Civil Engineering Reference
In-Depth Information
the effect of an action in the EQU limit state, whether it is a stabilising or a destabilising action, is a
function of the action, the material properties and the geometrical dimensions.
i.e.
E
= {
E F X a
;
;
}
dst d
;
d
d
d
dst
where
E
dst;d
is the design effect of the destabilising action, and
E
= {
E F X a
;
;
}
stb d
;
d
d
d
stb
where
E
stb;d
is the design effect of the stabilising action.
5.4.7 Design resistances
Equation 6.6 in EN 1990:2002 indicates that the design resistance depends on material properties and
the structural dimension. However, in geotechnical design, many resistances depend on the magnitude
of the actions and so EN 1997-1 redefines Equation 6.6 to include the contribution made by the design
action. The clause actually offers three methods of establishing the design resistance, R
d
:
R F X a
{
;
;
}
R F X a
{
;
;
}
d
k
d
d
d
d
R
= {
R F X a
;
;
}
or
R
=
or
R
=
d
d
d
d
d
d
γ
γ
R
R
Annex B of Eurocode 7 Part 1 offers guidance on which of the 3 formulae above to use for each
design
approach
(see Section
5.7)
.
5.5 Ultimate limit states
Eurocode 7 lists five limit states to be considered in the design process:
EQU
: the loss of equilibrium of the structure or the supporting ground when considered as a rigid body
and where the internal strengths of the structure and the ground do not provide resistance (e.g. Fig.
5.8a
). This limit state is satisfied if the sum of the design values of the effects of destabilising actions
(E
dst; d
) is less than or equal to the sum of the design values of the effects of the stabilising actions
(E
stb; d
) together with any contribution through the resistance of the ground around the structure (T
d
),
i.e. E
dst; d
≤
E
stb; d
+
T
d
. In most cases, the contribution to stability from the resistance of the ground
around the structure will be minimal so T
d
will be taken as zero.
GEO
: failure or excessive deformation of the ground, where the soil or rock is significant in providing
resistance (e.g. Figs
5.8b, 5.8c, 5.8d and 5.8e
). This limit state is satisfied if the design effect of the
actions (E
d
) is less than or equal to the design resistance (R
d
), i.e. E
d
≤
R
d
.
STR
: failure or excessive deformation of the structure, where the strength of the structural material is
significant in providing resistance (e.g. Fig.
5.8f
). As with the GEO limit state, the STR is satisfied if the
design effect of the actions (E
d
) is less than or equal to the design resistance (R
d
), i.e. E
d
≤
R
d
.
UPL
: the loss of equilibrium of the structure or the supporting ground by vertical uplift due to water pres-
sures (buoyancy) or other actions (e.g. Fig.
5.9a
). This limit state is verified by checking that the sum of
the design permanent and variable destabilising vertical actions (V
dst; d
) is less than or equal to the sum
of the design stabilising permanent vertical action (G
stb; d
) and any additional resistance to uplift (R
d
)
HYD
: hydraulic heave, internal erosion and piping in the ground as might be experienced, for example,
at the base of a braced excavation. This limit state is verified by checking that the design total pore
water pressure (u
dst; d
) or seepage force (S
dst; d
) at the base of the soil column under investigation is less
