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RESONANCE EFFECT DUE TO SOIL STRUCTURE
INTERACTION DURING EARTHQUAKES
Seminar Report
Submitted in partial fulfillment for the requirements of the degree
MASTER OF TECHNOLOGY
In
STRUCTURAL ENGINEERING
By
PAUL TOM P
13ST17F
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECNOLOGY KARNATAKA
SURATHKAL, MANGALORE – 575025
March 2014
CERTIFICATE
This is to certify that the P.G. Seminar Report entitled RESONANCE EFFECT
DUE TO SOIL-STRUCTURE INTERACTION DURING EARTHQUAKES
submitted by PAUL TOM P. (Register Number: 13ST17F) as the record of the work
carried out by him, is accepted as the P.G. Seminar Report submission in partial fulfillment
of the requirements for the award of degree of Master of Technology in Structural
Engineering in the Department of Civil Engineering.
Head of the Department & Seminar Guide
Dr: Katta Venkataramana
Department of Civil Engineering
National Institute of Technology Karnataka, Surathkal
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ACKNOWLEDGEMENT
I would like to express my sincere gratitude to Dr: Katta Venkataramana (H.O.D,
Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal)
for his valuable guidance and support during the course of this report as my guide and Head
of Department.
I would also like to thank Dr: K Swaminathan (Professor, Department of Civil
Engineering, National Institute of Technology Karnataka, Surathkal) for his timely
suggestions as faculty in charge of seminar.
I would also like to extend my appreciation towards all professors, research scholars
and friends for their encouragement throughout this venture.
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ABSTRACT
This seminar report discusses the effect of resonance on structures due to Soil –
Structure Interaction during earthquakes based on literature review of research work done
in the past. Contrary to popular belief and codal provisions, SSI actually amplifies the
seismic demand on structures by amplifying peak acceleration during an earthquake. This
effect is only valid in case of soft soils were the Soil – Structure Interaction is more
apparent as the support acts more flexible thereby vibrating with the soil around the
support. The ill-effects of resonance are more evident in heavy structures like tall buildings,
Nuclear Power Plants, etc. During an earthquake, in case of a structure founded in soft soil,
the extension of the natural period of vibration of the structure occurs as a result of Soil
Structure Interaction thus setting the system in resonance. During resonance all peak values
of motion are amplified thereby rendering the values considered for design inadequate.
Such structures although maybe considered to be well-designed but have a high probability
of failure when the system is in resonance. Study by various researchers have also shown
that the extent of amplification of the Seismic Response Spectra increases as the depth of
soil layer increases.
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CONTENTS
ACKNOWLEDGEMENT…………………………………….………………………. i
ABSTRACT…………………………………………………………………………... ii
CONTENTS…………………………………………………………………………... iii
CHAPTER 1: INTRODUCTION
1.1 Soil Structure Interaction………………………………………………………….. 2
1.2 Detrimental effects of SSI…………………………………………………………..3
1.3 Resonance between Soil and Structure……………………………………………..4
CHAPTER 2: EFFECTS OF NATURE OF SOIL ON RESONANCE
2.1 Soft and Hard Soil…………………………………………………………………..6
2.2 Work by Researchers…….……………………………………………………….....7
CHAPTER 3: SSI AND SEISMIC CODE SPECTRA
3.1 Seismic Response Spectra…………………………………………………………...9
3.2 Case Study of Various Past Earthquakes…………………………………………...10
3.2.1 Turkish Gediz Earthquake, 1970………………………………………….11
3.2.2 Mexico City Earthquake, 1985……………………………………………11
3.3 Seismic Response Spectra for Various Earthquakes ……………………………… 11
CONCLUSION………………………………………………………………………….14
REFERENCE……………………………………………………………………………15
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1. INTRODUCTION
As in the metropolitans, the building structures are built closely to each other over the soft-
soil deposit. Under such circumstances, the dynamic interaction among building structures
must occur through the radiation energy emitted from a vibrating structure to other
structures. Hence, the dynamical characteristics as well as the earthquake response
characteristics of a structure are unable to be independent of those of the adjacent
structures. In accordance with the parameterized study conducted, those two buildings with
distance less than 2.5 times of width of foundation are interacting with each other. And
when the distance was less than one time of width of foundation, the response of structures
may increase or decrease. Thus, the interactions between neighboring buildings have to be
investigated.
Figure 1: Cluster of high rise buildings in a tiny space.
