Some thougts on ship structural design

KONINKLIJK INSTITUUT VAN INGENIEURS

'Constructiedag' van de Sectie voor Scheepstechniek van het K. I. v. I.

op 8 maatt 1968 te Deift

Some thoughts on ship structural design

by

professor J. B CAL DWELL

Deift University of Technology

Ship Hydroméhanics Laboratory

'

Library

Mekelweg 2, 2628 CD Deift

The Nether1nds,

Phone: +31 15 2786873 - Fax: +31 15 2781836

Some thoughts on ship structural design')

by professor J. B. Caidwell

Introduction

It is both a pleasure and an honour to be invited to present a lecture on this important and auspicious occasion.

It is a pleasure because it enables me to continue and strengthen an association with Delft which dates back to 1946, since when, by correspondence and by subsequent visits, I have

enjoyed meeting and exchanging ideas with fiiends fr6m this

University.

It is an honour because many of us regard this Laboratory

as one of the fmest of its kind in Western Europe, and one, from which many important reports and advances in our

under-standing of ship structures, have come. And with the opening today of these new facilities and extensions to this Laboratory, we look forward to an increasing contribution from Deift to

the development of the science of marine structures.

The progress of ship structural design

I say science deliberately, because I believe we are now seeing

a rapid change from a situation on which the design of ship structures was - if not wholly an art - at least something in

which experience (and perhaps intuition) played a large part;

towards a situation in which we are compelled to try to develop

a more rational and a more scientific approach to the design

of ship structures.

Looking back over the past 100 or more years we find that

the development of improved ship structures has been very

largely a slow, but sure, process of evolution, involving accu-mulation of experience, the 'survival of the fittest', a gradual

refmemeht of designs, and the incorporation of the best practice into the Rules of the Classification Societies.

Theoretical analysis played little part in this process, despite

the early and notable contributions to structural mechanics which were inspired by problems in ships.

Too many things were unknown to enable designers to rely. on theory alone. Thus, accurate quantitative knowledge was lacking of the true loads on ships, and of the way in which the complex structu±e responded to these loads. Because of this there was no alternative but to base a new design very largely

on the experience in service of the' old. And this method of design worked well, so there was perhaps little need to change it. But various recent events force us to admit that this

'evolu-tionary' approach to the structural design of ships is no longer wholly adequate. This is principally because the evolutionary

method cannot be used where there is no similar previous experience. So that whenever there is a significant change from previous practice, the designer' may have no similar experience on which to draw.

In the marine world, as in many orhers, we live in a time of rapid change. Change not only from small to giant ships, and

from conventional to new materials, but also to new ship types,

Lecture delivered at the Symposium on Shipstructures. Section for Shiptechnology, 8th March 1968, Delft.

Department of Naval Architecture and Shipbuilding. The

Uni-versity of Newcastle-upon-Tyne.

629.12

such as' multi-hulled ships, articulating ships, submersibles, and air-cushioned vehicles and the like. We all know, I think,

of the structural difficulties brought about by the very rapid' increase in size of tankers and bulk carriers, and of the problems arising in new forms of marine vehicle such as oil rigs. There is

little doubt now that developments of this kind, as well as the continuing search for improved efficiency of ships (in, terms

of weight and cost) compel us towards a structural design procedure which is less dependent on previous experience.

Fortunately this kind of progress has been made easier 'by' three parallel developments. Firstly, by our greatly improved knowledge of the loads on ships; secondly, by advances in the understanding of structural responce to load; thirdly, by our improved power and scope of calculation brought about by the computer. So that it is becoming feasible to move towards a more scientific and more versatile design process in which this

new knowledge can be utilized and harnessed to relevant experience

I think it was this realisation - that ship struciures .was

emerging as a science in its own rightthat led to the formation

of ISSC in 1961. certainly in the minds of many present in

Glasgow at that first Congress was the thought that, as well as studying wave loads, ship response, materials, brittle fracture,

and other subjects, it would be a proper function of issç to

keep under review the whole process of ship structural design.

