5 ELASTOMERIC BASE ISOLATION SYSTEM FOR SEISMIC MITIGATION ... BASE ISOLATION... · Elastomeric...

Elastomeric Base Isolation System For Seismic Mitigation of Low-Rise Structures, Ganga Warrier A,

Balamonica K, Sathish Kumar K, Dhanalakshmi, Journal Impact Factor (2015): 9.1215 (Calculated by

GISI) www.jifactor.com

www.iaeme.com/ijciet.asp 33 [email protected]

1M.Tech Structural Engineering Student, Department of Civil Engineering,

Manipal Institute of Technology, Karnataka, India

2Senior Project Fellow, CSIR-Structural Engineering Research Centre,

Council of Scientific and Industrial Research, Chennai, India

3Senior Principal Scientist, CSIR-Structural Engineering Research Centre,

Council of Scientific and Industrial Research, Chennai, India

4Assistant Professor, Department of Civil Engineering,

Manipal Institute of Technology, Karnataka, India

ABSTRACT

Seismic base isolation is a passive way of achieving seismic response control by introducing

isolators between foundation and super structure. Isolator performs three functions: horizontal

flexibility, energy dissipation and rigidity against normal lateral loads. Lead Rubber bearing isolators

performs these functions efficiently. By reducing the horizontal stiffness of the system, it increases

the time period of the structure and decreases the spectral acceleration of the structure. The

superstructure acts like a rigid body, thus inter storey drift is reduced. Such type of isolators are used

in practice in India, yet a proper design procedure based on IS code is unavailable. The paper

presents design procedure for LRB adopting the procedure of IS 1893:2002 (Part-1) for earthquake

resistant design of buildings. The design procedure requires different input parameters like

fundamental period and damping of the fixed base structure, axial load on the column, seismic zone,

type of soil and shore hardness of rubber. Using this methodology, case study has been done using

SAP2000. Building displacement, acceleration and inter-storey drift are compared for model with

and without base isolator. Comparative study of linear and non-linear base isolators has also been

carried out. Linear and non-linear time history analysis has been done using El Centro earthquake.

Index Terms: Base Isolation, Laminated Rubber Bearing, IS Code, Seismic Protection, Time

History Analysis

I. INTRODUCTION

The seismic isolation system protects the structure from the damaging action of earthquake

by decoupling. The superstructure is partially decoupled from the horizontal component of

earthquake ground motion by interposing a layer with low horizontal stiffness and high damping

ELASTOMERIC BASE ISOLATION SYSTEM FOR SEISMIC

MITIGATION OF LOW-RISE STRUCTURES

Ganga Warrier A1, Balamonica K

2, Sathish Kumar K

3, Dhanalakshmi

4

Volume 6, Issue 6, June (2015), Pp. 33-45

Article Id: 20320150606004

International Journal of Civil Engineering and Technology (IJCIET)

© IAEME: www.iaeme.com/Ijciet.asp

ISSN 0976 – 6308 (Print)

ISSN 0976 – 6316(Online)

IJCIET

© I A E M E





characteristics. Seismic protection by base isolation is mainly used for hospitals, emergency

communication centers, fire stations, traffic management centers, historical buildings and other

buildings of importance. [1]Performance of the base isolated buildings in different parts of the world

during earthquakes in the recent past established that the base isolated technology is a viable

alternative to the conventional earthquake resistant design of a large category of buildings.

The essential concept is towards lengthening the period of the structure such that the spectral

acceleration is reduced. The super-structure essentially acts like a rigid body, thus reducing the inter-

storey drift. When the period is increased, pseudo-acceleration is decreased and hence the force in

the structure gets reduced (Figure 1[2]). However, the displacement of the system increases

drastically.

[3]Isolation systems used for seismic protection of buildings and bridges are mainly

elastomeric isolation systems or sliding isolation systems. Flexibility in elastomeric isolation systems

is provided by elastomeric bearings (laminated rubber bearings reinforced with steel plates). Energy-

dissipation capacity is provided by inherent damping capacity of the rubber, as in high-damping

elastomeric bearings. A typical low damping natural rubber bearing isolator is shown in Figure 2[4].

Fig 1. Elastic design spectrum [2]

Fig 2. Natural rubber isolator for the Foothill Communities Law and Justice Center showing

laminated construction [4]





II. DEVELOPMENT OF EQUATIONS

A. To find the thickness of isolator

Fig 3. Behaviour of isolator under shear

Shear strain of the isolator, γ is given by (Figure 3),

γ=∆

t

Where ∆ = Length of deformation

t=thickness of the isolator

γ is assumed to be 100% in this analysis.

