Loads for Rational Design

8/20/2019 Loads for Rational Design

1/121

A D - 7 6 7 3 8 9

L O A D

C R I T E R I A

F O R

S H I P

S T R U C T U R A L D E S I G N

WEBB

INSTITUTE O F

NAVAL

ARCHITECTURE

PREPARED

F O R

NAVAL

SHIP ENGINEERING

CENTER

MAY

1 9 7 3

D I S T R I B U T E D

B Y :

K I

a t i o n a l T e c h n i c a l

I n f o r m a t io n

S e r v i c e

U . .

E P A R T M E N T F

O M M E R C E


2/121

·•·

THIS

DOCUMENT IS BEST

QUALITY

AVAILABLE

THE

COPY

FURNISHED TO DTIC

CONTAINED

SIGNIFICANT

NUMBER OF

PAGES

WHICH

DO

NOT

REPRODUCE LEGI LYo


3/121

SSC-240

0*

00

LOAD

CRITERIA

OR SHIP

STRUCTURAL DESIGN

This

ocument as ee n

pproved

or

public

elease

nd ale

ts

distribution s

nlimited.

SHIP STRUCTURE COMMITTEE

1973

«t-produced b

NATIONAL ECHNICAL

INFORMATION

SERVICE

US

Department

of

Commerce

Springfield,

VA.

2215?

Y


4/121

MEMBER AGENCIES:

UNH

D

TATES

COAST

GUARD

"AVAI

HI P

YSTEMS

COMMAND

Mil

ITARY

SEALIFT

COMMAND

M A R I T I M E

DMINISTRATION

AMERICAN

B U R E A U

OF

SHIPPING

SHIP STRUCTURE

COMMITTEE

AN

NTERAGENCY

ADVISORY

COMMITTEE

DEDICATED

TO

IMPROVING

TH E STRUCTURE

OF

SHIPS

ADDRESS

CORRESPONDENCE

O:

S E C R E T A R Y

SHIP S T R U C T U R E

C O M M I T T E E

U.S.

C O A S T

GUARD

H E A D QU A R T E R S

W A S H I N G T O N , D.C.X96X&

20590

S R

198

1

8

J U L

1 9 7 3

T he development

of

a rational procedure for determining

th e loads which a ship's hull must withstand

is

a

primary

goal

of

the

Ship Structure Committee

program.

In

th e

last

several

years,

considerable research activity ha s

been

devoted to theoretical

studies on

the

prediction

of hull

loads a nd

to

measurement of response both

on

models and on

ships a t *

sea.

This

report describes

a first

effort into

th e

synthesis

of

the results of these

diverse

projects

into a rational design

procedure.

Comments on

this report would

be

welcomed.

W.

.

RE A, I I I

Rear

Admiral,

U . S .

Coast Guard

Chairman,

Ship

Structure

Committee


5/121

Unclassified

Sc


6/121

Unclassified

4S I

rity CUwtiflcaHon

KCV

WORD»

ROLE

I

WT

Bending moments

Wave

loads

Still

vater

loads

Thermal effects

Cvclic

loads

Dynamic

leads

Combining

loads

Design of

hull

girder

R

Jinrl

as.si fipri

Securit

Classification


7/121

SSC-240

F i n a l

Technical

Report

o n

Project SR-198, "Load

Criteria"

t o

t h e

Ship Structure

Committee

LOAD

CRITERIA

FOR SHIP STRUCTURAL DESIGN

b y

Edward

V .

Lewis

D a n Hoffman

Walter

M .

Maclean

Richard van Hooff, and

Robert

B .

Zubaly

Webb

Institute

of

Naval

Architecture

under

Department of

the

Navy

Naval

Ship Engineering Center

Contract N o .

N00024-71-C-5372

This ocument as

ee n

pproved

or

ublic elease nd

sale; its

istribution

s

nlimited.

U .

S .

Coast

Guard

Headquarters

Washington,

D . C .

1973

I


8/121

ABSTRACT

Consideration i s

given

t o

the critical

l o a c . «

-

n

. h i p s ' hulls,

a s

indicated

by

ossible

modes

of structural

damage

and/or

; a f • . . r e -

i s

recognized

that of

particular importance

i s

the

possibility of damage i n the

form

of

compression buckling or

plastic flow

i n

tension

of ne

r oth

flanges

which could

lead

t o ultimate failure. nother mode of

failure s

by

fatigue, which i s

important

because cracks

may

occur

which

ust

b e e-

paired before

they

propagate

t o a dangerous extent.

third

mode

of ail-

ure i s

brittle fracture, which i s

particularly

difficult t o d e a l ith ut

can

b e

minimized

b y control

of

material

quality

and

use nf

the

ustomary

"fail-safe"

approach

by using crack

arresters. inally, the

possibility

of

shear

and/or

torsional

buckling

requires

consideration.

Hence,

a n ultimate

load

criterion

i s set up involving he

ol-

lowing

bending

moments:

Quasi-static wave-induced,

vertical

and

lateral combined.

S t i l l

water,

including

effect

of

ship's

own wave.

Dynamic

loads,

including slamming, whipping, and springing

Thermal

effects.

The

determination

of

each

of

these

loads

i s

discussed

i n

detail,

nd

he

need

for

further

clarification

of

dynamic l o a d s i s

brought

out.

ethods

of

combining

these

loads,

i l l

expressed

i n probability

terms, are

considered.

