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INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 2, No 1, 2011

© Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN 0976 – 4399

Received on July 2011 published on September 2011 280

Performance of NonLinear Elastomeric BaseIsolated building structure Gomase O.P 1 , Bakre S.V 2

1Assistant Engineer, Irrigation Department, Nagpur, India. 2Associate Professor, Applied Mechanics Department, Visvesvaraya National Institute of

Technology, Nagpur, India. [email protected]

ABSTRACT

The seismic response of multistory building supported on base isolation is investigated under real earthquake time history motion. The force deformation behavior of an isolator is modeled as Bilinear hysteretic behavior which can be effectively used to model all isolation system in practice. Uniform Building Code (UBC97) is widely used in design of base isolation systems which contains provision according for near fault effect. In order to investigate the performance of base isolation systems, design according to UBC97 under near fault earthquake. Nonlinear time history analysis of a four storey base isolated building, located at close to an active fault. The isolation system is composed of high damping rubber bearing. Design displacements are estimated using UBC97 parameter. The building is subjected to 1979 ElCentro, 1995 Kobe and 1994 Northridge earthquakes. The analysis result shows that UBC97 predicts isolator displacements successfully. Floor acceleration and interstory drifts of the subject base isolated building are significantly reduced when compared to its fixed base counterpart.

Keywords: BaseIsolation, Earthquake, Elastomeric bearing, Bilinear hysteresis, Structural response, Relative displacement, Roof drift ratio.

1 Introduction

BaseIsolation is one of the most widely implemented and accepted seismic protection systems. Seismic base isolation is a technique that mitigates the effects of an earthquake by essentially isolating the structure and its contents from potentially dangerous ground motion, especially in the frequency range where the building is most affected. The goal is to simultaneously reduce interstory drifts and floor accelerations to limit or avoid damage, not only to the structure but also to its contents, in a costeffective manner. The main feature of the baseisolation technology is that it introduce between superstructure and his foundation a properly chosen flexible layer in order to shift the natural period of structure away from the dominant period of earthquake ground motion and thus to avoid the destructive effects given by the system resonance.

As a result of flexibilization, the natural period of the past fixedbase structure undergoes a jump and the new baseisolation structure has a new natural period. The flexibility of the interposing layers between structure and its foundation lead to a bigger fundamental period for structural ensemble. From this reason, base isolation technology is not suitable for all buildings. Structural response and isolator displacement are two key parameters to decide the characteristics of an isolation system. In nearfield area isolator displacement plays vital role in governing the design of an isolation system, as large isolator displacements leads failure of isolation system. To check isolator displacement, stiffness of isolation system is increased but such increase adversely affects the structural response, especially floor accelerations. Present

Performance of NonLinear elastomeric BaseIsolated building structure Gomase O.P, Bakre S.V

International Journal of Civil and Structural Engineering Volume 2 Issue 1 2011

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3 m

2 m

3 m

5 m 5 m 5 m 3.1 m

3.1 m

3.1 m

3.1 m

study aimed to explore the role of increase of isolation stiffness on structural response of a building. Bilinear isolation system is selected for the study. The bilinear model, used to express the relation between the shear force and the lateral displacement, can be defined by three parameters: initial stiffness, postyield stiffness, and characteristic strength. The characteristic strength, Q, is usually utilized to estimate the stability of hysteretic behavior when the bearing experiences many loading cycles. These three parameters properly reflect the mechanical properties of bearings and provide satisfactory estimations of a bearing’s nonlinear behavior. The specific objectives of the study are: (i) to investigate the effects of increase of initial stiffness on structural response, (ii) to analyze the effect of isolation period on structural response and, (iii) to investigate the effects of characteristic strength ratio of isolator on structural response.

