APPLICATION OF SEISMIC ISOLATION FOR AN
IMPORTANT BUILDING LOCATED IN A HIGH
SEISMIC ZONE IN INDIA
Tarun CHAUHAN
MEE17708

Supervisor: Tatsuya AZUHATA
Matsutaro SEKI

ABSTRACTThis paper aims to examine the effect of seismic isolation on an important building which is located ina high seismic zone of India. Firstly, the building designed as a fixed base (FB) building as per therequirements of Indian codes and its seismically isolated (SI) version were compared using the nonlinear static and dynamic analysis results. The damage states and the response parameters werecompared. Secondly, the same building was designed as a fixed base building in the lowest seismic riskzone in India and seismically isolated to check if it can be used in the highest seismic zone or not. The
results were positive which suggests that the member sizes can be reduced if seismic isolation is adopted,
and hence the initial cost can be offset. To check for the design of elements, the vertical distribution ofshear force coefficient was studied for the effect of higher modes and compared with Japanese code
equation. In conclusion, it was seen that seismic isolation could be effectively used, particularly inimportant public buildings which need to be protected against earthquake damage as they serve
important functions after an earthquake. Higher modal response using amplified shear coefficients canbe applicable for Indian structures with unreinforced masonry infill (URM) walls.
Keywords: Seismic isolation, shear coefficient, higher modal response.

1. INTRODUCTION
India is a highly earthquake-prone country, because of the collision of the Indian plate with the Eurasianplate. However, thousands of people die due to moderate earthquakes in India, because of poor seismic
resilience of the constructed environment. Also, seismic isolation is still not a popular construction
practice in India. Only a few SI buildings have been constructed to date. Also, the latest revision of thedesign code IS 1893(1):2016, recommends the use of base isolation and such advanced techniques toprevent loss of life and property. But, there are no specific guidelines or procedure prescribed to follow.
There are four seismic zones, viz., Zone II, III, IV, and V, which define the Zone factor, Z,
which is the value of the peak ground acceleration that is considered for the seismic design of structures
located in each seismic zones. Four input ground motions normalized to PGV of 50 kines are used foranalysis in this study. Seismic isolation design is done with the Japanese seismic design practice.

2. TARGET BUILDING
2.1. Outline
The target building is a telephone exchange building which is located in Guwahati, the capital of Assam,Central Public Works Department, Ministry of Housing and Urban Affairs, Government of India.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

Visiting Research Fellow, International Institute of Seismology and Earthquake Engineering, BRI, Japan.
1which is a state in the Northeastern part of the country. The seismic zone is V, which is the highest
seismic zone category in India. The importance factor of the building is taken as 1.5. The elevation andfloor plan of the building is shown in Figure 1 and Figure 2 respectively. The soil stratum is considered
to be hard soil.

Figure 1. Elevation of the target building.

Figure 2. Plan of the target building.

2.2. Non-linear static analysis of the building
Two models were made in STERA-3D software, with and without the effect of unreinforced masonry(URM) infill walls. X-direction was considered for the pushover analysis.
Base shear vs Top Displacement

Base Shear (kN)

60000
40000
20000
0
0

20
40
Top displacement (cm)Fixed base bare frame

60

Fixed base masonry infill

Figure 3. Pushover curves for fixed base buildings.

Figure 4. Damage state of FB building withmasonry infills after pushover analysis.

3. SEISMIC ISOLATION SYSTEM
The maximum displacement of the seismic isolation system was predicted using Figure 5, which wasdeveloped using the Japanese standard of seismic design for hard soils. In general Japanese design
practice, isolators with Tf = 3s are considered good enough. The shaded area in Figure 5, shows the target2

displacement and the shear coefficient, α1 forwhich the isolators were designed.
Yield shear force coefficient of elastoplastic damper is denoted by ɑs. Three values ofɑs (0.03, 0.05 and 0.10) were used for threedifferent combinations of isolators. The threedifferent bearings systems were designed, andthe best representative systems were chosenfrom the Bridgestone product catalogue, 2017.
Increasing values of ɑs means that the yieldforce is getting higher and the initial stiffness
increases too.

0.5
0.4

αs = 0.01

αs = 0.10

α1 (Shear coefficient)

0.6

0.3
0.2
0.1

Tf = 6s

0.0
0

20

40

Max Isolator Displacement, ∂max (cms)60

Figure 5. Prediction of maximum displacement.

4. NON-LINEAR TIME HISTORY ANALYSES
4.1. Input earthquake strong motions

Four strong ground motions mentioned in Table
1, were used for input, after normalizing the PGV
to 50 kines, to make it realistic in Indian context.
These earthquakes were selected because two ofthem are short-period earthquakes, and one islong-period.