As I have been a member, and later Chairman, of the Committee charged with this task, it was suggested that this might be a good

opportunity to review some Of the findings of the Committee

with respect to ship structural design, and especially to mention some of the points brought out in our Committee Report presented recently to the third Congress in Oslo.

So I would like to talk firstly about the ship structural design process as a whole, then about the central matter of structural

design criteria, and fmally to make some brief observations

about structural optimization.

The design procedure

The two alternative approaches to the structural design process are shown schematically in fig. 1. Having decided the basic

configuration of the structure and its principal dimensions,

one can proceed to fmd the detailed scantlings of the structure' either by using the rules of the Classification Societies, or via a step-by-step process in which relevant facts are quantified

and logical decisions taken. These steps are summarized in

fig. 1. The essential difference between design-by-rule and the

other method is that in the latter the facts and decisions are made explicit. In the former, most of these individual facts and

decisions (shown within the dotted line in fig. I) are implicit, ,but are often not known separately, and this makes it difficult

to apply such rules to problems outside their normal scope. The step-by-step .procedure is attractive, but of cOurse poses a number of problems.

What do we mean by 'design load'?

How does one establish a proper design criterion? What is the right basis for optimization?

As remarked earlier; recent research is helping to provide some of

OBTAIN SCANTLINGS DIRECTLY FROM CLASSIFICATION soanv RULES OVERALL DEMENSIONS AND CONFIGURATION MATERIALS DESIGN LOADS DESIGN CRITERIA DESIGN ANALYSIS WASTAGE ALLOWANCE

--DESIGN CPu MAZATION

Define L, 8, 0, d, Number of

decks and bulkhead spacings. Chose framing system. Select suitable materials. Determine relevant mechanical

properties I E,f,, fatigue); and

other relevant factors. Estimate sIgn values for primary, secondary,and local toads on structure. Define types and numerical values of safety margins against damage and coLlapse

Define criterion for optimisation; vary detailed scantlings,etc, tO

develop optimum desn.

the answers. The nature Of loading on marine vehicles is

beco-ming clearer. Analytical procedures for evaluating response,

and for optimization, are well developed. But there has not

yet been any comparable formal study of the structural design

criterion. Consider, for example, the design of a transverse bulkhead from first principles, using the step-by-step procedure shown in figure 1. One would first identifS' a design load,

deter-mine material properties, choose trial dimensions, carry out

a design analysis, and find the maximum stress, and then comes

the question - what stress is acceptable? Put more generally: what is the criterion by which the adequacy of the design shall

be judged?

It is important to try to answer this question carefully. It is

not difficult to show that the frnal design of any structure, and hence its weight and cost; depends very much on what

design áriterion is used, or what working stress is acceptable. The ISSC Committee on Design felt that this neglected subject

was of central importance, and its recent report is therefore

concerned very largely with a discussion of structural design

criteria in ships. The principal questions considered were:

Why must there be a margin of safety at all? How should it be defined and evaluated?

Margins of afety

To develop these points, it may be helpful to refer to fig. 2, which presents, in a slightly different form from fig. I, the essential stages ofan idealized design process.

W124

Fig. 1. Ship structural design procedure.

From the basic design data on operating conditions,

dimen-sions and material properties, a mathematical model of thefl

system to be designed is constructed. Analysis of this, together with a practical definition of failure and a theoretical failure hypothesis, enables two principal characteristics of the system to be determined. These are termed flist1y the DEMAND on the structure (which broadly refers to the level of stress or load

under the operating conditions); and secondly the CAPABILiTY

of the structure (which is a measure of the maximum demand

which it can sustain before it fails)..

