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Performance parameters and performance specification in architectural design
BuiM. Sci. Vol. 3, pp. 125-133. Pergamon Press 1969. Printed in Great Britain
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Performance Parameters and Performance Specification in Architectural Design" R. J. MAINSTONE'~ L. G. BIANCOI" H. W. HARRISON#
Performance specifications are concerned primarily with ends not means. They make explicit the performances required of an artefact and provide a basis for assessing objectively those that are achieved. The paper considers what is entailed in making requirements explicit in this way in architectural design. It pays particular attention to the writing of performance specifications for standard components, but it does so within the context of a much wider discussion of the cycles of choice and appraisal which make up the process of design. The use of performance clauses in building regulations and some related aspects of standardisation are also considered.
HIERARCHIES OF CHOICE AND PERFORMANCE
INTRODUCTION T H E D E S I G N of a building is never merely a doodle. All buildings, with the possible but questionable exception of the folly, serve a purpose, and it has always been part of the architect's professional skill to identify this purpose and to allow it to trigger off and discipline his creative work. Looked at in its simplest terms, a specification of performance requirements merely makes the purpose to be served or the needs to be met explicit in ways that do not unnecessarily restrict the designer's response to them. It may do so at any stage in the design process. It may, on the one hand, constitute a precise functional brief for a building or group of buildings or, on the other, define the performance requirements of a single component or material. It may be used for commissioning new products, for the assessment or selection of existing ones, or for regulation or standardization. In considering the writing of such specifications and their varied uses, this paper draws particularly on experience acquired at the Building Research Station over the past year and a half in advising on the drafting of a number of performance specifications for particular types of component and in monitoring the use of others. It also draws on wider and longer standing interests in setting performance standards, in devising appropriate tests, and in the mechanisms of the whole architectural design process.
The architectural design process can be considered from the present point of view as the fitting of a physical form to a complex pattern of human objectives, activities and requirements. This necessarily involves identifying the requirements, devising the form and finally appraising the "fit". The emphasis throughout the paper will be on the first of these activities, but it has to be recognised at the outset that the closeness of a "fit" can be judged objectively only when like is compared with like. This has an important bearing on the manner in which requirements can most usefully be stated. Consider, for instance, the design of a school. The building of this school will be one step in furthering an educational policy. It is, however, meaningless to speak of it fitting directly the ultimate objective of educating the young except in a purely arbitrary manner. Before the "fit" can be objectively appraised it is necessary to decompose the ultimate objective progressively into sub-objectives and to define appropriate activities conducive to the achievement of each and the requirements that these activities generate. At one level the activities might be a broad curriculum of training and study. At another they might include anything from writing or reading to swimming or making music. They will generate requirements ranging from supports on which to write to comfortable temperatures for physical relaxation or exertion, enclosed spaces, intercommunication between these, and freedom from excessive transmission of sound from one to
* Crown Copyright Reserved. Published by permission of the Director. t Building Research Station, Garston, Watford, Herts. 125
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
another. It is these, and only these, primary requirements which can be directly matched by the performance achieved or expected to be achieved by the design, though, for purposes of detailed design, some will have to be broken down further into secondary requirements relevant to each of the individual means selected by the designer to meet them. To achieve an overall "fit" it is necessary to meet each primary requirement. To do this it is necessary, in turn, to meet the secondary ones generated, in part, by the designer's decisions on how he will meet the primary ones. Figure 1 is a simple representation of the total design process seen in this way. Without the enclosing bars the right-hand part represents the characteristic single cycle of identifying a requirement, choosing the means to satisfy it and Technology ] Object ires'---'Activities ~ "'t Perf°rmonce ~- - J - 4,-4Specifications of requlrements~ ~ / f o r m .material land method of Performance ~ construction characteristicsl 1 J Science, experiment experience -
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Fig. I. The total design process.
appraising the choice. This representation of it is untypical of much present practice only to the extent that it introduces an explicit statement of the performance requirements in place of a tacit recognition, and an equally explicit statement of the performance achieved or expected to be achieved as a basis for the appraisal. Technology limits the range of reasonable choice and science, experience or experiment furnish the means for predicting or otherwise determining the performance achieved. The enclosing bars denote that the total process is concerned with complex systems of objectives, activities, requirements and performances and with many individual cycles of choice and appraisal. Because, in these complex systems, the individual requirements and performances are interrelated in various ways, the individual cycles of choice and appraisal are also interrelated. The arrows in the right-hand loop should therefore be read as denoting not merely multiple relationships of the kind that typify the single cycle, but complex systems of these relationships. It is in these interrelationships that much of the difficulty of understanding the logical structure of the design process lies. Fortunately, for the present purpose, it is unnecessary to consider all aspects. Nor is it intended to prejudice further study of them. It is necessary, though, to look a little further at the systems of requirements and performances and to touch more briefly on the ways in which these are related, by the designer's choices and by physical laws, to the specifications of form which
constitute the design. Even here, to simplify the discussion, the emphasis will be on satisfying individual primary requirements of the user--for a comfortable air temperature or for a certain rate o f air change for instance--and not on any interdependence of or interference between these requirements. These are subjects calling for considerable further study. The performances which are achieved by a particular design are clearly related in hierarchical sets which directly reflect the manner in which smaller units of construction are combined to form larger ones of recognisably different character from any one of them. In each set the performances at any level (corresponding to a particular level of physical organisation of the fabric and/or services of the building) are determined by those at the next lower level and by the manner in which these interact. The acoustical properties of a room, for instance, are determined by the relevant properties of the elements making up the spatial envelope, by those of their interconnections and by the geometry of the room. The characteristic strengths and stiffnesses of a floor assembled from precast components are similarly determined by the strengths and stiffnesses of these components and of their interconnections and by the geometry of the floor. Typical hierarchies of thermal and structural performances are compared in figure 2. They are described only as typical because they relate only to particular constructional means of meeting the primary requirements. For some types of structure. for instance, the hierarchy of performances might Spatial envelope + external environment+ heat source (air temperature )
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Fig. 2. Typical hierarchies of thermal performance (A) and
structural performance (B).
Performance Parameters and Performance Specification in Architectural Design include only the performances of materials, of elements and of the whole system. They are, nevertheless, unique hierarchies for the means envisaged. The levels are determined by these means and are, therefore, most easily denoted by them, but appropriate measures of performance are indicated in parenthesis. Four things in particular should be noted: (1) Each level in a hierarchy is usually distinguished by different appropriate measures of performance, though the measures may, as in the structural hierarchy, be of the same kind at different levels. The measures of strength and stiffness distinguished by the suffixes 1, 2, 3 and 4 will differ only in that they will be expressed in terms of different patterns of load and different types of deflection. (2) Hierarchies do not necessarily terminate in a performance characteristic of the whole building, though the level at which they do terminate is partly a matter of the precise definition of their scope. Primary requirements for the thermal environment, for instance, will, taken together, relate to the whole building. Individually, however, they relate to the spaces within it considered singly, and it seems preferable to terminate the hierarchy here. This means that the air temperature requirement in a room adjacent to the one considered is treated as part of the external environment of this room. (3) Hierarchies may be linear throughout or may branch as in the case of the example for the thermal environment. Only the branch corresponding to the spatial envelope is shown in full, but, to reach the highest level, it is necessary also to consider the heat input from the heating system which is the highest-level performance of another branch. (4) Though, in themselves, the different hierarchies are independent of one another, performances belonging to different hierarchies may be properties of the same physical unit of construction. There is, for instance, no direct dependence of thermal properties on structural strength and stiffness or vice versa. Both may, nevertheless, be properties of the same unit of walling. This is represented symbolically in figure 3 which is an expansion of the lower part of the loop at the right of figure 1. The suffixes a, b, c . . . . denote different hierarchies of performance (thermal, structural etc.) and the suffixes I, 2, 3. . . . denote different hierarchical levels. Gaps have been left in the numbered sequence of levels in some hierarchies to recognise Po~
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Fig. 3. Expansion o f the lower part o f the loop in figure 1. P = single performance characteristic. F S -~ specification of form, material etc.