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1.1 SOIL STRUCTURE INTERACTION
Most of the civil engineering structures involve some type of structural element with direct
contact with ground. When the external forces, such as earthquakes, act on these systems,
neither the structural displacements nor the ground displacements, are independent of each
other. The process in which the response of the soil influences the motion of the structure
and the motion of the structure influences the response of the soil is termed as soil-structure
interaction (SSI).
Conventional structural design methods neglect the SSI effects. Neglecting SSI is
reasonable for light structures in relatively stiff soil such as low rise buildings and simple
rigid retaining walls. The effect of SSI, however, becomes prominent for heavy structures
resting on relatively soft soils for example nuclear power plants, high-rise buildings and
elevated-highways on soft soil. (Wolf, 1985)
Damage sustained in recent earthquakes, such as the 1995 Kobe Earthquake, have also
highlighted that the seismic behavior of a structure is highly influenced not only by the
response of the superstructure, but also by the response of the foundation and the ground
as well. Hence, the modern seismic design codes, such as Standard Specifications for
Concrete Structures: Seismic Performance Verification JSCE 2005 stipulate that the
response analysis should be conducted by taking into consideration a whole structural
system including superstructure, foundation and ground.
Soil–structure interaction, one of the most major subjects in the domain of earthquake
engineering, has been paid comprehensive attention internationally in recent decades. Soil–
structure interaction phenomena concern the wave propagation in a coupled system, ie.,
buildings erected on the soil surface. Its origins trace back to the late 19th century, evolved
and matured gradually in the ensuing decades and during the first half of the 20th century,
and progressed rapidly in the second half stimulated mainly by the needs of the nuclear
power and offshore industries, by the debut of powerful computers and simulation tools
such as finite elements, and by the needs for improvements in seismic safety.
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1.2 DETRIMENTAL EFFECTS OF SSI
Using rigorous numerical analyses, Mylonakis and Gazetas have shown that increase in
natural period of structure due to SSI is not always beneficial as suggested by the simplified
design spectrums. Soft soil sediments can significantly elongate the period of seismic
waves and the increase in natural period of structure may lead to the resonance with the
long period ground vibration. Additionally, the study showed that ductility demand can
significantly increase with the increase in the natural period of the structure due to SSI
effect. The permanent deformation and failure of soil may further aggravate the seismic
response of the structure.
When a structure is subjected to an earthquake excitation, it interacts with the foundation
and the soil, and thus changes the motion of the ground. Soil-structure interaction broadly
can be divided into two phenomena: a) kinematic interaction and b) inertial
interaction. Earthquake ground motion causes soil displacement known as free-field
motion. However, the foundation embedded into the soil will not follow the free field
motion. This inability of the foundation to match the free field motion causes the kinematic
interaction. On the other hand, the mass of the super-structure transmits the inertial force
to the soil causing further deformation in the soil, which is termed as inertial
interaction.(Wolf, 1985)
At low level of ground shaking, kinematic effect is more dominant causing the lengthening
of period and increase in radiation damping. However, with the onset of stronger shaking,
near-field soil modulus degradation and soil-pile gapping limit radiation damping, and
inertial interaction becomes predominant causing excessive displacements and bending
strains concentrated near the ground surface resulting in pile damage near the ground level.
Observations from recent earthquakes have shown that the response of the foundation and
soil can greatly influence the overall structural response. There are several cases of severe
damages in structures due to SSI in the past earthquakes. Damage has occurred in number
of pile-supported bridge structures du e to SSI effect in Loma Prieta Earthquake in San
Francisco in 1989. Extensive numerical analysis carried out by Mylonakis and
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Gazetas have attributed SSI as one of the reasons behind the dramatic collapse of Hanshin
Expressway in 1995 Kobe Earthquake.
