Managing risk and achieving reliable geotechnical designs using Eurocode 7 - Risk and Reliability in Geotechnical Engineering - page 399

value of an action, F d for use in verification calculations is either assessed directly or derived
by multiplying the representative action, F rep by the appropriate partial action factor value, γ F :

F d = γ F F rep (10.4)

The representative value of an action may be the characteristic value F k or an accompany-
ing value ψF k in the case of a variable action when there is more than one variable action
and where ψ is the combination factor. The characteristic value of an action is its main
representative value and shall be specified as a mean value, an upper or lower value or a
nominal value. The characteristic permanent action G k derived from the weights of materi-
als, including soil and water, is normally calculated using the given nominal weight density
(unit weight) of the material and the nominal geometry of the ground and the structural
components, since the variability and hence the uncertainty in the weight density and geom-
etry are not normally significant.
The design effect of the actions E d for ultimate and serviceability limit states is derived
from the design value of the permanent action plus the design value of the leading variable
action and the design combination values of the accompanying actions obtained using the
appropriate combination rules and ψ values. EN 1990 provides rules for combining perma-
nent and variable actions in different design situations, for example, persistent or transient,
accidental or seismic, using three combination factors: ψ 0 , ψ 1 , and ψ 2 . The combination fac-
tor ψ 0 is applied to nonleading actions and takes account of the reduced probability of the
simultaneous occurrence of two or more independent variable actions. The combination fac-
tor ψ 1 is used to obtain the frequent value of variable actions, mainly for serviceability limit
state designs, while the combination factor ψ 2 is used to obtain the quasi-permanent value of
variable actions, mainly in the assessment of long-term effects, such as creep effects in pre-
stressed structures. Recommended values for ψ 0 , ψ 1 , and ψ 2 for different types of variable
actions, including imposed loads on buildings, snow, wind, and traffic loads, are presented in
tables in EN 1990. The tables are for different types of structures, for example, for buildings
and different types of bridges; as an example, for imposed loads in residential buildings and
offices, the recommended combination factor values are ψ 0 = 0.7, ψ 1 = 0.5, and ψ 1 = 0.3. The
recommended ψ 0 , ψ 1 , and ψ 2 values are based on the statistical analyses and chosen to achieve
structures with the required degrees of reliability for ultimate and serviceability limit states.

10.4.2.4 Design geotechnical parameters
The design value of a geotechnical parameter X d for use in verification calculations is either
assessed directly or derived by dividing the characteristic parameter value X k by the appro-
priate partial material factor value γ M :

X d = X k /γ M (10.5)

It should be noted that the partial material factor values to obtain the design values of
materials are divisors and always greater than unity, unlike in some other codes, such as, for
example, the Australian Standard for piling, AS 2159 (Standards Australia, 2009), which
has strength reduction factors less than unity that multiply the “design geotechnical ulti-
mate strength” to obtain the reduced (i.e., conservative) “design geotechnical strength.”
The requirements in Eurocode 7 for selecting the characteristic values of geotechnical
parameters and some guidance on their selection are presented in the following sections.
Sometimes in geotechnical designs, there is insufficient information or it is not sufficiently
precise to enable characteristic values to be selected with confidence. An example of such a