Structural Safety 21 (1999) 349±356
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What researchers do and what practitioners need J.B. Menzies* 42 Sheepcot Lane, Garston, Watford, WD2 6DT, UK
Abstract Progress in the development of engineering risk analysis has been substantial over the past 20 years. The present state of analytical reliability techniques provides a powerful starting point for considering uncertainties in structural engineering. Researchers have gained considerable understanding of uncertainties in the design of structures and systems. However the techniques only address some of the uncertainties which are of interest to the practising engineer and which are crucial to structural safety. Substantial diculties have to be overcome before the needs of practising engineers for dealing with uncertainties in decision making can be met fully by risk-based structural safety analyses. The diculties relate mainly to modelling, human error and trends in construction practice. This paper reviews hazards and risks in structural engineering and the fundamental engineering approaches to reducing risk. The nature of research in this area is described and the main problems in further development of risk-based analysis identi®ed. A wide gap is found between what researchers do and what practitioners need. The role of other mechanisms in meeting practitioners needs is brie¯y discussed. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The task of practitioners of structural engineering is to synthesise a solution which meets their clients requirements. Not only must a structure be designed or assessed to be safe but it must meet functional, performance and environmental requirements and be delivered at an acceptable cost. Uncertainties abound in the engineering and in all the activities associated with it. The engineer however must progress his task. Action is required based on predictions followed by decisions taken despite uncertainty. This is the essence of engineering. The researcher in structural engineering, on the other hand, is concerned with observation and modelling of the physical world and the processes associated with it and with human endeavours to shape that world. The task of the researcher is primarily to observe, understand and describe
* Tel.: +44-(0)1923-675-106; fax: +44-(0)1923-680-695. E-mail address:
[email protected] (J.B. Menzies) 0167-4730/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0167-4730(99)00029-6
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physical phenomena and interactions and to provide information and tools of prediction and management for the practitioner. For this purpose the researcher must be focussed and speci®c in order to understand. The role of the researcher is essentially to support the engineer in his task. The needs of practitioners and the researchers role in the area of structural safety and riskbased structural safety analysis are discussed below against this background of the essential nature of engineering and research. For the purposes of this paper, risk-based structural safety analysis is de®ned as the combination of statistical distributions of loadings and resistances of structures to enable engineering decisions on structural design or assessment based on criteria of risk of failure. 2. Hazards in structural engineering The hazards that threaten the safety of structures are many and varied. Engineers are generally keenly aware of the associated risks and spend much of their time considering how to control them to an acceptable degree. Some of the principal hazards involved in the construction and use of a structure are: . . . . . .
design inadequate to provide the required structural performance over time gross error in the structural design materials of construction and structural components not up to speci®cation error in construction of structure collapse during construction unforeseen combinations of hazards or circumstances in service jeopardising the safety or serviceability of the structure . unforeseen or more rapid deterioration in service than expected. The practising engineer has therefore to consider all of these possibilities and, by means of design and/or provisions for control of use and maintenance decide and specify what is to be built and how it is to be maintained. For this purpose, it is necessary to view the structure, its use and the environment in which it operates as a whole, i.e. as a system. The approach is essentially to use conservatism. The aim is to predict and reduce the risks associated with the hazards as far as is practical. Traditionally in structural engineering this has been done largely implicitly, but more recently engineers have begun to examine hazards and risks in a more systematic and explicit way. Risk-based structural safety analyses have been, and continue to be, developed to assist in the control of risks to structural safety. 3. Reducing risks Prediction is fundamental to the practice of structural engineering. Good structural engineering includes, where possible, artful decisions to avoid a predicted hazard or reduce a recognised risk. The location of a structure at some distance from the highway or the use of protection to supporting
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columns may be decided to avoid the hazard of vehicle collision. Where variable or poor ground is found near to the surface, a structure may be founded on piles to avoid the uncertainties of the upper ground strata. Such decisions associated with the structure/environment system exclude having to consider situations determined by the probabilities of extreme values, ie essentially the hazard is avoided. They are risk based in the sense that the engineer recognises and avoids a signi®cant but unquanti®ed risk to the structure. Engineering decisions relating to the overall structural concept have a major in¯uence on reducing risks to structural safety. Perhaps the most important are those decisions which lead to redundancy in a structure. Risks of collapse in service are reduced by ensuring as far as possible that the stability and load-carrying capacity of a structure is not dependent on a single structural element or connection. The use of continuity and interconnection between elements provides capability for load