Therefore, ∆= � �� is the spectral displacement of the structure as per IS 1893-2002 Part I. We assume that the

isolator gets deformed keeping the structure intact. Hence the maximum displacement of isolator

is��

i.e, �� = � [1]

But, Sd=Sa

ω2 [2]

Therefore t= ��ω2

[3] Where,

Sa= Spectral acceleration of the isolator

ω = frequency of the structure

By considering the zone factor (Z), importance factor (I) and response reduction factor (R), the

above equation can be written as

t= Ah

ω2 [4]

Where, the horizontal acceleration coefficient,

for DBE, Ah=Z

2

I

R

Sa

g

for MCE Ah=ZI

R

Sa

g

B. To find diameter of isolator

The linear frequency of the isolator is given by,

f= 1

2ᴫ� k

m [5]

Where k= Stiffness of the isolator

m= Mass of the isolator





2ᴫf=� k

m

ω2

=k

m [6]

ω= Natural frequency of the isolator

Horizontal stiffness of the isolator,

KH=

Load

Deflection [7]

Load = Shear stress � Cross sectional area From eq (7), �� = �� [8]

Substituting the value of stiffness in Eq.(6), we get

ω2=GA

tm

GA

ω2t=

W

g [9]

Using Eq(1) the above equation can be modified as,

Gg�� =W

A [10]

Also Sd=Sa

ω2 [11]

Substituting Eq(11) in Eq(10),

�(!"# ) = %&

Therefore,

A=W (

Sa

g)

G [12]

D = �(%(!"# )ᴫ� [13]

Incorporating the Importance factor, Response reduction factor and Zone factor, the above equation

can be modified as

D=�4Wf⍺1⍺2⍺3

πG [14]

Where

⍺1=I

R





⍺2= *+ for DBE

Z for MCE

⍺3= 1.0 for hard soil

1.36 for medium soil

1.67 for soft soil

C. Layering of Isolator

To keep the ratio of the horizontal and vertical stiffness equal for the different isolator sets, a

parameter called Shape factor, S is introduced, which is a dimensionless measure of the aspect ratio

of the single layer of the elastomer. For a single pad in the form of a complete circle, the

compression modulus EC is given by

,- ≈ 6/�+ [15]

Also,

�0 = 12�� [16] Considering Eq (8) and Eq(16), the following relation can be obtained.

Ec

G=

Kv

KH [17]

From Eq(6),

EC

G=ωv

2

ωH2 [18]

Or, it can be further modified as

14� = 56�57� [19] Therefore Eq(15) can be re-written as,

6S2=

fv2

fH2 [20]

It is assumed that 89 = 20 8�

Therefore, � ≈ 10

� = Loaded area Force free area

For circular isolator,

� = Cross section area

Curved surface area

� = :;�/(:;� [21]

� = ;(� [22] Therefore,

t= D

4S [23]

The above equation shows that by varying the number of sandwich layer, quite a large

variation in vertical stiffness of the individual isolator element could be achieved.





III. CASE STUDY

A. Description of the structure analysed

The structure analysed is a two bay three storied reinforced concrete framed structure

consisting of three frames (Frame-1 to Frame-3). The centre to centre distance between the columns

is 3000mm and the height of each storey is 3600mm. The column section is of 300mm X 400mm

size consisting of two 16mm bars at top and bottom. The beam section is also of 300mm X 400mm

size consisting of two 12mm bars on either side.

Fig 4. Elevation and plan view of the frame

Column has 8mm diameter ties spaced at 150mm centres and beam has 8mm diameter two

legged stirrups at 100mm centres. The slab is 150mm thick with 12mm bars at 200mm centres with

reinforcement for support moment provided separately. Figure 4 shows the elevation and typical plan

view of the frame tested for verification of time history analysis methodology.

B. Design of base isolator

Models were created in SAP 2000 without base isolator (Figure 5), with linear base isolator

and with non-linear base isolator. Dead load analysis was done for the self weight of the system and

axial load acting on each column was obtained (Table 1). The structure was assumed to be in Zone

V, built on soft soil. The rubber used for the isolator is assumed to have a shear modulus of

0.35MPa.

Elastomeric Base Isolation System For Seismic Mitigation of Low



www.iaeme.com/ijciet.asp

Fig 5.