The design problem is thus essentially to ensure that capa-bility of a ship structure (or any part of it) exceeds the demand on it by an appropriate margin. Fig. 2 shows four typical ways

of defining this margin by 'safety factors' relating the value of some characteristic (such as stress or deflection) in the.

failing condition to its value in the operating condition. The most common way of defining the safety margin is by using

a factor of safety based on stress. The working stress is not allowed to exceed an. agreed proportion of the yield, or ultimate,

stress. In certain situations, for example where fatigue or brittle fracture are possible causes of damage or failure, the use of the

stress factor of safety is justifiable. But it is better to regard the stress factor as a particular case of a more general load

factor of safety, which may'be defined as the ratio of the load causing the structure to become unserviceable to the working load. It thus represents the margin between the working

condi-non (however that may be defined) and the condition which brings

about unserviceabiity through damage or collapse. But how. big should the safety margin be? Consider a structure for which

DE INGENIEIJR / JRG. 80 / NR. 24 / 14 JLJNI 1968

Determine response of trial design to design loads. Modify if design criteria are violated.

Add margin for corrosion, depending on exposure and on protect ion applied.

OPERATING CONDITIONS M,O,p,W,N,T STRESS ANALYSIS "DEMAND" DESIGN DATA DIMENSIONS L,B.4,E MATHEMATICAL MODEL

STRESS SAFETY FACTOR

DELECTION FACTOR

RELIABILITY

the demand is 10 tons. Should it be designed to have a capability (i.e. a failing load) of Ii tons, or 15 tons, or 20 tons?

It is the selection and evaluation of such factors, or safety

margins, which constitutes the choice of structural design

criteria. One way of doing this would be to analyse (or test)

existing structures using the most modern knowledge of loading

and response and then to deduce what margins of safety are currently acceptable. Another way is to consider carefully why

it is necessary to have a safety margin at all. There are two

main reasons. Firstly, because of incertainties, or of ignorance,

of certain factors affecting the behaviour of the structure. For example, there are uncertainties about loading, because

we cannot yet reduce the highly variable dynamic environment of a ship to simple quantities. There are uncertainties in deter-mining capability, because the true strength of a structure is affected by material variability, dimensional tolerances and so

forth, and there are uncertainties in accuracy of analysis, which

must inevitably contain some assumptions and idealizations. And secondly, safety margins are needed because of the con-sequences of failure, which may involve injury or loss of life,

and possibly economic loss of capital or revenue.

In most design situations it should be possible to make a judgement in broad qualitative terms about these sources of

uncertainty, and about the consequences of failure. So that if it is considered that the uncertainties are small and the con-sequences of failure are not-serious, it would be justifiable to use a smaller margin of safety than if there were considerable

uncertainties and the consequences of failure were serious.

Hence by considering in this way the constituent reasons for

MATERIAL. PROPORTIES

E, a, Si,, _{t/' 5N}

Fig. 2. Stages of an idealized design process.

DEFINITION OF"FAILURE"

THEORY OF FAILURE

having a safety margin, it should be possible to prepare a lqgical basis for deciding the appropriate numerical value of the

Factor of Safety. The ISSC Committee found that a method for evaluating Load Factors had been developed in this way

by a group of Structural Engineers in the United Kingdom,

and included in our Report are their tables from which factors can be derived. It has since been found that these Tables can be expressed in simple formulae, and fig. 3 summarizes the

principal results, and the use of this method of evaluating margins of safety.

Briefly, the procedure is to make broad judgements regarding

three factors A, B, and C which are considered to influence the

probability of failure; and thereby to assign a value to each:

This enables an x-factor to be found from the formula shown. Similarly an assessment is made of the two factors influencing the seriousness of failure, and a y-factor deduced. The Imal Load Factor of Safety is then simply the product, xy.

The advantages of using this kind of approach in deciding

the margins of safety in ship structural design are:

I. It makes the designer consider carefully each area of igno-rance or uncertainty;

It enables improved knowledge (of loads, materials or res-ponse) to be rewarded by lower safety margins and hence

lighter scantlings, but without detriment to structural

performance;

It shows where research is most needed.

For these reasons the Committee felt that this method of assessing structural design criteria, and its application to ships, were well worthy of further study.

