548
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
motion on the near side of the artificial boundary and the
motion in the free field (Wolf 1985).
The present research aims to study the effects of SSPSI on
the seismic response of the superstructure by employing the
fully nonlinear method in which main components of the
interaction including subsoil, pile foundation, and superstructure
are modelled simultaneously. For this purpose, a three-
dimensional explicit finite-difference program, FLAC3D (Itasca
2009), is used to numerically model and examine the influence
of the soil-structure interaction on the seismic response of a 15-
storey moment resiting building. Two types of foundations
including shallow foundations and end-bearing pile foundations
have been considered. The proposed numerical soil-structure
model has been verified and validated against experimental
shaking table test results.
2 SHAKING TABLE EXPERIMENTAL TESTS
2.1 Prototype characteristics and scaling factors
In order to provide a calibration benchmarks for the numerical
simulation and to make quantitative predictions of the prototype
response several of shaking table tests have been conducted.
Previous researchers (e.g. Meymand 1998, Chau et al. 2009)
modeled the superstructure as a simplified single degree of
freedom oscillator in which the behaviour of the soil-structure
system may not be completely conform to reality and the higher
modes would not be captured. In the current model tests, unlike
the previous efforts, a multi-storey frame for the superstructure
is adopted representing most of the dynamic properties of the
prototype structure such as natural frequency of the first and
higher modes, number of stories, and density. The experimental
model tests have been carried out utilising the 3×3 m shaking
table facilities located at structures laboratory of the University
of Technology Sydney (UTS).
The selected prototype structure is a fifteen-storey concrete
moment resisting building frame with the total height of 45 m
and width of 12 m consisting of three spans, representing the
conventional types of mid-rise moment resisting buildings. The
spacing between the frames into the page is 4 m. Natural
frequency of the prototype building is 0.384 Hz and its total
mass is 953 tonnes. The soil medium beneath the structure is a
clayey soil with the shear wave velocity of 200 m/s and density
of 1470 kg/m3. The horizontal distance of the soil lateral
boundaries and bedrock depth has been selected to be 60 m and
30 m, respectively. The building is resting on a footing which is
4 m wide and 12 m long. For the pile foundations case, a 4×4
reinforced concrete pile group with equal spacing and pile
diameter of 1.25 m and 30 long are considered. The piles are
embedded into the bedrock representing typical end-bearing pile
foundations.
In order to achieve a reasonable scale model, a dynamic
similarity between the model and the prototype is applied as
described by Meymand (1998). Dynamic similarity governs a
condition where homologous parts of the model and prototype
experience homologous net forces. Although small scale models
could save cost, the precision of the results could be
substantially reduced. Considering the specifications of UTS
shaking table, scaling factor of 1:30 is adopted for experimental
shaking table tests on the scale model which provides the largest
achievable scale model with rational scales, maximum payload,
and overturning moment meeting the facility limitations.
2.2 Shaking table tests model components
The developed soil-structure model for shaking table tests
possesses four main components including the model structure,
the model pile foundations, the laminar soil container, and the
soil mix. Employing geometric scaling factor of 1:30, height,
length, and width of the structural model are determined to be,
1.50 m, 0.40 m, and 0.40 m, respectively. In addition, the
required natural frequency of the structural model is 2.11 Hz.
The model structure has been designed employing SAP2000
(CSI 2010) software to meet the required characteristics, and
finally a 500×500×10 mm steel plate as baseplate, fifteen
400×400×5 mm horizontal steel plates as the floors and four
500×40×2 mm vertical steel plates as the columns are adopted.
The completed structural model is shown in Figure 1.
Control room
Model structure
Displacement
transducers
Shaking table
Laminar Soil container
Soil mix
Figure 1. Final setup of the shaking table tests for the structure with
end-bearing pile foundation
Similar to the model structure, the model pile is subjected to
the competing scale model criteria. The model piles have a
diameter of 40 mm with L/d ratio of 25. By selecting a
commercial Polyethylene high pressure pipe with Standard
Dimension Ratio (SDR) of 7.4 the model piles fall in the range
of acceptable criteria with 5% deviation from the target value
for EI.
The ideal soil container should simulate the free field soil
response by minimising boundary effects. Since the seismic
behaviour of the soil container affects the interaction between
the soil and structure, the performance of the soil container is of
the key importance for conducting seismic soil-structure
interaction model tests successfully (Pitilakis et al. 2008). A
laminar soil container with final length, width, and depth of
2.10m, 1.30m, and 1.10m, respectively, are designed and
constructed for this study. The employed laminar soil container
consists of a rectangular laminar box made of aluminium
rectangular hollow section frames separated by rubber layers.
The aluminium frames provide lateral confinement of the soil,
while the rubber layers allow the container to deform in a shear
beam manner.
A synthetic clay mixture was designed to provide soil
medium for the shaking table testing considering required
dynamic similarity characteristics. Several mixtures were
examined and finally the desired soil mix (60% Q38 kaolinite
clay, 20% Active-bond 23 Bentonite, 20% class F fly ash and
lime, and water, 120% of the dry mix) produced the required
scaled shear wave velocity of 36 m/s at the second day of its
cure age. Accordingly, the soil density and undrained shear
strength on the second day were determined to be 1450 kg/m
3
and 3.14 kPa, respectively.
The shaking table tests have been carried out in three stages:
fixed-base condition, shallow foundations, and end bearing pile
foundations. Since the properties of the designed soil mix is
time depended, the second and third stages should be carried out