Figure 2: Earthquake damage due to SSI
1.3 RESONANCE BETWEEN SOIL AND STRUCTURE
Resonance effect is an important subject in earthquake engineering practice. It is the result
of making the frequency of super structure to the frequency of supporting soil closer. This
fact has been experienced in the several past earthquakes where tuning of the natural period
of a building structure with that of a surface ground caused significant response
amplifications on the buildings and resulted significant damage (The earthquakes of 1970
Gediz, 1985 Mexico City, 1998 Adana-Ceyhan, etc.). Soil–structure interaction (SSI) is a
major topic that deals with the resonance phenomenon in detail. It refers to the relationship
between the characteristics of both the structure and the soil stratum and is usually
represented by modifying the dynamic properties of the structure. This interaction causes
energy dissipation and changes the natural modes of vibration of the structure such as
natural frequencies and the corresponding mode shapes (Wolf, 1985; Gullu, 2014))
An importance factor in predicting earthquake damage is relationship between the
fundamental frequency of a building and the fundamental frequency of the ground on
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which building is constructed. If the building’s frequencies are close to a nearby the
fundamental frequencies of the material on which it is built, or if it equals some whole-
number multiple of the material’s fundamental frequencies, then seismic motion will create
a resonance with building that can greatly increase the stresses in the structure.(Warnana,
2011)
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2. EFFECTS OF NATURE OF SOIL ON RESONANCE
2.1 SOFT AND HARD SOIL
Investigations of soil–structure interaction have shown that the dynamic response of a
structure supported on flexible (soft) soil may differ significantly from the response of the
same structure when supported on a rigid base. One of the important reasons for this
difference is that part of the vibrational energy of the flexibly mounted structure is
dissipated by radiation of stress waves in the supporting medium and by hysteretic action
in the medium itself. Analytical methods to calculate the dynamic soil–structure interaction
effects are well established. When there is more than one structure in the medium, because
of interference of the structural responses through the soil, the soil–structure problem
evolves to a cross-interaction problem between multiple structures and the problem is
magnified. (Lou Menglin, 2011)
It is increasingly desired to take into account the soil effects on the design of structures
particularly those located in active seismic zones. In recent years, numerous researchers
have performed studies on the effects of SSI on the dynamic seismic response of buildings.
The analysis and design process for dynamic loading generally assumes structures to be
fixed at their bases. However, supporting soil medium actually allows a movement to some
extent due to flexibility. This may reduce the overall stiffness of the structural system and
may increase the natural periods of the system. Considerable change in spectral
acceleration with natural period can be observed from the response spectrum curve. Such
change in natural period may considerably alter the seismic response of any structure.
Despite this dynamic SSI effects should be taken into account for stiff and/or heavy
structures supported on a relatively soft soil. These are generally small and may be
neglected for soft and/or light structures founded on stiff soils. Based on the latest study, it
is apparently inferred that SSI effects directly alter the resonance characteristics of the soil–
structure system. (Gullu, 2011)
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The presence of deformable soil supporting a structure affects its seismic response in many
different ways, as illustrated in Fig. 3. Firstly, a flexibly-supported structure has different
vibrational characteristics, most notably a longer fundamental period, p, than the period T
of the corresponding rigidly-supported (fixed-base) structure. Secondly, part of the energy
of the vibrating flexibly-supported structure is dissipated into the soil through wave
radiation (a phenomenon with no counterpart in rigidly-supported structures) and hysteretic
act ion, leading to an effective damping ratio, p, which is usually larger than the damping
P of the corresponding fixed- base structure.
Figure 3 (Mylonakis 2008)
2.2 WORK BY RESEARCHERS
Although many works have been attempted about the SSI effects on soil and structures,
there is limited number of work that particularly involved the resonance effects from
resonance models. Moreover, there is a lack of resonance study for models in resonance
that systematically examine the effects of soil layer thickness on the dynamic response of
plane frame structures under strong ground motion.
The aim of the work was to gain some insights into the reasons for earthquake damage to
engineered buildings due to the resonance effect on the basis of dynamic SSI methodology.
Investigation was carried out using some hypothetical SSI models which were adjusted so
the soil and structure were in resonance. The soil layer thickness in these models was
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varied; however, a constant structure (a midrise building) resting on soil surface was used
throughout the study. The reason for choosing the midrise building is that majority of them
in previous severe earthquakes did not demonstrate good performance. Direct method
configuration was used for the SSI analysis. The SSI model was constructed by 2D finite
element method with rectangular meshes employing a common method of SAP2000. Site
response analysis of soil layers to the interaction from FEM modeling was carried out by
the method of SHAKE. This study is believed to contribute to engineers in practice when
designing structures against resonance.
He concluded that the resonance effect (i.e., the amplitudes, shear force and moment) on
the RC structure increases with the increased soil layer thickness. Even though the soil
layer has good engineering characteristics the ground floor of the RC structure under the
resonance can be considerably damaged from the larger soil layer thicknesses. The rate of
shear force increments are more pronounced on the mid-storeys as compared with the ones
of remaining storeys.
The overall evaluation of this investigation reveals that the resonance effects estimated
from the SSI analysis fairly produce greater responses on the RC structure. The practical
relevance of the findings obtained in this study can be considered to be high. They can be
beneficial for gaining an insight into code provisions as well.