WERKTUIG- EN SCHEEPSBOUW 11 / 14 JUNI 1968 W 125

STRENGTH

ANALYSIS

FACTORS INFLUENCING PROBABILITY OF FAILURE A - QUALITY OF CONSTRUCTION

8 - ACCURACY OF LOADING DATA C - ACCURACY OF ANALYSIS

Rate each of these as

Verygood - valueO

good 7/3

fair - 2/

poor-1,O

I,x=1,10,324I.28c)+0,45(48+8CC4 I

FACTORS INFLUENCING SERIOUSNESS OF'FAILURE

0 - DANGER TO PERSONNEL E - ECONOMIC CONSIDERATiONS Rate, each of these as

Not serious - O serious

DESIGN LOAD

-Fig. 3. Factors- influencing probability of failure.

Structural reliability

The load factor method described above i's attractively simple,

but 'it has an underlying weakness; The working load on 'a -ship's structure cannot be uniquely defined. It varies with time, for example. Much is now known about the statistical variations in primary wave loading on ships hulls, and it is also becoming

clear that the still water loads likewise have a statistical

dis-tributidn. Similaily, the strength of a number of nominally

identical structures will show 'substantial \'ariations, because

of slight variations in material properties, in fabrication stfesses

and in dimensional accuracy.

- It follows that demand and capability are likewise not single

unique values, but are quantities which will also exhibit statistical

variation. The design problem thus emerges as an attempt to ensure that the highest demands on a structure do not exceed the lowest capabilities, and thereby cause failure. Because of the statistical nature of thevariations in the demand on, and the

capabilities of, ships' structures, the concept of structural

reliability may eventually come to be used as the principal design criterion.

-Fig. 4 shows in a diagram form the basic approach to structu-'

ral reliability. Consider a structural element such as a bullth&d

stiffener. Suppose that the curve forD represents the probability distribution of the load on this element. The normal design load is 3 tons, this being the average value of the expected

load on the element. In fact,.because of variations in the

opera-ting conditions, this-design load may vary in the manner shown. Suppose also that the curve for C represents the probability

distribution of the capability of the element,', that is, its failing load. The calculated strength (using nominal values) may be

4 tons. But among a large number of nominally identièal elements

there miy'i,e one having a strength, for example, of only' 3.5 tons, and it is possible that this element may experience a

load-of 3.6 tons, and hence failure would oCcur. Evidently the design problem is to find the risk that the demand D exceeds the.;

capability C of the element concerned; or to find the risk that (C - D) is less than zero. Now the margin (C - D)-can also be' presented as a statistical distribution, and it should therefore' be possible to determine the area under this distribution which lies to ,the left of the zero axis. It is the size of this arda which

is important, because it determines the risk of failure.

Provided therefore that the distributions of the 'demand on, and the capability of, ships' structures can be determined, then

it will be possible to deduce the risk of failure. It is evident from fig. 4 that this risk will depend primarily on

- the relative positions of the mean values C and of capability

and demand. The ratio of these two values is of course very

Similar to the load factor referred to previously.

the spread (or variance) of the distribtitions of demand and

capability.- - ' '

-It is shown in our Committee Report that the load factor can be

related to the risk of failure provided that these statistical distributions of demand and capability are known. If the demand on a structure is specified, then the-designer can control the risk

of failure either by ensuring a tufficient margin between the

mean values of demand and capability, or by reducing the variance of demand or capability and thereby reducing the area' of overlap between the curves. Limitation of the variance of the

-demand curve implies control of loading, and, of the capability

curve, implies quality control of materials and fabrication procedures, and a limitation'on tolerances.

-There are at present many difficulties in applying this sta-' tistical approach to structural safety in ships. 'More needs to

be known about the' variations of loads on ships structures, (particularly local loads), and also abáut the variations in

strengths from the aVerage calculated values. The latter may

--Fig. 4. Values of C, D and M.

(i=point of inflection of Gaussian curve)

require, for example, extensive surveys of existing structures to establish their true geometries and the extent to which they

depart from the dimensions assumed in design calculations. Information of this kind will help to determine representative distributions of demand and capability of structural elements. It will also be necessary to determine what risks of failure are acceptable, and this might be done either by considering the accident statistics and the risks of failure which are tolerated at present,or by analysing,using similar statistical procedures, existing structures to find the risk of failure implicit in their design. It is probably ultimately for the Classification Societies

to decide what are acceptable margins of safety, or risks of failure, in ships' structures; but it is important to, recognize

that if the kind of scientific design procedure outlined above is to be applied to ship structures, it will be necessary to defme clearly the safety margins or risks of failure which are

accept-able.

