AECEF NEWSLETTER 1/2001

From Deterministic to Probabilistic Way of Thinking
in Structural Engineering

by Professor Pavel Marek
Department of Civil Engineering VŠB TU, Ostrava
Czech Republic

and

Professor Jacques Brozzetti
Ecole Nationale des Ponts et Chaussées, Paris
France

Fall or Rise of the Prestige of Structural Engineering?

One of the speakers at the 1997 Congress of the American Society of Civil Engineers pointed out a significant fall of prestige of structural engineering [1]. In the history of engineering education at the civil engineering faculties in the U.S.A. this field used to occupy the highest echelon; therefore, the speaker was trying to find the explanation of this fall. He stated inter alia that the number of causes might include also the preference granted to other specialisation’s due to social interests concerned e.g. with the development of transport networks and environment protection. In his opinion, however, the said fall might be connected also with the development of computers and structural design codes, as it was possible to hear that the education of structural engineers concentrated gradually on the application of sophisticated software, requiring from the structural engineer merely the introduction of adequate input data, while the computer spouted almost immediately the dimensions of the structure, its reliability assessment and the whole documentation required by respective standards. In this process the designer need not even know the details of dimensioning or the substance of the reliability assessment. Is this opinion justified? Does the development of computer-aided structural design really belong among the causes of the above mentioned fall of prestige observed not only in the U.S.A., but in other countries as well?

Is the Structural Engineer the Creator of Structures
or merely an Interpreter of Codes ?

Together with the manufacturer and erector the designer has been, is and will always remain the creator of the structure. His activities are based on professional knowledge, experience and cooperation with related professions. Although the computer revolution is providing ever more powerful instruments facilitating and accelerating his work, these instruments and how ever perfect standards can never replace the designer-creator, responsible for effectiveness and reliability of his work.

The design codes and standards cannot cover all situations, loading alternatives, performance conditions, etc. the designer may encounter. Often he has to decide himself on the basis of his own knowledge and experience in accordance with the “rules of the game” of reliability assessment. In respect of safety, serviceability and durability of structures the development of codes and standards in the past few decades has resulted in a certain damping of the creative role of the designer who has become merely an interpreter of the rules and criteria formulated in the standard. “The rules of the game” (i.e. the theoretical foundations of reliability assessment) of the Partial Factor Design (PFD) are given in the standards excessively simplified and the designer during his education is not acquainted with their substance. The teachers often use the wording “the code states..”, thus avoiding the explanation of the problems with which they are often not thoroughly acquainted themselves. To be accurate, they cannot be thoroughly acquainted with them, as the commentaries and data explaining fully and consistently the background of the codes, various simplifications and the influence of calibration, are not available. The consequences of this development are that the students of building faculties are educated primarily on the basis of the interpretation of standards and codes and not in the engineering way of thinking and conceptual work. This fact must be afforded full attention.

Let us recall the related experience with the introduction of the PFD concept into US design codes. In the field of steel structures a standard based on the LRFD (Load and Resistance Factor Design) method was issued in the U.S.A. in 1986. This method is actually an analogy of the PFD method – see the Eurocodes. The coded was to replace the standard based on the deterministic method of Allowable Stress Design (ASD). Although the issue of the LRFD standard was preceded by an extensive explanatory campaign and training courses emphasising the advantages of the LRFD method, today – 15 years after it has been introduced – it is applied merely by one quarter of designers [2] while the prevailing majority of designers in the country of tallest buildings, biggest bridges and other unique structures, has remained faithful to the excessively simplified, but understandable deterministic ASD method. This seemingly conservative attitude of designers is usually explained by unsatisfactory teaching of the LRFD method at universities. However, the number of principal causes of reserve on the part of experienced US designers may include their feeling that the LRFD method, developed in the pre-computer era, has been submitted to the designers too late and that it no longer provides qualitatively new possibilities corresponding with the computer era in respect of reliability assessment.