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the fact that there is not necessarily a direct one-toone correspondence in all cases. In the examples given in figure 2 of thermal and structural performance hierarchies, for instance, a level corresponding to the whole building is missing from the first and a level corresponding to the single space is missing from the second. If the performance requirements are to be capable of realisation in practice, it must be possible to relate them to one another in similar hierarchical sets. Assuming that the building to which figure 1 relates perfectly meets all the requirements, it must be possible to expand the upper part of the loop at the right in exactly the same way, as shown in figure 4, as the lower part. To each achieved performance characteristic will correspond directly a performance requirement, and these performance requirements will be related to one another and to the chosen forms in the same ways as the performance characteristics are. If the building does not perfectly meet the requirements figure 4 will still correctly represent the necessary pattern of hierarchical relationships of the requirements PRol
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Fig. 4. Expansion o f the upper part o f the loop in figure 1. PR = single performance requirement. FS = specification o f form, material etc.
because the lower part of figure 1 and its expansion in figure 3 can equally be read as relating to a possible building which does meet them. Two important differences between the hiera rchical relationships of performances and of performance requirements and between the relationships linking both to the chosen forms are indicated by the opposite directions of the arrows in figures 3 and 4. The horizontal arrows in figure 3 denote the dependence of the achieved performances on the chosen forms. Those in figure 4 denote choices by the designer to satisfy particular groups of requirements by means of particular forms. The grouping of different requirements belonging to the same hierarchical level is, in other words, a design choice in itself, whereas the common dependence of a group of achieved performances on a single form is a consequence of such a choice. The downward directed arrow in figure 4 denotes a logical sequence of the choices typified by those represented by the horizontal arrows, assuming that in general the requirements that are of direct concern to the client or user are the higher level ones. It is described as a logical sequence because it is essentially a hierarchy of the choices that are finally effective. Design, in practice, is rarely a
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simple unbroken linear process so that the actual sequence is likely to have a number of superimposed loops. The upward directed arrow in figure 3 denotes, on the other hand, a simple physical dependence stemming from the previous choices of forms in the manner illustrated in detail in figure 2. In what follows the emphasis is on the specification of performance requirements. The chief implications for this of what has been said so far are that all requirements should be stated in terms of performances at the appropriate hierarchical level and that before this can be done at the lower levels it will usually be necessary to make explicit design choices about the ways in which higher level requirements in the same hierarchies are to be broken down. BRIEFS FOR DESIGN Explicit statements of performance requirements may be used both to initiate a design process and at intermediate stages within the process as attention is progressively focussed on more detailed aspects. The term performance specification will be reserved for the statement that serves to initiate a process, though it may do so at any level in the hierarchies .just considered. The traditional responsibility of a single designer for the entire design of a building including all its details is becoming a thing of the past and the most important use for performance specifications at present is probably in the commissioning of new components. The pure performance specification may be defined as one which is stated solely in performance terms. It is concerned solely with the functions to be served rather than with the means whereby they are to be served. These functions, in general, will be both geometrical and non-geometrical. The former will be concerned with ability to occupy, span or enclose a defined space rather than with the precise form of the product and the latter with all other requirements. A pure performance specification, sufficiently comprehensive to ensure that any product which meets it in all respects will fully satisfy the client or user, is often, however, an unattainable ideal at present. This is particularly true at the higher levels such as that of the complete building, because higher level objectives and their associated requirements are, in general, less materialistic and less readily defined in unequivocal terms than lower level ones. Clients and users buildings are, moreover, rarely skilled in and frequently quite incapable of expressing their needs in purely functional terms. At best a skilled professional may be able to assist them in doing so, but he will often be at a disadvantage because he lacks direct experience of similar needs. Even at lower levels, such as that of the component, some requirements may still be difficult to define precisely in performance terms and an imperfect understanding of the technical possibilities may lead to difficulties of another kind.
In practice, therefore, performance specifications will usually be impure according to the present definition or incomplete or both. This, in itself, need not be a fault. It is assumed that the prime objective of any design commission is to draw on the designer's special skills in meeting the client's needs and that the prime purpose of any design brief is to do so as effectively as possible, within the available budget. What this entails will vary with the nature of the commission. On the one hand the client may wish to limit the choice of means whereby the functions are served, by specifying, for instance, the use of materials or other products in which he has a proprietary interest. On the other he may wish to leave certain choices completely open. In a performance specification to serve as a basis for competitive tendering for the supply of a new component, the objective may, with respect to some performances, be simply to get the best that manufacturers can offer at an acceptable price. It may then be sufficient to state that these pertk~rmances will be considered in choosing between tenders with, preferably, some indication of the basis of choice. In the brief for a building the aim may, in a rather similar way, be to give an architect chosen on the basis of his past work a fairly free rein in both identifying and satisfying the client's needs. This is in effect putting the onus of completing the specification on the architect himself and asking him (to paraphrase Denis Lasdun) "to give the client something he did not know he wanted until he gets it". The best design has probably always been the product of a creative dialogue between client and designer, and the deliberately incomplete performance specification may be the best starting point for this. All such impure or incomplete performance specifications may still be referred to as performance specifications, but they will then be so described primarily in terms of their intention rather than their form. The intention of a performance specification is always a delegation of responsibility for further design which allows the designer the maximum freedom in deploying his own experience and skills in meeting the client's requirements. The difficulties that arise should be considered in the light of this intention. Among them are costs, the ranking or weighting of requirements, and the specification of requirements that it is not possible to define precisely in performance terms. One objective of any delegation of responsibility for design, and hence of any performance specification, is to obtain good value for the available money. The specification might therefore be looked upon as a definition of what is considered to be "good value". The difficulty is that it cannot be known, even in performance terms, what this really is until the design or several designs have been completed. Clearly there is no point in dressing up a preconceived design as a performance specification. Some reference to the performances achieved in existing designs and to their costs is, nevertheless,
Performance Parameters and Performance Specification in Architectural Design the only basis for setting the values of the performances asked for reasonably. Usually the aim will be to stimulate some improvement on existing designs and the problem, in other words, is to know how much improvement is possible. In practice the problem can be solved only by a certain looseness in the specification. The form that it can best take will depend on the nature of the commission and the manner in which responsibility for design is delegated. The more possibility there is of a creative dialogue, for instance, the more readily can performances be upgraded or downgraded in the course of design and the less need there is therefore to cater for all conceivable possibilities in advance. Where the specification is to serve as a basis for competitive tender, on the other hand, it should be as unequivocal as possible from the outset. This will usually mean, in effect, some sort of ranking of performances and weighting of values. Ranges of values might, for instance, be specified for some performance requirements with indications of the value that the client would place on achieving more than the minimum. Other performances might be left unquantified with an indication merely of the sort of weight that the client will place on them in comparing alternative tenders. The problems presented by requirements that cannot be stated precisely in performance terms are rather similar. Again, where there is a possibility of a creative dialogue, they need not be very troublesome. Where there is no such possibility it may be necessary either to illustrate the sort of solution that would be acceptable or to indicate in some similar manner the limits between which an acceptable solution should lie. Some attempt, at least, should be made to indicate how designs will be assessed and compared. C O N T R O L S ON DESIGN Building regulations, departmental circulars, some BSI codes of practice and some recommendations of other national bodies may be regarded as special and incomplete briefs for design. Their purpose is primarily to safeguard the generally recognised interests of the individual user and those of society as a whole by restricting the freedom of action of clients and designers. It is desirable though that they should be as little restrictive as possible consistent with serving this purpose. Originally most of these controls, though incomplete design briefs, were almost wholly restrictive in those aspects of design which they did cover. They were complete specifications of minimum acceptable forms of construction. Today such specifications frequently survive as illustrations of acceptable interpretations of imprecisely defined or even of precisely defined minimum performance standards. The forms illustrated are "deemed to satisfy" the requirements. They make design easy but they do not encourage fruitful innovation. They may easily result in serving the client or user less well or less economically than might have been possible.
