# SHIP STRUCTURES by rcr14802

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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

– The shape is optimized for reasons other than

– Ships operate in a wide variety of
environments.
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.
SHIP STRUCTURES
• Similarly, weight is a distributed force.
• But is rarely uniformly distributed.
SHIP STRUCTURES
• Nonuniform distributions produce shear planes
Shear plane

• 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.

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

Compression

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.
Sagging

Trough Amidships
SHIP STRUCTURES

• If sagging loads get too large...
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• Hogging - Buoyancy Support in the Middle
SHIP STRUCTURES
• Sagging - buoyancy support at the ends
SHIP STRUCTURES
• 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?

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

V(L/2) = -/2   Length
SHIP STRUCTURES
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
• 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
• 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

– 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
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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