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3. SSI AND SEISMIC CODE SPECTRA
3.1 SEISMIC RESPONSE SPECTRA
With little exception, seismic codes today use idealized smooth design spectra which attain
constant acceleration up to a certain period (of the order of 0.4 s to 1.0 s at most, depending
on soil conditions), and thereafter decrease monotonically with period (usually in
proportion to T-1 or T-2/3) . As a consequence, consideration of SSI leads invariably to
smaller accelerations and stresses in the structure and its foundation.
Figure 4 :Reduction in design base shear due to SSI according to NEHRP – 97 seismic code.
Thus, frequently in practice dynamic analyses avoid the complication of accounting for
SSI - a supposedly conservative simplification that would lead to improved safety margins.
This beneficial effect is recognized in seismic provisions. For example, the NEHRP-97
seismic code states (Commentary, p. 111):
"The (seismic) forces can therefore be evaluated conservatively without the adjustments
recommended in Sec. 5.5 (i-e. for SSI effects)."
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Since design spectra are derived conservatively, the above statement may indeed hold for
a large class of structures and seismic environments. But not always. There is evidence
documented in numerous case histories that the perceived beneficial role of SSI is an
oversimplification that may lead to unsafe design for both the superstructure and the
foundation. (Mylonakis, 2008)
Additional concerns come from the fact that Soil-Structure Interaction (SSI) has been
traditionally considered beneficial for seismic response. Apparently this perception stems
from oversimplifications in the nature of seismic demand adopted in code provisions. This
conservative simplification is valid for certain class of structures and soil conditions, such
as light structures in relatively stiff soil. Unfortunately, the assumption does not always
hold true. In fact, the SSI can have a detrimental effect on the structural response, and
neglecting SSI in the analysis may lead to unsafe design for both the superstructure and the
foundation.
In fact, damage in structures associated with SSI effects has been proven or suspected in
many cases in the past. For instance, the Mexico City earthquake of 1985 was particularly
destructive to 10 to 12-story buildings (founded on soft clay) whose period increased from
about 1.0 sec (for the fixed-base structure) to nearly 2.0 seconds due to SSI.
3.2 CASE STUDY OF VARIOUS PAST EARTHQUAKES
The fundamental periods of structures can be crudely estimated from a rule of thumb
method in which the fundamental period of N-story building is approximately N/10 s. They
may range from about 0.05 s for a well-anchored piece of equipment, 0.1 s for a one story
simple bent or frame, 0.5 s for a low structure up to about four stories and between 1 and
2 s for a tall building from 10 to 20 stories. The fundamental periods of soils usually have
values varying from 0.1 s (rock, stiff or dense soils) to 1 s (soft or loose soils). If the two
fundamental periods are matched each other, there is a high probability for the building
will approach a state of partial resonance. Experiences from historical earthquakes reflect
that long-period seismic waves from large-magnitude earthquake events can be amplified
by some four- to sixfolds due to resonance with flexible soil layers. The amplified motion
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may be subjected to further resonance with flexible tall buildings where torsional inertia
generated by dynamic coupling can create significant horizontal rotation and result in a
significant increase in the drift demand on individual lateral load resisting elements. This
torsional coupling effect of resonance is particularly apparent in structures that respond
elastically to an earthquake prior to initiation of damage.
3.2.1 TURKISH GEDIZ EARTHQUAKE, 1970
The 1970 Turkish Gediz Earthquake demolished the paint workshop building of the Tofas-
Fiat automobile factory in Bursa, located 135 km away from the epicenter, while no other
building in Bursa was damaged. The main reason for the demolished structure was found
that the predominant periods of the structure and underlying soil were approximately equal
around a value of 1.2 s.
3.2.2 MEXICO CITY EARTHQUAKE, 1985
The 1985 Mexico City earthquake is one of the instructive earthquakes where the resonance
is well defined in many damaged buildings. The greatest damage occurred in the Lake Zone
underlain by soft soil (38–50 m depth) where the characteristics of site periods were
estimated from 1.9 to 2.8 s. Buildings less than five stories and modern buildings greater
than 30 stories were exposed to slight damage within this area. However, most of the
buildings in range from 5 to 20 stories, those fundamental periods were nearly equal to or
somewhat less than the characteristics site period, either were collapsed or badly damaged.
The possible reason for the damage was the resonance.
