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In current design office practice, a commonly used modelling assumption is that the base of a building structure can be idealised with fixed support conditions, thereby neglecting any effects from soil-structure interaction (SSI). Various recent studies, however, have shown that the explicit consideration of SSI effects in seismic analysis of buildings structures may significantly affect the predicted seismic demands and resulting structural performance. This study addresses some key issues and practices in the area of SSI and its effects on the dynamic response and seismic performance of buildings. It is also intended to demonstrate the significance of considering SSI effects in structural modelling and analysis while providing key insights into practical applications in real projects. Using a forty storey example building, the effect of considering SSI on the predicted seismic performance is demonstrated. For the purpose of comparison, five detailed computer models (one without considering any SSI effects, two models with SSI modelled using indirect approach, and two models with SSI modelled using direct approach) of the example building were constructed and subjected to various input ground motions. It is observed that depending upon the modelling approach used, the consideration of SSI effects may affect the predicted seismic performance in varying degrees. Moreover, the direct modelling approach presented in this study may provide improved results compared to various approximate methods.
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In current design office practice, a commonly used modelling assumption is that the base of a
building structure can be idealised with fixed support conditions, thereby neglecting any effects
from soil-structure interaction (SSI). Various recent studies, however, have shown that the explicit
consideration of SSI effects in seismic analysis of buildings structures may significantly affect the
predicted seismic demands and resulting structural performance. This study addresses some key
issues and practices in the area of SSI and its effects on the dynamic response and seismic performance
of buildings. It is also intended to demonstrate the significance of considering SSI effects in structural
modelling and analysis while providing key insights into practical applications in real projects.
Using a forty storey example building, the effect of considering SSI on the predicted seismic
performance is demonstrated. For the purpose of comparison, five detailed computer models (one
without considering any SSI effects, two models with SSI modelled using indirect approach, and
two models with SSI modelled using direct approach) of the example building were constructed and
subjected to various input ground motions. It is observed that depending upon the modelling approach
used, the consideration of SSI effects may affect the predicted seismic performance in varying
degrees. Moreover, the direct modelling approach presented in this study may provide improved
results compared to various approximate methods.
Keywords: soil-structure interaction; inertial interaction; kinematic interaction; high-rise buildings;
direct approach; substructure approach; soil modelling.
1. INTRODUCTION
One of the challenging aspects in structural modelling of building structures is the accurate idealisation
of below-grade components including foundation, soil and their interaction. The characteristics and
mechanical properties of medium on which a structure is founded may significantly influence the
seismic responses of structure. If the structure is supported on a solid rock, the seismic behaviour
of the structure will be similar to a fixed-base structure subjected to free field motion. However, if
the soil is deformable, the structure responds to the soil dynamics and in return, the soil also may
respond to the dynamics of the structure - a phenomenon referred to as the soil-structure interaction
(SSI). This phenomenon may ultimately cause the response of structure to be governed by the
interaction of soil and structure as well as the characteristics of input ground motions.
SIGNIFICANCE OF SOIL-STRUCTURE INTERACTION IN SEISMIC RESPONSE
OF BUILDINGS
NED UNIVERSITY JOURNAL OF RESEARCH-SPECIAL ISSUE ON FIRST SOUTH ASIA CONFERENCE ON EARTHQUAKE ENGINEERING (SACEE'19), Vol. 1, 2019
Naveed Anwar1, Abinayaa Uthayakumar2, Fawad Ahmed Najam3
1 Executive Director, AIT Solutions, Asian Institute of Technology, Bangkok, Thailand, Ph. +66819206569, Email: nanwar@ait.ac.th.
2 Structural Engineer, AIT Solutions, Asian Institute of Technology, Bangkok, Thailand, Ph. +66614251071, Email: abinayaa.u@gmail.com.
3 Assistant Professor, National University of Sciences and Technology (NUST), Islamabad, Pakistan, Ph. +923345192533,
Email: fawad@nice.nust.edu.pk.
ABSTRACT
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Naveed Anwar is an Executive Director at Asian Institute of Technology (AIT) Solutions,
Thailand. He is also the Director of Asian Center for Engineering Computations and Software
(ACECOMS) and heads the Habitech Building Technology Center at AIT, Thailand. His
professional carrier spans over 35 years. He is an author of over one hundred publications
including books, conferences and journal papers.
