The JCSS probabilistic model code

Structural Safety Vol. 19, No. 3, pp. 245-251, 1997

ELSEVIER

© 1997 Elsevier Science Ltd. All rights reserved Printed in The Netherlands 0167-4730/97 $17.00 + .00

PII: S0167-4730(97)00008-8

The JCSS probabilistic model code Ton Vrouwenvelder

*

Delft University of Technology~ TNO Bouw, Delft, The Netherlands

Abstract The JCSS is developing a model code for full probabilistic design. This note gives an overview of the set up and contents of this code. © 1997 Elsevier Science Ltd

1. Introduction Most present day building codes are based on the limit state approach and the partial factor method. The partial factors are, to some extent, based on a probabilistic background. For this reason these documents are referred to as "probability based codes". However, when estimating the reliability of structures designed according to those codes, it turns out that the obtained failure probabilities show a fairly wide scatter. In a JCSS project on this topic reliability indices for concrete structures, designed according to the Eurocodes, were found to vary from 3 to 6, see e.g. [1]. Such scatters are partly due to the fact that present day codes are a mix of probabilistic arguments on the one hand and calibration to design procedures of the past on the other [2,3], but even in the case of fully calibrated codes, the limitations of semi-probabilistic design procedures will always give rise to fluctuations in the theoretical failure probabilities. Generally, as the probabilistic design method may be considered as more rational and consistent than the partial factor design, there is a tendency to use probabilistic methods not only as a background for codes, but also directly in the assessment of special of important structures existing, as well as under design. Both the researcher, evaluating codes of practice, and the designer, assessing structures, however, face the problem that they have to find out many things by themselves, in particular in the fields of the statistical modelling of random variables and in the field of accepted approximative methods of calculation. In many cases there is insufficient data to make objective estimates and, consequently, subjective elements have to enter the analysis. This may bring the engineer to a difficult position and, so, there is * Correspondence address: TNO Building and Construction Research, Lange Kleiweg 5, Rijswijk, PO Box 49, 2600 AA Delft, The Netherlands. 245

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a need for a code which gives sufficient guidance for the engineer who wants to do a full probabilistic analysis. Such a model code should: • provide a complete and consistent set of models and procedures for probability based decision making and design; • be written in a code-type manner, i.e. unique solutions for each problem are anticipated; where physical or statistical evidence is not sufficient to provide unique solutions, rules are specified on the basis of agreement; be an operational " c o d e " intended for application in the context of probability based expertise. A document, meeting these issues, is considered simultaneously fit to serve as a guidance document for future code-writing of "everyday codes". The JCSS has considered it as its task to write such an operational probabilistic model code. After a number of preliminary studies in the past years the actual drafting activities have started now. Of course, such a first attempt may show many shortcomings, but the idea is that "somebody has to set the first step". The document is planned to be built up out of four main parts: Part 1. General principles; Part 2. Modelling of loads; Part 3. Modelling of structural properties; Part 4. Existing structures. Note that the code is not intended to be text book on structural or reliability engineering. The code is written in a condensed way and little or no physical or probabilistic explanations are given. The readers are supposed to be experienced engineers, familiar with probabilistic concepts. Where necessary, of course, short notes a n d / o r references are given. The four parts will now be discussed in some detail.

2. Part 1: basis of design For this part the third draft is already available. The table of contents is presented in Table 1. The basic principles for this Part 1 were already available from earlier work in the JCSS [4-6]. In the elaboration, however, this part has been written as closely as possible to the present draft of ISO 2394 (general principles on reliability of structures). The idea behind this choice is that it is important that the probabilistic code looks familiar to the designer.

Table 1 Table of contents for Part 1 Part 1: basis of design 1.1 Definitions and notations 1.2 General requirements 1.3 Concept of the adverse state 1.4 Basic variables and uncertainty modelling 1.5 Structural models and reliability analysis 1.6 Reliability measures, risk analysis, target reliability

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As the present ISO draft gives proper attention to probabilistic methods, this task was not too difficult. In fact, the main difference between ISO 2394 and the JCSS probabilistic model code is that the latter does not present any concepts like design values, characteristic values or partial factors. In the JCSS code random quantities (variables, processes, fields) are characterised only by probabilistic models. The ISO code leaves both the probabilistic and the semi-probabilistic options open. Apart from definitions and general concepts, the important operational information in Part 1 are the target reliabilities for the various limit states. These targets are in part the result of a calibration process that should lead to the situation that the probabilistic model code renders about the same structural dimensions as the present Eurocodes. The targets are also, by and large, compatible with observed failure rates and with outcomes of cost-benefit analyses. The present third draft of Part 1 contains a number of paragraphs that need some further development. In particular this holds for serviceability aspects, robustness requirements, interaction with quality assurance, system reliability and durability.

