Importance of Structural Safety and Reliability

by H. Ishikawa

Department of Reliability-based Information Systems Engineering
Faculty of Engineering
Kagawa University

2217-20 Hayashi-cho, Takamatsu City,
Kagawa 761-0396 JAPAN

Phone: +81-(0)87-864-2001 Fax: +81-(0)87-864-2030
e-mail: ishikawa@eng.kagawa-u.ac.jp
http://www.eng.kagawa-u.ac.jp/

(Last updated: March 7, 1998)


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Importance of Structural Safety and Reliability

Recently, almost all of integrated structures are required to perform their expected functions with high level of reliability during the prescribed service time. In particular, in case that their failure or fracture might cause fatal loss of lives of human beings, structural reliability assurance is indispensable from a viewpoint of safety. In addition, this reliability assurance becomes of crucial importance also from an economical point of view such as savings of energy as well as natural resources, decrease in expenses and degradation of societal capital to be assured by protecting them against failure or fracture.

In general, most of practical structures are usually subjected to randomly varying external loads, and the strengths of their identical components will never be the same even under the same loading conditions. In other words, both the load and the strength are of an indeterministic nature. In addition, a variety of uncertainty factors will inevitably arise in the processes of their construction and maintenance. Hence, in order to perform their rational design and maintenance, these uncertainties have to be properly evaluated on a probabilistic basis. This is why reliability should be emphasized and the reliability-based design methodology plays a role of vital importance in the rational design [Ref (1)-(7)].

It appears, on the other hand, that in the conventional design method such as the allowable stress design method and the load factor design method, these uncertainties are partly dealt with on the basis of an empirical design factor termed as the safety factor. Although this method pays some attention to the variabilities both of the load and of the strength by introducing the safety factor, the selection of its value will depend upon designers' engineering know-how or experiences, and hence, is lacking in quantitative evaluation of the reliability or safety. In this respect, this type of empirical method will become insufficient to deal with the present and future engineering problems from the following reasons: First, the demand of designing structures by use of advanced or low-cost materials under unexperienced severe service conditions will be inevitably increased from now on. In such cases, the determination of the safety factor based upon individual experiences as well as subjective ideas is very risky and therefore the corresponding uncertainties must be treated on the theory of probability. Secondly, in reference to the increasing severe demand for safety and reliability as well as cost performance, the current safety factor must sufficiently reflect the statistical properties of uncertainty factors of interest. Finally, although the uncertainty factors of interest are successfully modelled in a probabilistic manner, it is often the case that their distribution parameters are still remained unknown and they have to be determined empirically from a sample of small size in most cases. Hence, a careful attention must be paid to estimation errors in order to assure the necessary structural safety and reliability.

As is well known, A. M. Freudenthal first introduced the concept of failure probability to handle this problem in 1947 [Ref (8)]. Following his creative research work, a number of studies have been carried out in the field where safety and reliability both play an important role. Needless to say, safety and reliability play a crucial role in a variety of engineering fields such as material science, mechanical engineering, civil and architectural engineering, naval architecture, aerospace engineering and nuclear engineering, to name but a few. The concept of structural safety and reliability has become of vital importance, which is reflected by increasing engineering concern to a considerable extent and by the accomplishment and publication of a number of research works and books in the field of structural safety and reliability [Ref (9)-(85)].

At this point, a brief schmatic representation of the importance of structural safety and reliability is given in the following figure. Unfortunately, however, because of the lack of preparation time, no explanation is made here so far. Eventually this will be made. Please be patient to wait for the time of completion of this explanation. Sorry for this incompletion.

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ReliabSchem.jpeg

Fig.1. Schematic Representation of Importance of Structural Safety and Reliability

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References

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