2. Mathematical Modeling of Fixed Base Building Structure

Typical floor plan and elevation of base isolated 4 storey reinforced concrete structure building, which is used as the subject structure in this study as shown in Figure1 and Figure2 respectively. All columns are 30 x 55 cm. and beams are 40 x 50 cm with floor heights are 3.1 m. There are 3 bays of 5m in Xdirection, 3 bays of 2m, 3m and 2m in Ydirection, i.e. plan dimensions are 15 m x 8 m. The total mass of the building is 1400 tons corresponding to the weight of W=14250 kN. All structural members are of concrete with Fck=20 N/mm 2 and FY= 415 N/mm 2 . The fixedbase periods of superstructure in each direction are 0.75 seconds and the superstructure modal damping ratios are assumed to be constant for each mode as 5%.The superstructure is placed on an isolation system consisting of highdamping rubber bearings placed under each column. Since, it is considered that the weight is equally transfer to each bearing under the column. There exists a rigid slab at the base level that connects all isolation elements. The threedimensional model of the baseisolated building and the nonlinear time history analyses are made using a wellknown software program SAP2000 version (11).

Figure 1: Typical floor plan Figure 2: Elevation of building

The building is assume to be located in high seismicity region, i.e. Zone 4, and assigned a seismic zone factor Z=0.4 according to Table 16I of the UBC97. The actual time history data has been carried out specifying closest distance to a known fault that is capable of producing large magnitude events and that has high rate of seismic activity (Class B seismic source according to Table: 16U of UBC97).



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Table 1: Time history record for different types of Earthquake

EARTHQUAKE MAGNITUDE RECORD/COMPONENT PGA

ELCENTRO 1979/10/15 M (6.5)

IMPAVAL/HAEP 045 Closest to fault rupture 16

km 0.327 g

KOBE 1995/01/16 M (6.9)

KOBE/KAK 000 Closest to fault rupture26.4

km 0.251 g

NORTHRIDGE 1994/01/17 M (6.7)

NORTHR/ORR 360 Closest to fault rupture 22.6

km 0.514 g

The recording stations are just near to an active fault, it is likely to be subjected to the near fault effects. The UBC97 takes these effects into account by defining the near source factor NV, based on the closest distance to the known seismic source. The near source factor NV is obtained from Table: 16T of UBC97 as 1. Based on the seismic zone factor and soil profile type for soft soil, stiff soil and hard rock, the seismic coefficient CVD=CV is obtained from Table :16R of the UBC97 as CVD=CV= 0.96 NV (Soft soil) . The figure has shown the nature of time history With its acceleration (g) and time (t).

Figure 3: Actual Time history record for ElCentro, Kobe and Northridge Earthquakes

3. Mathematical Modeling of BaseIsolated Building Structure

For the present study, the force deformation behavior of isolator is modeled as nonlinear hysteretic presented by the bilinear model. A comparison is made for the response of fixed base and Base isolated structure also the effect of increase in time period with different soil profile. A four story RCC fixed base and base isolated (Elastomeric rubber bearing) building model prepared with design software SAP 2000 (Figure3). In analysis the isolator are attached at the plinth level of the structure



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Figure 4: Front View and 3D view of BI building structure model

Figure 5: ForceDeformation behavior of lead rubber bearing

3.1 Displacement criteria as per UBC97

High damping rubber bearings are composed of rubber layers and thin steel sheets. The damping is increased by adding oils, resins, or other fillers and a damping around 10%~15% can be obtained. The stiffness of the bearing is high in case of small displacements and low in case of high displacements. In this project work has follows the standard design procedure for high damping rubber bearing at MCE level effective isolation period TM at different increasing values selected are (TM=2, 2.5, 3, 3.3, 3.5 Sec.) with effective damping βD=0.20 has taken for study. The effective horizontal stiffness of isolation bearing is given by the equation [Naeim. F and Kelly. J. M ,1999] :

Where, W is total weight carried by isolation bearing and TM is effective isolation period assumed for MCE Level. Providing an effective isolation period



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This is nearly equal to the target period. Here g is gravitational force and taken as 9.81 m/Scc 2 . The damping coefficient corresponding to βD=0.20 is BD=1.5 according to Table A16C of the UBC97. The design displacement of an isolation system along each main horizontal axis at maximum capable earthquake (MCE) level for soft soil at ElCentro earthquake is calculated according UBC97