Table 1: Axial Load on Column

Column Number Axial Load

(kN)

1 81.45 2 122.8 3 81.45 4 127.1 5 195.1 6 127.1 7 81.45 8 122.8 9 81.45

Columns 1, 3, 7 and 9 have same axial load. Also, columns 2, 4,

Therefore, 3 isolators were designed for the whole structure.

Fig 6. Detail design of isolator for central column

The detail design of isolator below the central column is given in Figure 6.

Elastomeric Base Isolation System For Seismic Mitigation of Low-Rise Structures


www.iaeme.com/ijciet.asp 39

Fig 5. SAP 2000 model of the frame

Axial Load on Column and Dimensions of Base Isolator

Thickness of Rubber

(mm)

Diameter of

Isolator (mm)

150 220 150 280 150 220 150 280 150 420 150 280 150 220 150 280 150 220

Columns 1, 3, 7 and 9 have same axial load. Also, columns 2, 4, 6 and 8 have similar loads.

Therefore, 3 isolators were designed for the whole structure.

Detail design of isolator for central column

The detail design of isolator below the central column is given in Figure 6.

Rise Structures, Ganga Warrier A,


[email protected]

f Base Isolator

Thickness of Layer

(mm)

5.5 7

5.5 7

11 7

5.5 7

5.5

6 and 8 have similar loads.





C. Linear time history analysis

Initially a gravity load analysis and modal analysis is carried out the system. Linear time

history analysis of the frame is carried out using SAP

7) is used in the time history analysis. Figure 8 shows the SAP

isolator.

Fig 7. North-South component of El Centro earthquake, May 18, 1940

Fig 8. SAP model of the frame with base isolator

Acceleration on the building and displacement of the building, before and after installatio

base isolator is compared. Graphs are plotted for the same (Figure 9 and 10).




Linear time history analysis

tially a gravity load analysis and modal analysis is carried out the system. Linear time

history analysis of the frame is carried out using SAP-2000 software. El Centro earthquake (Figure

7) is used in the time history analysis. Figure 8 shows the SAP-2000 model of the frame with base

South component of El Centro earthquake, May 18, 1940

SAP model of the frame with base isolator

Acceleration on the building and displacement of the building, before and after installatio

base isolator is compared. Graphs are plotted for the same (Figure 9 and 10).



[email protected]

tially a gravity load analysis and modal analysis is carried out the system. Linear time

2000 software. El Centro earthquake (Figure

model of the frame with base

South component of El Centro earthquake, May 18, 1940

Acceleration on the building and displacement of the building, before and after installation of





Fig 9. Comparison of acceleration of building with and without base isolator

Fig 10. Comparison of displacement of building with and without base isolator.

Inter-storey drifts for the building are calculated before and after installation of base isolator,

the values of which are given in Table 2. (Figure 11)

Table 2. Inter-storey Drift of building

Floor

Ground level

First Floor

Second Floor

Third Floor

Fig 11. Inter-storey drift of building (a) without base isolator (b) with base isolator




Comparison of acceleration of building with and without base isolator

Comparison of displacement of building with and without base isolator.

ey drifts for the building are calculated before and after installation of base isolator,

e given in Table 2. (Figure 11)

storey Drift of building before and after installation of base isolator

Inter-storey drift

Without Base

Isolator With Base Isolator

Ground level 0 5.4 mm

First Floor 1.2 mm 0.2 mm

Second Floor 1.3 mm 0.2 mm

Third Floor 0.6 mm 0.1 mm

storey drift of building (a) without base isolator (b) with base isolator



[email protected]

Comparison of acceleration of building with and without base isolator

Comparison of displacement of building with and without base isolator.

ey drifts for the building are calculated before and after installation of base isolator,

before and after installation of base isolator

With Base Isolator

storey drift of building (a) without base isolator (b) with base isolator





D. Non-linear time history analysis

The dissipation of kinetic energy during seismic ground motions put into the conventional

fixed base structures takes place by internal damping, friction damping at the supports, and radiation

damping trough the base and side soils. In base isolated structures, additional damping should be

provided wherever necessary. One of the effective means of providing a substantial level of damping

is through hysteretic energy dissipation.

The typical hysteresis loop of a laminated rubber bearing can be modelled as bilinear i.e., the

nonlinearity in the bearing is assumed to be bilinear as shown in the figure 13. The parameters d1,

F1, d2, and F2 are defined in the bilinear curve as the yield displacement, yield force, ultimate

displacement and ultimate force respectively.