A

criterion

for

cyclic

loading

i s

discussed,

nvolving the

re-

diction

of

t h e

expected number

of

combined

loads of

different levels,

a s

w e l l

a s

t h e

expected

shifts

of

mean value.

A

criterion f o r brittle fracture i s

also

discussed.

Attention

i s

given

t o estimating

an

acceptable robability

f

failure for u s e

i n

design.

inally, calculations of

loads

re

carried ut

for

a

typical

cargo

ship,

the

S

O L V E R I N E

TATE

he

oads

re

hen

combined

i n

accordance

with

the

proposed

ultimate

load

criterion

and

compared

with t h e

stanoards under which the

ship

was

designed.

-n-


9/121

CONTENTS

Chapter

age

I .

INTRODUCTION Edward V .

Lewis

...

I I . CRITICAL

LOADS

Edward

V .

Lewis

and

...

Walter

M .

Maclean

I I I .

STILL

WATER

LOADS

Robert

B .

Zubaly

...

4

I V . WAVE

LOADS Dan

Hoffman ...

1

V . DYNAMIC

LOADS

Walter

M . Maclean

... 2

V I . PHASING OF

SLAM

AND

WAVE

LOADS

Richard

van

Hooff ... 3

VII.

THERMAL

EFFECTS Robert

B . Zubaly

... 7

VIII. COMBINING

LOADS

FOR

DESIGN Edward V .

Lewis

... 9

I X . SAMPLE LOAD CALCULATIONS Richard van Hooff and .. 3

Robert

B .

Zubaly

X .

CONCLUSIONS

AND

RECOMMENDATIONS

Edward V . Lewis

...

1

ACKNOWLEDGEMENTS

3

REFERENCES

4

APPENDIX A

- BIBLIOGRAPHY 2

-in-


10/121

L I S T

O F TABLES

TABLE

A G E

I MEASURED

FULL-SCALE

- S L A M

L O A D S A N D MIDSHIP STRESSES

7

I I DETERMINATION O F U L T I M A T E H U L L GIRDER B E N D I N G L O A D S 4

I I I

APPROXIMATE

PROBABILITY

O F

F A I L U R E :

F O U N D E R I N G ( 1 2 6 )

8

I V

FRACTURES I N

STRENGTH

D E C K

A N D S H E L L P L A T I N G

9

V

STILL-WATER B E N D I N G M O M E N T S , 5 . S

WOLVERINE

ST TE

5

V I D A T A O N C L O U D COVER

A N D

ESTIMATED TEMPERA

T

U

RE DIFFERENTIALS,

...

7

N O R T H

ATLANTIC

V I I

CALCULATED

C H A N G E S

I N

S T R E S S AND

EQUIVALENT BENDING

MOMENT,

....

1

3 S

WOLVERINE

ST TE I N N O R T H ATLANTIC S E R V I C E

V I I I WEATHER

D A T A

F O R N O R T H ATLANTIC 9

I X

S U M M A R Y

O F LIFETIME

H U L L

L O A D S

5 S WOLVERINE ST TE 3

X

COMBINED L I F E T I M E H U L L L O A D S

S S

WOLVERINE

ST TE

4

X I CALCULATED T O T A L C O S T O F F A I L U R E

8

X I I

CALCULATED

T O T A L

C O S T

O F

F A I L U R E

A N D D A M A G E

0

-IV-


11/121

L I S T O F F I G U R E S

F I G U R E

A G E

1

Y P I C A L

V O Y A G E

V A R I A T I O N

O F M I D S H I P V E R T I C A L

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S. S.

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.

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1

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D I S T R I B U T I O N S I N

D I F F E R E N T

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6

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- v -


12/121

SHIP STRUCTURE COMMITTEE

The

SHIP

STRUCTURE COMMITTEE i s constituted

t o

prosecute a

research

program t o

improve

the

h u l l

structures of ships by an extention of nowledge

pertaining

t o

design,

materials

and

methods of

fabrication.

RÄDM

W .

F .

Rea,

III,

USCG,

Chairman

Chief, Office of Merchant

Marine

Safety

U.S.

Coast

Guard Headquarters

CAPT

J .

E .

Rasmussen,

U^ N

Head,

Ship Systems

Engineering

and Design Department

Naval

Ship

Engineering Center

Naval Ship

Systems

Command

Mr.

K .

Mori and

Vice

President

American

Bureau of

Shipping

Mr.

E .

S .

Dillon

Deputy Asst. Administrator

fo r

Operations

Maritime

Administration

CAPT

L . L .

Jackson, USN

Maintenance

and Repair

Officer

Military

Sealift

Command

SHIP

STRUCTURE

SUBCOMMITTEE

The

SHIP STRUCTURE SUBCOMMITTEE acts fo r the Ship Structure Committee

on

technical

matters

by providing

technical coordination

for

he

determination

of goals

and objectives of the

program,

and by

evaluating and interpreting he

results

i n

terms of

ship

structural

design, construction

and

operation.

NAVAL

SHIP ENGINEERING

CENTER

AMERICAN BUREAU

OF SHIPPING

Mr. . .

Palermo

-

Chairman

r .

S .

Stiansen

- Member

Mr.

.

