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Reliability of structural members designed with the Eurocodes NDPs selected by EU and EFTA Member States

JRC Report: Eurocodes Scientific and Technical Report by Markova, J., Sousa, M.L., Dimova, S., Athanasopoulou, A., Iannaccone, S.

Published in Luxembourg,
Publications Office of the European Union, 2018

Reliability is defined in the Eurocodes as the “ability of a structure or a structural member to fulfil the specified requirements, including the design working life, for which it has been designed”. The reliability levels are expressed by reliability indices, which are defined by probabilistic analysis considering the uncertainties in the actions and material properties, and the uncertainties in the modelling of action effects and structural resistance. The Eurocodes recommend reliability indices for three reliability classes which are associated to the consequences of failure or malfunction of the structure.

The study is developed under Administrative Arrangements between DG GROW and the JRC. The analysis of the reliability levels achieved with the Nationally Determined Parameters (NDPs) chosen by the EU/EFTA Member States (MS) allows to cluster the national choices related to the design of particular type of structures and to compare their joint impact on the level of safety achieved.

The reliability analysis is performed for selected basic structural members, namely a beam, a column, a slab, a tie and a wall, considering different materials, i.e., concrete, composite steel concrete, steel, timber and masonry. The imposed loads considered in the analysis correspond to commonly used categories of loaded areas A to D in buildings, as specified in EN 1991-1-1. Typical buildings whose prevailing type of area corresponds to these categories of loaded areas are: residential buildings and houses (category A); office buildings (category B); schools (category C1); churches, theatres, cinemas (category C2); museums, exhibition centres (category C3); sports facilities (category C4); concert halls, sports halls (category C5); retails shops (category D1); department stores (category D2).

The reliability level of structural members in buildings designed according to the Eurocodes was assessed for 20 EU and EFTA MS using the NDPs uploaded in the European Commission NDPs Database by the end of 2017 by 16 EU MS and consulting the National Annexes of four other EU and EFTA MS.

The main conclusions of the study are the following:

- the reliability of structural members which were designed according to the national choice of the NDPs varies in a rather broad range. The reliability levels of the structural members for most common categories of imposed loads match the reliability indices recommended in EN 1990.

- in some cases the reliability levels according to the country choices of the NDPs are below the CEN target values and therefore should be further analysed and calibrated. Special attention should be given to country choices related to composite members, for the categories of imposed loads C2 and C5, to steel members for the category C5, and to timber structural members for the categories C2, C4 and C5, especially for fundamental combination of actions defined with the twin expressions (6.10a, 6.10b) of EN 1990.

- the reliability levels achieved using CEN recommended values should also be studied further, in order to delineate eventual needs for calibration of the recommended values, especially when expressions 6.10a & 6.10b of EN 1990 are used for the fundamental combination of actions.

- the reliability levels of composite, steel and timber members designed according to the lower bound of imposed loads recommended in EN 1991-1-1, are commonly lower than the reliability levels recommended in EN 1990, when expressions 6.10 & 6.10b, and expressions 6.10amod & 6.10b of EN 1990 are used for the fundamental combination of actions. The rather broad interval of values of imposed loads for categories A to D presently recommended in the Eurocodes, should be further analysed by CEN and narrowed down.

The reliability indices calculated in the present study, for the typical load ratio equal to 0.4, vary from 2.9 to 6.8, the former value was obtained by Latvia for the composite steel concrete slab for category of use C2 and for the combination procedure b and the latter value was obtained by the United Kingdom for the masonry wall for category of use C4 and for the combination procedure a. It should be noted, that the reliability of whole structure is normally higher than the one of a structural member. Thus, the presented results show that a generally good level of structural reliability has been achieved with the country choices of the NDPs.

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