Fig 12. Bilinear hysteresis loop model

The hysteretic behaviour of a laminated rubber bearing can also be modelled linear, by using

the effective stiffness Ke and the equivalent viscous damping coefficient ζe that depends on the

ultimate displacement d2 and on the corresponding force F2 of the bilinear system. The linearization

of the bilinear model can be made by using the Pre-yield stiffness (K), Post-yield stiffness (Ke) and

the Ratio of pre-yield and post-yield stiffness (α). The effective stiffness from the bilinear model is

calculated as equation 24

�= = >�� [24]

The main parameter for the bilinear model is the effective stiffness i.e., the post yield

stiffness value of the isolator and the additional damping value given by the hysteresis loop model.

The formulation for the values of the effective stiffness and the additional damping is given below in

equation 25.

?+ = ?@ + B� (C+ − C@) [25] = �C@ + B� (C+ − C@)

= �C@[ 1 + B(E − 1)] [26]

By simplifying and solving further,

KG = Hμ

[ 1 + α(μ− 1)] [27]

Finally,

KG = K[ α + I@Jαμ

K] [28]

Additional damping provided by the hysteresis area,

ζe = +: (>M��J >��M>��M ) [29]

= +π

(NMN� − OMO�) [30]





The additional damping in terms of the effective stiffness is,

ζe = +: [ (@JP)(QJ@)Q� RRS] [31]

The additional damping incorporated in the system and the effective stiffness of the system

can be determined from the equations 28 and 31. The post yielding stiffness ratios (α) are assumed

to be 0.05, 0.1 and 0.15 for the isolator. The yield strength of non-linear case (F1) is taken as one-

tenth, one-fifth, one-third, half and three-fourth of yield strength of linear case (F1, elastic). Fifteen

cases are considered in total. Bilinear hysteresis loops were obtained. A typical bilinear hysteresis

loop for α=0.05 and F1=0.1F1,elastic is shown in Figure 13.

Fig 13.Bilinear hysteresis loop for α=0.05 and F1=0.1F1,elastic

From the hysteresis loop, the additional damping and the effective stiffness values are

calculated. The values are given in Table 3.

Table 3: Additional Damping and Effective Stiffness of Non-Linear Isolator

Cases Additional Damping

ζe (%)

Effective Stiffness Ke

(kN/m)

α =0.05, F1=0.10F1,elastic 22.12 25.5

α =0.10, F1=0.10F1,elastic 17.53 38.9

α =0.15, F1=0.10F1,elastic 11.86 53.1

α =0.05, F1=0.20F1,elastic 25.04 23.3

α =0.10, F1=0.20F1,elastic 15.85 36.4

α =0.15, F1=0.20F1,elastic 17.19 52.2

α =0.05, F1=0.33F1,elastic 19.48 20.5

α =0.10, F1=0.33F1,elastic 15.91 35.5

α =0.15, F1=0.33F1,elastic 13.97 51.1

α =0.05, F1=0.50F1,elastic 12.98 18.7

α =0.10, F1=0.50F1,elastic 9.83 34.4

α =0.15, F1=0.50F1,elastic 8.01 50.3

α =0.05, F1=0.75F1,elastic 6.60 17.8

α =0.10, F1=0.75F1,elastic 5.67 33.8

α =0.15, F1=0.75F1,elastic 5.82 49.9





E. Comparison of linear and non-linear analysis

Displacement of building with linear and non-linear base isolators is compared. Displacement

is seemed to be reduced when isolators are designed as non-linear. The comparison is given in table

4.

Table 4: Displacement Reduction In Non-Linear Analysis

Cases Displacement (m) Reduction (%)

α =0.05, F1=0.10F1,elastic 0.0789 32.61

α =0.10, F1=0.10F1,elastic 0.0853 27.13

α =0.15, F1=0.10F1,elastic 0.0947 19.10

α =0.05, F1=0.20F1,elastic 0.0885 24.40

α =0.10, F1=0.20F1,elastic 0.0783 33.09

α =0.15, F1=0.20F1,elastic 0.0806 31.12

α =0.05, F1=0.33F1,elastic 0.0915 21.84

α =0.10, F1=0.33F1,elastic 0.0852 27.15

α =0.15, F1=0.33F1,elastic 0.0850 27.33

α =0.05, F1=0.50F1,elastic 0.0927 20.81

α =0.10, F1=0.50F1,elastic 0.0930 20.52

α =0.15, F1=0.50F1,elastic 0.0926 20.86

α =0.05, F1=0.75F1,elastic 0.0963 17.70

α =0.10, F1=0.75F1,elastic 0.0954 18.44

α =0.15, F1=0.75F1,elastic 0.0946 19.11

IV RESULTS

• For the same soil condition and time period, thickness of the isolator decreases with increase

in damping percentage.