The consideration of SSI effects in modelling and analysis of structures is a complex and tedious
task which requires significant expertise and skills. Moreover, the problem is more complicated due
to several possible scenarios of foundation embedment in soil. Consider, for example, the case of
foundation for a simple reinforced concrete (RC) column. Figure 1 shows some of the practical
configurations of columns embedded in soil. Depending on the size of columns, type of soil and
presence of lateral loads, the column may become effectively fixed or flexible against rotation
(Figure 1(a)). In case of columns or piers in waterways, the soil erosion, silting and scour may keep
changing the SSI effects with time. Another factor which may make the problem more complex is
the amount of restraint or fixity provided by back fill and compacted soils. Similarly, consider case
c where the column size below the soil level is larger than the main column. So, the effect of larger
cross-section as well as the larger restraint due to soil needs to be taken into account. At the same
time because of the larger column near the footing, it is not immediately clear as to which column
dimensions should be considered in determining the effective length for simplified modelling
approaches. In this case, one can use the concept of inverse stiffness (or flexibility) to determine
the stiffness of the variable column. Case (d) is further complicated by the presence of a concrete
floor which is not rigidly connected to the column but nevertheless provides significant lateral
restraint. The compact and well constrained compact filling under the floor provide further restraint
which is numerically difficult to evaluate. One simplified option in this case can be to consider the
column as hinge-supported. However, this hinged condition should also be considered in modelling
of the frame to obtain consistent and compatible moment in the column. An even more complex
case is that of a column supported on a pile cap and pile foundation. If the piles are driven into soft
top layer or are exposed above the firm soil layers, then the column-pile cap-pile-soil system will
interact together to determine the effective bending length or effective length of the whole system.
The solution to such problems is to carry out a P-
D
analysis of the complete system or to estimate
the effective length from a first order analysis. This example shows the complexities involved in
the practical cases of considering SSI effects even for the case of a simple column foundation.
Abinayaa Uthayakumar is a Structural Engineer at Asian Institute of Technology (AIT) Solutions,
Thailand. She received her Masters in structural engineering from AIT, Thailand. Her research
interests include soil-structure interaction, seismic performance assessment of buildings and
geotechnical earthquake engineering.
Fawad Ahmed Najam is an Assistant Professor at National University of Sciences and Technology
(NUST), Pakistan. He received his PhD from Asian Institute of Technology (AIT), Thailand.
His research interests include earthquake engineering, structural dynamics, seismic performance
evaluation of tall buildings, and seismic hazard and vulnerability assessment.
Figure 1. Levels of complexity involved considering SSI effects-various practical cases
and configurations of RC columns embedded in soil.
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2. EFFECTS OF SSI ON DYNAMIC RESPONSE OF BUILDINGS
There are two types of interactions which can influence the response and performance of structures
due to soil-foundation-structure interaction. These are referred to as the inertial interaction and
kinematic interaction. The inertial interaction is mainly related to the base shear and base moment
and may result in the elongation of natural period and additional damping effects. The most important
parameter controlling the inertial SSI effects is the structure-to-soil stiffness ratio as shown in Eq. (1).
where h is the height of the centre of mass of building in first vibration mode shape, Vs is shear
wave velocity and T is the fundamental period corresponding to fixed-base structural model. If the
structure-soil stiffness ratio is greater than 0.1, a strong inertial interaction may exist. This ratio is
typically low for high-rise buildings [1].
Figure 2(a) shows the typical variation of natural period (normalised to fixed-base period) and
foundation damping ratio (normalised to fixed-base damping ratio) with structure-soil stiffness ratio
for different values of h/B (where h is structure height and B is half-width of foundation). It can be
seen that the foundation damping increases with structure-soil stiffness and decreases with increasing
value of h/B, indicating that energy dissipation is more through foundation's lateral movement than
rocking. The natural period also elongates significantly with increasing structure-soil stiffness ratio.
Figure 2(b) illustrates the effect of inertial interaction on base shear for elastic response of structures.
It can be observed that for positive slope the base shear tends to increase and for negative slope the
base shear decreases. Hence, the long-period structures tend to have reduced base shear due to the
effect of inertial interaction.
The kinematic interaction, on the other hand, deals with the ground motion and deems the difference
between free-field motion (motion unaffected by wave scattering or any structural vibration signature)
and foundation input ground motion (the theoretical motion at base slab if the structure and foundation
had no mass). The difference is due to structures configurations such as the rigidity and size of the
base (base-slab averaging), and the depth of foundation below ground (embedment). Kinematic
interaction models are often interpreted as transfer functions of free-field motions to foundation
input motions [1].
