Geoscience Reference
In-Depth Information
is simply that the human side of the process is organised to realise the best output from
the numerical modelling.
Theabovesetsaverybroadscenario.Thispaperisconcernedonlywithshallowfounda-
tions.Afurtherlimitationisthatitwillnotbeconsideringliquefactioneffectsonshallow
foundations.
The paper starts with consideration of the bearing strength of shallow foundations under
earthquake loading. The conventional wisdom is that the design of shallow foundations
is controlled by considerations of settlement and differential settlement rather than bear-
ing strength. This may be true enough for foundations subject only to static vertical
load, but when cyclic moment loading is involved, such as in earthquake, wind and
wave loading, then the stability is extremely sensitive to small increases in the applied
moment. The designer needs to take account of this. Next the effect of soil variability
relative to the variability of the properties of structural elements is discussed. Then the
limitations of bed-of-spring models for shallow foundations is discussed. Finally, an
example of the integrated design of three-story framed structure on shallow foundations
is presented.
2. Ultimate limit state design of shallow foundations in Eurocode 8
Bearing strength theory gives us a way of estimating what combinations of vertical load,
horizontal shear, and moment mobilise all the available shear strength of the soil under-
lying and surrounding a shallow foundation. The sum total of these combinations forms
a bearing strength surface in athree-dimensional space.
Aconvenientwayofpresentingthesurfacesistouseaxesdefinedintermsofdimension-
lessparameters,oneforverticalload,anotherforhorizontalshearandathirdformoment
applied tothe foundation. The suiteof dimensionless parameters isdefined as
V
V
uo
,
H
V
uo
,
M
V
uo
B
V
=
H
=
M
=
(10.1)
where:
B
is the width of thefoundation
V
uo
istheultimatevertical thatmaybeappliedtothefoundation, intheabsence ofshear
and moment loading, evaluated usingconventional bearing capacity equations
V
,
H
and
M
are acombination ofactions that induce an ultimate limitstate, i.e. the
coordinates of apoint on thebearing strengthsurface
V
,
H
and
M
arethe normalised foundation actions.
To account for the effect of seismic inertia in the material beneath the foundation the
following twoadditional dimensionless parameters areused.