Optimization

Let me turn finally to an aspect:of structural design in which the decisions lie with' the shipbuilder or shipownér, rather than with the Classification Societies. An infinite variety of designs can be produced to satisfy any given set of requirements

re-garding operating conditions and structural criteria. The

question then arises: which of these possible designs is best?

In its study of thisproblem of optimization the ISSC

Commit-tee concluded that two aspects should be distinguished:

- The criterion of optimization - the way in which the best design is distinguished from the rest - must be clearly defined. - When the criterion is decided, the development of an optimum

design becomes largely an analytical procedure for which a number of techniques have been described in recent papers.

'This analytical aspect of optimization is therefore not.

considered further here.

In discussing optimization criteria for ship structures it must first be recognized that thcre is probably no such thing 'as a qesign which is truly 'optimum'; because what is 'best' for the

shipbuilder, is unlikely to be 'best' for the shipowner. The former may aim for minimum cost ofproduction, the latter for maximum profitability according to some economic criterion. A further dif-ficulty arises.from the fact that the information required, and the

analytical procedures involved, vary in complexity depending on the 'object function' which it is required to minimize. Thus weight-optimization is generally much more straightforward than cost-optimization, and because there is thought to exist a fairly consistent relationship between the weight of material in a structure' and the total cost of building it, many authors. have argued that design for, least weight is a good objective, of

optimization. But more detailed studies do not support this

view. For example, the 'effect of increasing the frame-spacing in transversely-framed ships was found to increase the weight,

but to reduce the cost, of construction. In such cases it becomes

necessary to balance the relative advantages of cost-reduction

against weight increase, and this requires that both factors

should be expressed in terms of some other single parameter. This can sometimes be done by relating structural weight and initial cost'to an economic criterion, such as. Capital Return Factor, but the optimum thus obtained by minimizing CRF is i'nlilcely to represent the design with the minimum production. cost, and will therefore not be optimum from the standpoint

of the shipbuilder.

Another criterion which has been investigated is the

minimi-zation of the number of parts required to build the structure. Here the intention is clearly to reduce production cost, on the

assumption that this is directly related to the amount of welding, edge-preparation and general complexity of the structure.

Clearly when carried to the limit, this criterion will generally

lead to much heavier structures, and the method is of value only'

as a temporary expedient until more detailed costing data is

available upon which a more thorough cost-optimization procedure can be based. Furthermore, experience in certain

shipyards shows that the production cost of a typical structural

intersection (between say, a ,transverse and a longitudinal

stiffening member), is less if the members are coñnectèd by additional lugs, than if welded together direct, because of the more accurate fit required in the latter method. Such conside-

-rations are difficult to quantify in any optimization procedure; so the development of designs, involving minimum production costs may have to rely for some time to come on the mingling

of experience and analysis which has characterized the progress of ship structures generally.

If the emphasis in optimization studies moves, as seems

likely, from least-weight to least-cost design, 'tlen one should logically include not only the initial cost of construction but

also the subsequent costs involved in maintaining, and if

neces-sary, repairing, the structure. The . latter would include any.

consequent costs arising from loss of' revenue while repair

is being carried out. In seeking to minimi7e the total cost, the

choice would thus be made from a.spectrum of possible solutions ranging from designs having high initial costs but low total

repair and maintenance costs,. over .the life of the structure, to those having low initial costs but, high repair costs. Assuming,

that the cost of repair does not vary much among different

designs, then the total repair costs will depend primarily on the' number of times repairs are likely to be needed during the life

of the structure. This in turn depends 'on the probability of failure of the structure, as discussed earlier. Hence the least' overall cost design is the one for which (C + PR) is minimized,

C being the initial cost, P the probability, of'failure, and R the'

cost of failure. ' ' .'