• Buildings situated on soft soil conditions require thicker isolator than those situated on hard

soil conditions, provided the required time period and damping are the same.

• For a particular value of axial load which is to be transferred, the diameter of the isolator

increases, as isolator with rubber of lesser value of shear modulus is used.

• The diameter of isolator decreases as the desired value of time period increases, provided the

soil condition and the rubber used are the same.

• Buildings situated on soft soils need isolator with bigger diameter than situated on hard rocky

soils.

• Both thickness and diameter increases as greater earthquake prone area is chosen.

Linear time history analysis shows that acceleration on the building decreases after the installation of

base isolator, whereas the building displacement increases after the installation. The maximum value

of acceleration on the building before the introduction of base isolator is 10.96m/s2 and after the

installation is 1.12m/s2. The value of building displacement is 40.68mm before installing isolator and

117.7mm after the installation. Inter-storey drift of building decreases but base displacement

increases after the installation of base isolation.

Results of non-linear analysis shows that a maximum additional damping of 25.04% was

obtained for post yielding stiffness ratio of 0.05 and non-linear yield strength taken as one-fifth of

linear yield strength.





On comparing linear and non-linear isolators, a maximum displacement reduction of 33.09% in the

case of post yielding stiffness ratio of 0.10 and non-linear yield strength taken as one-fifth of linear

yield strength was obtained, when the isolators are designed as non-linear.

V CONCLUSIONS

The guidelines for designing laminated rubber bearing (LRB) isolators are developed.

Different isolator parameters were compared with respect to fundamental period and damping of the

fixed base structure, axial load on the column, seismic zone, type of soil and shore hardness of

rubber.

Acceleration on building and displacement are altered by the installation of isolator.

Acceleration on the building decreases and displacement of the building increases, whereas inter-

storey drift decreases. The increase in displacement can be reduced if the isolator is designed as non-

linear. Additional damping is also introduced by the non-linear isolator.

ACKNOWLEDGEMENT

The authors thank Director, CSIR-SERC, Chennai, for the support and encouragement

provided in carrying out the above research work and for the kind permission to present the paper in

the National Conference on Quest for Advancement in Civil Engineering-QACE2015 at SRM

University, Chennai.

REFERENCES

[1] D. Bratosin, “Non-linear effects in seismic base isolation”, Proceedings of the Romanian

academy, Series A, Volume 5, Number 3/2004.

[2] Indian Standard Criteria for earthquake resistant design of structures Part 1: General provisions

and buildings (Fifth revision), IS 1893:Part1(2002)

[3] S. Nagarajaiah, A.M. Reinhorn, and M.C. Constantinou. “Nonlinear dynamic analysis of 3-d

base- isolated structures”, Journal of Structural Engineering, Vol. 117, No. 7, July 1991, pp.

2035-2054.

[4] J.M. Kelly. NISEE Online Archive, University of California, Berkeley

[5] A.S. Arya, “Concepts and techniques for seismic base-isolation of structures”, Earthquake

Engineering, Tenth World Conference 1994, Balkema, Rotterdam pp 6639-6648

[6] F. Naeim and J.M. Kelly, “Design of seismic isolated structures: From Theory to practice”,

John Wiley & Sons, 1999

[7] T. Andriono, and Carr, A. J (1991), “A simplified earthquake resistant design method for base

isolated multi storied structures”, Bulletin of the New Zealand National Society for Earthquake

Engineering. Vol 24, No 3 pp238-250

[8] A.K. Chopra, “Dynamics of structures: Theory and practice to Earthquake engineering”,

Prentice Hall, 1995.

[9] Lekshmy D, Renjith S and Dr. Laju Kottalil, “Design of Composite Gas Bottle with

Elastomeric Layer” International Journal of Civil Engineering & Technology (IJCIET),

Volume 5, Issue 10, 2013, pp. 96 - 103, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

[10] Islam M. Abo El-Naga, “Performance of Asphalt Mixes Containing Rubber” International

Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 7, 2014, pp. 7 - 16,

ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

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