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Controls framed in terms of minimum standards of acceptable performance do allow the maximum freedom in design consistent with safeguarding society's interest in such matters as safety and health. There seems to be no question, therefore, that every effort should be made to extend their use as widely as possible. The only questions that do arise relate to the choice of level in each hierarchy of performance at which to frame the requirement and to the choice of the values to be specified and the manner of their specification. The lower the level of a performance requirement, the more any decision about it necessarily implies prior decisions about the manner in which higher level requirements are to be met, as has just been shown. Since it has been suggested that controls should be as little restrictive as possible, it follows that, ceteris paribus, requirements should be specified as nearly as possible at the levels where they are socially most important. At the same time it has to be remembered that mandatory requirements are of little use unless they can be enforced. They must therefore be unequivocal and so framed that compliance can be readily checked when plans are submitted for approval, and this is likely to favour specification in terms of lower-level performances. The ideal form of control is possibly best exemplified at present by the current revisions of the main structural codes of practice. These revisions, and previous development of the codes leading up to them, have been possible, however, only on the basis of the prior development of adequate and workable methods for the prediction of performances in the course of design. Before all controls can take the same form more effort needs also to be devoted to developing analogous methods for the prediction of other performances. It should also be remembered that there may be valid social reasons for influencing the manner in which some higher level requirements are met. These may range from a national need to encourage some forms of construction at the expense of others to wider economic considerations which make it desirable to strike particular balances between expenditure on thermal insulation and on fuel or between expenditure on making buildings fire-resistant and on firefighting and rescue services. Such considerations are not, however, immutable. If they are allowed to influence the specifications of performance requirements at lower levels without being explicitly recognised there is a danger that they will be forgotten when a need for revision arises. The difficulties of specifying values for particular performances reasonably are similar to those that arise in specifying values for a complete design brief. The main difference is that acceptability and costs must always be considered on a much wider scale than that of the single project, this scale being determined by the-intended range of application of the control whether international, 'national or regional,,~ Acceptabilityis always largely a matter of past experience and ~Off-expectation based on that
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experience. Cost, a little less directly, is also a function of past and present practice, though one which is susceptible of change with developments in technology. The normal standard product or something similar to it is always, ipsofacto, cheaper than any marked deviation from it, unless this situation is radically changed by a technological breakthrough. Both acceptability and cost are therefore strongly influenced by what is generally specified and by what has commonly been specified hitherto. Indirect controls designed to encourage technical innovation without exerting a positive influence on its direction will naturally emphasize this aspect of acceptability in setting values for performances, as did the certification of new products for insurance purposes by the original French Agr6ment procedure. There may be a natural tendency, for instance, to assess the fire resistance of a floor within a dwelling largely in relation to that of the common low-cost timber joisted floor. More direct controls, such as building regulations, departmental circulars and specified codes of practice, must also recognise it, but they can more easily and effectively upgrade performances when this seems to be desirable. Since also they are not tied to specifying performances in terms of particular types of product, they can do so at any level in the relevant hierarchies. In drafting such controls it is important therefore to bear in mind the more effective encouragement that can be given to technical innovation by specifying performances at those higher levels which are of direct concern to the user and less tied to current techniques of construction. In specifying values, however the balance between performance and the cost of achieving it is struck, there is frequently also a choice between doing so for the " n o r m a l " condition of use or for a "limit" state in which the product ceases to function adequately. There is a choice, in other words, between legislating for a comfortable margin in performing acceptably under average conditions or for an acceptably high limit of failure under extreme conditions. Where performance is a linear function of the relevant variables (such as external temperature or imposed load) throughout their range, it does not really matter whether the average or the extreme condition is considered. Where, on the other hand, it is a non-linear function, the choice will be significant. Careful consideration should then be given to the alternatives bearing in mind their implications for the definition of appropriate measures for the performance and the possibilities of prediction and test. P E R F O R M A N C E SPECIFICATIONS FOR STANDARD PRODUCTS The performance specifications considered here are a special case of the brief for design. Like other design briefs they are a means to an end--in this case standard product specifications in terms of
particular forms, materials and methods or construction or manufacture. Standardisation has as its primary aim the achievement of economy. It may also have connotations of "maintaining standards", but, for the present purpose, this aspect has already been adequately covered in the previous section and it will not be considered further. Economy may be aimed at in design time, in production, in stock holding and distribution, in erection or in subsequent maintenance or in most or all of these. It will be achieved, however, only at the cost of a reduced variety of choice in meeting particular requirements. If, in general, a standard product is to meet a variety of particular requirements it must, in each of its relevant performances, meet the most onerous. In relation to the average, therefore, an extra cost will be incurred which must be offset against the economies achieved. An attempt must be made to strike a favourable balance between the savings and the costs, both by a correct choice of products to be produced in quantity to a standard design and by correct specification of their performance requirements. Writing a performance specification to be used for commissioning a standard product or range of products calls, therefore, for all choices to be made in a much wider context than that of the product destined for a single use only. It is necessary throughout to consider the intended variety of use in relation to both geometric and non-geometric performances. With units as small as the brick or as large as the dwelling there is relatively little need for co-ordination with other products. For units of intermediate size intended to be used alongside other standard units of similar size with some measure of interchangeability, co-ordination of geometrical performances becomes essential. The extent to which it is achieved will place an upper limit on the potential market for any particular unit. Co-ordination of non-geometrical performances is mostly less essential because of the different manner in which these interact, It is also potentially undesirable to the extent that it is restrictive, because it may stand in the way of technical advances. As a basis for present attempts to develop component building on a national scale, some national co-ordination of geometrical performances (including some aspects of jointing and tolerances) is therefore desirable, but a strictly analogous (and therefore similarly restrictive) national co-ordination and standardization of other performance requirements seems less desirable. At present it seems preferable, in considering for this purpose the more important of these other requirements, to aim merely at agreed methods of test and perhaps standard grades of performance. Any co-ordination must take as its starting point the client and user requirements at the highest levels and it must involve certain design choices about how these are to be met. These choices will then be .part of the framework within which
Performance Parameters and Performance Specification in Architectural Design particular specifications are written and they will, in part, ensure that the wider context is taken into account. In illustrating in more detail the approach that is recommended for arriving at a detailed specification it will be assumed that the specification is to be written for a component. It has to be admitted that there is, as yet, little rational basis for choosing to standardize components rather than elements, whole rooms or whole buildings. Hitherto there has been insufficient study either of the economics of scale of production in relation to the whole building process or of user requirements and their significant variations and probable changes over the lifetime of a building. The assumption is made chiefly in deference to current interest in components and in recognition of the fact that they belong more to the English tradition of building than, for instance, the standard dwelling. The choice of what should constitute a component nevertheless remains. In practice it will usually be made initially on the basis of a simple extrapolation of recent practice and writing of the specification will start on this basis. It may however have to be questioned when subsequently weighing the costs of overprovision in some respect to meet extremes in the ranges of performances against the economies expected to accrue from standardization. The economical solution might be to redefine the original function so that it is performed by ranges of two or more smaller components used additively in various combinations. A performance specification for a standard component should usually be as comprehensive as possible, even to the extent of calling explicitly for properties that are normally taken for granted, if it is to be used as a basis for competitive tendering. It is therefore highly desirable that it should be drafted as systematically as possible. The following procedure is recommended: (1) Prepare a list of all relevant properties. (2) Attach a degree of importance to each of these, bearing in mind the whole range of intended use of the component. Some imprecision is inevitable here but it has been found useful in recent work at BRS to distinguish between (a) properties which are essential, (b) properties which are reasonably important but not absolutely essential, and (c) properties which are relevant but normally of m i n o r significance. (3) Decide, at the next higher level above that of the element in each hierarchy of performance, what requirements are to be met, again bearing in mind the whole range of intended use of the component and if necessary tabulating ranges of performances corresponding to different uses. The need for the last might arise, for instance, when preparing a performance specification for a range o f windows to be used under different conditions o f exposure. (4) Translate these requirements into requirements at the element and component levels,