From Slide-Rule Era to Computer Era in Structural Design

In the courses of steel, concrete and timber structures the students of civil engineering faculties may hear from some of their instructors that due to the introduction of the Eurocodes “nothing much will happen in the field of reliability assessment in the next few decades”. This statement must be debated. The expansion of fast improving computers to the desk of every structural designer has produced profound qualitative and quantitative changes, which have no analogy in the whole history of this field. The growing computer potential improves the prerequisites for the “re-engineering” of the whole design process (i.e. its fundamental re-assessment and reworking) to adapt it to entirely new conditions and possibilities. We can follow with admiration the fast development of software for the analysis and for dimensioning of structures according to standards and for the production of their respective drawings. At the same time it is necessary to emphasise that entirely unsatisfactory attention has been afforded so far to the preparation of concepts and corresponding standards based on the qualitatively improved method of reliability assessment corresponding to computer potential available.

Since the early Sixties, a “semi-probabilistic” Partial Factors Design (PFD) such as that found in the Eurocodes has replaced many national and international specifications for structural design based on deterministic concept. The PFD concepts have been developed using statistics, reliability theory and probability, however, without considering the computer revolution. The interpretation of the assessment format in codes is somewhat similar to the fully deterministic scheme applied in earlier codes except there are applied two partial factors instead of a single factor. The application of PFD does not require the designer to understand the rules hidden in the background of the codes. The semi-probabilistic background of the reliability assessment procedure has been considered by those writing the codes, however, the calibration and numerous simplifications introduced in the final format of codes affected the concept in such a way that the concept is better to be called ”prescriptive” instead of ”semi-probabilistic”, see [2]. The designer’s activities are limited to the interpretation of equations, criteria, instructions, factors, and “black boxes” contained in the codes. The reliability check can be conducted using a calculator, slide rule or even long-hand, while the modern computer serves only as a ”fast calculator”. The actual probability of failure and the reserves in bearing capacity cannot be explicitly evaluated using PFD codes. From the designer’s point of view, the application of PFD in practice is still deterministic. A designer‘s direct involvement in the assessment process is not assumed and, therefore, his/her creativity is suppressed.

Has the computer potential created the prerequisites for a qualitative improvement of the partial factors method? The answer can be illustrated by the following analogy: Is it sufficient to attach a high-efficient jet engine to the gondola of a balloon in order to achieve its incomparably higher velocity and efficiency? It can be concluded that the combination of the balloon and the jet engine will not create a higher quality means of transport. Analogously it is possible to conclude that the partial factors method based on numerous limitations and simplifying assumptions cannot be raised to a qualitatively higher level of the structural reliability assessment concept by its combination with computer potential. It can be concluded that the computer revolution opens the door to qualitatively new fully probabilistic structural reliability concepts.

Application of Elite Research Results to Structural Design Codes

The results of elite research are usually applied to specific fields (offshore structures, space programmes) by top-level experts. The conferences, however, lack the papers by research scientists explaining their ideas of the application of their concepts to standards and codes used by hundreds of thousands of designers in their everyday work. Who will bring the message from the elite researchers to the rank and file designers? An understandable explanation of scientific methods of reliability assessment used in the standards accepted by structural designers worldwide is a highly challenging task. However, without its solution the results of elite research remain merely the object of articles in scientific periodicals and the designer remains merely an interpreter of “prescriptive” codes.

Research affords attention to the development of risk engineering, “fuzzy sets” and other methods, while the designer lacks a fundamental, understandable and consistent method of determination of failure probability. Therefore, it is necessary to reassess “the rules of the game” of the reliability assessment, beginning with the load definition: the present day load expression in standards and codes by the characteristic value and load factors must be replaced with a qualitatively higher form enabling to take into account also the loading history (such as the “load duration curves” [3]). With reference to bearing capacity it is necessary to provide a “reference level” applicable to the computation of failure probability. Reliability must be expressed by a comparison of the computed failure probability with design probability.

The awareness of designer is the necessary prerequisite for the practical application of the probabilistic concept of reliability assessment. Let us turn our attention to the education of students of civil engineering faculties and designers in post-graduate courses.

Deterministic or Probabilistic Approach in the Education?