This philosophy 'of optimization was prOposed originally for

engineering situations in which a genuie choice exists between

inexpensive, unreliable designs, and expensive, reliable designs. It would. only become applicable to ship structures if the reliability, or risk of failure, of the structure was not only known

explicitly, but was something about which a choice could be made. Although there are indications that thoughts are turning towards the. short-life, inexpensive design as ,an alternative to

long-life and reliability, it is too early for this approach to

opti-:mization to have any direct impact on ship structural design.

Nevertheless, as the only method which the 'author has

encount-ered which considers the total cost of a structure over its life, it seems worthy of mention in the context of this paper..

It is likely that there will always be certain design problems in which weight-saving is of overriding importance. It is therefore'

worthwhile to determine the limits to which structural weight' can be reduced if no regard is taken of production costs. This leads to the development of structures in which the weight is saved by, for example, so proportioning the members that all parts of the structure are stressed to a similar level. This

gene-rally precludes stiffeners having prismatic sections and requires

that such members be fabricated with a continually varying cross-sectional shape. AlthOugh such constrUction is bound to be more costly, the concept of designing structuies by this

kind of 'inverse' method, has many attraions, and seems to merit further study. In this methOd one postulates a certain

distribution of stress or deflection which the structure should develop under load, and then determines the necessary

scant-lings and shapes of the structure in order to achieve this desired response.

It was this approach which led to the discovery of the 'neutral hole', in which the geometry of a hole in a stressed sheet, and the disposition of the reinforcing material around the rim of the hole, are so arranged that the state of stress in the sheet is the same as if there were no hole present at all. The objective in this application

of the inverse design technique was to defme a desired state of

stress (zero stress concentration factor) and then to find the shape and reinforcement of the hole to produce this stress

distribution. In minimum-weight design the method would be used to prescribe the state of stress corresponding to the most effective utilization of all parts. of the structure, and then to generate the required scantlings of the structure which would

ensure this response. In applying this method to the elastic design of grillages under lateral load it was found that very substantial reductions in weight could be effected by using

non-prismatic members. An interesting. consequence of the

method was that in certain cases the requirement that the

flange stress in the grillage members should be constant, led to the surprising result that the weight of the grillage was quite independent of the magnitudes of the mutual reactions at the

intersection of the members. This led to coniderable

sixnplifica-tion of the mathematics of grillage respOnse, But quite apart

from such incidental benefits, the study. of ship structures

of absolute minimum-weight seems worthy of further study, particularly if improvements in production techniques make-possible the economical construction of structures of unusual geometry..

Conclusions

One of the principal 'benefits to be derived from the study and review Of ship structural design procedures, and the effort to develop' more logical procedures, is that by identifying all, the facts and 'decisions needed in such a process, it helps to show

where research is most needed.

Since design is essentially an attempt to ensure that there is

an acceptably small risk that the dçmand on a structure, or

any part 'o,f it, will exceed its capability, hence the three basic

areas in which research is needed are:

Determination of demand. Although the primary loading

on ship structures has been the sibject of much research

there is likely to be a growing need for more information regarding the secondary and local loads arising particularly froiri, the direct action of water pressure, or cargo, on the

structure.

Determination of capability. Most research on structural response of ships has concerned its elastic behaviour. Further study is needed of the ability of structural elements to sustain

loading without becoming unserviceable through damage or collapse. Strength analysis, involving bothexperimental and theoretical research', rather than stress analysis, may inèreasingly be required.

Determination of the appropriate margin of safety between

demand and capability, or of acceptable risks of failure. It is in this area that operational experience is the most

valuable by indicating what margins f safety have proved

adequate in the past. As knowledge of demand and capability improves, the margin of uncertainty will be narrowed. In this.

way the experience of the past can be synthesized with new

knowledge to produce a scientific method of designing marine structures which are reliable, economical, and efficient.

Discussion

The rational design philosophy, presented by professor Caidwell in such a 'lucid manner, deserves very careful! attention.

Professor Caldwell first introduces the, from a practical point

or view, very attractive concept of arriving at a safety factor

by. a process of numerical judgement of fctors influencing

probability of collapse and seriousness of the consequences of collapse. As professor Caldwell points out, it is here assumed that demand and capability are uniquely defined properties.