. O'Brien -

Contract

Administrator

r. I . L .

Stern

-

Member

Mr. . orkin -

Member

M r .

.

.

P o h l

er

-

Member

U .

S .

COAST

GUARD

LCDR

C .

S .

Loosmore

-

Secretary

CAPT H . H .

BELL

-

Member

CDR J . L .

Coburn

- Member

CDR W . M .

Devlin

- Member

MARITIME

ADMINISTRATION

Mr. . .

Nachts

h e

i m -

Member

M r .

.

ashnaw - Member

M r .

.

a i liar -

Member

Mr.

. .

Coombs -

Member

M r . . e i bold

-

Member

MILITARY

SEALIFT COMMAND

M r . R . .

Askren - Member

M r .

T .

.

Chapman - Member

CDR

A .

cPherson, USN

-

Member

M r . A . . Stavovy - Member

NATIONAL

ACADEMY

OF

SCIENCES

Ship

Research

Committee

M r . R . W .

Rumke

- Liaison

Prof.

R . A . Yagle

-

Liaison

SOCIETY OF

NAVAL ARCHITECTS

& MARINE

ENGINEERS

Mr.

T . M .

Buerman

-

Liaison

BRITISH

NAVY

STAFF

D r .

V . F l

in

t -

Liaison

WELDING

RESEARCH

COUNCIL

Mr. K . H .

Koopman

-

Liaison

INTERNATIONAL

SHIP

STRUCTURE

CONFESS

M r .

J . Vasta

-

Liaison

-vi-


13/121

I .

INTRODUCTION

RATIONAL

DESIGN

For many

years the goal of

truly rational design of ship structures has been

discussed

v

and

a great

deal

of

research

bearing

on

this objective

has

been

car-

ried out.

he

concept

was

describe.£, to r

example, in

an

early

planning

document

of the Ship Structure Committee (1)

and

since the establishment

of

the Inter-

national Ship Structures Congress

(I.S.S.C.)

in 1961

it

has

been

regularly dis-

cussed

on

a

worldwide

basis

by

Comm-'ttee

No .

10,

Design

Philosophy.

lthough

this

report

is

intended

only

tc

indicate

progress

to

date, it is

hoped that

it will

assist

in

the advance

toward the

ultimate

achievement

of

rational design

of the

main

hull girder.

The concept

of

rational design

involves

the complete determination of all loads

o r ;

the

basis

of

scientific

rather

than empirical

procedures,

in

order

that

uncertain-

ties may be

reduced

to

a

minimum.

his approach

carries

with it

the

idea that

the

response

of the

stru:ture can

also

be

accurately

determined and

that

arbitrary

large factors of safety,

or

"factors of ignorance," can be

avoided.

The concept is

consistent with

the

modern

approach

to

structural

design

that

considers

the "de-

mand"

upon

and "capability" of the

structure.

In short,

instead

of insuring

that

a simple calculated design stress is below the ultimate strength of

the

material by

an

arbitrary

factor

of safety,

an

attempt

is

made

to

determine

the

demand of all

loads

acting

on

the

structure

and

then

the

capability

in

terms

of

load-carrying

ability — the load the structure can

withstand

without failure. Of course, this

approach

requires

a definition of failure, which may be a serious

buckle, a major

crack,

complete

collapse,

or

a tensile

failure

(Chapter

II).

The

concept

of

ration-

al design

of

a ship

hull

is

believed

to

be consistent with a probabilistic

approach,

which has already been found t o be essential fo r

dealing

with random seaway load-

ings. Both demand

and

capability can be

expressed

in

terms

of

probabilities,

and a

satisfactory design is then

one in which the

probability of failure is reduced to

an acceptably lo w value. The problem

of

determining

local loads or

stresses

fo:

detailed structural design is much more complex

and

is

no t

discussed

here.

This particular report deals only

with the demand

—or

loading —

on

the

hull

girder, bu t

an

attempt

has

been made tc

formulate

it

in

a

manner that

is

consistent

with

the above approach.

In

due

course,

with

the

cooperation of the ship

structural

designer,

it

is

anticipated

that

a

rational

design

procedure

will

evolve

(2).

^Numbers

in

parentheses refer

to

References

listed

at the

end

of

this

report.


14/121

» 2 -

It is

no t

intended

to minimize

the importance

of

the

conventional

empirical

approach to ship structural design

which

has served the

designer,

builder

and

operator

well

through

the

years. ut there is

currently

a

substantial

need fo r

a fully rational

approach because

of such

ne w

maritime

developments

as

larger

ships,

faster

ships,

unusual

hull

configurations

(such

as

the

catamaran),

an d

ne w

materials.

omplete and comprehensive

load

criteria

can

facilitate the

extrapola-

tion

of ship design into

new

configurations, using

new concepts

and

materials.

LON

G-RANGE PATTERN OF LOAD

VARIATION

It

may

be

useful

at this

point

to

describe

the

typical

long-range pattern

of

load

variations

on

typi.

.

merchant ships as background fo r

the

detailed discussion

of

the various types of xoads in subsequent

chapters.