Let us ask these questions: Is the approach applied in our courses, to the solution of technical problems in structural design, deterministic or probabilistic? Are instructors infusing a deterministic understanding into the ”knowledge-base” of their students, or is the fact that we are living in a world defined by random variables already accepted and applied in the educational process? In courses such as Statistics and Probability Models in Structural Engineering, the common textbooks are based on a ”classical” approach to statistics and probability theory [4]. Such an approach is limited to analytical and numerical solutions, and does not allow for transparent analysis of reliability functions that depend on the interaction of several random variables. The textbooks mostly remain silent on common real-world problems, such as the probability of failure of a structural component exposed to variable load combinations in which one might consider the contributions of variable yield stress, variable geometrical properties and random imperfections. In structural design courses, the interpretation and application of the existing codes are emphasised; however, students are using the codes without a full understanding of the actual reliability assessment rules and of the meanings of the factors used to express safety, durability and serviceability of structural components.

Teaching Reliability

Advances in computer technology allow for using simulation, see for example [3]. The direct Monte Carlo simulation technique has been applied to basic problems in statistics and structural reliability assessment problems has been taught, for example, since 1989 at San Jose State University, California, and since 1996 at the Department of Civil Engineering, VŠB TU Ostrava, Czech Republic, at the graduate and undergraduate levels. The positive response of the students, and their understanding encouraged the instructors. A team of undergraduate students developed, for example, a study proving that the PFD method is not leading to a consistent level of safety (see Probabilistic Engineering Mechanics 14, 1999, p.109 to 118, ”Parametric Study: LRFD vs. SBRA”). The new generation of civil engineers seems to be anxious to apply advanced computer technology to its fullest including application of simulation techniques in the analysis of multi-variable problems.

TERECO Project

With reference to the improvements expected in the field of structural reliability assessment the training of students and designers ranks among the most important tasks. What starting point should be chosen? A transition to the qualitatively higher probabilistic concepts will require the designer to change his way of thinking, i.e. to replace his current ”deterministic thought-process” with the probabilistic one. The professional EC Committees consider the training of designers in this respect highly desirable. For this reason it has been sponsored, through the Leonardo da Vinci Agency in Brussels, the long-term TERECO Project (TEaching REliability COncepts using simulation techniques, see [5]). The resulting product of the Project involving the work of 33 authors from eight European countries and from the U.S. is the textbook ”Probabilistic Assessment of Structures Using Monte Carlo Simulation”[6]. The book acquaints the reader with the basis of a fully probabilistic structural reliability assessment concept, using as a tool the transparent SBRA method (Simulation-based Reliability Assessment, see the textbook [3], and home pages

The concept allows for bypassing the ”design-point” approach as well as the load and resistance factors, and leads to the reliability check expressed by P_f < P_d comparing the calculated probability of failure P_f with the target design probability P_d given in codes. The application of SBRA is explained in the book using 150 solved examples. On the attached CD-ROM the reader will find the input files and computational tools enabling the duplication of the Examples on a PC, the pilot data-base of mechanical properties (expressed by histograms) of selected structural steel grades, selected histograms (loads, imperfections, and more), manuals for computer programs and selected presentations of Examples (Microsoft PowerPoint). The book should serve in teaching undergraduate and graduate students as an aid introducing the students to the strategy of the fully probabilistic reliability assessment of elements, components, members and simple systems using direct Monte Carlo simulation and modern PC computers.

Summary and Conclusions

The structural engineering profession needs new approaches if we want to provide the best possible service to society. We have to consider the transition from the deterministic ”way of thinking” to open-minded probabilistic concepts accepting the random character of individual variables involved as well as their interaction. Tools such as simulation techniques and powerful personal computers will contribute to reaching such goals. Students find these techniques easy to learn and thus they do not require the instructor to take a great deal of classroom time to explain the theoretical background. Once in the computer lab, students can explore to their hearts content and gain a fuller understanding of the effects of each parameter on the variability of the final answer. With this understanding students are better informed to make decisions about tradeoffs that need to be made, for example, between service life and safety. The simulation technique should be included in the program of undergraduate and graduate students and in corresponding textbooks to prepare them for the types of problems they will encounter in the real world, especially for the application of probabilistic structural reliability assessment concepts in the new generation of codes that is expected to be introduced in the near future. Such approach will make the engineer the creator of the structure and may bring the prestige of structural engineering back to one of the highest positions.

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