sharing between elements. Alternative load paths are thereby created. Weaker elements in such structures can shed load and the load is then successfully carried to the ground. Structural redundancy through continuity and interconnection between elements may be provided by deliberate design decision or incidentally. Examples of the latter case are the bene®cial eects of masonry walls in traditional steel-framed building structures and of masonry jack arches in nineteenth century bridges. Design decisions deliberately recognising qualitatively the bene®cial contributions from secondary eects have resulted in a large proportion of structures built in the ®rst half of the twentieth century being stier and stronger than predicted by structural calculations. The prime reason has been the simpli®cation of the modelling of structural behaviour necessitated by the limited capability to complete calculations in an economic way. Perhaps the most common examples have been assumptions in calculation that all beams are simply supported and that elements are pin connected. The practitioners who created the designs knew very well these were assumptions and that the eects of moment capacity at supports and connections, although not quanti®ed, would be bene®cial both in terms of serviceability and ability to resist overload and accident. The advent of computers in structural engineering has enabled more complex calculations to be carried out and more design options to be examined routinely. As a result, engineers are now able to take redundancy and secondary eects into account quantitatively and directly. The ability to undertake complex structural calculations has increased the structural engineer's con®dence, while at the same time created a dependence on the validity of such calculations. Consequently there appears to be a trend for designers to produce structures with less redundancy than previously. For any given structure there is no established basis for determining how much redundancy is required to maintain the risks of unserviceability or collapse in service at an acceptably low level. Hence the engineer may be tempted to reduce redundancy too far in pursuit of economy of construction by using re®ned and detailed calculations to validate a design. The use of redundant forms of structure is not appropriate, and does not need to be insisted upon, in all circumstances. For example, where the ground is likely to be subject to mining subsidence, the engineer in designing a bridge may decide to use simple spans. This form of structure, if appropriately designed, can accommodate a degree of dierential vertical movement between supports. Overstressing which would occur with a continuous deck structure is thereby avoided. At the same time, should subsidence become unexpectedly large, damage to the bridge is likely to be more limited than for a continuous structure. There is little systematic research in this crucially important area.
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Where structures are determinate, the engineering design is usually, but not always, made so that the critical structural elements(s) have a higher margin of safety against failure and damage, e.g. suspension bridge cables. Essentially redundancy and toughness are built into the critical element. Similar considerations are traditionally applied to connections between structural elements. For example in structural steelwork, dependence on single bolts is avoided and several bolts are always used in tension connections. The use of such forms has the eect of reducing risks of failure. In these ways risks of failure are kept very low and are usually maintained at a low level by the inspection regime for the structure. Inspection of structures in service is a powerful in¯uence in the structural/environment system for reducing risks of structural failure. 4. What practitioners need The foregoing discussion has indicated that prediction of the behaviour of the structure/environment system is fundamental to successful structural engineering practice. The practitioner needs to predict all aspects of the behaviour of a system and to control the risks to the performance of the structure in service that may originate in the design and construction processes, in order to synthesise the engineering solution. Traditionally, structural predictions were based more on a qualitative understanding of behaviour than on quantitative analysis. Clearly understanding at the qualitative level is a minimum requirement for engineering success. The large qualitative element in much engineering since the Industrial Revolution up to the middle of the twentieth century was not a matter of choice. Quantitative information was not available or, at best, was only based on simple and imperfect models of behaviour. For design today more realistic models are available and computational resources enable sophisticated calculations to be carried out routinely. Some of the main aspects where predicted values of physical quantities over the required life of a structure are needed by practitioners include: . Actions expected on the structure due to self-weight, imposed loads due to occupancy or trac, environmental loads due, for example, to wind, snow, icing and thermal eects. . Bearing capacities and other properties of the ground on which the structure is to be founded. . Strength, stiness and other properties, eg. thermal expansion coecients, of the structural materials to be used. Modern construction uses increasingly higher strength materials leading to structures that are relatively less ductile and more prone to dynamic response. . The durability of structural materials in the expected service environment and, where they are known not to have the required durability, the performance of available protection systems, e.g. paint, their expected life and the feasibility of maintenance during the service life of the structure. . The dimensional accuracy of manufacture of the structural elements and construction of the structure. . The behaviour of the structure under dierent combinations of circumstances likely to occur in service. Essentially the engineer needs to predict the occurrence of onerous design situations, both from the structural safety and serviceability points of view, so that the design can be made so that risks of unsafety or unserviceability are kept acceptably small. The structure
