the Framing. The ship plating is attached to the framing.
Framing 骨架，支撑外板、甲板底板等的一切桁材和型材的统称

3. Typical Transverse and Longitudinal Strength Members

4. Superstructure: poop, forecastle, deck house, etc
1.1.1 Structural components
Shell plating：keel, bottom plating, bilge strake, bilge keel, side plating, sheer strake, etc
Bottom: single bottom, double bottom, center(side) girder, center (side) keelson, bottom frame, floor, bottom longitudinal, etc
Side framing: frame, web frame, side stringer, side transverse, side longitudinal, etc
Deck: deck, deck framing, beam, deck girder, deck transverse, side longitudinal, web beam, etc
Bulkhead: transverse bulkhead, longitudinal bulkhead, horizontal girder, vertical girder, etc
Superstructure: poop, forecastle, deck house, etc

5. Keel
A large center plane girder running longitudinally along the bottom of the ship.
Plating
Thin pieces closing in the top, bottom, and sides of the structure. Plating makes a significant contribution to longitudinal hull strength, and resists the hydrostatic pressure load.

6. Frame (肋骨)
A transverse member running continuously from the keel to the deck. Resists transverse loads (ie - waves hitting the side of the ship)
Floor （肋板）
Deep frames running from the keel to the turn of the bilge. Frames may be attached to floors - the frame would be that part above the turn of the bilge.

7. Longitudinal Stringers
Girders which run parallel to the keel along the bottom of the ship. Longitudinals intersect floors at right angles, and provide longitudinal strength.
Stringers
Girders running along the sides of the ship. Typically smaller than longitudinals, they also provide longitudinal strength.

8. Deck Beams
Transverse members of the deck frame.
Deck Girders
Longitudinal members of the deck frame.

9. 1.1.2 Framing systems
There is a compromise between the requirements of strength and the conflicting but equally important requirements of buoyancy, space utilization and cost.
Longitudinal framing
Longitudinal strength members: keel, longitudinals, stringers, deck girders, plating

10. The longitudinal elements have a primary role in combating the longitudinal bending stress.
Ships that are longer than about 300ft (91.44m) which is a typical wave length associated with an ocean wave tend to have a greater number of longitudinal elements to their structures than transverse elements.

11. A practical solution for modern cargo ships is to frame the bottom shell, inner bottom and strength deck longitudinally utilizing transverse floors every third frame. Transverse framing is then applied to the side shell and between decks.
Longitudinal framing is generally used on warships

12. Transverse framing
The transverse elements such as frames and hull plating have a primary role to combat the hydrostatic load.
For ships shorter than 300 ft (91.44m) and those designed to operate at large depths, this is the primary load of concern. Hence short ships and submarines have structures consisting of many frames and fairly thick plating.

13. Combination Framing System
Modern Naval vessels typically use a Combination Framing System which combines the other two methods in some creative manner. A typical combination framing network might consist of longitudinals and stringers with shallow web frames. Every third or fourth frame would be a deep web frame. The purpose of such a system is to optimize the structural arrangement for the expected loading, while minimizing weight and cost.

14. Double bottom construction

15. They are strong and can withstand the upward pressure of the sea in addition to the bending stresses.
Double bottoms provide a space for storing fuel oil, fresh water (not potable), and salt water ballast.

16. The structure can withstand considerable bottom damage caused by grounding or underwater blasts without flooding the ship provided the inner bottom remains intact .
Also, a double bottom provides a smooth inner bottom. This makes it easier to arrange cargo and equipment while providing better accessibility for cleaning..

17. Small vessel single-bottom construction

18. Watertight Bulkheads
Large bulkheads split the hull of a ship into separate sections. In addition to their stiffening of the overall ship structure, they have a primary role in reducing the effects of damage on a ship .

19. 1.2 Characteristics of ship structures
A ship structure is a complicated three-dimensional shape. The shape is determined more by resistance, powering, and internal arrangement considerations than by the desire to optimize the structure's shape for load carrying capability.
A ship's structure possesses certain distinctive features not found in other man-made structures .

20. Main dimensions: length, breadth, depth, draft, displacement, block coefficient
Lines plan, General arrangement
Stability, low resistance and high propulsive efficiency, and navigational limitations on draft

21. 1.2.1 Size and complexity of ships
Ships can be gargantuan in their proportions.
400m (1312 ft) in length, 63m (207 ft) in breadth,
35.9m (118 ft) in depth, a loaded draft of 28.5m (93 ft)

23. Knock Nevis Length: 458.45m breadth : 68.8m Draft: 24.6 m
Knock Nevis is a floating storage and offloading unit (FSO)
Knock Nevis was built in 1979
Length: m
breadth : 68.8m
Draft: 24.6 m
DWT: 564,650 t
From Wikipedia

24. USS Enterprise (CVN-65) From Wikipedia Length: 342 m
Beam: 40.5m(waterline), 78.4m(extreme)
Draught: 12 m
Displacement: 73,858 tons standard 92,325 tons loaded

25. The external surface of the hull, or shell, must be a complex three-dimensional curved surface.
Difficulties in analysis and construction.

26. 1.2.2 Multipurpose function of ship structural component
The shell plating
The principal strength member
A watertight envelope of the ship
Bulkheads
Contributing substantially to the strength of the hull.
Serving as liquid-tight boundaries of internal compartments.

