SHIP STRUCTURES

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SHIP STRUCTURES - PowerPoint PPT Presentation

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

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Presentation Transcript

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
• Ships operate in a wide variety of environments.

What are they optimized for?

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

Stern

Bow

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

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).

SHIP STRUCTURES
• Overall force distributions are Load Diagrams

SHIP STRUCTURES

For simplicity, we often model ships as simple beams.

• Longitudinal Bending Moments are the principle load of concern for ships >100 ft.

SHIP STRUCTURES
• If the beam sags, the top fibers are in compression and the bottom fibers are in tension.

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.

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.

SHIP STRUCTURES
• If sagging loads get too large...

SHIP STRUCTURES
• Hogging - Buoyancy Support in the Middle

SHIP STRUCTURES
• Sagging - buoyancy support at the ends

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.

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

SHIP STRUCTURES
• The bending stress at the neutral axis is zero.

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.”

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.

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

Ship Structure (6.3)

SHIP STRUCTURES
• Transverse Framing
• 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.

Ship Structure (6.3)

SHIP STRUCTURES

frame

plate

DDG-51 DC Mat’l and Structure

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.

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.

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.

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

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

Modes of Structural Failure (6.4)

SHIP STRUCTURES
• Fatigue & Endurance Limits (Revisited)

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.

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?

Surprisingly common!