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must be designed so that it will carry the most onerous load combinations likely to occur with a `margin of safety', i.e. so that the probability of `failure' (not meeting performance requirements) is small. In addition to the control of risks to structural performance, the practitioner today is increasingly required by regulations relating to health and safety to carry out risk assessments covering the structure/environment system as a whole. Risk management has become an explicit feature of the practitioner's task. Assessment of organisational and human factor issues are an important part of risk management. The practitioner needs better tools for this work. The control of risk through feedback mechanisms during design, construction and use should receive a high research pro®le through collaboration between engineers and social scientists. 5. What researchers do 5.1. General Over recent decades, most structural engineering safety and risk research has been devoted to providing a basis for designing safe structures using traditional `factors of safety' or `partial factors' to achieve a low probability of failure. Such research can be considered to provide the engineer with a quantitative basis for decision that is only marginally supported by `analysis' of risk of failure. The `analysis' does not generally go beyond consideration of statistical distributions of data on loads or structural materials properties and the provision of appropriate extreme values. The assessment of risk usually remains largely implicit within the engineer's judgement and decision-making process subsequently. Sometimes the aim of such research is related to a speci®c structural engineering project. The results may then be used directly in the engineering of that project. Ground investigations are a common form of speci®c project-related research. More usually structural engineering research aims to develop basic data on loads, material properties, or general models of structural behaviour that can be used by engineers when they are considering that part of the structure/environment system to which the research relates. Basic data and general models are often used as general rules in codes of practice. Such rules incorporate statistical adjustments to reduce probabilities of exceedence of actions or performance criteria to very low values. Examples of research topics of this type are: . . . . . .
the occurrence of extreme winds the variability in the strength properties of structural materials eg. reinforcing steel or timber the bending capacity of prestressed concrete beams the fatigue strength of welded steel beams the behaviour of composite bridge decks under simulated trac load the behaviour of steel building frames under load.
Topics of research relating to the environment, e.g. wind, in which structures operate are usually based on measured observations of the phenomenon. This work provides the statistics
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from which mean or extreme values are calculated. Similar forms of research are used to provide engineers with predictive tools relating to mean or extreme values for properties of materials, or tolerances of manufacture or construction. Research relating to the behaviour of structural elements or complete structures under load may consist of experimental or ®eld observations serving as benchmarks for the development of calculation methods. Once a good agreement between experiment and analysis is demonstrated, calculation may be used in parametric studies to explore the eects of larger changes of key variables than is possible, for economic and time reasons, to determine experimentally. In this way sensitivities to changes in particular variables may be investigated. Such research investigations may provide a basis for immediate engineering decision only in cases where the researcher is aiming to assist the engineering of a speci®c construction project. Generally these types of research are too limited in scope to be immediately useful to engineers. Usually they have the wider aim of in¯uencing engineering practice. For this purpose it is necessary ®rst for the results from several separate researchers to be pooled and evaluated through peer review involving both researches and practitioners in order to derive general design data or models that can be accepted by practitioners with con®dence. This process may culminate in recommendations for design in codes of practice or other guidance documents. Even though the recommendations are general in character, it is important to recognise that invariably they are useful to the practitioner only for a part of his task. Whilst some codes of practice, e.g. the Structural Eurocodes [1], are quite extensive in scope, they do not wholly cover the practitioner's needs. They provide a framework for the practitioner which he has to adjust and extend to the context of his project, often on the basis of qualitative judgements of risks, in order to make decisions to progress his task as a whole. Where the recommendations/guidance are limited or do not directly cover the particular situation of interest, the practitioner must make decisions despite the uncertainty arising from the paucity of his knowledge. This is generally the case. Practitioners would be assisted by clearer articulation of the philosophy and goals of structural design. They are increasingly turning to risk assessment techniques to assist decision making by providing indications of the relative magnitudes of risks. 5.2. Risk-based structural safety analysis The development of reliability theory has provided the foundation for risk-based structural safety analysis. Researchers in this area have not addressed the risks in the structure/environment system and design and construction processes as a whole. They generally have investigated one small part of the whole and sought to understand it in detail and to create models which can be used to determine its behaviour. The reasons for the apparent reluctance of researchers to work across the whole domain of interest to practitioners have been discussed elsewhere [2]. Essentially conceptual and analytical diculties during the development of risk-based approaches to structural safety have constrained research to parts of the whole. Research on reliability theory has been focused in two main areas, structures and systems. For structures the emphasis has been on codes of practice, whilst in the systems area developments in electronics and aerospace have led to applications in nuclear, chemical and oshore engineering. Early research on the calculation of failure probabilities of structures achieved only limited application mainly because of the high sensitivity of the small calculated risks to distribution