Table 1. List of EQ strong motions used for NTHA.
S. No. Ground Motion
Scaled to PGV
1El Centro NS, 1940
50cm/s2
Hachinohe EW, 1968 50cm/s
3Uttarkashi NS, 1991 50cm/s
4Kobe NS, 1995
50cm/spSv or Ve (cm/s)

300

200

100

0
0

1
pSv hard soils
Hachinohe pSv
Hachinohe Ve

Time (s)2
pSv medium soils
Kobe pSv
Kobe Ve

3

4
pSv soft soils
Uttarkashi pSv
Uttarkashi Ve

5
El Centro pSv
El Centro Ve

6

Figure 6. Pseudo velocity response and energy spectra for EQs considered with Indian code pSv.
The four earthquakes that are used in this study are plotted for their spectral velocity response
spectrum along with the defined spectra in the Indian code for hard soils, medium soils and soft soils(Figure 6). Also, the equivalent velocity of earthquake input energy, VE for the strong ground motions
are plotted. Now, the VE was assumed to be 100 cm/s for hard strata, as per the Japanese design code.
But, as per Indian code, for hard soils, pSv ≈ 55 cm/s. Therefore, the assumption was appropriate forpreliminary design, but in some cases, it was an underestimation. It is an interesting observation thatafter 4 s, the pSv increases. It is because the defined spectral acceleration is constant after T=4 s.
Therefore, to convert to pSv, we need to multiply by T/2π, and hence it becomes linear.

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4.2. Non-linear Time History Analysis (NTHA) Results

Displacement profile

10
0

4

15
30
Displacement (cm)Figure 7. Displacement profile
comparison for El Centro.

Seismic shear coefficient

4
0

0
0

8

Story

20

Acceleration response

8

Story

Height (m)

30

0

450
900
Acceleration (gals)
Figure 8. Accelerationcomparison for El Centro.

0
0.3
0.6
0.9
Seismic shear coefficient
Figure 9. Shear coefficientcomparison for El Centro.

The designed isolator bearings for target period, Tf=3s for the three ɑs values, were modeled in the fixedbase building above the first story columns, for both bare frame and with URM infill walls and nonlinear time history analyses (NTHA) were performed for the FB and the SI buildings. Figures 7-9 show
the comparisons for El Centro EQ only. The analyses were conducted for all the 4 EQ motions.
4.3. Low seismic risk zone-II building
In this section, the same target building was considered to be located in zone-II and designed as a fixedbase building, to ascertain if this reduced-section building could be applied with seismic isolation and if
it could be used in the highest seismic zone V. From an economic point of view, a possibility of reductionof the section sizes can be considered. A quick comparison is made between the Zone-V and Zone-IIbuildings for only one earthquake ground motion, i.e., Hachinohe EW (PGV-50kines). Both thebuildings were seismically isolated at the same first story level as before.

Story

8

Seismic shear coefficient- Hachinohe

4
0
0

0.2
0.4
0.6
Seismic shear coefficient

FB zone V
SI zone V

0.8

FB zone II
SI zone II

Figure 10. Zone-V and Zone-II comparison.

Figure 11. Damage state of SI zone-II building.

5. VERTICAL DISTRIBUTION OF SHEAR FORCE COEFFICIENT
To check for the damage in superstructure, the accurate calculation of shear force coefficient is required.
In this section, the comparison is made between the non-linear time history analysis (NTHA) results,
with the theoretical Japanese standard predictions. But for the contribution of higher mode responses,
one more proposal by Kobayashi and Matsuda (2012) is studied and compared with the above two.
The proposed equation by Kobayashi and Matsuda is given below:
𝛼𝑖 = 𝛼𝑓 + 𝛽𝑖 . 𝐴𝑖 . 𝛼𝑠4(1)where αi, αf, and αs are the shear force coefficient of i-th story, elastomeric isolators and elasto-plastic
dampers respectively. βi is amplification factor for Ai distribution to take into account response
amplification. βi is formulized by isolation ratio, I and equivalent viscous damping ratio, heq.

For αs = 0.10 El Centro NS

4
0

α1
0
0.1
0.2
0.3
0.4
Figure 13. αi distribution for Hachinohe (αs=0.10).

For αs = 0.10 Kobe NS

For αs = 0.10 Uttarkashi NS

8

Story

Story

4
0

α1

0
0.1
0.2
0.3
0.4
Figure 12. αi distribution for El Centro (αs=0.10).
8

For αs = 0.10 Hachinohe EW

8

Story

Story

8

4
0

0
0.1
0.2
0.3
0.4
Figure 14. αi distribution for Kobe (αs=0.10).