For

completeness,

we

should

perhaps

begin with

the

construction

of

the

ship

on

the

building

berth.

trictly speaking the

only

loads

present ar e

those

induced by

the weight of the

structure

itself.

owever,

there

are

residual

stresses

in

the

plating and locked-in

stresses due to welding, often of

considerable

magnitude

an d

sometimes sufficient

to

lift

the bo w and/or stern

off th e

keel blocks. he locked-

in

stresses are

of

particular concern where

they

may

exist in combination

with

other

stresses

at

a weld defect

or

notch and under certain conditions could help to pro-

duce a

brittle

fracture.

o r other

types

of

failure it seems

reasonable

to

consider

them

to

be of

minor

significance to longitudinal strength, since they tend

to be

eliminated

by

"shakedown" or adjustment

in

service.

ha t

is,

an

occasional high

longitudinal wave bending

load

—in combination

with

other loads

—

may

be expected

to cause

local

yielding in

an y

of

the high residual stress region.

po n

determina-

tion of this high wave

load

the structure will tend to return to

a

condition

of

re -

duced

residual stress.

During launching

a high longitudinal bending moment may

occur,

but this is

usually calculated

and

allowed fo r by

the

shipyard. uring

outfitting a con-

tinual change

of still

water

shear

and

bending moment

can

be expected

as

vari-

o us items

of

machinery

and outfit are

added.

he

longitudinal

still water

bend-

ing

moment

on

the

ship can

always be calculated,

but

the

midship

stress

will

probably no t correspond

exactly

to

this

calculated value because

of

possible

built-

in

ho g

or

sa g residual stresses, and

departures

of

the

hull behavior from

simple

homogeneous beam

theory.

In short,

the ship

is

never in a simple

no-load condition

no r even

in

a

condition where

the

absolute value

of even the

longitudinal

'bending

moment is exactly

known.

uc h

a

built-in

bending moment

will

no t be considered in

this

report

since it

is

believed

that

changes in

load

while

th e

ship is in

service

are

of

primary significance.

In

general the still

water

hull loadings vary quite slowly.

hen

a ship is in

port

there are gradual changes in

the

bending

moments, shears, an d perhaps

th e

tor-

sional

moments

as

cargo

is

discharged

and loaded, fuel oil

and stores ar e tc?ken

aboard, etc.

uring

the voyage there

are even more

gradual changes in

mean

loadings

as

fuel

is

consumed, and ballast is added or shifted. ypical

changes

of this kind

are shown in Figs.

1 and 2(3). inally, at the end of voyage changes resulting

from

cargo

discharging

and loading, plus

possible

fuel

oil and ballast rhanges, will

again

modify

the

bending

moments,

shearing

forces and

torsional

moments. he

lead-

ing changes in

port may

be

considerable and

depend

on

the

nature and

quantities of

cargo carried on various

legs

of the

voyage. hese changes do

no t show up in Figs.

1 and

2 because

the recording

equipment zero

was

customarily

readjusted at

every

mm t̂Knm Hm imim

imA^^


15/121

-3-

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16/121

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17/121

- 5 -

port visit. s explained in an SSC report, "This capability is necessary

to

pre-

vent the

dynamic

stress

range

from exceeding

the limits of the instrumentation

system" (4).

When

the

ship

gets

under

w ay

to go to

sea, the

first

new

hull

loading to

be

experienced

—

especially

if the ship

is

a high-speed

vessel

—

is

the sagging

bending moment induced by

the

ship's

own

wave

train.

hi s longitudinal

bending

moment is

a functi-

a

of ship

speed, an d will be

superimposed with little

change

onto other bending moments (5).

Another load variation results

from

diurnal

changes in ai r temperature^and in

raaiant

heating

from

the sun.

he

effect

is

clearly shown in Figs. 1 an d

2

Such thermal stresses can

be

explained

on

the

basis

of

irregular or uneven thermal

gradients, which can

perhaps be

considered

as

the

"loads." n general, if a beam

is

subject to

heating

that

produces

a uniform thermal

gradient from to p

to

bottom

It

will

deflect an d there will

be

no resulting

stresses. ut, if

the

gradient

is

no t

uniform, stresses will be induced.

In

th e

case of a floating

ship,

the temper-

ature

of

all the

steel

in contact

with

the

water

will

be

at

the nearly

uniform

water

temperature,

and

there

will

be

very

little

change

from

day

to

night.

ut

the portion of

the

hull above water will usually be at a different temperature

that

changes

continually

and

depends

on

both the air

temperature

and

the

amount of su n

radiation

(extent

of

cloudiness,

duration

of

sunlight,

altitude

of

su n

at

noon).

In

respect

to

the

latter

factor, the color of the deck

is

important

also. here is

usually

a

marked

change

in stress in the

vicinity

of

the

waterline.

especially

on

the

sunny side

of

the ship,

but

from the

point

of

view

of

longitudinal

strength

the

temperature

change

of

the

weather

deck

—in

relation to

the

underwater

hull tempera-

ture —is

significant.

Another

large

load at se a is

that

induced

by

the

encountered waves

(Fig.

3) .

This load usually varies in

an

irregular

fashion

with an average period

of

5-10

seconds, depending

on

the ship. ot only

is

there irregularity

in

wave-induced

loads

from

one

cycle

to

the next, but there

is

a

pronounced

variation in

average

level

with

ship

heading

and

with

weather

changes

during

a

voyage

an d

from

one

season

to

another.

he

irregularity

of

these

loads is,

of

course,

due

to

the irregularity

of the waves at

sea.

owever, the

baffling irregularity

of

ocean

waves

ha s

yielded

to

modern analytical

techniques.

hi s

was explained by Dr.