3. SSI EFFECTS-LOW-RISE VERSUS HIGH-RISE BUILDINGS
The inertial and kinematic interactions may not affect the seismic performances of low- and high-
rise buildings in the same manner. For the low-rise buildings, the most important parameter governing
their dynamic response is the natural period for fundamental mode of vibration. The design base
shear is a function of natural period in the corresponding direction (Figure 2(b)). For the purpose
of simplified structural design, the design codes often provide smooth response spectrum curves
to estimate the design base shear coefficient as shown in Figure 3 [2]. The seismic loading in terms
of spectral acceleration or base shear coefficient (base shear normalised to the total seismic weight
of the structure) can be directly determined from this response spectrum corresponding to the natural
periods of vibration of any structure.
To demonstrate the effect of different soil properties, the results of two example buildings (a five
storey low-rise building and a forty storey high-rise building) supported on a soft soil, stiff soil and
rock (from NEHRP soil profile type classification) is compared with that of a fixed-base condition.
Figure 2. Response of square footing: (a) period lengthening and foundation damping
versus structure-soil stiffness; (b) effects on spectral acceleration.
Structure-soil stiffness ratio = h/VST(1)
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Figure 3. Effect of soil type on natural periods and resulting base shear coefficient.
The effect on time period and associated alteration in base shear coefficient is considered and is
shown in Figure 3. It is observed that due to the elongation in natural period, the base shear
coefficient is reduced by twenty percent compared to the fixed-base condition for the low-rise
building. However, there is not much reduction in base shear coefficient for high-rise building. The
comparison of natural periods and base shear coefficients for other soil types is tabulated in Table 1
T/T is the ratio of flexible-base period to fixed-base period, while C/C is the corresponding ratios
of base shear coefficients). It can be seen that rock base is resulting in a periods almost equal to
fixed-base condition. However, for other soil types, there is an elongation in natural period (resulting
in reduction in base shear coefficient) in varying degrees. Therefore, for low-rise buildings, the
consideration of SSI is often regarded as beneficial due to time period elongation and the associated
reduction in design base shear. As mentioned earlier, the design engineers make use of this phenomenon
in retrofitting and design of low-rise buildings [1]. However, studies such as by Maheshwari and
Sarkar [3] and Saez et al. [4] urge the importance of including SSI for low-rise buildings due to its
potential influence on overall seismic responses.
4. CONSIDERATION OF SSI EFFECTS-EXISTING RESEARCH AND CURRENT
PRACTICE
In current design office practice, the most widely used approach is to idealise the structures base
with fixed support condition, while ignoring any base flexibility. The base is then subjected to free-
field ground motion records for the purpose of dynamic analysis. Similarly, in case of high-rise
buildings having deep subterranean levels, the SSI effects along the deep basement walls are generally
ignored and the ground motion records are applied at fixed base. However, several recent studies
have shown that the explicit consideration of SSI effects in seismic analysis of buildings structures
may significantly affect the predicted seismic demands and resulting structural performance. As a
result of this recent realisation, the inclusion of SSI is gradually trending as a part of the performance-
based evaluation in retrofitting of existing buildings.
The 1971 San Fernando earthquake first caught the attention towards SSI effects. Since then, several
studies have been conducted in light of understanding the effects of SSI and its importance on
seismic responses of various types of structures. For example, Turek et al. [5] and Fatahi et al. [6]
conducted laboratory experiments including shake table tests, ambient vibration tests and micro-
tremor tests to study building behaviour due to SSI. Various techniques considering equivalent-
linear soil behaviour were employed by Naeim et al. [7], Li et al. [8], Ellison et al. [9] and Lu et al.
[10] to study the effect of SSI on behaviour of high-rise buildings. Naeim et al. [7] observed that
though inclusion of SSI may not result in a significant increase in natural time period, it does
Table 1. Effect of different soil types on fundamental natural periods and base shear
coefficients of example low-rise and high-rise buildings
Soil type
Soft
Stiff
Rock
Fixed-base
Shear wave
velocity,
m/sec (ft/sec)
100 (328)
300 (984)
1000 (3281)
-
Low-rise building
Natural
period
(sec)
0.581
0.504
0.494
0.494
T/T
1.18
1.02
1
1
~C/C
0.8
0.95
1
1
~High-rise building
Natural
period
(sec)
4.601
4.558
4.482
4.424
T/T
1.04
1.03
1.01
1
~C/C
0.96
0.97
1
1
~
~ ~
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NED UNIVERSITY JOURNAL OF RESEARCH-SPECIAL ISSUE ON FIRST SOUTH ASIA CONFERENCE ON EARTHQUAKE ENGINEERING (SACEE'19), Vol. 1, 2019
significantly affect the vertical distribution of inter storey drift. Ellison et al. [9] considered the
nonlinear soil behaviour but focused on the interaction between a deep, long embedded trainbox
and the adjacent deep basement of a tall building. This study emphasised the need to consider SSI
in dense, urban areas as the adjacent embedded trainbox had a significant effect on the demands
of basement walls.