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making the necessary decisions, in general terms, about the ways in which the higher level requirements are envisaged as being met. This again calls for consideration of the range of intended use and for an attempt to forsee the sort of decisions that it would be reasonable to make in designing a variety of future buildings. (5) Select the values to be specified for all those properties which can be quantified. Indicate which are obligatory and which are, to some extent discretionary. With the latter indicate also the basis that will be adopted in assessing tenders, perhaps by suggesting different cost limits for different performances. (6) Indicate criteria for the assessment of properties which cannot be quantified and any constraints on design. The latter are any requirements, like the specification of the use of a particular material or finish, which are stated not in terms of performances but in terms appropriate to the final specification of the product. The distinction between geometrical constraints and geometrical performance requirements is not a sharp one, but it is normally convenient to regard as performance requirements all specifications of the space to be occupied or spanned or enclosed and as constraints only those geometrical characteristics which uniquely determine the final geometric form of the product. The difference again is inherent partly in the intention. (7) In parallel with 1, 3, 4 and 5, attention must be given to appropriate measures for the performances. As part of 5 it is necessary also to decide and to define how the properties achieved are to be evaluated. This may be by calculation, by inspection of the final specification or of a prototype, or by physical test in a prescribed manner. The number of independent checks required can often be reduced by an appropriate choice of the measures to be adopted. National Standards and Codes of Practice and Agr6ment Methods of Test may provide guidance here. A corollary of this approach to design is an equally systematic declaration of the properties of standard products offered either in response to specific commissions or on the open market. This is desirable both for the initial assessment of tenders and for the subsequent selection of components for particular uses. The CIB Master List of Properties of Materials and Products (1) was devised as a comprehensive general framework for the latter purpose. Derived lists are now being prepared of the properties relevant to particular classes of component. These provide a useful starting point for this declaration. They have also been used as an initial guide in listing the properties to be included in performance specifications. Not having been conceived initially with this use in mind, they have, however, some deficiencies here. It therefore seems desirable that revised lists should now be prepared giving performance
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
requirements and properties in parallel columns. Different lists will be required for different hierarchical levels of performance. It is debatable whether details of, for instance, materials should be listed as properties, but they might usefully be listed in separate parallel columns of design constraints and design details. Among the properties there should, however, be provision for the declaration of details of recommended applications of the products. The use of such declarations of properties in selecting products for use in normal design is considered briefly in the next section. MECHANISMS OF CHOICE This section considers in more detail some of the design choices represented symbolically in figure 4. It assumes that many or most of the choices at the lower hierarchical levels are selections from "catalogues" of standard components and that the properties of these components have been systematically listed in the manner just described. A typical sequence of decisions might be then stated formally as follows: (1) Assumptions are made about the manner in which the stated performance requirements at the next higher level (usually of the element or space) are to be met by assembling a number of components. For each component a performance specification is then prepared. (It is convenient here to use this term less restrictively to denote any explicit statement of performance requirements). (2) The listed properties of available components are assembled for comparison. (3) If one component only fully satisfies the specification it is chosen. If several components satisfy it a choice is made either by upgrading some performances in the original specification or by comparing others not included in this specification (such as appearance or contractual arrangements). If no component satisfies the specification either a new one must be designed or the specification must be downgraded until a match is obtained. (4) The properties of the chosen component are fed back into the total design process and its cycles are repeated if necessary until acceptable matches are obtained throughout. If the original specification has been upgraded or downgraded to obtain an acceptable match, consequential changes in other specifications at the same level may, for instance, be desirable or necessary to meet the requirements originally stated at the next higher level. Alternatively, the requirements at this higher level may be revised, leading to other consequential changes. It is not suggested, of course, that this procedure should, in practice, be rigorously followed throughout every design. This would be impossibly tedious and probably professionally suicidal. It is, however, implicit already whenever a designer turns to those existing products that are familiar to him and adapts the properties he seeks to those possessed by these products. The skiiful designer will continue to select frequently on the basis of experience,
reserving a more rigorous approach for those choices where it is essential and for those properties and performances which call for special attention. He should also be able to recognise in advance those details which are most likely to be troublesome if the hierarchical sequence of choice is followed throughout. He will then give prior consideration to these details on the basis of a preliminary consideration only of the higher level requirements. Any decisions so made will then be fed back, as design constraints, into the further consideration of these levels. Only in this way, for instance, may it be possible to ensure the aesthetic coherence of a design without too much retracing of steps already taken. CONCLUSIONS Performance specifications may be considered finally from two points of view. They may be regarded first as formalizations of those parts of any design process which are concerned with identifying needs and selecting criteria for appraisal. Secondly, in the sense to which the term has generally been restricted in this paper, they may be considered as formal means for the delegation of responsibilities for design. From the first point of view, their chief practical applications are in the selection of standard products, for which they provide a rational basis, and in a more quantitative approach to those aspects of design where objective criteria are relevant and important. The latter include most of those which are the concern of building regulations and codes of practice and it is highly desirable that they should increasingly take precedence there over illustrations of details which are "deemed to satisfy". From the second point of view, their principal applications at present are in the commissioning by large clients of standard components. Here it is important that, while providing an adequate and equitable basis for choosing between alternative tenders, they should allow manufacturers and their designers the maximum freedom to deploy their own skills and knowledge in profitable innovation. Their chief value to the client is probably in enabling him to stimulate this innovation in directions which are profitable to him. Inevitably he relinquishes his positive control over the manner in which any performance requirement is met. There need, however, be no hard and fast rule about the extent to which this control should be relinquished. The pure performance specification is not obligatory even where it is feasible. What is important is that constraints on design should be introduced only with good reason. Whenever such reason exists, though, it is important that they should be stated. In analysing the place of performance specifications in the design process, the heirarchical relationships of performances and performance requirements have been stressed. In writing perfor-
Performance Parameters and Performance Specification in Architectural Design mance specifications they must be respected as follows: (1) A specification prepared at any level constitutes the brief for the design of specifications of the next lower level. (2) As the converse of this, a performance specification other than a basic design brief cannot be designed until the requirements have been established for the next higher level. (3) The design of a performance specification at any level should consider all the interrelations with other specifications belonging to the same level. (4) The specification of performance requirements
133
at any level should be restricted to what is considered to be the "whole" at that level. Present work at the BRS on performance specifications is concentrated on case studies in close association with Development Groups in central government. Through these an attempt is being made to discover difficulties which may arise --for example in setting appropriate values for particular performances and in deciding on suitable methods of evaluation--and to develop a systematic methodology for writing specifications. The need for further study of the logic and methodology of design decisions is, however, also very apparent and it is hoped soon to pay more attention to this.
REFERENCE
1. A master list of properties for building materials and Products. CIB Report No. 3, 1964, International Council for Building Research, Studies and Documentation, Rotterdam. Les sp6cifications relatives ~t la performance ont trait principalement aux r6sultats et non aux moyens d'y parvenir. Elles rendent explicites les performances requises d'un ouvrage et fournissent une base pour 6valuer objectivement ceUes qui sont r6alis6es. L'expos6 consid6re ce qui est entraln6 en rendant les besoins explicites de cette fa~on dans la conception architecturale. I1 porte une attention particuli6re ~t la description des sp6cifications relatives h la performance pour les pi6ces standard, mais il le fait scion une discussion beaucoup plus 6tendue des cycles de choix et d'6valuation qui constituent le processus de conception. On consid6re aussi l'usage de conditions portant sur la performance dans les r~glements de construction et certains aspects de standardisation qui s'y rattachent.
Leistungsspezifikationen besch/iftigen sich in erster Linie mit den Endresultaten und nicht mit den Mitteln und Wegen. Sie umreissen klar die von einem Werkzeug geforderte Leistungsf~ihigkeit und bilden eine Grundlage zur objektiven Bewertung der erzielten Leistungen. In dieser Abhandlung wird erwogen, was auf diese Art mit der Klarlegung der Erfordernisse fiJr Architekturentwiirfe verbunden ist. Besondere Aufmerksamkeit wird der Abfassung yon Leistungsspezifikationen fiir Standardbestandteile geschenkt, allerdings innerhalb des Rahmens einer viel umfassenderen Diskussion des Wahlkreises und der Bewertung, worin der Entwurfsvorgang besteht. Die Verwendung von Leistungsvorbehalten bei Bauvorschriften und einige damit verbundene Gesichtspunkte der Standardisierung werden gleichfalls in Betracht gezogen.
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721
I
Performance Parameters and Performance Specification in Architectural Design" R. J. MAINSTONE'~ L. G. BIANCOI" H. W. HARRISON#
Performance specifications are concerned primarily with ends not means. They make explicit the performances required of an artefact and provide a basis for assessing objectively those that are achieved. The paper considers what is entailed in making requirements explicit in this way in architectural design. It pays particular attention to the writing of performance specifications for standard components, but it does so within the context of a much wider discussion of the cycles of choice and appraisal which make up the process of design. The use of performance clauses in building regulations and some related aspects of standardisation are also considered.