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assumptions, the inability of the mathematics to take into account workmanship and other factors which are a major in¯uence on actual risks, and the problem for practitioners of explicitly recognising risks of structural failure. These problems were tackled by researchers through use of: . a `safety index' to overcome the sensitivity of risk calculations to the type of distribution function adopted . subjective or Bayesian variables whose means and variances could be estimated by judgement to re¯ect the eect of variables such as workmanship or judgement factors such as structural analysis simpli®cations . `calibration' procedures to estimate the level of safety in existing design procedures as a basis for establishing more uniform safety levels. The resulting methods for establishing structural reliability have been widely accepted by practitioners despite the remaining conceptual and analytical diculties. A persuasive argument put forward for this success is that safety indices look like the safety factors traditionally used in engineering practice [2]. The methods have been adopted for code calibration relating to the design of simple structural elements. However, individual structures are generally made up of many elements. Their reliability analysis is inevitably complex especially as they usually have many possible failure modes. Whilst this problem has been addressed by researchers in structural reliability techniques, complete structures are complex systems and their reliability needs to be addressed by systems reliability techniques. Research on systems reliability began with problems of the performance of networks of electronic components whose individual reliability was known. The reliability of the whole was found by dividing the system into groups of components in series and parallel. Practical diculties were experienced in taking redundancy into account, especially for systems of any complexity. The emergence of fault and event tree approaches from the aerospace industry was another strand of development of systems reliability theory. Although these approaches have been re®ned, for example to deal with non-independent events and with repairability, problems remain. Amongst the many problems identi®ed in a 1992 review [2] of the limitations of reliability theory, major ones were: . Human values are an essential element of risk analysis but there is only limited understanding of acceptance of risk. There is also particular diculty in cases where the normal requirement for a very low probability of occurrence of failure is combined with consequences that are very severe. The resulting risk estimate is generally not considered credible. . Since human factors are more important causes of structural failure than technical considerations, the omission of human factors, e.g. errors in management, design or construction, in structural reliability theory raises doubt about the credibility of carrying out complex technical analyses of structural risk. . Since the structure/environment system is generally complex, many failures in practice occur due to highly unlikely combinations of circumstances coming together. Although each combination may have only an exceedingly small chance of occurrence, the number of such combinations can be exceedingly high. . The incompleteness of the models used by reliability theory, particularly in relation to the above diculties, and the constraints arising from the underlying assumptions of the models.
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. The diculty of recognising and taking into account technological and organisational changes which may in¯uence uncertainties in design, construction and use. 6. Other mechanisms assisting risk reduction The above problems remain a challenge for researchers. Practising engineers, however, must continue to practise. They use other mechanisms to oset human factor and other risks that cannot be taken into account analytically. Particularly important are education and continuing professional development including feedback of experience from practice and peer group review. Education and continuing professional development provide the essential background to enable the practising engineer to control uncertainty and avoid structural failure. Research and development of learning processes continues to be needed, particularly for aspects of practice where quantitative techniques do not exist, e.g. the use of case-based learning using information technology to feedback experience on avoidance of ¯awed structural concepts. An important mechanism for review and feedback on current structural safety practice is provided by the Standing Committee on Structural Safety. It is an independent body in the United Kingdom established by the professional civil and structural engineering institutions to maintain a continuing review of building and civil engineering matters aecting the safety of structures. It provides feedback on trends which may adversely aect safety, thereby alerting practising engineers to uncertainties which hitherto may have been unrecognised and which may be the trigger for structural failures [3]. At the same time, it helps to identify for researchers risks that may warrant their attention. References [1] Menzies JB, Smith BW. The structural eurocodes. Paper P320-4: Structural engineers world congress, San Francisco. Elsevier, 1998. [2] Elms DG, Turkstra CJ. A critique of reliability theory. In: Blokley D, editor. Engineering safety. McGraw±Hill, 1992. [3] Standing Committee on Structural Safety. Structural safety 1997±99: review and recommendations. Twelfth report, London: SETO, February 1999.