There exists a gap between the state of existing knowledge and the state of practice in the area of SSI
and its consideration for seismic analysis of high-rise building structures. Beside the need of improved
understanding, more skills and expertise, an important reason for this gap can be the unavailability
of appropriate practical recommendations and guidelines by standards and codes [11, 12]. For high-
rise building structures with basements, the consideration of SSI can be an important factor in seismic
performance as it can substantially affect their seismic responses, as observed by [7] Naeim et al. and
Ellison et al. [9]. Given the modern computational capabilities, it is favourable to implement SSI in
seismic design of high-rise buildings. Nevertheless, an ambiguous question is whether adopting
tedious approaches of modelling SSI actually produce significantly more accurate results.
5. MODELLING APPROACHES FOR SSI-CHALLENGES AND OPPORTUNITIES
Currently, the modelling approaches to account for the SSI effects in the analysis of building
structures can be classified under two categories. The first approach, referred to as the direct approach,
represents the soil continuum surrounding the building foundation with three-dimensional finite
elements. This continuum is truncated at a certain distance from foundation by using special absorbing
boundary conditions. The second approach, sometimes referred to as the simplified substructure
approach, represents the surrounding soil with a series of springs and dashpots. The mechanical
properties, modelling parameters and recommendations for using this approach are available in
various guidelines including NIST [1] and PEER [13]. Since the direct approach is based on an
explicit finite element modelling of surrounding soil, it is considered a relatively more accurate
method than the simplified substructure approach to consider SSI effects and is widely adopted in
SSI research studies.
Although several guidelines and recommendations for modelling SSI effects using simplified
approach are available [1, 13], it is seldom considered in practical applications. Even when the SSI
is implemented, it is often limited to the modelling of vertical foundation springs. Among the projects
that included SSI, greater applications are generally found in retrofitting of existing low-rise buildings
as a part of the performance-based assessment than in the design of new buildings. One major reason
is that even simplified modelling of SSI requires significant expertise and skills which most of the
practicing designers may not be able to develop under conventional capacity building trainings and
programs. The consideration of SSI effects in practice also requires a close collaboration between
the structural and geotechnical engineers. However, in many projects, the geotechnical engineers
are generally not a part of the design team meetings that are set up by the architect and project
owner. Also, the consideration of SSI effects may significantly increase the analysis and post-
processing time, which the practicing engineers may sometimes not afford. Considering the modelling
phase of SSI, existing studies [7] reported several difficulties which may be encountered while
modelling the SSI. The detailed finite element modelling of SSI is considered to be tedious. However,
several studies have managed to implement and test this approach by excluding various SSI effects
such as base-slab averaging and by scaling down the complexity involved with soil medium and
wave motions such as assuming only linear behaviour of soil, and depth invariable motions, etc.
In this study, while discussing some key issues and practices in the area of SSI and its effects on
the dynamic response and seismic performance of low- to medium-rise buildings, the significance
of considering these effects in high-rise buildings is also demonstrated using an example building.
As mentioned earlier (and is also demonstrated in Figure 3), the base shear coefficient for high-
rise building supported on soft soil is almost same as that of a fixed-base condition. Similarly,
Table 1 showed that the maximum reduction in base shear coefficient is only about four percent.
Due to such negligible effects on natural periods, generally it is believed that the SSI effects are
not significant for high-rise buildings. However, as stated earlier, several recent studies have indicated
the potential influence of SSI on seismic responses of high-rise buildings. The case of high-rise
buildings will therefore be evaluated here in detail using a case study building. For this purpose, a
forty storey RC core wall building is selected and subjected to detailed analysis. The effect of
considering SSI on the predicted seismic performance of this case study building is demonstrated.
The objective is to study the effect of SSI on a high-rise case study building structure by comparing
seismic responses of the model without SSI and the code-based models with SSI. For simplicity,
only the linear behaviour of the structure is considered in this study.
