Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 417

γ f

Uncertainty in representative values of
actions

γ F

γ Sd

Model uncertainty in actions and action
effects

Model uncertainty in structural resistance γ Rd

γ M

Uncertainty in material properties

γ m

Figure 10.7 Uncertainty and the Eurocode partial factors.

The methods that can be used to calibrate the partial factors and the relation between
them are presented diagrammatically in EN 1990, as shown in Figure 10.8. The probabi-
listic methods are subdivided into Level I, semi-probabilistic methods, Level II, first-order
reliability methods (FORM) and Level III, full probabilistic methods. Although shown in
Figure 10.8 , the semi-probabilistic method is not defined in EN 1990. However, in the ISO
standard 2394 for the reliability of structures (ISO, 1998), the semi-probabilistic method
is described as the method where design calculations are carried out, as in the Eurocodes,
using consequence class categorizations (see Section 10.5.3), design situations, design values
of actions, and material properties with partial factors applied to characteristic values to
achieve the appropriate degree of reliability.
When a probabilistic method is used, EN 1990 states in Annex C §C3(3) that the ulti-
mate limit states partial factors for material and actions should be calibrated such that the

Probabilistic methods

Deterministic methods

Procedure 1 Procedure 2

Historical methods FORM Full probabilistic

Empirical methods (Level II) (Level III)

Calibration Calibration Calibration

Semi-probabilistic
methods

Method c

Method b

Method a

Partial factor
design

Figure 10.8 Overview of the reliability calibration methods in EN 1990.