SHIP STRUCTURES by rcr14802

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									           SHIP STRUCTURES
             Unique Structures (6.1)
• 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.  What are they optimized for?

  – Ships operate in a wide variety of
    environments.
        SHIP STRUCTURES
        Ship Structural Loads (6.2)
• Up until now we have used Resultant (single
point) Forces through “G” (s) and “B” (FB)




Stern                                 Bow
         SHIP STRUCTURES
         Ship Structural Loads (6.2)
• Buoyancy is actually a distributed force. (LT/ft)
• Often it is uniformly distributed.
        SHIP STRUCTURES
        Ship Structural Loads (6.2)
• Similarly, weight is a distributed force.
• But is rarely uniformly distributed.
         SHIP STRUCTURES
         Ship Structural Loads (6.2)
• Nonuniform distributions produce shear planes
    at areas of unequal loading.
                              Shear plane




• Overall force distributions are Load Diagrams
           SHIP STRUCTURES
           Ship Structural Loads (6.2)
   For simplicity, we often model ships as simple beams.

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

                                LOAD APPLIED


      Recall M=F x D!          LOAD APPLIED
          SHIP STRUCTURES
          Ship Structural Loads (6.2)
• If the beam sags, the top fibers are in compression
       and the bottom fibers are in tension.



                      Compression


                       Tension
          SHIP STRUCTURES
          Ship Structural Loads (6.2)

• 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
           Ship Structural Loads (6.2)

• 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.
                               Sagging



                    Trough Amidships
           SHIP STRUCTURES
           Ship Structural Loads (6.2)

• If sagging loads get too large...
          SHIP STRUCTURES
          Ship Structural Loads (6.2)
• Hogging - Buoyancy Support in the Middle
      SHIP STRUCTURES
      Ship Structural Loads (6.2)
• Sagging - buoyancy support at the ends
            SHIP STRUCTURES
            Ship Structural Loads (6.2)
• A ship can be modeled as a simple beam with a
     distributed load for weight, and point forces
     representing the wave buoyancy.

  Load Diagram    Ques. Is this hogging or sagging?
  Load = (x)/L

                      Buoyancy = 
                                          Recall F=0!
  Shear Diagram
                         V(L/2) = /2

                                        V(L/2) = -/2   Length
           SHIP STRUCTURES
           Ship Structural Loads (6.2)
                                           Neutral
                                           axis
               Compression side (-)



                Tension side (+)

• 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
            Ship Structural Loads (6.2)
• The bending stress at the neutral axis is zero.

                  Compression
                                Stress at top of beam

              y
Distance from 0
neutral axis of                               Neutral axis
beam


                                            Stress at bottom of beam
                                Tension
                            0     Stress
            SHIP STRUCTURES
            Ship Structural Loads (6.2)
• 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
               Ship Structure (6.3)

• 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 STRUCTURES
  Ship Structure (6.3)
           SHIP STRUCTURES
               Ship Structure (6.3)
• 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 STRUCTURES
               Ship Structure (6.3)

• Longitudinals:     Parallel to keel on ship bottom,
                     provide longitudinal strength.



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




                                frame
                                 plate
DDG-51 DC Mat’l and Structure
          SHIP STRUCTURES
              Ship Structure (6.3)
• 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 STRUCTURES
              Ship Structure (6.3)
• 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 STRUCTURES
  Ship Structure (6.3)
          SHIP STRUCTURES
              Ship Structure (6.3)
• 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.
          SHIP STRUCTURES
       Modes of Structural Failure (6.4)
• The five basic modes of failure are:

  – Tensile or compressive yield

  – Compressive Buckling/Instability

  – Fatigue

  – Brittle Fracture

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

   

   S
                   Steel
   t
   r                                  Endurance Limit
   e
   s
   s                 Aluminum

   (psi)

                           Cycles N
           SHIP STRUCTURES
        Modes of Structural Failure (6.4)
• 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.
          SHIP STRUCTURES
       Modes of Structural Failure (6.4)

• 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.

								
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