27. Decks
Governed by the arrangement of internal spaces
Resisting local distributed and concentrated loads (Local strength)
Contributing to longitudinal and transverse strength

28. 1.2.3 Variability of ship structural loads
Static components (the weight and buoyancy of the ship in calm water)
Dynamic components (simple wave-induced loads, slamming or springing, by the pro­pellers, ice break­ing, thermally-induced loads, underwater explosions, gunfire, blasts and projectiles, etc.)

29. The loads imposed by the sea, like the sea itself, are random in nature.
Time-dependent or time-varying

30. 1.2.4 Probabilistic nature of structural behavior
In ship structural performance prediction there are at least three sources of uncertainty.
Computational modeling
Properties of the materials
The quality of workmanship during construction and maintenance

31. 1.3 Modes of ship structural failure
The four basic modes of failure that we will consider are:
Tensile or compressive yield
Yield Buckling/Instability
Fatigue
Brittle Fracture .

32. 1.3.1 Tensile or compressive yield
“Slow plastic deformation of a structural component due to an applied stress greater than yield stress.”
A factor of safety is applied during the design of a ship’s structure so that the largest expected stress is only 1/2 or 1/3 of the yield strength .

33. 1.3.2 Buckling/Instability
“An unstable condition caused by the compression of long slender columns resulting in substantial dimensional changes and a sudden loss of stiffness”

34. Instability failure of a structural member loaded in compression may occur at a stress level that is substantially lower than the material yield stress.
The load at which instability or buckling occurs is a function of member geometry and material modulus of elasticity rather than material strength.
Buckling is likely to occur on cross-stiffened deck panels on a ship due to large compressive stresses from longitudinal bending.

35. 1.3.3 Fatigue
“The failure of a material from repeated applications of stress, such as from vibration”
The Endurance Limit is the stress below which the material will not fail from fatigue.
Fatigue Characteristics

36. Fatigue failure in a real structure is greatly affected by such things as material composition (impurities, carbon content, internal defects), surface finish (smooth surfaces are best), environment (salt water is worse than air, moist air is worse than dry air), geometry (sharp corners and discontinuities are bad), and workmanship.

37. The most common consequence of fatigue in ships is the development and propagation of cracks. If such cracks are not repaired, they can result in catastrophic failure.

38. 1.3.4 Brittle Fracture
“The sudden catastrophic failure of a structure with little or no plastic deformation”
The risk of brittle fracture occuring depends on the material, temperature, geometry, and rate of loading.

39. Material
A material with low toughness is susceptible to brittle fracture. Low carbon steels are less brittle than high carbon steels.
Temperature
A material operating below its transition temperature is much more susceptible to brittle fracture because the toughness is very low.

40. Geometry
Cracks having sharp edges are worse than those which are rounded. A smaller crack is better than a big one. Even the orientation of the crack with respect to the loading is a factor.
Rate of Loading
Impact loads are more likely to cause brittle fracture than loads applied gradually and smoothly.

41. 1.4 Loads acting on ship structures
It is convenient to divide the loads acting on the ship structure into four categories.
1.4.1 Static loads
1.4.2 Low-frequency dynamic loads
1.4.3 High-frequency dynamic loads
1.4.4 Impact loads

42. 1.4.1 Static loads
Loads that change only when the total weight of the ship changes. (loading or discharge of cargo, consumption of fuel, or modi­fication to the ship itself)

43. Weight of the ship and its contents.
Static buoyancy of the ship at rest or moving.
Thermal loads resulting from nonlinear temperature gradients within the hull.
Concentrated loads caused by dry docking and grounding.

44. 1.4.2 Low-frequency dynamic loads
Loads that vary in time with periods ranging from a few seconds to several minutes, and therefore occur at frequencies that are sufficiently low compared to the frequencies of vibratory response of the hull and its parts that there is no appreciable resonant amplification of the stresses induced in the structure.

45. Wave-induced hull pressure variations.
Hull pressure variations caused by oscillatory ship motions.
Inertial reactions resulting from the acceleration of the mass of the ship and its contents.

46. A ship’s axes and degrees of freedom
surge （船舶）纵荡
Sway 横摆
Heave 升沉
Yaw 摇艏
A ship’s axes and degrees of freedom

47. 1.4.3 High-frequency dynamic loads
Time-varying loads of sufficiently high frequency that they may induce vibratory response of the ship structure. Probably loads with small magnitudes can give rise to large stresses and deflections due to resonant amplification.

48. Hydrodynamic loads induced by propulsive devices on hull or appendages.
Loads imparted to the hull by reciprocating or unbalanced rotating machinery.
Hydroelastic loads resulting from interaction of appendages with the flow past the ship.

49. 1.4.4 Impact loads
Loads resulting from slamming or wave impact on the forefoot, bow flare and other parts of the hull structure, including the effects of green water on deck.
bow flare艏外飘

50. 1.5 Design philosophy and procedure
The issue of structural design involves the selection of material types, framing forms (longitudinal framing, transverse framing, mixed framing), frame spacing, frame and stiffener sizes, plating thickness.

51. Development of the initial configuration and scantlings
Knowledge, experience, imagination, intuition, creativity
Analysis of the performance of the assumed design
Loads and responses

52. Comparison with performance criteria
Safety, serviceability, durability, technology, economy
Redesign the structure by changing both the configuration and scantlings in such a way as to effect an improvement
Repeat the above as necessary to approach an optimum

53. Main contents of structural strength calculation
The determination of loads
Magnitudes, properties
The responses of the structure (Load effects)
Stresses, buckling loads, defections
Strength criteria
Allowable stresses or permissible stresses; Allowable deflections or permissible stresses