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Seismic Response of Superstructure on Soft Soil Considering Soil-Pile-Structure
Interaction
Influence de l'Interaction sol- pieu- structure sur la réponse sismique de la superstructure
sur sol mou
Hokmabadi A.S., Fatahi B., Samali B.
School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Broadway NSW 2007, Australia
ABSTRACT: This paper presents results of shaking table tests and three dimensional numerical simulations to investigate the
influence of Soil-Pile-Structure Interaction (SPSI) on the seismic response of mid-rise moment resiting buildings supported by end-
bearing pile foundations. Three different cases have been considered, namely: (i) fixed-base structure representing the situation
excluding the soil-structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by
end-bearing pile foundation in soft soil. Comparison of the numerical predictions and the experimental data shows a good agreement
confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil-structure interaction
induces significant increase in the lateral deflections and inter-storey drifts of the structures on both shallow and end-bearing pile
foundations in comparison to the fixed base structures. This increase in the lateral deformations and in turn inter-storey drifts can
change the performance level of the structure during earthquakes which may be safety threatening.
RÉSUMÉ : Cet article présente les résultats des essais sur table vibrante et trois dimensions simulations numériques pour étudier
l'influence de l'Interaction sol-pieu-structure (ISPS) sur la réponse sismique des bâtiments pris en charge par les fondations sur pieux.
Trois cas différents ont été examinés, à savoir: (i) la structure de base fixe sans interaction sol-structure; (ii) la structure soutenue par
la fondation superficielle sur sol mou; et (iii) la structure soutenue par la fondation sur pieux dans le sol mou. Les prédictions
numériques et les données expérimentales montrent un bon accord. Résultats expérimentaux et numériques indiquent que l'interaction
sol-structure augmente les déflexions latérales et les dérives inter étage des structures en comparaison avec les structures de base
fixes. Cela peut changer le niveau de performance de la structure lors de tremblements de terre qui peuvent être un problème
d'innocuité.
KEYWORDS: soil-pile-structure interaction, seismic response, shaking table test, FLAC3D, end-bearing pile foundation
1 INTRODUCTION
The problem of soil-pile-structure interaction in the seismic
analysis and design of structures has become increasingly
important, as it may be inevitable to build structures at locations
with less favourable geotechnical conditions in seismically
active regions. Influence of the underlying soil on seismic
response of the structure can be ignored if the ground is stiff
enough, and the structure can be analysed considering fixed-
base conditions. However, the same structure behaves
differently when it is constructed on the soft soil deposit.
Earthquake characteristics, travel path, local soil properties, and
soil-structure interaction are the factors affecting the seismic
excitation experienced by structures. The result of the first three
of these factors can be summarised as free-field ground motion.
However, the foundation is not able to follow the deformation
of the free field motion due to its stiffness, and the dynamic
response of the structure itself would induce deformation of the
supporting soil (Kramer 1996).
Over the past decades, several researchers (e.g. Tajimi 1969,
Gazetas 1991, Shiming and Gang 1998, Hokmabadi et al. 2011,
Carbonari et al. 2011, Tabatabaiefar et al. 2013) have studied
the seismic soil-pile-structure interaction (SSPSI) and the effect
of this phenomena on the response of the structures. The
developed analytical methods for studying the soil-pile-structure
interaction may be categorised into three groups: (i)
Substructure Methods (or Winkler methods), in which series of
springs and dashpots are employed to represent the soil
behaviour (e.g. Hokmabadi 2012); (ii) Elastic Continuum
Methods, which are based on Mindlin (1936) closed form
solution for the application of point loads to a semi-infinite
elastic media; and (iii) Numerical Methods. The substructure
methods are the simplest and most commonly used methods,
however, these methods adopting the substructuring concept
rely on the principle of superposition, and consequently, are
limited to either the linear elastic or the viscoelastic domain
(Pitilakis et al. 2008).
The dynamic equation of motion of the soil and structure
system can be written as:
[M]{ü}+[C]{ů}+[K]{u}= -[M]{m}ü
g
+{F
v
}
(1)
where, {u}, {ů}, and {ü} are the nodal displacements, velocities
and accelerations with respect to the underlying soil foundation,
respectively. [M], [C] and [K] are the mass, damping, and
stiffness matrices of the structure, respectively. It is more
appropriate to use the incremental form of Equation (1) when
plasticity is included, and then the matrix [K] should be the
tangential matrix and {ü} is the earthquake induced acceleration
at the level of the bedrock. For example, if only the horizontal
acceleration is considered, then {m}=[1,0,1,0,....1,0]
T
. {Fv} is
the force vector corresponding to the viscous boundaries. This
vector is nonzero only when there is a difference between the