Norbert Wiener, who

developed

the

necessary

statistical techniques fo r

another purpose.

How

could one bring

to

a

mathematical

regularity th e study of

the mass of ever shifting ripples and

wsves

..,.?," he wrote ( 6 ) .

" At

one time ehe waves

ran

high,

flecked

with

patches

of

foam,

whils at another, they were barely noticeable ripples What descrip-

tive

language

could I us e that would portray

these

clearly visible facts without in-

volving

me in the inextricable complexity of a

complete

description of

th e

water

surface, i ' h i s

problem of the

waves

was

clearly

one

fo r

averaging

and

statistics

.

In

time Wieaer

evolved

his mathematical tcol,

spectrum

analysis

--a

means

of

break-

ing

down

complex

patteins

into

a

large

number

of measurable components.

In

recent

years

wave-induced

bending

moments

have

been

extensively

studied,

so

that

a

good statistical picture

is

beginning to emerge. esearch over

a

number

of

years ( 4 ) ( 7 ) has

provided

a bank of

statistical

stress data on

four

cargo

ships

in

several

services.

sing

so m

f

these, data

it

has

been

found (8) that

two dif-

ferent mathematical

models

can

b t used

to

extrapolate such results to much

longer

*

Gages

were temperature

compensated.


18/121

- 6 -

F i a u r e

l y p i c a l

R e c o r d

o f

Midship V e r t i c a l

B e n d i n g S t r e s s w i t h S l a m m i n g ,

M . - V . FOTW L.

term

probabilities. urthermore, it

has been shown

that

using

the

same mathemati-

cal

models

—

combined with model tests in regular waves and ocean

w ve

spectra —

short-term

( 9 ) and

long-term trends (10)

can

be predicted

with

a

precision

that

depends

only on the reliability-

of

the data. t the same

time, computer programs

have

been developed

fo r applying ship

motion

theory

to

the

calculation

of

loads

in

regular

waves

as

a substitute fo r model tests.

Finally, o

of which result

that follows

it

difficult

to

de

mentioned

loads

(13)

have been

bu t no t

solved

other

sources

o

ceaugoing

ships experience dynamic loads, th e

most

troublesome

from impact

(slamming)

and the

vibratorv

response

(whipping)

(Fig. 3) . In

general these loads

are

transient

and

therefore are

al

with

statistically.

he y

are

superimposed

on

the

previously

Both full-scale

measurements

(11)

and

theoretical studies (12)

carried

out

o n slamming and whipping,

and

these

have clarified

the problem.

Shipping of water

on

deck

and

flare

immersion are

f

transient

dynamic

loading.

Recent

attention has been focused on another

dynamic

phenomenon, springing,

which under

certain

conditions

seems

to be excited more-or-less continuously

in

flexible-hulled

ships,

without the need fo r

wave

impact.

Considerable

progress

has been

made

toward solution by

means

of theoretical

and experimental

studies

(14).

All of the

above

loads will

be

discussed in

detail

in

subsequent

chapters.

HULL LOAD CRITERIA

In

general

treatises

on structural design

(15)

tw o types

of

loading ar e usually

distinguished:

controllable an d

uncontrollable.

In

the

first

case

one

can

speci-

fy design

loads

with

instructions

to

insure

that

these are never exceeded. n ex -

ample is a highway

bridge designed

on

the basis of a posted load limit.

In

the

second

case, usually involving natural forces, one must

make a statistical analy-

sis and

endeavor to

design on the basis of the expected loads, with

no limitation

on

the

structure

or

its

operation.


19/121

- 7 -

In the design of ships,

still

water

loads are generally

controllable

and

wave

loads

are not.

If

calculations

of

typical

conditions of loading

indicate

that ex -

cessive

still water

bending

moments might occur, specific

operating

instructions

may

be

issued

to make

sure

that

certain

limits ar e no t exceeded. he

possibility

has

been

discussed of

specifying limiting wave bending

loads,

as

well

—

somewhat

in

the

same

manner

that

wing

loads

on

an

aircraft

are

limited

by

requiring

certain

performance restrictions. Such a limit on wave loads

fo r ships

could only be ap -

plied

if

special

instrumentation

were

available

to advise

the

officer

on watch when

and

if the

limiting

bending

moment

is

reached, since

there

is

no

wa y fo r

him

to

judge this loading unaided. Furthermore, he must have

guidance information

at

hand

that

will enable

him

to

take

steps

to

reduce

the

bending

moment if

it should ap -

proach the

safe

limit.

Dynamic

loads

are partially controllable,

since

the

vibratory spon.*«

of the

hull girder ca n be

felt by

the

Master

on

the

bridge. y a

change in ship speed

and/or course

he ca n reduce

the magnitude

of the

exciting forces and thus

in-

directly

reduce the loads

to

levels

that he has found

by

experience

to

be

acceptable,

In

this

report a compromise

approach

ha s

been

adopted

regarding statistical

dynamic wave loads. n effort

is

made to determine

all

the loads

acting on the

ship's hull to

provide load

cri

eria from wnich a satisfactory but economical

structural

design

can

be developed.

owever,

to

guard

against

the

possibility

of

some unforeseen

extreme

load condition, it is recommended that suitable

stress

in-

strumentation be provided

as

a warning device

fo r added safety

(16)

.