HIERARCHIES OF CHOICE AND PERFORMANCE
INTRODUCTION T H E D E S I G N of a building is never merely a doodle. All buildings, with the possible but questionable exception of the folly, serve a purpose, and it has always been part of the architect's professional skill to identify this purpose and to allow it to trigger off and discipline his creative work. Looked at in its simplest terms, a specification of performance requirements merely makes the purpose to be served or the needs to be met explicit in ways that do not unnecessarily restrict the designer's response to them. It may do so at any stage in the design process. It may, on the one hand, constitute a precise functional brief for a building or group of buildings or, on the other, define the performance requirements of a single component or material. It may be used for commissioning new products, for the assessment or selection of existing ones, or for regulation or standardization. In considering the writing of such specifications and their varied uses, this paper draws particularly on experience acquired at the Building Research Station over the past year and a half in advising on the drafting of a number of performance specifications for particular types of component and in monitoring the use of others. It also draws on wider and longer standing interests in setting performance standards, in devising appropriate tests, and in the mechanisms of the whole architectural design process.
The architectural design process can be considered from the present point of view as the fitting of a physical form to a complex pattern of human objectives, activities and requirements. This necessarily involves identifying the requirements, devising the form and finally appraising the "fit". The emphasis throughout the paper will be on the first of these activities, but it has to be recognised at the outset that the closeness of a "fit" can be judged objectively only when like is compared with like. This has an important bearing on the manner in which requirements can most usefully be stated. Consider, for instance, the design of a school. The building of this school will be one step in furthering an educational policy. It is, however, meaningless to speak of it fitting directly the ultimate objective of educating the young except in a purely arbitrary manner. Before the "fit" can be objectively appraised it is necessary to decompose the ultimate objective progressively into sub-objectives and to define appropriate activities conducive to the achievement of each and the requirements that these activities generate. At one level the activities might be a broad curriculum of training and study. At another they might include anything from writing or reading to swimming or making music. They will generate requirements ranging from supports on which to write to comfortable temperatures for physical relaxation or exertion, enclosed spaces, intercommunication between these, and freedom from excessive transmission of sound from one to
* Crown Copyright Reserved. Published by permission of the Director. t Building Research Station, Garston, Watford, Herts. 125
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
another. It is these, and only these, primary requirements which can be directly matched by the performance achieved or expected to be achieved by the design, though, for purposes of detailed design, some will have to be broken down further into secondary requirements relevant to each of the individual means selected by the designer to meet them. To achieve an overall "fit" it is necessary to meet each primary requirement. To do this it is necessary, in turn, to meet the secondary ones generated, in part, by the designer's decisions on how he will meet the primary ones. Figure 1 is a simple representation of the total design process seen in this way. Without the enclosing bars the right-hand part represents the characteristic single cycle of identifying a requirement, choosing the means to satisfy it and Technology ] Object ires'---'Activities ~ "'t Perf°rmonce ~- - J - 4,-4Specifications of requlrements~ ~ / f o r m .material land method of Performance ~ construction characteristicsl 1 J Science, experiment experience -
- -
--
- - ~ -
Choice Appraisal
Fig. I. The total design process.
appraising the choice. This representation of it is untypical of much present practice only to the extent that it introduces an explicit statement of the performance requirements in place of a tacit recognition, and an equally explicit statement of the performance achieved or expected to be achieved as a basis for the appraisal. Technology limits the range of reasonable choice and science, experience or experiment furnish the means for predicting or otherwise determining the performance achieved. The enclosing bars denote that the total process is concerned with complex systems of objectives, activities, requirements and performances and with many individual cycles of choice and appraisal. Because, in these complex systems, the individual requirements and performances are interrelated in various ways, the individual cycles of choice and appraisal are also interrelated. The arrows in the right-hand loop should therefore be read as denoting not merely multiple relationships of the kind that typify the single cycle, but complex systems of these relationships. It is in these interrelationships that much of the difficulty of understanding the logical structure of the design process lies. Fortunately, for the present purpose, it is unnecessary to consider all aspects. Nor is it intended to prejudice further study of them. It is necessary, though, to look a little further at the systems of requirements and performances and to touch more briefly on the ways in which these are related, by the designer's choices and by physical laws, to the specifications of form which
constitute the design. Even here, to simplify the discussion, the emphasis will be on satisfying individual primary requirements of the user--for a comfortable air temperature or for a certain rate o f air change for instance--and not on any interdependence of or interference between these requirements. These are subjects calling for considerable further study. The performances which are achieved by a particular design are clearly related in hierarchical sets which directly reflect the manner in which smaller units of construction are combined to form larger ones of recognisably different character from any one of them. In each set the performances at any level (corresponding to a particular level of physical organisation of the fabric and/or services of the building) are determined by those at the next lower level and by the manner in which these interact. The acoustical properties of a room, for instance, are determined by the relevant properties of the elements making up the spatial envelope, by those of their interconnections and by the geometry of the room. The characteristic strengths and stiffnesses of a floor assembled from precast components are similarly determined by the strengths and stiffnesses of these components and of their interconnections and by the geometry of the floor. Typical hierarchies of thermal and structural performances are compared in figure 2. They are described only as typical because they relate only to particular constructional means of meeting the primary requirements. For some types of structure. for instance, the hierarchy of performances might Spatial envelope + external environment+ heat source (air temperature )
t
Spatial envelope + external environment ' q j (net rate of heat loss and solar heatgain) ]
{
Heat source ] rate of heat input ]
t
Element of spatial envelope eg wall | (thermal admittance end transmittance) J
t
Component of spatial envelope e g wollunit or oaf (thermal admittance and conductance Material of spatial envelope (density, specific heat, thermal conductivity)
Whole structural system ( strength~, stiffness I )
]
Structural sub-system-e.g floor system,vertical frame, lift shaft ( strength2, sti.ffness2 , weight ) J Structural element-eg beam, slab, column, shear wall (strength~, stiffness~, weight)
I
J
[strength4, stiffness4, weight/unit length) I
Structural material (unit strength, elasticity, density)
]
Fig. 2. Typical hierarchies of thermal performance (A) and
structural performance (B).
Performance Parameters and Performance Specification in Architectural Design include only the performances of materials, of elements and of the whole system. They are, nevertheless, unique hierarchies for the means envisaged. The levels are determined by these means and are, therefore, most easily denoted by them, but appropriate measures of performance are indicated in parenthesis. Four things in particular should be noted: (1) Each level in a hierarchy is usually distinguished by different appropriate measures of performance, though the measures may, as in the structural hierarchy, be of the same kind at different levels. The measures of strength and stiffness distinguished by the suffixes 1, 2, 3 and 4 will differ only in that they will be expressed in terms of different patterns of load and different types of deflection. (2) Hierarchies do not necessarily terminate in a performance characteristic of the whole building, though the level at which they do terminate is partly a matter of the precise definition of their scope. Primary requirements for the thermal environment, for instance, will, taken together, relate to the whole building. Individually, however, they relate to the spaces within it considered singly, and it seems preferable to terminate the hierarchy here. This means that the air temperature requirement in a room adjacent to the one considered is treated as part of the external environment of this room. (3) Hierarchies may be linear throughout or may branch as in the case of the example for the thermal environment. Only the branch corresponding to the spatial envelope is shown in full, but, to reach the highest level, it is necessary also to consider the heat input from the heating system which is the highest-level performance of another branch. (4) Though, in themselves, the different hierarchies are independent of one another, performances belonging to different hierarchies may be properties of the same physical unit of construction. There is, for instance, no direct dependence of thermal properties on structural strength and stiffness or vice versa. Both may, nevertheless, be properties of the same unit of walling. This is represented symbolically in figure 3 which is an expansion of the lower part of the loop at the right of figure 1. The suffixes a, b, c . . . . denote different hierarchies of performance (thermal, structural etc.) and the suffixes I, 2, 3. . . . denote different hierarchical levels. Gaps have been left in the numbered sequence of levels in some hierarchies to recognise Po~
P,,z
%~
-..
............
FS I
P~z . . . . . . . .
po~ p~ Po3~-
. . . . . .
Don
Pi~
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FSn.I
........
