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

i

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.

ii

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

1

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

4

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

5

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

8

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)."

10

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

11

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.

12

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.

13

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.

15

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