The Eurocodes generally use a partial factor approach as a means of regulating safety levels, and for geotechnical design a range of alternative formulations is allowed, including some that are similar to LRFD. Although the possible use of more direct reliability methods was acknowledged, their application in practice has been limited to a few examples of verifying that particular sets of partial factors are appropriate for use. One significant advantage of partial factor methods, compared with former global factors, was that safety margins could more effectively reflect the known uncertainties of leading parameters in calculations. These are usually material strengths, or resistances of structural members or bodies of ground, and actions (the European word for loads) or the effects of actions within structural members or the ground. Generally, expert judgement was used to relate factor values to parameter uncertainties in an approximate way, supported by calibration of calculation results against past experience. When failures occur it is often not because of unexpectedly severe values of the known lead variables, but rather because an unforeseen event or action has taken place. Sometimes these relate to "human errors" by designers or constructors. It is therefore considered that structures should be designed to accommodate events and actions whose nature may be unforeseen, up to a magnitude that is acceptable to society. This issue, termed robustness, also affects the selection of appropriate values for partial factors and is arguably more difficult to accommodate in reliability calculations.
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