3. Parts 2 / 3 : models for loads and resistance Parts 2 and 3 present the basic information on load and resistance modelling and a number of subsections describing probabilistic models for the various individual loads and materials. The intended tables of contents for the Parts 2 and 3 are presented in Table 2. Most of the subsections will contain 5-10 pages. Only the information that is relevant for the probabilistic modelling is presented. For information as shape factors, material densities, mechanical calculation models (like e.g. buckling models), reference is made to CEN or ISO codes or to special annexes and background documents. In principle, for all random quantities time and spatial variability are considered. For the spatial variability the hierarchical modelling procedure is followed. Consider, as an example, the live load on office floors. In the hierarchical model the load is comprised of three contributions:

q(x,

y)=m+

v+

u(x, y)

(1)

where m = the overall load intensity for a building of a particular user category; v = a zero mean variable which may differ from floor to floor; u = a zero mean fluctuating field. This way the correlation between the various buildings in one category of use, between the floors in a building and between the various parts of the floor can be easily accounted for. A similar model holds for the time variably. A distinction is made between time invariant variables, slowly varying variables and rapidly varying variables [7]. For instance, as far as the wind load is concerned we may have:

q(t)=O.5CppV2(1 + 2c/V)

(2)

where Cp = time invariant shape coefficient; V = a slow mean value process (10 min average wind velocity); v = a rapidly varying gust speed process; q = air density. The time variability of the slow process is usually modelled by the simple well-known Borges Catanheta models. For many cases such a model is sufficient, also taking into account the often limited amount of information. The rapid process may be of varying nature. Gust velocities, for

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Table 2 Table of contents for Parts 2 and 3 Part 2: loads

Part 3: materials and structures

2.0 General

3.0 General

2.1 Selfweight 2.2 Live load 2.3 Industrial storage 2.4 Cranes 2.5 Traffic 2.6 Car parks 2.7 Silo load 2.8 Liquids and gasses 2.9 Manmade temperature 2.10 Earth pressure 2.11 Water and groundwater 2.12 Snow 2.13 Wind 2.14 Temperature 2.15 Wind waves 2.16 Avalanches 2.17 Earth quake 2.18 Impact 2.19 Explosion 2.20 Fire 2.2l Chemical/physical agencies

3.1 Concrete 3.2 Reinforcement 3.3 Prestressing steel 3.4 Steel 3.5 Timber 3.6 Aluminium 3.7 Soil 3.8 Masonry 3.9 Model uncertainties 3. l0 Dimensions 3.11 Imperfections

instance, are usually m o d e l l e d as a Gaussian process. Finally, for intermittent processes the Poisson m o d e l is often adequate. For the detailed specifications o f the loading part use is m a d e of the preparatory w o r k done b y CIB W81 [8]. As an example, for the sustained (slow) part o f the live load model presented above, the models shown in Table 3 have been chosen in the present draft (offices only). The last c o l u m n represents the time that one occupant is present (time variability parameter). M a t h e m a t i c a l l y it is the duration o f one realisation o f m, v and u. The reference area is a measure for the spatial correlation of the u-field. For the details o f the resistance models use is being m a d e o f the classical JCSS Basic notes on

Table 3 Proposed parameters for the live load on office floors (see Eq. (1)) Parameter Distribution type

Mean value (kN/m 2)

Standard deviation (kN/m 2)

Reference area (m 2)

1/A (year)

m v u

0.50 0 0

0 0.30 1.10

--20

5 5 5

deterministic normal gamma

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resistance [9] and other more recent JCSS work [10-12]. However, in some cases additional work has to be done, especially in the fields of fatigue; foundations; model uncertainties; effects of quality control procedures. As stated already in Section 2, the intention is that the models presented in Parts 2 and 3 of the probabilistic model code, together with the target reliabilities of Part 1 lead to structures that are to some extent similar to the structures as will be designed on the basis of characteristic values and partial factors specified in the present Eurocodes. In other words, the probabilistic code will be calibrated to present practice. This calibration process is now in full swing. A first publication presenting reliability calculations for a concrete column, may be found in [1].