Fig. 3. Expansion o f the lower part o f the loop in figure 1. P = single performance characteristic. F S -~ specification of form, material etc.
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the fact that there is not necessarily a direct one-toone correspondence in all cases. In the examples given in figure 2 of thermal and structural performance hierarchies, for instance, a level corresponding to the whole building is missing from the first and a level corresponding to the single space is missing from the second. If the performance requirements are to be capable of realisation in practice, it must be possible to relate them to one another in similar hierarchical sets. Assuming that the building to which figure 1 relates perfectly meets all the requirements, it must be possible to expand the upper part of the loop at the right in exactly the same way, as shown in figure 4, as the lower part. To each achieved performance characteristic will correspond directly a performance requirement, and these performance requirements will be related to one another and to the chosen forms in the same ways as the performance characteristics are. If the building does not perfectly meet the requirements figure 4 will still correctly represent the necessary pattern of hierarchical relationships of the requirements PRol
PRbl
...........
PRo2
....
PRc2 . . . . . .
PRo3
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t ..............
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Fig. 4. Expansion o f the upper part o f the loop in figure 1. PR = single performance requirement. FS = specification o f form, material etc.
because the lower part of figure 1 and its expansion in figure 3 can equally be read as relating to a possible building which does meet them. Two important differences between the hiera rchical relationships of performances and of performance requirements and between the relationships linking both to the chosen forms are indicated by the opposite directions of the arrows in figures 3 and 4. The horizontal arrows in figure 3 denote the dependence of the achieved performances on the chosen forms. Those in figure 4 denote choices by the designer to satisfy particular groups of requirements by means of particular forms. The grouping of different requirements belonging to the same hierarchical level is, in other words, a design choice in itself, whereas the common dependence of a group of achieved performances on a single form is a consequence of such a choice. The downward directed arrow in figure 4 denotes a logical sequence of the choices typified by those represented by the horizontal arrows, assuming that in general the requirements that are of direct concern to the client or user are the higher level ones. It is described as a logical sequence because it is essentially a hierarchy of the choices that are finally effective. Design, in practice, is rarely a
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
simple unbroken linear process so that the actual sequence is likely to have a number of superimposed loops. The upward directed arrow in figure 3 denotes, on the other hand, a simple physical dependence stemming from the previous choices of forms in the manner illustrated in detail in figure 2. In what follows the emphasis is on the specification of performance requirements. The chief implications for this of what has been said so far are that all requirements should be stated in terms of performances at the appropriate hierarchical level and that before this can be done at the lower levels it will usually be necessary to make explicit design choices about the ways in which higher level requirements in the same hierarchies are to be broken down. BRIEFS FOR DESIGN Explicit statements of performance requirements may be used both to initiate a design process and at intermediate stages within the process as attention is progressively focussed on more detailed aspects. The term performance specification will be reserved for the statement that serves to initiate a process, though it may do so at any level in the hierarchies .just considered. The traditional responsibility of a single designer for the entire design of a building including all its details is becoming a thing of the past and the most important use for performance specifications at present is probably in the commissioning of new components. The pure performance specification may be defined as one which is stated solely in performance terms. It is concerned solely with the functions to be served rather than with the means whereby they are to be served. These functions, in general, will be both geometrical and non-geometrical. The former will be concerned with ability to occupy, span or enclose a defined space rather than with the precise form of the product and the latter with all other requirements. A pure performance specification, sufficiently comprehensive to ensure that any product which meets it in all respects will fully satisfy the client or user, is often, however, an unattainable ideal at present. This is particularly true at the higher levels such as that of the complete building, because higher level objectives and their associated requirements are, in general, less materialistic and less readily defined in unequivocal terms than lower level ones. Clients and users buildings are, moreover, rarely skilled in and frequently quite incapable of expressing their needs in purely functional terms. At best a skilled professional may be able to assist them in doing so, but he will often be at a disadvantage because he lacks direct experience of similar needs. Even at lower levels, such as that of the component, some requirements may still be difficult to define precisely in performance terms and an imperfect understanding of the technical possibilities may lead to difficulties of another kind.
In practice, therefore, performance specifications will usually be impure according to the present definition or incomplete or both. This, in itself, need not be a fault. It is assumed that the prime objective of any design commission is to draw on the designer's special skills in meeting the client's needs and that the prime purpose of any design brief is to do so as effectively as possible, within the available budget. What this entails will vary with the nature of the commission. On the one hand the client may wish to limit the choice of means whereby the functions are served, by specifying, for instance, the use of materials or other products in which he has a proprietary interest. On the other he may wish to leave certain choices completely open. In a performance specification to serve as a basis for competitive tendering for the supply of a new component, the objective may, with respect to some performances, be simply to get the best that manufacturers can offer at an acceptable price. It may then be sufficient to state that these pertk~rmances will be considered in choosing between tenders with, preferably, some indication of the basis of choice. In the brief for a building the aim may, in a rather similar way, be to give an architect chosen on the basis of his past work a fairly free rein in both identifying and satisfying the client's needs. This is in effect putting the onus of completing the specification on the architect himself and asking him (to paraphrase Denis Lasdun) "to give the client something he did not know he wanted until he gets it". The best design has probably always been the product of a creative dialogue between client and designer, and the deliberately incomplete performance specification may be the best starting point for this. All such impure or incomplete performance specifications may still be referred to as performance specifications, but they will then be so described primarily in terms of their intention rather than their form. The intention of a performance specification is always a delegation of responsibility for further design which allows the designer the maximum freedom in deploying his own experience and skills in meeting the client's requirements. The difficulties that arise should be considered in the light of this intention. Among them are costs, the ranking or weighting of requirements, and the specification of requirements that it is not possible to define precisely in performance terms. One objective of any delegation of responsibility for design, and hence of any performance specification, is to obtain good value for the available money. The specification might therefore be looked upon as a definition of what is considered to be "good value". The difficulty is that it cannot be known, even in performance terms, what this really is until the design or several designs have been completed. Clearly there is no point in dressing up a preconceived design as a performance specification. Some reference to the performances achieved in existing designs and to their costs is, nevertheless,
Performance Parameters and Performance Specification in Architectural Design the only basis for setting the values of the performances asked for reasonably. Usually the aim will be to stimulate some improvement on existing designs and the problem, in other words, is to know how much improvement is possible. In practice the problem can be solved only by a certain looseness in the specification. The form that it can best take will depend on the nature of the commission and the manner in which responsibility for design is delegated. The more possibility there is of a creative dialogue, for instance, the more readily can performances be upgraded or downgraded in the course of design and the less need there is therefore to cater for all conceivable possibilities in advance. Where the specification is to serve as a basis for competitive tender, on the other hand, it should be as unequivocal as possible from the outset. This will usually mean, in effect, some sort of ranking of performances and weighting of values. Ranges of values might, for instance, be specified for some performance requirements with indications of the value that the client would place on achieving more than the minimum. Other performances might be left unquantified with an indication merely of the sort of weight that the client will place on them in comparing alternative tenders. The problems presented by requirements that cannot be stated precisely in performance terms are rather similar. Again, where there is a possibility of a creative dialogue, they need not be very troublesome. Where there is no such possibility it may be necessary either to illustrate the sort of solution that would be acceptable or to indicate in some similar manner the limits between which an acceptable solution should lie. Some attempt, at least, should be made to indicate how designs will be assessed and compared. C O N T R O L S ON DESIGN Building regulations, departmental circulars, some BSI codes of practice and some recommendations of other national bodies may be regarded as special and incomplete briefs for design. Their purpose is primarily to safeguard the generally recognised interests of the individual user and those of society as a whole by restricting the freedom of action of clients and designers. It is desirable though that they should be as little restrictive as possible consistent with serving this purpose. Originally most of these controls, though incomplete design briefs, were almost wholly restrictive in those aspects of design which they did cover. They were complete specifications of minimum acceptable forms of construction. Today such specifications frequently survive as illustrations of acceptable interpretations of imprecisely defined or even of precisely defined minimum performance standards. The forms illustrated are "deemed to satisfy" the requirements. They make design easy but they do not encourage fruitful innovation. They may easily result in serving the client or user less well or less economically than might have been possible.
