Geoscience Reference
In-Depth Information
designing a bridge foundation to uplift in rocking has been proposed as an effec-
tive seismic isolation method by Kawashima and Hosoiri (2005). Moreover, even
with very slender and relatively rigid structures, uplifting would not lead to over-
turningexceptinratherextremecasesoflittleconcerntotheengineer(Makrisand
Roussos, 2000; Gerolymos et al.,2005).
In soft and moderately-soft soils much of what was said above is still valid, but inelastic
actioninthesoilisnowunavoidableunderthesupportingedgeoftheupliftingfootingin
rocking. At the extreme, inelastic deformations in the soil take the form of mobilization
of failuremechanisms, as discussed below.
(c)
Mobilisation of bearing capacity failure mechanisms in the supporting soil
: Such
inelasticactionunderseismicloadingwouldalwaysbeaccompaniedwithuplifting
of the foundation. In static geotechnical analysis large factors of safety are intro-
duced to ensure that bearing capacity modes of failure are not even approached.
In conventional seismic analysis, such as in the EC8 - Part 5 bearing capacity is
avoided thanks to an “overstrength” factor of about 1.40. The oscillatory nature of
seismic shaking, however, allows the mobilisation (for a short period of time!) of
the maximum soil resistance along a continuous (“failure”) surface. No “collapse”
oroverturningfailureoccurs,astheapplied(causative)moment“quickly”reverses,
and a similar bearing-capacity “failure” mechanism may develop under the other
edgeofthefoundation.Theproblemagainreducestocomputingtheinelasticdefor-
mations, which in this case means
permanent rotation
. The designer must ensure
that itsconsequences are not detrimental.
The concept of allowing mobilization of bearing capacity mechanisms in foundation
design may represent a major change in foundation design philosophy (FEMA, 1997;
Pecker, 1998). However, for analysis of the ultimate response of a structure-foundation
system to extreme earthquake shaking, accounting for such a possibility is necessary.
Martin and Lam (2000) illustrate with an example of a hypothetical structure contain-
ing a shear wall connected with a frame how dramatically different are the results of
analyses in which inelastic action in the soil is considered or is ignored. With inelastic
action (including uplifting) the shear wall “sheds” some of its load onto the columns of
the frame, which must then be properly reinforced; the opposite is true when linear soil-
foundationbehaviourisassumed.Thus,computingtheconsequencesof“plastichinging”
in shallow foundation analysis may be anecessity.
The interplay between uplifting and mobilization of bearing capacity mechanisms is
governed
primarily
bythe following factors:
theverticalfoundationloadNincomparisonwiththeultimateverticalcapacityN
ult
,
expressed through the ratio
•
χ =
N
/
N
u
,
theheight, h, ofthemasscentre ofgravity fromthe base compared withthefounda-
tion dimensions (width B, length L) and
•
•
theintensity, frequency content and sequence ofpulses of the seismicexcitation.