A great

deal of

research

ha s

been

done

in

recent

years

on the

ship

hull

load-

ings mentioned in

the previous section,

much of

it

in

the

Ship

Structure Committee

(SSC)

program. esearch under other

sponsorship

has

also

contributed

to

an

under-

standing of hull loads,

including

particularly

that supported

directly by the U.S.

Navy, the Society of Naval Architects and Marine

Engineers

and

th e American Bureau

of

Shipping

in this country,

and

by various organizations in

Great

Britain,

Nor-

way, the

Netherlands,

and

Japan,

as

reported to the

International

Ship Structures

Congress

(I.S.S.C).

A

partial

bibliography

is

given

at

the

end

of

this

report

(Appendix

A) .

Some typical loads have received

more

attention

than others, however,

leaving

gaps

in

the

overall picture.

t is

the purpose

of

this

report

to

present

a compre-

hensive and reasonably complete picture

of

the

hull loads and

hence

load criteria

fo r

ship

design,

with particular emphasis

on

dr y

cargo ships. ence, considera-

tion

will

be

given

in

the

next

chapter to identifying

the

critical

loads

of

inte-

rest to the

ship structural

designer.

In

succeeding

chapters

each

of the

various

loads will be

discussed

in turn, and consideration of typical magnitudes

and

of

pro-

cedures

fo r detailed calculations

will

* - .

included. Finally the piobiems

of

combin-

ing these loads fo r

hull

girder design

purposes will be taken up .

here

important

j ? ; a p s in

o ur

knowledge

appear, they

will

be

identified and recommendations

made fo r

lurcher

research. numerical

example fo r the S.S.

Wolverine State

will

be pre-

sented

.

A number of attempts have

been

made to consider ho w the

available material

on

loads

ca n

be combined and applied to the rational design of

ships.

Of these,

particular mention

might

be made of the

work

of

Caldwell

(17), Aertssen (18),

Abrahamsen,

Nordenstr^m,

and R^ren

(19 ),

and

of

Committee

10

of the I.S.S.C. (20).


20/121

II. RITICAL LOADS

INTRODUCTION

Before discussing hull loads

in detail

it

is

necessary to consider the dif-

ferent

ways that th e structure

ca n

suffer

damage

or

fail. he

object

is to

in -

vestigate structural aspects o f th e problem only to th e

extent

necessary to be

sure that

all

of the

necessary information

on loading

—or demand

—

will be made

available

to the structural

designer.

n

short,

we must ask, what

ar e

th e criti-

cal

loads

and how do

they

combine?

eanwhile,

it is hoped that work

will

continue

toward developing

a

completely

rational

approach

to

ship structura

1

analysis and

determination

of

th e

capability of the structure.

Discussion

of

critical

loads

can

be facilitated

by defining

structural failure.

Caldwell (17) considers

ultimate

failure

as

the

complete

collapse by

buckling

of

the

compression

flange

and

simultaneous

tensile

failure

of

the

tension

flange.

o w-

ever,

it

is

clear

that a considerably less severe

damage

would

be a serious matter,

as indicated

by such

factors

as

necessity fo r major repairs, interference

with rior-

mal ship

operation and

non-watertightedness.

s pointed

out

by

C. S .

Smith

in

dis-

cussion of (17), In designing

a midship

section,

th e

designer

should consider the

various

levels

of

damage which a hull girder may experience

between th e

limits

of

initial

yield

and final

collapse,

and

should attempt

to

relate

each level

of damage

to

an

applied bending moment."

Hence,

fo r our purpose we may define

damage

as

a structural occurrence

that

interferes with

th e

operation of th e ship to

th e extent

that

withdrawal

from service

fo r

repair

is

required.

ailure

is

then

a

severe

damage

that

endangers the safety

of

the

ship.*

Further

study

of

the

subject

of

critical

loads

during

this

project

has

re-

sulted

in

no

basic improvement

in

Gerard's

analysis of

specific

ways

in

which

tu e

hull girder

could f il, as given

in "A

Long-Range

Research

Program

in

Ship

Structural

Design" (1), e considered overall damage by compressive buckling, overall ten-

sile

yielding,

low-cycle fatigue cracking

and

brittle fracture.

o

these should be

added

combined normal

and

shear

stress

buckling, an d

it is possible to

elaborate

somewhat

on

hi s scheme

and

in certain respects

to

obtain more definite statements.

Th e

types

of

damage

that

should

be considered then in

connection

with

critical

loads

might consist of

an y

of

the

following:

Damage

•Excessive

hull

deflection associated with buckling

and/or permanent

set.

•Fatigue

cracking.

•Brittle

fracture,

minor

or

extensive.

•Shear or

torsional buckling.

Failure

•Collapse and/or fracture

of

the

hull

giraer.

This

is

sometimes referred

to

as

"collapse" (20), but we

feel

that

this

term

connotes buckling

failure to

the

exclusion

of

tensile

failure or

perma-

nent

set

and

therefore

prefer "failure."

t tum am

i n i iM i üir-


21/121

-9-

Although

only

the last

is considered to

be structural failure,

all of

these types

of

damage ar e

important

fo r a longitudinal strength criterion. lari-

fying

the

nature

of these

potential

damages will

assist

us

in

providing the

neces-

sary information on loads.