4. Part 4: assessment of existing structures The assessment of existing structures differs in a number of ways essentially from the structures to be designed. One key point is that the structure can be inspected and another key point is that improving the safety is relatively more expensive than in the design stage. Part 4 should contain the principles for a probability based assessment of existing structures. The intended table of contents is presented in Table 4. In order to write this chapter the JCSS is now preparing a working document in this field that may serve as a basis and background for the model code chapter. The working document should be of a more educational nature and contain practical and operational rules. These operational rules may also be based on present practical experience. This working document should have the following characteristics: (1) it should explain how to derive specific inspection and repair decisions for existing structures from the general decision making concepts; (2) it must be generally applicable for various materials and various structure types; (3) it should lend itself as a basis for material specific elaboration. The document may also give information about common practice with respect to repair decisions, inspection schemes, inspection techniques, check lists, how to handle in the cases of incomplete or

Table 4 Table of contents for Part 4 Part 4: existing structures 4.1 Definitionsand concepts 4.2 Inspectionand updating techniques 4.3 Maintenance 4.4 Decisioncriteria 4.5 Practicalprocedures

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absent information and so on. In order to ensure that the theoretical part is meaningful for the practice, the JCSS has first gathered a number of practical examples and their solutions. Next, the common denominators of those individual cases have been looked for and written down. Some of the practical questions that the guideline addresses are: How to process specific information about the existing structure and its previous loads for deriving assessment rules. • What to do if structures fulfil the rules of the code on the basis of which they have been designed, but fail to obey new codes. • Should repair restore the original state or is partial repair acceptable? • What are the typical problems in setting up mechanical models for structures to be repaired. • What should be concluded about other parts of the structure, or even other structures, if some low strength elements have been observed. How to deal with implicit durability requirements in the codes? What parts of the design codes still apply for existing structures, which do not apply and which parts should be modified; and so on. The document should answer these questions on the basis of theoretical arguments as far as possible. Examples clarify the theory and show applications in a number of practical cases.

5. Closure The work that is going on is extremely interesting, but progress, unfortunately, is slow. Partly this has to do with the voluntary nature of the work. Additionally, we should admit that the task to write a probabilistic model code is quite an ambitious one and that we still face a number of unsolved problems and lack of data. On the other hand, it is the JCSS's firm belief that the research and experiences of the past should now be brought together in a document that standardises models and methods to facilitate the use of probabilistic methods in practice. Maybe such a document will have many flaws and shortcomings, but even then, writing the document is the only way to find out.

References [1] Holicky, M. and Vrouwenvelder, A., Reliability of a reinforced column designed according to the Eurocode. 1ABSE Colloquium Basis of Design and Actions on Structures, Delft, 1996. [2] ENV 1991-1, Eurocode 1: basis of design and actions on structures, Part 1: basis of design, CEN 1994. [3] Background document Eurocode 1, Part 1: basis of design. JCSS working document, ECCS Publication 94, March 1996. [4] Ditlevsen, O. and Madsen, H., Proposal for a code for the direct use of reliability methods in structural design. JCSS working document, November 1989. [5] Ditlevsen, O., Bayesian decision analysis as a tool for structural engineering decisions. JCSS working document, February 1991. [6] Ostlund, L., Structural performance criteria. JCSS working document, January 1991. [7] Rackwitz, R., Load models and load combinations. 5th Probabilistic Summer School and Workshop, Poland, June 1992. [8] CIB W81, Action on Structures. CIB reports, 115, 116, 141, 166, 167, 193, 194, 195, 201.

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[9] JCSS, Basic notes on resistance. JCSS, 1980. [10] Kersken Bradley, M. and Rackwitz, R., Stochastic modelling of material properties and quality control. JCSS working document, March 1991. [11] Schueller, G., Design for durability including deterioration and maintenance procedures. JCSS working document, November 1990. [ 12] Casciati, F., Negri, I. and Rackwitz, R., Geometrical variability in structural members and systems, a critical review of available data on buildings. JCSS working document, January 1991.