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6. SSI EFFECTS IN HIGH-RISE BUILDINGS-A CASE STUDY
6.1 Site Characteristics and Modelling
An existing forty storey case study building with RC core wall is selected for the detailed performance
evaluation with and without considering the SSI effects. The building has four basement levels and
is resting on a mat foundation (Figure 4: Model 1). The soil profile chosen is located in Makati
City, Manila, Philippines. The soil profile at the site of building (shown in Figure 4) is derived from
the actual soil profile studies in Philippines. Figure 4 shows that the profile consists of two layers
(soft-to-medium stiff clay up to 15 m (49 ft) and weathered rock up to 40 m (130 ft). For the detailed
performance evaluation, a total of five detailed three-dimensional (3D) finite element models were
developed. The baseline model (designated as Model 1) represents the typical fixed support idealisation
which commonly used in design practice (completely ignoring any SSI effects). Generally, this
model is recommended to be used for the service-level performance evaluation [13]. The models
designated as 2A and 2B are constructed as per the recommendations by PEER [13] and NIST
[1] for considering the SSI effects on seismic performance of high-rise building structures. The
model 2A considered SSI at foundation level only yet accounting for the cumulative stiffness of
the embedded foundation. The model 2B is also known as bathtub model. The major difference
between the two is that the latter considered SSI along basement walls (Figure 4). This model
doesnt consider depth-variable motions along the embedment, hence avoiding multi-support
excitation. For both the models the spring and dashpots coefficients were determined as per the
recommendations of NIST [1].
In the models designated as 3A and 3B, the soil is explicitly modelled using detailed finite element
modelling. In Model 3A, the structure is assumed to be bounded perfectly with the soil with no
interface elements. While for Model 3B, the nonlinear compression-only interface elements were
introduced between structure and soil (Figure 5). For the modelling of soil, an equivalent linear
3D model is used in this study. For this purpose, the modulus reduction curves and equivalent
damping curves for individual soil layers are required. To develop the equivalent linear soil model,
ground response analysis is also required. The soil profile at the site of case study building was
modelled in the geotechnical software DEEPSOIL V6.1 [14] with bedrock assumed to be rigid. For
the soft layer, the widely used reference curve by Vucetic and Dobry [15] was employed. For the
weathered rock layer, the expression proposed by Schnabel [16] (1973) was used. The effective
shear strain was assumed as sixty five percent and the method for complex shear modulus was
specified as recommended.
One dimensional (1D) equivalent linear ground response analysis was performed for various input
ground motion records. The reduced shear modulus and damping values were then calculated. The
lateral boundaries were chosen as three times the foundation width from the centre of the structure
[17]. The dimensions of the soil medium were 300×275×40 m (985×900×130 ft). The infinite soil
medium was bounded by viscous boundary in all four lateral directions by placing viscous dampers
along the boundaries in normal and shear directions [18, 19]. The horizontal boundary at the bottom
was fixed at bedrock as it is regarded as the most suitable way to represent bedrock. Also, the elastic
modulus at 40 m (130 ft) depth was more than ten times the elastic modulus of the top layer. The
soil elements were represented by eight node brick elements with three degrees of freedom. The
chosen dimensions of the soil medium and dampers were verified by comparing the time history
plot and response spectrum at free surface and at the top of second layer of the model with another
model with half-width that is ten times the width of the foundation with free lateral boundary
(Figure 6). Also, two types of damping elements (linear damper and nonlinear Maxwell damper)
Figure 4. Characteristics and Vs
30 profile of soil.
Note: 1 m = 3.28 ft; 1 m/sec = 3.28 ft/sec
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Figure 5. Detailed finite models of case study building.
were used along the boundary. After comparison, the linear dampers were found to be computationally
efficient and less expensive compared to nonlinear dampers and therefore, were chosen for further
detailed analysis. The comparison of time histories (five hundred thirty one ground motion) and
response spectra at the free surface and at top of layer 2 obtained from DEEPSOIL and SAP2000
is shown in Figure 7. A good match was seen for all selected ground motions, implying that the
use of 1D results in the 3D FE models is adequate.
6.2 Selection of Ground Motions and Analysis Procedures
In this study, three ground motions were selected from a detailed ground motion development study
conducted for the site of case study building. These ground motions are in the form of ground
acceleration histories recorded from three different earthquake events generated by seismic sources
with three different faulting mechanisms (Table 2).