Minimum design displacement permitted for dynamic analysis

Where ‘T’ is the fixed base time period of building structure. Finally the total design displacement including additional displacement due to accidental torsion is calculated according to UBC97 as follows:

Where b=8 m is the shortest plan dimension of the structure measured perpendicular to the longest plan dimension of the structure, which is d=15 m. Here y is the distance between the center of rigidity of the isolation system and isolation bearing placed at the sides of the plan, measured perpendicular to the direction of seismic loading under consideration, thus y=b/2=4 m in this study. Finally, e is the actual eccentricity plus the accidental eccentricity which is taken as 5 percent of the maximum building dimension perpendicular to the direction of force under consideration [2, 25]. The total design displacement calculated above satisfies the minimum criteria; DTD=0.626 m> 1.10 DD=0.612 m.

3.2 Bilinear Hysteric model of Isolator

The non linear force deformation behavior of the isolation system is modeled through the bi linear hysteresis loop characterized by three parameters namely (i) Characteristic strength Q (ii) Initial stiffness K1 (iii) Post yield stiffness K2, and (iii) Yield displacement Dy (Figure 4). The bilinear behavior is selected because this model can be used for all isolation systems used in practice. The forceDisplacement relationship of high damping rubber bearing shows the yield force, Fy, the design displacement DD, the effectives stiffness, Keff, and characteristic force, Q. Post yield to preyield stiffness ratio (n=K2/K1) depends on the material used and



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considered n=0.10 for rubber isolator [Naeim. F and Kelly. J. M, 1999 & Pradeep T. V. and D. K. Paul, 2006]. The elastic stiffness K1 is difficult to measure and is usually taken to be an empirical multiple of K2, which can be accurately estimated from the shear modulus of the rubber and the bearing design. The Postyield stiffness of the isolation systems, K2 is 'generally design in such a way to provide the specific value of the isolation period, Tb expressed as [Jangid R.S. and Matsagar V.A.,2004]:

Where, M is the total mass of the base isolated structure.

4. Numeric Study

Seismic response of 4Story RC fixed base and baseisolated building structure are investigated under various real earthquake time history ground motions for nonlinear isolator characteristics. The earthquake motions are selected for the studies are 1979 ElCentro, 1995 Kobe and 1994 Northridge recorded at different stations as the details are given in (Table1). The maximum displacement calculation for the dynamic calculation has done at maximum capable earthquake level as criteria given in UBC97.

4.1 Comparison of FixedBase and BaseIsolated Building Structures

In this section a comparison of earthquake response of fixed base structure with the base isolated structure is made with baseisolated building structure. The bilinear behavior is selected in a way to represent the forcedeformation behavior of the commonly used isolation system such as elastomeric bearing (i.e. lead rubber bearing).

Table 2: Output result for ElCentro, Kobe and Northridge Earthquake

Fixed Base Structure Base Isolated Structure

Earthquake El Centro Kobe Northridge El

Centro Kobe Northridge

Base Shear (kN) 5202 3102 10710 2052 1713 2723 Acceleration (m 2 /sec) 2.8 2.81 3.816 2.38 1.7 3.3

Displacement (m) 6.5x10 3 4.2x10

3 1.386x10 2 0.13 0.049 0.131

The structure analyzed for above time history for soft soil condition. For the analyses structural time period has assumed 2.5 Sec. at MCE level. As result output it is found that the response of Base Isolated Structure is predominantly lower than Fixed Base Structure. Acceleration response at base somewhat lesser in case of isolated structure. Base displacement has increased drastically to make the structure flexible and lower damage. Represent the Base Shear response for ElCentro, Kobe and Northridge Earthquake time histories. Here earthquake response comparisons have plotted for fixed base and Baseisolated building structures. The responses are plotted for the assumed Time Period T2.5 Sec. at the MCE level (soft soil) as per UBC97 design criteria. The peak values for fixed and baseisolated structure are given in Table5. During first 6 second the base shear for fixed structure gets instantly increased in ElCentro earthquake showing the undulating response but in case of baseisolated structure it shows the



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less and smooth response. The same behavior is obtained in Kobe earthquake during 5 to 10 th second and for Northridge earthquake it happened during 10 to 15 th second.

