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foundation the state paths lie within the bearing strength surface. With the reduced
moment demand on the footings itisalso possibletouse smaller footings.
Onequestionthatcomestomindwhenthecolumn-footingconnectionsarepinnedisthe
deformation of the structure during the earthquake shaking. Figure 10.19 gives the peak
horizontal displacement envelopes for the two cases. It is clear that the profiles are well
within the driftlimitsspecified inNZS 1170.5.
6. Conclusions
In this paper we have discussed four aspects of the design of shallow foundations to
resist earthquakes and as a sub-theme have indicated that the way ahead is to develop
theintegrateddesignofstructure-foundationsystemswhichwillrequiretightinteraction
between the structural and geotechnical communities.
Firstly the shallow foundation bearing strength surface shows that the combinations of
verticalload,horizontalshearandmomentthatleadtobearingfailurearequitecomplex.
To assess footing behaviour under earthquake loading requires more than a conventional
bearingstrengthcalculation.HowevertheRuaumokocalculationsattheendofthepaper
show that there is useful insight to be gained by looking at the response of a complete
model of the structure-foundation system.
Variability of the soil properties is much greater than the variability of structural mem-
ber properties. This means that careful checking is necessary to ensure that the design
considers themost critical case.
Figures 10.10 and 10.13 indicate that, although a simple representation of shallow foun-
dation behaviour, the bed-of-springs model is of limited value in situations where both
the vertical load and moment are changing during the course of the earthquake.
Finally, as a demonstration of the potential of integrated design of structure-foundation
systems, an example of a low-rise structure on shallow foundations indicated that
moment-freecolumn-footingconnectionsgivemoreeconomicalfootingdesign,evenso,
thestructure-foundationsystemstillachievessatisfactorylateraldisplacementbehaviour
of the structureduring earthquake loading.
Clearlythereismuchscopeforfuturedevelopmentoftheintegrateddesignofstructure-
foundation systems.
REFERENCES
Ahmed-Zeki AS, Pender MJ, Fitch NR (1999) Strain-rate effects on the undrained shear strength
of Waitemata residual clay. Proc. of 8th Australia-NZ Conference on Geomechanics, Hobart,
Vol. 2, pp 791-796
Andersen KH, Pool JH, Brown SF, Rosenbrand WF (1980) Cyclic static and laboratory tests on
Drammen clay. Journal of Geotechnical Engineering, 106(GT5):499-529