Figure 6. Comparison of response spectra and time histories between models with x10
width, linear damper and NL damper.
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Figure 7. Comparison of response spectra and acceleration time histories of five
hundred thirty one ground motions in X direction.
Table 2. Ground motions used in this study
Earthquake event
1989 Loma Prieta
1999 Duzce, Turkey
1985 Michoacan, Mexico
Earthquake
ID
531
LGP
UNIO
Faulting
mechanism
Reverse Oblique
Strike slip
Subduction
Magnitude
(Mw)
6.93
7.14
8.1
Vs
30, m/sec/site class
(ft/sec)/site class
478 (1568)
660 (2165)
Rock
For the response history analysis of models developed in this study, three types of earthquake
motions are required: (a) free-field ground motion for Model 1; (b) foundation input motion for
Models 2A and 2B; and (c) bedrock motion for Models 3A and 3B. The response spectrum of the
foundation input motion differs from that of the free-field motion. With the use of transfer functions
[1], the modified response spectrum was developed. The free-field ground motions were then scaled
to the modified response spectrum to obtain the foundation input motions. The ground motions were
converted to bedrock motions using DEEPSOIL software by the deconvolution process [20]. In this
process, the input motion was specified at top of Layer 1, and since the bedrock was assumed as
rigid, the within bedrock motion was obtained. For the models except Model 3B, the Linear
Response History was performed. For Model 3B, Nonlinear Response History Analysis (NLRHA)
procedure was performed since the interface elements were nonlinear.
6.3 Results and Discussion
It is observed that as expected the consideration of soil-structure interaction (SSI) did not result in
a significant increase in the vibration time periods of the selected case study high-rise building. The
natural period of Model 1 was 4.4 sec whereas for the models with SSI it was about 4.6 sec. The
consideration of SSI effects using simplified (substructure) approaches had a moderate effect on the
storey displacements and storey drifts. The other considered responses such as storey shear, storey
moment, roof acceleration were also not significantly affected by the consideration of SSI effects.
Both of the substructure approaches produced comparable results, suggesting that the consideration
of SSI at the base is an important factor compared to consideration at the basement walls.
The detailed comparison of seismic demands (between the models considering SSI using simplified
approaches (Models 2A and 2B) and those using the direct continuum approaches (Models 3A and 3B))
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is performed. The latter is widely regarded as the most realistic technique to model SSI and hence,
mostly employed in SSI related research studies. The direct approach is generally not used to study
the effect of SSI on tall buildings. A relatively recent study carried out by Ellison et al. [9] have
attempted to use the direct method, however, the research interest was restricted to the performance
evaluation of basement alone.
Figure 8 compares the storey displacement in X and Y directions reported by the four models for
each of the three ground motions. A moderate difference in storey displacement can be seen from
the plots for all three ground motions. Similar observation can be made from the storey drift
comparison. Although, some deviation can be seen for UNIO ground motion, the difference is not
significant. As mentioned before, Naeim et al. [7] observed that storey drift significantly varies
along the height of the structure especially in subterranean levels compared to fixed-base models.
However, the difference was not observed to be significant in this study in both X and Y directions
of the case study building. One possible reason could be the inconsideration of nonlinearity in the
case study building (only linear elastic behaviour of the building components was considered).
Figures 9 and 10 shows increased shear and moment demands along the height of the building in
case of direct modelling approach to account for SSI effects. To compare the deviation with the
baseline model, the shear and moment values of the models with SSI were normalised with respect
to Model 1 as presented in Figure 11. It can be seen that shear differs along the height whereas
moment differs mostly along higher stories only. To further look into the deviation of shear, floor
19 was chosen as it experienced higher shear and shear response history was compared as shown
in Figure 12. The comparison shows a higher shear throughout the excitation in direct modelling
approach compared to the simplified procedure. To the best of the authors knowledge, this observation
is new. Figure 13 presents the comparison of roof acceleration experienced by the building due to
different modelling techniques for considering SSI effects. The comparison between the results of
model 2A and Model 3A were compared here as an example. For all three ground motions, the finite
element direct based models (for SSI) had higher roof acceleration than the substructure-based
approximate models in both X and Y directions.
Figure 8. Comparison of storey displacement for models 2A, 2B, 3A and 3B.
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Figure 9. Comparison of storey shear for models 2A, 2B, 3A and 3B.
Figure 10. Comparison of storey moment for models 2A, 2B, 3A and 3B.