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Controls framed in terms of minimum standards of acceptable performance do allow the maximum freedom in design consistent with safeguarding society's interest in such matters as safety and health. There seems to be no question, therefore, that every effort should be made to extend their use as widely as possible. The only questions that do arise relate to the choice of level in each hierarchy of performance at which to frame the requirement and to the choice of the values to be specified and the manner of their specification. The lower the level of a performance requirement, the more any decision about it necessarily implies prior decisions about the manner in which higher level requirements are to be met, as has just been shown. Since it has been suggested that controls should be as little restrictive as possible, it follows that, ceteris paribus, requirements should be specified as nearly as possible at the levels where they are socially most important. At the same time it has to be remembered that mandatory requirements are of little use unless they can be enforced. They must therefore be unequivocal and so framed that compliance can be readily checked when plans are submitted for approval, and this is likely to favour specification in terms of lower-level performances. The ideal form of control is possibly best exemplified at present by the current revisions of the main structural codes of practice. These revisions, and previous development of the codes leading up to them, have been possible, however, only on the basis of the prior development of adequate and workable methods for the prediction of performances in the course of design. Before all controls can take the same form more effort needs also to be devoted to developing analogous methods for the prediction of other performances. It should also be remembered that there may be valid social reasons for influencing the manner in which some higher level requirements are met. These may range from a national need to encourage some forms of construction at the expense of others to wider economic considerations which make it desirable to strike particular balances between expenditure on thermal insulation and on fuel or between expenditure on making buildings fire-resistant and on firefighting and rescue services. Such considerations are not, however, immutable. If they are allowed to influence the specifications of performance requirements at lower levels without being explicitly recognised there is a danger that they will be forgotten when a need for revision arises. The difficulties of specifying values for particular performances reasonably are similar to those that arise in specifying values for a complete design brief. The main difference is that acceptability and costs must always be considered on a much wider scale than that of the single project, this scale being determined by the-intended range of application of the control whether international, 'national or regional,,~ Acceptabilityis always largely a matter of past experience and ~Off-expectation based on that
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
experience. Cost, a little less directly, is also a function of past and present practice, though one which is susceptible of change with developments in technology. The normal standard product or something similar to it is always, ipsofacto, cheaper than any marked deviation from it, unless this situation is radically changed by a technological breakthrough. Both acceptability and cost are therefore strongly influenced by what is generally specified and by what has commonly been specified hitherto. Indirect controls designed to encourage technical innovation without exerting a positive influence on its direction will naturally emphasize this aspect of acceptability in setting values for performances, as did the certification of new products for insurance purposes by the original French Agr6ment procedure. There may be a natural tendency, for instance, to assess the fire resistance of a floor within a dwelling largely in relation to that of the common low-cost timber joisted floor. More direct controls, such as building regulations, departmental circulars and specified codes of practice, must also recognise it, but they can more easily and effectively upgrade performances when this seems to be desirable. Since also they are not tied to specifying performances in terms of particular types of product, they can do so at any level in the relevant hierarchies. In drafting such controls it is important therefore to bear in mind the more effective encouragement that can be given to technical innovation by specifying performances at those higher levels which are of direct concern to the user and less tied to current techniques of construction. In specifying values, however the balance between performance and the cost of achieving it is struck, there is frequently also a choice between doing so for the " n o r m a l " condition of use or for a "limit" state in which the product ceases to function adequately. There is a choice, in other words, between legislating for a comfortable margin in performing acceptably under average conditions or for an acceptably high limit of failure under extreme conditions. Where performance is a linear function of the relevant variables (such as external temperature or imposed load) throughout their range, it does not really matter whether the average or the extreme condition is considered. Where, on the other hand, it is a non-linear function, the choice will be significant. Careful consideration should then be given to the alternatives bearing in mind their implications for the definition of appropriate measures for the performance and the possibilities of prediction and test. P E R F O R M A N C E SPECIFICATIONS FOR STANDARD PRODUCTS The performance specifications considered here are a special case of the brief for design. Like other design briefs they are a means to an end--in this case standard product specifications in terms of
particular forms, materials and methods or construction or manufacture. Standardisation has as its primary aim the achievement of economy. It may also have connotations of "maintaining standards", but, for the present purpose, this aspect has already been adequately covered in the previous section and it will not be considered further. Economy may be aimed at in design time, in production, in stock holding and distribution, in erection or in subsequent maintenance or in most or all of these. It will be achieved, however, only at the cost of a reduced variety of choice in meeting particular requirements. If, in general, a standard product is to meet a variety of particular requirements it must, in each of its relevant performances, meet the most onerous. In relation to the average, therefore, an extra cost will be incurred which must be offset against the economies achieved. An attempt must be made to strike a favourable balance between the savings and the costs, both by a correct choice of products to be produced in quantity to a standard design and by correct specification of their performance requirements. Writing a performance specification to be used for commissioning a standard product or range of products calls, therefore, for all choices to be made in a much wider context than that of the product destined for a single use only. It is necessary throughout to consider the intended variety of use in relation to both geometric and non-geometric performances. With units as small as the brick or as large as the dwelling there is relatively little need for co-ordination with other products. For units of intermediate size intended to be used alongside other standard units of similar size with some measure of interchangeability, co-ordination of geometrical performances becomes essential. The extent to which it is achieved will place an upper limit on the potential market for any particular unit. Co-ordination of non-geometrical performances is mostly less essential because of the different manner in which these interact, It is also potentially undesirable to the extent that it is restrictive, because it may stand in the way of technical advances. As a basis for present attempts to develop component building on a national scale, some national co-ordination of geometrical performances (including some aspects of jointing and tolerances) is therefore desirable, but a strictly analogous (and therefore similarly restrictive) national co-ordination and standardization of other performance requirements seems less desirable. At present it seems preferable, in considering for this purpose the more important of these other requirements, to aim merely at agreed methods of test and perhaps standard grades of performance. Any co-ordination must take as its starting point the client and user requirements at the highest levels and it must involve certain design choices about how these are to be met. These choices will then be .part of the framework within which
Performance Parameters and Performance Specification in Architectural Design particular specifications are written and they will, in part, ensure that the wider context is taken into account. In illustrating in more detail the approach that is recommended for arriving at a detailed specification it will be assumed that the specification is to be written for a component. It has to be admitted that there is, as yet, little rational basis for choosing to standardize components rather than elements, whole rooms or whole buildings. Hitherto there has been insufficient study either of the economics of scale of production in relation to the whole building process or of user requirements and their significant variations and probable changes over the lifetime of a building. The assumption is made chiefly in deference to current interest in components and in recognition of the fact that they belong more to the English tradition of building than, for instance, the standard dwelling. The choice of what should constitute a component nevertheless remains. In practice it will usually be made initially on the basis of a simple extrapolation of recent practice and writing of the specification will start on this basis. It may however have to be questioned when subsequently weighing the costs of overprovision in some respect to meet extremes in the ranges of performances against the economies expected to accrue from standardization. The economical solution might be to redefine the original function so that it is performed by ranges of two or more smaller components used additively in various combinations. A performance specification for a standard component should usually be as comprehensive as possible, even to the extent of calling explicitly for properties that are normally taken for granted, if it is to be used as a basis for competitive tendering. It is therefore highly desirable that it should be drafted as systematically as possible. The following procedure is recommended: (1) Prepare a list of all relevant properties. (2) Attach a degree of importance to each of these, bearing in mind the whole range of intended use of the component. Some imprecision is inevitable here but it has been found useful in recent work at BRS to distinguish between (a) properties which are essential, (b) properties which are reasonably important but not absolutely essential, and (c) properties which are relevant but normally of m i n o r significance. (3) Decide, at the next higher level above that of the element in each hierarchy of performance, what requirements are to be met, again bearing in mind the whole range of intended use of the component and if necessary tabulating ranges of performances corresponding to different uses. The need for the last might arise, for instance, when preparing a performance specification for a range o f windows to be used under different conditions o f exposure. (4) Translate these requirements into requirements at the element and component levels,
