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5. Integrated design of structure-foundation systems
In recent years there has been a rising demand for superior performance under earth-
quake loading of both existing and new infrastructure. Assessing accurately the existing
state of foundation systems is particularly demanding. By considering the structure and
foundation as an integrated system, new opportunities may arise for achieving superior
performance; an example of this is the purpose of this part of the paper which extends
initial work in thisarea (Wotherspoon et al., 2004a, b).
ComputermodellingwasundertakenusingRuaumoko(Carr,2004),anonlineardynamic
structural analysis program. Yielding of beams and columns in the structure can be
included.Bothyieldingofthefoundationsandupliftcanbemodelled.Variousfoundation
and structural characteristics were investigated to demonstrate effects on the behaviour
ofthewholesystem.ThepurposeofusingasoftwarepackagesuchasRuaumoko,which
is intended for structural analysis, is to investigate what can be achieved with existing
facilities and also to develop an environment to enhance communication between struc-
tural and geotechnical specialists. Discussion is confined to low-rise framed structures
on shallow foundations where foundation uplift is the main challenge for the numerical
model.
Existing Ruaumoko elements allow uplift when the vertical load on the foundation
becomeszero,butdonotallowdetachmentoftheshearandmomentspringsattheinstant
of uplift. A detaching foundation element that uplifts when the vertical load on the foun-
dationiszeroandalsodetachesshearandmomentspringsatthesametimehasnowbeen
developed for Ruaumoko.
5.1. STRUCTURE DESCRIPTION
The design of a three-storey framed structure with shallow foundations is considered,
with the details of the structure illustrated in Figure 10.14. As can be seen, the structure
is five bays long and three bays wide, each bay is 7.5m by 9.0m and the storey heights
are3.65mwiththeexceptionofthefirststoreywhichis4.50m.Theshallowfoundations
were located ina layer of clay withan undrained shear strengthof 100kPa.
The seismic load on each floor was equivalent to 8.65kPa, the roof seismic weight
was comprised of a 6.75kPa distributed load and 1000kN of plant. The basis of these
loads was the imposed load required by current New Zealand structural design actions
standard, NZS 1170.1 (Standards New Zealand, 2002), and the permanent load resulting
from reinforced concrete frames supporting prestressed precast concrete floor slabs with
65mm of sitepoured concrete topping.
Following NZS 1170.5 (Standards New Zealand, 2004) and NZS 3101 (Standards New
Zealand, 1995), structural models were designed such that all members contributed to
the seismic resistance of the structure and each frame parallel to earthquake propaga-
tion had an identical member configuration. Both nominally elastic and limited ductility
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