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Figure 11. Comparison of storey shear and storey moment for models 2A, 2B, 3A and
3B (normalised to Model 1).
Figure 12. Shear response history at 19th floor.
Figure 13. Comparison of roof acceleration in X direction of case study building.
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7. CONCLUSIONS
Some key issues and practices in the area of SSI and its effects on the dynamic response and seismic
performance of buildings are discussed in this study. The significance of considering SSI effects
in structural modelling and analysis is demonstrated while providing key insights into practical
applications in real projects. After discussing the common design office practices in the area of SSI
and its effects on seismic performance of low- to medium-rise buildings, the significance of
considering these effects in high-rise buildings is also evaluated using a forty storey example
building. For this purpose, the analysis results obtained from the response history analysis of five
detailed computer models (one without considering any SSI effects, two models with SSI as
recommended in the literature and two models using direct approach) were compared. It is observed
that the consideration of SSI effects using simplified (substructure) approaches had a moderate
effect on the storey displacements and storey drifts of the case study building. However, the effect
on storey shears, storey moments and roof accelerations were not significant under various input
ground motions. Also, a reasonable difference in predicted responses were observed between the
substructure and direct modelling approaches with the latter producing higher shear and moment
demands. It is therefore proposed that the dynamic behaviour of a structure can be more accurately
studied by considering SSI effects and 3D modelling of surrounding soil instead of idealising the
base of structure with rigidly fixed support condition.
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Article
Full-text available
Recent field and laboratory studies have shown that the effect of Soil-Structure Interaction (SSI) can be not conservative to a structure if not considered. In evaluating the SSI effect along basement walls on the seismic response of moment-resisting frame buildings with multiple basements, a parametric study is performed on multiple story buildings resting on flexible grounds. The number of basements is varied and UBC suggested site conditions are considered. Comparing results obtained by Plaxis that considers SSI along basement walls and Robot that consider it only at foundation level (by mean of springs), one can observe that SSI along basement walls greatly affects the seismic performance of buildings subjected to ground movements. Results obtained from Plaxis and integrated into the Robot model by means of springs along basement walls with specific extracted stiffness values proved the accuracy of the method. These stiffness values are calculated using force-displacement relationships, and charts that relate stiffness to depth of basements are generated.
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Several structure-soil-structure interaction analyses were recently conducted using the finite element program LS-DYNA to assess the interaction between the deep, long embedded trainbox of the Transbay Transit Center and the adjacent deep basement of the 245-meter high 181 Fremont Tower when subjected to bedrock-propagating ground motions. Key features of this model, including the 181 Fremont basement and the soil domain, are removed from the SSSI framework in order to compare with the equivalent linear recommendations of NEHRP (2012) for the classical scenario of a basement embedded in an infinite soil domain. Discrepancies in the predictions of soil springs between the methods can largely be explained by the inability of the equivalent linear method to distinguish the significance of a thin, relatively soft clay layer immediately underlying the base mat. Good agreement is obtained for the spectral ratio between response spectra at the free field surface and base mat at periods greater than about 1.0 seconds. Finally, a soil-structure interaction analysis of the 181 Fremont Tower without the TTC is performed and compared with the results of the SSSI analysis. The presence of the TTC has a significant effect on the demands of the basement walls, diaphragms and drilled shafts.
Article
Seismic damage surveys and analyses conducted on modes of failure of structures during past earthquakes observed that the asymmetrical buildings show the most vulnerable effect throughout the course of failures (Wegner et al., 2009). Thus, all asymmetrical buildings significantly fails during the shaking events and it is really needed to focus on the accurate analysis of the building, including all possible accuracy in the analysis. Apart from superstructure geometry, the soil behavior during earthquake shaking plays a pivotal role in the building collapse (Chopra, 2012). Fixed base analysis where the soil is considered to be infinitely rigid cannot simulate the actual scenario of wave propagation during earthquakes and wave transfer mechanism in the superstructure (Wolf, 1985). This can be well explained in the soil structure interaction analysis, where the ground movement and structural movement can be considered with the equal rigor. In the present study the object oriented program has been developed in C++ to model the SSI system using the finite element methodology. In this attempt the seismic soil structure interaction analysis has been carried out for T, L and C types piled raft supported buildings in the recent 25th April 2015 Nepal earthquake (M = 7.8). The soil properties have been considered with the appropriate soil data from the Katmandu valley region. The effect of asymmetry of the building on the responses of the superstructure is compared with the author’s research work. It has been studied/observed that the shape or geometry of the superstructure governs the response of the superstructure subjected to the same earthquake load.
