Figure 2.1. Geometric representation of the reliability index in the r-s plane

Figure 2.1. Geometric representation of the reliability index in the r-s plane

The level 2 method is also difficult to apply in practical design because the necessary data are often not available, so that another method is used: the partial factor method or semi-probabilistic method (level 1 method).

This method is based on the compliance with a set of rules that ensure the required reliability of the structure by using "characteristic values" of the problem variables and a series of "safety elements". These are represented by partial safety factors, y which cover the uncertainties in actions and materials, and by additional elements A for uncertainties in geometry, e.g. to allow for the randomness of cover to reinforcement and therefore of the effective depth of a reinforced concrete section.

This method does not require that the designer has any probabilistic knowledge, because the probabilistic aspects of the question of safety are already taken into account in the method calibration process, i.e. in the choice of characteristic values, partial safety factors etc., fixed in the Standards. The method is based on the following assumptions:

1. strength and stress are independent random variables;

2. characteristic values of strength and stress are fixed as fractiles of given order of the respective distributions, on the basis of a given probability;

3. other uncertainties are taken into account by transforming characteristic values into design values, by applying partial safety factors and additional elements;

4. the assessment of safety is positive if the design action effects don't exceed the design strengths.

It has to be pointed out that the characteristic values of actions are fixed as those values with a given probability of being exceeded during the service life of the structure only if statistical data are available. Otherwise, characteristic values are fixed as the nominal values prescribed in standards or specifications, or as target values, for example in the case of accidental actions such as impacts from road vehicles, explosions etc.

2.1.3 Design working life, C2.1.3 Design working life, durability and quality management durability and quality Independently from the method used for safety evaluation, a structure can be defined as reliable if management positive safety measures have been provided for all its limit states during the whole design working life Tu. Tu is defined as the period for which a structure is assumed to be usable for its intended purpose with anticipated maintenance but without major repair being necessary. Table 2.1 gives the indicative values of design working life for different types of structures.

Table 2.1. Indicative design working life [Table (2.1) - EN1990]

Design working life category

Indicative design working life (years)


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