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making the necessary decisions, in general terms, about the ways in which the higher level requirements are envisaged as being met. This again calls for consideration of the range of intended use and for an attempt to forsee the sort of decisions that it would be reasonable to make in designing a variety of future buildings. (5) Select the values to be specified for all those properties which can be quantified. Indicate which are obligatory and which are, to some extent discretionary. With the latter indicate also the basis that will be adopted in assessing tenders, perhaps by suggesting different cost limits for different performances. (6) Indicate criteria for the assessment of properties which cannot be quantified and any constraints on design. The latter are any requirements, like the specification of the use of a particular material or finish, which are stated not in terms of performances but in terms appropriate to the final specification of the product. The distinction between geometrical constraints and geometrical performance requirements is not a sharp one, but it is normally convenient to regard as performance requirements all specifications of the space to be occupied or spanned or enclosed and as constraints only those geometrical characteristics which uniquely determine the final geometric form of the product. The difference again is inherent partly in the intention. (7) In parallel with 1, 3, 4 and 5, attention must be given to appropriate measures for the performances. As part of 5 it is necessary also to decide and to define how the properties achieved are to be evaluated. This may be by calculation, by inspection of the final specification or of a prototype, or by physical test in a prescribed manner. The number of independent checks required can often be reduced by an appropriate choice of the measures to be adopted. National Standards and Codes of Practice and Agr6ment Methods of Test may provide guidance here. A corollary of this approach to design is an equally systematic declaration of the properties of standard products offered either in response to specific commissions or on the open market. This is desirable both for the initial assessment of tenders and for the subsequent selection of components for particular uses. The CIB Master List of Properties of Materials and Products (1) was devised as a comprehensive general framework for the latter purpose. Derived lists are now being prepared of the properties relevant to particular classes of component. These provide a useful starting point for this declaration. They have also been used as an initial guide in listing the properties to be included in performance specifications. Not having been conceived initially with this use in mind, they have, however, some deficiencies here. It therefore seems desirable that revised lists should now be prepared giving performance
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R. J. Mainstone, L. G. Bianco and H. W. Harrison
requirements and properties in parallel columns. Different lists will be required for different hierarchical levels of performance. It is debatable whether details of, for instance, materials should be listed as properties, but they might usefully be listed in separate parallel columns of design constraints and design details. Among the properties there should, however, be provision for the declaration of details of recommended applications of the products. The use of such declarations of properties in selecting products for use in normal design is considered briefly in the next section. MECHANISMS OF CHOICE This section considers in more detail some of the design choices represented symbolically in figure 4. It assumes that many or most of the choices at the lower hierarchical levels are selections from "catalogues" of standard components and that the properties of these components have been systematically listed in the manner just described. A typical sequence of decisions might be then stated formally as follows: (1) Assumptions are made about the manner in which the stated performance requirements at the next higher level (usually of the element or space) are to be met by assembling a number of components. For each component a performance specification is then prepared. (It is convenient here to use this term less restrictively to denote any explicit statement of performance requirements). (2) The listed properties of available components are assembled for comparison. (3) If one component only fully satisfies the specification it is chosen. If several components satisfy it a choice is made either by upgrading some performances in the original specification or by comparing others not included in this specification (such as appearance or contractual arrangements). If no component satisfies the specification either a new one must be designed or the specification must be downgraded until a match is obtained. (4) The properties of the chosen component are fed back into the total design process and its cycles are repeated if necessary until acceptable matches are obtained throughout. If the original specification has been upgraded or downgraded to obtain an acceptable match, consequential changes in other specifications at the same level may, for instance, be desirable or necessary to meet the requirements originally stated at the next higher level. Alternatively, the requirements at this higher level may be revised, leading to other consequential changes. It is not suggested, of course, that this procedure should, in practice, be rigorously followed throughout every design. This would be impossibly tedious and probably professionally suicidal. It is, however, implicit already whenever a designer turns to those existing products that are familiar to him and adapts the properties he seeks to those possessed by these products. The skiiful designer will continue to select frequently on the basis of experience,
reserving a more rigorous approach for those choices where it is essential and for those properties and performances which call for special attention. He should also be able to recognise in advance those details which are most likely to be troublesome if the hierarchical sequence of choice is followed throughout. He will then give prior consideration to these details on the basis of a preliminary consideration only of the higher level requirements. Any decisions so made will then be fed back, as design constraints, into the further consideration of these levels. Only in this way, for instance, may it be possible to ensure the aesthetic coherence of a design without too much retracing of steps already taken. CONCLUSIONS Performance specifications may be considered finally from two points of view. They may be regarded first as formalizations of those parts of any design process which are concerned with identifying needs and selecting criteria for appraisal. Secondly, in the sense to which the term has generally been restricted in this paper, they may be considered as formal means for the delegation of responsibilities for design. From the first point of view, their chief practical applications are in the selection of standard products, for which they provide a rational basis, and in a more quantitative approach to those aspects of design where objective criteria are relevant and important. The latter include most of those which are the concern of building regulations and codes of practice and it is highly desirable that they should increasingly take precedence there over illustrations of details which are "deemed to satisfy". From the second point of view, their principal applications at present are in the commissioning by large clients of standard components. Here it is important that, while providing an adequate and equitable basis for choosing between alternative tenders, they should allow manufacturers and their designers the maximum freedom to deploy their own skills and knowledge in profitable innovation. Their chief value to the client is probably in enabling him to stimulate this innovation in directions which are profitable to him. Inevitably he relinquishes his positive control over the manner in which any performance requirement is met. There need, however, be no hard and fast rule about the extent to which this control should be relinquished. The pure performance specification is not obligatory even where it is feasible. What is important is that constraints on design should be introduced only with good reason. Whenever such reason exists, though, it is important that they should be stated. In analysing the place of performance specifications in the design process, the heirarchical relationships of performances and performance requirements have been stressed. In writing perfor-
Performance Parameters and Performance Specification in Architectural Design mance specifications they must be respected as follows: (1) A specification prepared at any level constitutes the brief for the design of specifications of the next lower level. (2) As the converse of this, a performance specification other than a basic design brief cannot be designed until the requirements have been established for the next higher level. (3) The design of a performance specification at any level should consider all the interrelations with other specifications belonging to the same level. (4) The specification of performance requirements
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at any level should be restricted to what is considered to be the "whole" at that level. Present work at the BRS on performance specifications is concentrated on case studies in close association with Development Groups in central government. Through these an attempt is being made to discover difficulties which may arise --for example in setting appropriate values for particular performances and in deciding on suitable methods of evaluation--and to develop a systematic methodology for writing specifications. The need for further study of the logic and methodology of design decisions is, however, also very apparent and it is hoped soon to pay more attention to this.
REFERENCE
1. A master list of properties for building materials and Products. CIB Report No. 3, 1964, International Council for Building Research, Studies and Documentation, Rotterdam. Les sp6cifications relatives ~t la performance ont trait principalement aux r6sultats et non aux moyens d'y parvenir. Elles rendent explicites les performances requises d'un ouvrage et fournissent une base pour 6valuer objectivement ceUes qui sont r6alis6es. L'expos6 consid6re ce qui est entraln6 en rendant les besoins explicites de cette fa~on dans la conception architecturale. I1 porte une attention particuli6re ~t la description des sp6cifications relatives h la performance pour les pi6ces standard, mais il le fait scion une discussion beaucoup plus 6tendue des cycles de choix et d'6valuation qui constituent le processus de conception. On consid6re aussi l'usage de conditions portant sur la performance dans les r~glements de construction et certains aspects de standardisation qui s'y rattachent.
Leistungsspezifikationen besch/iftigen sich in erster Linie mit den Endresultaten und nicht mit den Mitteln und Wegen. Sie umreissen klar die von einem Werkzeug geforderte Leistungsf~ihigkeit und bilden eine Grundlage zur objektiven Bewertung der erzielten Leistungen. In dieser Abhandlung wird erwogen, was auf diese Art mit der Klarlegung der Erfordernisse fiJr Architekturentwiirfe verbunden ist. Besondere Aufmerksamkeit wird der Abfassung yon Leistungsspezifikationen fiir Standardbestandteile geschenkt, allerdings innerhalb des Rahmens einer viel umfassenderen Diskussion des Wahlkreises und der Bewertung, worin der Entwurfsvorgang besteht. Die Verwendung von Leistungsvorbehalten bei Bauvorschriften und einige damit verbundene Gesichtspunkte der Standardisierung werden gleichfalls in Betracht gezogen.