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This paper presents the results of a series of vibration tests conducted on tall reinforced concrete shear-wall buildings located in Downtown Vancouver, British Columbia, Canada. The buildings tested were the Sheraton Wall Centre, The City Tower building, The Heritage Court Tower and The Melville. Ambient vibration tests were performed on each building to obtain its dynamic characteristics. Overall vibration levels, mode shapes, modal frequencies and modal damping were determined. In addition, detailed ground measurements were taken on the Melville and the City Tower buildings to examine potential rocking behaviour and soil-structure interaction effects of the raft foundation. Interesting results of the study show that some lateral modes of vibration of the buildings include base rocking, even at low levels of vibration, and that modal coupling between the lower modes of vibration is significant. These findings indicate that the seismic response of these buildings during severe shaking may be different than that predicted during the design process.
Article
The seismic excitation experienced by structures is a function of the earthquake source, travel path effects, local site effects, and soil-structure interaction (SSI) influences. The result of the first three of these factors is referred to as free-field ground motion. The structural response to free-field motion is influenced by the SSI. In particular, accelerations within structures are affected by the flexibility of the foundation support and variations between the foundation and free-field motions. Consequently, an accurate assessment of inertial forces and displacements in structures can require a rational treatment of SSI effects. In the current study, to depict these effects on the seismic response of moment-resisting building frames, a 10-story moment-resisting building frame resting on a shallow foundation was selected in conjunction with three soil types with shear-wave velocities of less than 600 m/s, representing Soil Classes Ce, De, and Ee according to an existing Australian Standard. The structural sections were designed after applying dynamic nonlinear time-history analysis, based on both the elastic method, and inelastic procedure using the elastic-perfectly plastic behavior of the structural elements. The frame sections were modeled and analyzed using the finite-difference method andthe FLAC 2D software under two different boundary conditions: (1) fixed-base (no SSI) and (2) considering the SSI. Fully nonlinear dynamic analysis under the influence of various earthquake records was conducted and the results of the two different cases for elastic and inelastic behavior of the structural model were extracted, compared, and discussed. The results indicate that the performance level of the model resting on Soil Class Ce does not change substantially and remains in the life safe level while the performance level of the model resting on Soil Classes De and Ee substantially increase from the life safe level to near collapse for both elastic and inelastic cases. Thus, considering SSI effects in the elastic and inelastic seismic design of concrete moment-resisting building frames resting on Soil Classes De and Ee is essential. Generally, by decreasing the dynamic properties of the subsoil such as the shear-wave velocity and shear modulus, the base shear ratios decrease while interstory drifts of the moment-resisting building frames increase relatively. In brief, the conventional elastic and inelastic design procedure excluding the SSI is not adequate to guarantee structural safety for moment-resisting building frames resting on Soil Classes De and Ee.
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This paper presents the corrected development of viscous boundary efficiency initially proposed by Lysmer and Kuhlemeyer. The expressions of the energy ratio are given, in accordance with the original numerical results, and confirm the authors' recommendations to minimize the reflected energy of body waves that impinge on the artificial boundary. DOI: 10.1061/(ASCE)EM.1943-7889.0000250. (C) 2011 American Society of Civil Engineers.
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This paper investigates the influence of the inelastic dynamic soil–structure interaction (DSSI) on the response of moment-resisting frame buildings. A modeling strategy related to a modified 2D in-plane approach allowing faster finite element analyses is introduced. Two regular buildings founded on sandy soil in dry and fully saturated conditions are analyzed for a suite of motions in order to assess the influence of the inelastic DSSI on the structural response.Examination of results reveals that influence of DSSI on dry soil case is highly erratic, however tends to be invariantly favorable or negligible when the soil is in saturated condition. For motions inducing extensively inelastic structural behavior and mean period close to first elastic period of the soil deposit, inelastic DSSI seems to be very important.
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Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide range of frequencies of harmonic excitations. The inertial interaction attributable to a structure is analyzed for a system consisting of a four-storied portal frame on the pile group-soil subsystem. The responses of the structure are investigated for harmonic excitation and transient excitations. The importance of consideration of nonlinearity and liquefaction of the soil medium for analysis and design of a pile-supported structure is highlighted. Results from an analysis considering a practical soil-pile problem are presented to demonstrate the applicability of the developed algorithm for a practical problem.