3.3 SEISMIC RESPONSE SPECTRA FOR VARIOUS EARTHQUAKES
It is shown from the past events that tuning of the natural period of a building structure
with that of a surface soil causes significant amplifications that result in the increasing of
inertial forces acting on the structure with a considerable damage. So, it is very important
to check the interactions between the vibration periods (or frequencies) of structures and
the supporting soil in order to determine how close they are to resonance.
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Figure 5 : Comparison of a typical seismic code design spectrum to actual spectra from catastrophic
earthquakes with strong long-period components, ζ = 5% (Mylonakis 2008)
To elucidate this, the ordinates of a conventional design spectrum for soft deep soil, are
compared graphically in Fig. 5 against four selected response spectra:
Brancea (Bucharest) 1977, Michoacan [Mexico City (SCT)] 1985, Kobe (Fukiai, Takatori)
1995, presented in terms of spectral amplification. Notice that all the recorded spectra attain
their maxima at periods exceeding 1.0 s. The large spectral values of some of these records
are undoubtedly the result of resonance of the soil deposit with the incoming seismic waves
(as in the case with the Mexico City SCT record). Another phenomenon, however, of
seismological rather than geotechnical nature, the "forward fault-rupture directivity"
(Somerville, 1998), may be an important contributing factor in the large spectral values at
T > 0.50 s in near-fault seismic motions (e.g. in Takatori and Fukiai). As noted by
Somerville, an earthquake is a shear dislocation that begins at a point on a fault and spreads
outward along the fault at almost the prevailing shear wave velocity. The propagation of
fault rupture toward a site at very high velocity causes most of the seismic energy from the
rupture to arrive in a single long-period pulse of motion, at the beginning of the recording.
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The radiation pattern of the shear dislocation on the fault causes this large pulse of motion
to be oriented in the direction perpendicular to the fault, causing the strike-normal peak
velocity to be larger than the strike-parallel velocity. The effect of forward rupture
directivity on the response spectrum is to increase the spectral values of the horizontal
component normal to the fault strike at periods longer than about 0.5 s.
It is therefore apparent that as a result of soil or seismological factors, an increase in the
fundamental period due to SSI may lead to increased response (despite a possible increase
in damping), which contradicts the expectation incited by the conventional design
spectrum. It is important to note that all three earthquakes presented in Fig. 5 induced
damage associated with SSI effects.
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4. CONCLUSION
The role of soil in the collapse of the structures in the case study taken was double and
detrimental.
Forward directivity of the fault rupture is one of the reasons for the amplified
seismic demand. This reason is more seismological rather than geotechnical but
still is directly influenced by resonance of seismic waves due to elongation of
natural period.
It modified the incoming seismic waves such that the resulting motion at the surface
become detrimental for the structure at hand by the amplification of spectral
accelerations due to resonance.
The presence of compliant soil at the foundation resulted to an increased natural
period of the structure which moved to a region of stronger response.
Of course, the above said phenomena might simply worsen an already dramatic situation
for the structure due to its proximity to the fault and inadequate structural design.
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REFERENCES
1. Wolf, J. P. (1985). Dynamic Soil-Structure Interaction. Prentice-Hall, Inc.,
Englewood Cliffs, New Jersey
2. Mylonakis, G., Gazetas, G., Nikolaou, S., and Michaelides, O. (2000). The Role
of Soil on the Collapse of 18 Piers of the Hanshin Expressway in the Kobe
Earthquake, Proceedings of 12th World Conference on Earthquake Engineering,
New Zealand, Paper No. 1074
3. George Mylonakis & George Gazetas (2008), Seismic Soil-Structure Interaction:
Beneficial Or Detrimental, Journal of Earthquake Engineering, 4:3, 277-301
4. Japan Society of Civil Engineers. Standard Specifications for Concrete Structures
– 2002: Seismic Performance Verification. JSCE Guidelines for Concrete No. 5,
2005
5. Dwa Desa Warnana, Triwulan, Sungkono, Widya Utama (2011) ,Assessment to
the Soil-Structure Resonance Using Microtremor Analysis on Pare -East Java,
Indonesia, Asian Transactions on Engineering, Volume 01 Issue 04
6. Hamza Gullu, Murat Pala, (2014)On the resonance effect by dynamic soil–
structure interaction: a revelation study, Nat Hazards,DOI 10.1007/s11069-014-
1039-1
7. Lou Menglin, Wang Huaifenga N., Chen Xib, Zhai Yongmeic (2011), Structure–
soil–structure interaction:Literature Review, Soil Dynamics and Earthquake
Engineering 31(2011)1724–1731