ship structures unique structures (6.1) ship’s structures are unique for a variety of reasons. for...

SHIP STRUCTURESUnique 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.

– Ships operate in a wide variety of environments.

What are they optimized for?

SHIP STRUCTURESShip Structural Loads (6.2)

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

Stern Bow


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


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


• Nonuniform distributions produce shear planes at areas of unequal loading.

• Overall force distributions are Load Diagrams


For simplicity, we often model ships as simple beams.

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


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


• 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.


• 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


• If sagging loads get too large...


• Hogging - Buoyancy Support in the Middle


• Sagging - buoyancy support at the ends


• 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.


• 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 Iwhere, = bending stress in tons per ft2

M = bending moment in ft-tonI = second moment of area of structural cross section in ft4

y = distance of any point from the neutral axis in ft


• The bending stress at the neutral axis is zero.

SHIP STRUCTURESShip 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.”


• 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.


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

• Stringers: Parallel to keel on sides of ship, also provide longitudinal strength


• 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.


DDG-51 DC Mat’l and Structure

frame

plate


• 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.


• 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.


• 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 STRUCTURESModes of Structural Failure (6.4)

• The five basic modes of failure are:

– Tensile or compressive yield (often from bending)

– Compressive Buckling/Instability

– Fatigue

– Brittle Fracture

– Creep


• 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


• Fatigue & Endurance Limits (Revisited)


• 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.


• 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!

ship structures unique structures (6.1) ship’s structures are unique for a variety of reasons. for...

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