He then introduces the uncertainties attached to the determinati-on of load and strength and, rightly, stresses that statistical 'methods must be used to determine a probability of failure. An acceptable level of probability of failure would then be

determined by reference to the results of structural testing and

past experience.

This situation would undoubtedly be ideal, but it presupposes that failure can be uniquely defined. In most cases .in ship structures, a clear definition of failure or unserviceability cannot

be given however, since long' before total collapse occuts, as defined by 'limit design considerations, a state of affairs will be reached, which is found unacceptable. This will usually be

because an unacceptable level of local failures is reached. There

is, in fact, no need to define precisely what' constitutes failure and what we are going .to call safe as. long as the yardstick by which the criterion of structural adequacy is measured is based

on a general level of strength which has, in practice, been associa-ted with an'acceptable risk of damage.

The general principles outlined by professor Caldwell should most certainly form the basis for a rational approach.to

structur-al design. My intention was only to point out that for a 'puke' application of these principles a clear definition of what we are

going to consider unacceptable is required and this, to my mind, is virtually impossible.

Reply to discussion

Mr. Starink: It is not possible to give a precise answer to this question since the causes of some ship failures are unknown,

and complete records have not been published..

Mr. de Jong I would not agree that wastage allowance shoid

be included in the design analysis, if by, this is meant the

deter-mination of the response of the structure to beds. The designer must be sure that the structure is adequate after wastage has occurred, i.e. at the end of the life of the ship. But optimization could equally well be carried out for the structure at the start

of its life, and in this respect Mr. de Jong's proposal is accepted

as a valid alternative.'

Mr. Soejadi: The equations for x and y were derived by Dr. Masuda (a member of ISSC Committee 10) by studying the tables of factors given in the report of the Committee on

Struc-tural Safety of the Institution of StrucStruc-tural Engineers (see

Appendix B of the report of Committee 10 at ISSC '67). These

factors were obtained by agreement among the Committee after studying acceptable design standards for a wide,range

of engineering structures, and thereby deducing suitable values

for each factor A, B, C, D, Eaccording to its rating. The process W128 DE INGENIEUR / JRG. 80 / NR. 24 / 14 JUNI 1968

thus depended On experience, rather than any mathematiOal analysis. (For fullei details See. referenb 2 of'oür Committee

10 report.)

Mr. Rorn f/n: As indicated in the reply to Mr. Soejadi, the factor 1.1 emerged as a consensus of the Committee's views as a suitable load factor of safety in cases where all factors A, B, and C were rated 'very good', and factors D, E were rated 'not serious'.

The 10 % margin beyOnd the design load was regarded as reasonable insurance against uncertainty even in such cases. Evidently 1.0 was coiisidered too risky, and 1.2 unnecessarily cautious!

Mr. Herfst: This is agreed, but I would add that the

'judge-ment should be good and good judge'judge-ment is itself best based

on knowledge and experience.

Mr. Antonides: This is an, important comment on the use of

(c + PR) as a basis for optimizatiOn. The method is only

useful in cases where the designer (Or buyer) has a choice

be-.tween low C and high PR (which geneilly means high, risk

of failure) Or high Cand ldw PR (low risk of failure) For primary ship structure, failure may cause loss of life, and the risk of

failure should therefore be decided quite independently of the

designer or owner's requirements. For minor structural

ele-ments, however, the method may be applicable..

Mr. de Wilde: Mr de Wilde rightly points out that in many

cases, before total collapse occurs 'a state of affairs will be reached which is found unacceptable but then argues that it is not 'possible' to define what is unacceptable; The logic of.'

this is elusive if unacceptability cannot be defmed how can it be stated that it will occur? The author prefers the view that it should be possible to define unserviceabiity in ship Structures,

just as is being done in other areas of structural engineering

(e.g. reinforced concrete, aircraft structure, etc.). The Classifi-cation Societies, with their vast experience, of damage and of assessing when repairs are 'needed, would be well .placed tO

forthulate standards of unserviceabiity, which would then define the required capability of the elements of ship structures.