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Unique Structures (6.1). SHIP STRUCTURES. Ship’s Structures are unique for a variety of reasons. For example: Ships are BIG! Ships see a variety of dynamic and random loads The shape is optimized for reasons other than loading.

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ship structures

Unique Structures (6.1)

SHIP STRUCTURES
  • Ship’s Structures are unique for a variety of reasons. For example:
    • Ships are BIG!
    • Ships see a variety of dynamic and random loads
    • The shape is optimized for reasons other than loading.
    • Ships operate in a wide variety of environments.

What are they optimized for?

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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Up until now we have used Resultant (single point) Forces through “G” (s) and “B” (FB)

Stern

Bow

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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Buoyancy is actually a distributed force. (LT/ft)
  • Often it is uniformly distributed. The distribution follow the Curve of Areas.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Similarly, weight is a distributed force.
  • But it is rarely uniformly distributed. Many of the weights, such as the engines, are concentrated (point loads).
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Nonuniform distributions produce shear planes at areas of unequal loading.
  • Overall force distributions are Load Diagrams
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Ship Structural Loads (6.2)

SHIP STRUCTURES

For simplicity, we often model ships as simple beams.

  • Longitudinal Bending Moments are the principle load of concern for ships >100 ft.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • If the beam sags, the top fibers are in compression and the bottom fibers are in tension.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • A ship has similar bending moments, but the buoyancy and many loads are distributed over the entire hull instead of just one point.
  • The upward force is buoyancy and the downward forces are weights.
  • Most weight and buoyancy is concentrated in the middle of a ship, where the volume is greatest.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Buoyant force is greater at wave crests.
  • If the wave crest is at the bow and stern, the vessel is said to be sagging. The net effect is that the middle has less support.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • If sagging loads get too large...
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Hogging - Buoyancy Support in the Middle
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • Sagging - buoyancy support at the ends
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • The location where the beam remains its original length is called the neutral axis and marks the transition between tension and compression in a section.
  • The neutral axis is located at the geometric centroid of the cross section.
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • The maximum bending moment and simple beam theory enables us to determine the bending stress anywhere in the beam. The expression for bending stress is:
  •  = My
  • I
  • where,
  •  = bending stress in tons per ft2
  • M = bending moment in ft-ton
  • I = second moment of area of structural cross section in ft4
  • y = distance of any point from the neutral axis in ft
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Ship Structural Loads (6.2)

SHIP STRUCTURES
  • The bending stress at the neutral axis is zero.
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Ship Structure (6.3)

SHIP STRUCTURES
  • A ship structure usually consists of a network of frames and plates.
  • Frames consist of large members running both longitudinally and transversely. Think “picture frame.”
    • Plating is attached to the frame providing transverse and longitudinal strength. Think “dinner plate.”
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Ship Structure (6.3)

SHIP STRUCTURES
  • Keel: Longitudinal center plane girder along ship bottom “Backbone”.
  • Plating: Thin skin which resists the hydrostatic pressure.
  • Frame: Transverse member from keel to deck.
  • Floor: Deep frames from keel to turn of the bilge.
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Ship Structure (6.3)

SHIP STRUCTURES
  • Longitudinals: Parallel to keel on ship bottom, provide longitudinal strength.
  • Stringers: Parallel to keel on sides of ship, also provide longitudinal strength
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Ship Structure (6.3)

SHIP STRUCTURES
  • Transverse Framing
    • Combats hydrostatic loads
    • Consists of closely spaced continuous frames with widely spaced longitudinals.
    • Best for short ships (lengths less than typical ocean waves: ~ 300ft) and submarines.
    • Thick side plating is required.
    • Longitudinal strength is relatively low.
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Ship Structure (6.3)

SHIP STRUCTURES

frame

plate

DDG-51 DC Mat’l and Structure

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Ship Structure (6.3)

SHIP STRUCTURES
  • Longitudinal Framing
    • Consists of closely spaced longitudinals and widely spaced web frames.
    • Longitudinal framing resists longitudinal bending stresses.
    • Side plating is thin, primarily designed to keep the water out.
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Ship Structure (6.3)

SHIP STRUCTURES
  • Modern Naval vessels typically use a “Combination Framing System”
    • 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.
    • Optimizes the structural arrangement for expected loading, minimize weight and cost.
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Ship Structure (6.3)

SHIP STRUCTURES
  • Double Bottoms
    • Double bottoms are two watertight bottoms with a void (air) space in between.
    • They are strong and can withstand the upward pressure of the sea in addition to the bending stresses.
    • Provide a space for storing fuel oil, fresh water (not potable), and salt water ballast.
    • Withstand U/W damage better, but rust easier.
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Modes of Structural Failure (6.4)

SHIP STRUCTURES
  • The five basic modes of failure are:
    • Tensile or compressive yield (often from bending)
    • Compressive Buckling/Instability
    • Fatigue
    • Brittle Fracture
    • Creep
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Modes of Structural Failure (6.4)

SHIP STRUCTURES
  • Tensile or Compressive Yield
    • Plastic deformation due to applied > yield.
    • Failure criteria for many structures is that no stress shall exceed yield.
    • Factor of Safety included in design to decrease liklihood of failure.
    • allowable < 1/2 yield or 1/3 yield
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Modes of Structural Failure (6.4)

SHIP STRUCTURES
  • Fatigue & Endurance Limits (Revisited)
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Modes of Structural Failure (6.4)

SHIP STRUCTURES
  • Brittle Fracture
    • Catastrophic failure, generally by rapid propagation of a small crack into a large crack. (All metals have initial small cracks.)
    • Cracks grow from fatigue.
    • Brittle fracture dependent on (1) material, (2) service temp, (3) flaw geometry, and (4) load application rate.
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Modes of Structural Failure (6.4)

SHIP STRUCTURES
  • Creep
    • Time dependent plastic deformation of a material due to continuously applied stresses that are below the yield stress.
    • Not a primary concern for failure of metals.
    • Important for wood and some composites.
ship s breaking
Ship’s Breaking?

Surprisingly common!