Ine magnitude

of

elastic hull

deflection

is

usually considered in the design

criteria

of classification societies,

and it will also

be

discussed in this chap-

ter.

Finally,

consideration should

be given

to other minor effects, such as the

forces generated

by

rudders and

anti-rolling

fins.

nd

other

types

of

service

loading, such as berthing, drydocking,

and

grounding, which may have direct

effects

or ,

primary hull

girder

structure,

cannot

be

overlooked.

ocal

damage

to structure

that

is no t

part

of

the

main

hull

girder

is

excluded

from

consideiation.

An important consideration

in

structural design

is corrosion. owever, since

this

is

ne t

a

load

it

will

no t

be

considered

in this

report.

PERMANENT

SET

AN D

ULTIMATE

FAILURE

We

may

first

consider

overall

static

damage

to one of the

"flanges"

(deck or

bottom)

in either compression or

tension,

i.e.,

buckling

or elasto-plastic yield-

ing.

he

effect

of lateral as well

as

vertical longitudinal

bending and

torsion

must

be

included

here.

onsideration

must

be

given

to

the

combined effect of

still water

bending, wave bending and

thermal

loads. In

addition, a

basic

ques-

tion is whether or no t

the superimposed dynamic

effects of

high

frequency

"whipping"

following

a

slam

and/or

flare

entry

should

be

considered,

as

well

as

the

effect

of wave impacts

on the

side

of

the

ship

and

continuously

excited

springing. t

is

quite

possible that the short duration

of

dynamic bending moments

—

and

stresses —

1"mits the

amount of

permanent

set or buckling

that they

ca n produce. s noted by

Spinelli, "It

should

be

borne

in

mind that the short time

in which

the

wave

mom-

ents

due

to slamming develop

their

maximum

values, and the

entity of the total

de-

flection that

would be consequent on them,

make

the probability of its realization

extremely

scarce"

(21).

And

in referring to

plastic deformation,

Nibbering states, " In

practice

these

deflections

will no t

develop

the

very first

time

an extreme load

of

the

required

magnitude occurs.

Th e

time

during

which the

load

is

maximum

is

to o

short,

especi-

ally

when

a

part

of

the

load is

due

to

slamming"

(22).

hi s

is

a

problem in

struc-

tural

mechanics no t within the scope

of this project, and

therefore

we

shall

attempt

merely

to identify and

evaluate

dynamic

as

well

as static

loads.

Finally,

local loads (not due

to

longitudinal

bending) on

which all of

the

above

are superimposed must no t be

overlooked.

hese include deck loads,

cargo

loads on

innerbottom,

liquid pressures within tanks,

and external water pressures.

Although there seems to

be

general

agreement on

the

importance

of

ultimate

strength,

involving extensive

plastic yielding

and/or buckling,

there seems to

be

some

doubt

as

to

how

to

deal with it in design.

From the

point

of view

of the

present study,

however,

definite conclusions ca n be

drawn

regarding

the

load in-

formation

needed fo r

designing

against

potential damage of

this

type.


22/121

-10-

FATIGUE

Second is

the possibility of fatigue

cracking, which

seldom constitutes fail-

ur e

but

is

important fo r

two reasons:

atigue

cracks ca n grow to the point that

they

must be repaired,

and fatigue

cracks

ar e

notches that under certain circum-

stances

ca n

trigger rapid propagation

as brittle fracture. ibbering notes, "It

is

a

favorable

circumstance

that fatigue cracks propagate very

slowly

in

ships'

structures'*

(22) .

The possibility

of

fatigue

cracking

is

increased

by

the

presence

of stress

concentrations —as

fo r

example,

at

hatch

corners

(23) (24),

and it involves con-

sideration

of

the

magnitude

of

still water

bending

—i.e., th e

shift of mean

value —as

well

as the range of variation of wave bending

moments.

or example,

a

ship

may operate with a large still water sagging moment (loaded) on

its

outward

voyage

and with a

large

still

water

hogging

moment

(ballast) on

its

return,

and

such a

large

variation in

mean

value

needs to

be

considered

in

relation to

fatigue.

As before,

consideration of

lateral

as

well

as

vertical

bending

must

also be given,

Dynamic loads

and vibratory

stresses

may

be

expected to

contribute to

the

fatigue

loading.

It appears

that

the

fatigue loading histories

of

actual

ships

show

considera-

bl e variety. ence,

the

objective fo r

this

study is felt

to

be

simply

t : > obtain

clear

statistical

or

probabiliscic

pictures

of

each

of

the

types

of

loading

in-

volved

:

1 )

robability

density

of

mean

still water

bending

moments,

which

tenta-

tively

and

approximately

appears

to

be two

normal curves, one

representing

outbound

and

the other

inbound

conditions.

2 ) ong-terra cumulative distribution

of

wave-induced bending

moment,

which

together

with 1) can

be

interpreted

as

a

low-frequency

loading

"spectrum."

3 )

robability

density of high-frequency bending moments

associated

with

dynamic

loads

(slamming,

whipping

a n < ^

springing).

he

combination

of

these

effects with low-frequency

loads

is a difficult problem, as

discussed in

later chapters.

4)

hermal

stress conditions,

which

cause a diurnal change

in

stress level.