4
0

α1

0
0.1
0.2
0.3
0.4 α1Figure 15. αi distribution for Uttarkashi (αs=0.10).

6. DISCUSSIONS
30000
Base Shear (kN)The SI buildings showed good behavior whensubjected to the input strong ground motions.
It was seen that the migration of the
response took place from the non-linear rangeof the pushover curve to the linear range whensubjected to seismic isolation (Figure 16).
Firstly, the base shear and the response
acceleration are greatly reduced, which reduces
the seismic demand for the members and hence,the possibility of reduction of the member sizes
can be considered.
Secondly, the effect of the masonry
infill walls is evident, and it is an important
aspect that should be taken into account.
Thirdly, there was significant reduction
in story drift in SI building, which means thesuperstructure behaved elastically.

20000

10000

0
0

20
40
Top displacement (cm)Fixed base bare frame

ElCentro FB

Hachinohe FB

Kobe FB

60

Figure 16. FB bare frame pushover curve. (αs = 0.03).

αs, also affect the behavior of the structures. The lower αs values showed good behavior andmore reduction of base shear and the acceleration response compared to the higher αs values.
Lastly, with such good performance of SI buildings, structural damage is altogether prevented.
Especially, for important buildings like hospitals, etc., even non-structural damage can be minimized.

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For vertical distribution of shear force coefficients, Figures 12-15 show when masonry infill
walls were considered, and particularly for higher values of αs, the proposed Kobayashi method gave aproper or conservative evaluation of the shear forces, while the Japanese code method underestimated.
So, this comparison confirms that if the isolators have high stiffness, then the contribution of higher
modes is increased. Now, higher mode contribution is seen in Uttarkashi and Kobe earthquakes only.
The reason being that these two are short-period earthquakes, and the second natural period of theisolated structure for αs=0.05, is 0.351 s. and for αs=0.10, is 0.340 s, which almost matches the time
period for the peak of the response spectrum for Uttarkashi and Kobe earthquakes (Figure 6).

7. CONCLUSIONSApplication of SI system is hugely beneficial for important buildings in the high seismic zone because
all critical response parameters are reduced significantly.
Important buildings when designed with the codal provisions may still suffer some structural
damage and a lot of non-structural damage in the event of a strong earthquake, because of the design
philosophy adopted in the Indian codes. But in Seismic Zone-V in India, especially important buildings
which serve critical post-disaster functions need to be protected against even rare earthquakes. Seismic
isolation is one such idea, which to a large extent, prevents structural and non-structural damage.
αs =0.03 was selected as the preferred choice for installing the isolation system because of thehigh reduction in base shear, acceleration response and story drifts. Higher αs values are suitable inplaces which have space constraints, but the target building has no such restrictions. Hence 0.03 αs value
can be used even though the displacement of the isolation layer is relatively high. The effect of masonry
infill walls should be included in all analysis results as seen in the study.
For Zone-V building, it was seen that the seismic base shear coefficient was reduced to the zoneII levels after application of seismic isolation. But for Zone-II building, the seismic isolation works well,
but the superstructure doesn’t behave elastically or like a rigid body.
It is important to check for the amplification of the vertical distribution of the shear force
coefficients. The Japanese design code method gives acceptable results in some cases, however, forcertain situations like high αs value, and superstructure not behaving like a rigid body, it may give
underestimated results. Kobayashi method is thus, applicable to Indian structures with URM infill walls.

ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Dr. Tatsuya Azuhata and Dr. Eng. Matsutaro Seki for
their supervision, continuous support, and suggestions during my study.

REFERENCES
AIJ, 2015, Design Recommendations for Seismically Isolated Buildings.
Bridgestone Product Catalogue, version 2017, volume-1.
Criteria for Earthquake Resistant Design of Structures IS1893(1), 2016, Bureau of Indian Standards.
CSI, 2016, ETABS User's Manual.
Iiba, M., & Inoue, N., 2017-18, IISEE Lecture Note.
Jain S.K., 2016, Bull Earthquake Eng (2016), Springer.
Kobayashi, M., and Matsuda S., 2012, 15th WCEE, Lisboa 2012.
Naeim, F., & Kelly, M. J. (1999). John Wiley & Sons Inc.
Saito, T., 2017, STERA-3D User Manual, Version 9.6.
Seki, M., 2017-18, IISEE Lecture Notes.
The Japan Society of Seismic Isolation, 2013., Ohmsha.
Website: Center for Engineering Strong Motion Data(CESMD), https://www.strongmotioncenter.org/.

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