At

first

glance

it appears to be

a

hopeless task to

collect

all

the

necessary

statistical

data on

the

various loads fo r

ships

of

different

types

and

to develop

ways

of

combining

them that

are

no t

only

sound

by

probability theory standards,

but are

meaningful

from

the

viewpoint of the mechanics of fatigue and

of

the

properties

of

the

materials

used.

short-cut

answer,

as

proposed

by

Gerard

( 1 )

would

be

simply

to

design to avoid overall combined

loads as

listed above

that

ex -

ceed

the

yield

point of the material

anywhere In

the structure,

including

areas

of

stress concentration.

esign of the structure

on this basis would virtually

in-

sure

the ship against low-cycle fatigue,

but would possibly lead to heavier struc-

ture

than

in

present designs.

Since

fatigue

cracks

can

be detected

and

repaired, it

is

no t felt

that

It

is

necessary

to limit stresses to yield point level in

this way,

Attempts

should be made to

understand

and

evaluate

al l

components

of cyclic loading.


23/121

-11-

B R I T T L E F

RACTURE

Third, is the possibility of

failure

by

brittle

fracture.

his mode of

failure

wa s common in early days of welded

ship

construction, bu t has been

greatly

reduced

in recently

built

ships.

t

cannot

be overlooked in a comprehensive scheme,

how-

ever.

ll

of the

above-mentioned

loads

apply,

including

residual

and

thermal

stresses

and

ehe

notch

effect

of

weld defects.

It

has been pointed out

that

a

low-

cycle fatigue

crack

can be the initiation point fo r

brittle

fracture

(24)

.

It

is generally recognized that

the following factors

are involved in

brittle

fracture:

( 1 )

mbient

temperature.

(2)

teal

characteristics (transition temperature) ,

(3)

otches or

stress

raisers,

including

weld

defects.

( 4 )

tress ( or load)

level.

( 5 )

train

rate.

Secondary

factors

include strain as well

as

stress

fields,

corrosion

effects,

metal-

lurgical

effects

of

welding,

structural

details that

introduce constraint,

an d

residual

stresses

.

Because

of

improvements in design

and materials,

brittle fracture no w

seldom

occurs in

actual

service. owever, it is

conceivable

that

if, as

a

result

of

more

rational approaches to design, working stress levels are increased we may again

have trouble

with

brittle

fracture.

urthermore,

it

is

important

to recognize

that brittle fracture

has

been brought under control by careful attention to

mate-

rial qualities, selection and control of fabrication techniques,

and

inspection

at

all stages

of construction.

iligence cannot be relaxed, especially as new

materials, new fabrication techniques, and more rational design procedures are

introduced.

Nibbering maintains

"that

90%

of all

ships

in

the

world

move

regularly and

un-

damaged in conditions where the temperature is

lower

than

the

crack-arrest

tempera-

ture

of

their

steels

.... he nominal stresses

mostly

are

so

lo w

that

with

present

day quality of design

and

workmanship brittle fractures cannot initiate"

(22)

.

For

design

purposes the load information needed

is

generally the same as fo r

u l t i r . c i

-

; - - bending, as discussed

in

a

preceding section, including all dynamic

loads,

except that only tensile loads need be considered. at e of application of dynamic

l o a d . *

and

ambient

temperature

conditions

should

also be

specified.

Of the various dynamic loads, it is believed

that

consideration

should

be

giver

particularly to the midship

stress

following

a bottom

impact

slam.

Since higher

modes than the hull fundamental are involved, the strain rate may

be quite high.

Se e

Chapter

VI

fo r

further

discussion.

SH

E

AR

AND

TORSION

Fourth

is

the

possibility of

shear failure in

the

hull

girder "web." lthough

this

is

a problem

in

the design of light

naval

vessels,

it

has no t been

of

much

concern

in

more

heavily

built

merchant

vessels. This

is

no t

to

say

that

shear load-

ing

on

the side shell

or

longitudinal bulkheads

is unimportant, bu t rather that

other

types

of side shell

loadings

probably

constitute more

severe

criteria

of

satisfactory

design.

Though

there

is

a

possibility that

the

side

shell

of

merchant

ships

is

excessively

heavy, safe reductions

in

these

scantlings

can only

be made

by


24/121

-12-

developing more precise w a y , 5

of determining the

hull

girder torsion and

shear

load-

ings, as

well as

lateral loadings

due to such aspects

of

operation as

bumping

into

dock structures, being handled by

powerful tugs, etc.

Another aspect of conceri here arises from th e recent development of large

bulk

carriers

which

are

frequently

loaded

only

in

alternative

holds

with

high

den-

sity ores.

The result

of

such loading

is

that

large

shear and

moment

variations

ar

:

experienced along

the vessel's

length which must

be allowed fo r

in th e

design

of

hull girder

structure.

Further

definition of

this

problem

area

is

needed, since it

ca n

be

expected that

large

shear and moment, coupled with reduced structural effect-

iveness

of

the hull girder

material,

can

lead

to combined

loadings

of

critical

magnitude.

Torsion is important

in

relation

to

both shear

and

deflection, especially

in

wide-hatch

ships

(25).

xcessive hatch

distortion

ha s

become

the

major

area

of con-

cern as progressively larger ha

Documents

Loads for Rational Design