Figure 6: Base Shear Response comparison, fixed base and BI structure for ElCentro, Kobe and Northridge Earthquakes

Figure6, represent the base acceleration response for fixed and baseisolated structures for El Centro, Kobe and Northridge earthquake time history at MCE level for soft soil. The acceleration values given in Table5. The acceleration values vary as the nature of time history changes

Figure 7: Acceleration Response for Fixed base and BI base structure for ElCentro, Kobe and Northridge Earthquakes.

4.2 Performance Criteria

Displacements, accelerations, and base shear discussed here are all in Xdirection. In order to assess the performance of the structure when subjected to different magnitude of earthquakes, a



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performance criteria is established. These include peak base displacement (P1), peak roofdrift ratio (P2), peak roof top acceleration (P3), and peak base shear (P4). Since there exists a rigid slab at the base level that connects all isolation elements, relative displacement of the base with respect to the ground also represents the deformations of the isolators [Alhan C. and Altun M. ,2009]:

1. Maximum Relative Displacement at Base (P1) P1 = maxt (|db|) (Eq.1) Where, t = Time db = Relative displacement of base with respect to ground

2. Roof Drift Ratio (P2) Roof drift ratio is defined as the difference between the displacement of top

floor (roof) and the base floor divide by the total height of building. Thus, Peak roof drift ratio given by P2 = maxt (|d6 – db) / H|) (Eq..2) Where, d6 = Relative displacement of roof with respect to ground. H = Total height of building.

3. Peak Floor Acceleration (P3) Peak floor acceleration is the acceleration measured at the roof top to study the

earthquake impact on structure. P3 = maxt (| a3 | ) (Eq.3) Where, a3 = Total acceleration at top floor

4. Peak Base Shear P4 = maxt (| Vb |) (Eq.4)

Table 3: Performance Criteria calculation for ElCentro, Kobe and Northridge Earthquake for soft site soil condition

ElCentro Earthquake

Kobe Earthquake

Northridge Earthquake Performance

Criteria Fixed Base

Base Isolated

Fixed Base

Base Isolated

Fixed Base

Base Isolated

P1 (Cm) 12.98 49.13 11.3 P2 (x 10 3 ) 5.103 2.22 2.90 1.08 11.51 1.172 P3 (m/sec 2 ) 5.986 2.60 5.69 2.02 11.31 4.172 P4 (KN) 5202 2052 3102 1713 10710 2723 P3/ag 18.30 7.95 22.67 8.04 22 8.11

Fig 5 shows the Xdirection base shear time history of baseisolated building and its fixedbase counterpart for ElCentro, Kobe and Northridge Earthquakes. As it can be seen from figure, the period of the baseisolated building (T2.5 Sec.) is much higher and its base shear is much smaller than its fixed base building structure. Evidently, the seismic effects are reduce significantly. Tabe3 showing the performance criteria for fixedbase and baseisolated building, soft soil condition as per UBC97 at ElCentro, Kobe and Northridge Earthquakes.



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4.3 Hysteresis Loop

The hysteresis loop associate with viscous damping is the result of dynamic hysteresis since it is related to the dynamic nature of loading. The loop area is proportional to excitation frequency. The nonlinearity is well studied by hysteresis loop. In Figure15, shown hysteresis loop for ElCentro, Kobe and Northridge earthquake at target time period T=2.5 Sec. at MCE level for soft, stiff and hard rock soil condition as prescribed in UBC97. The amount of energy dissipated by bearing is equal to the area covered by the hysteresis loop as shown.

Figure 8: Comparisons of Energy Dissipation of bearing for soft soil condition at (MCE) level at T = 2.5 Sec.

The force displacement loop for elastomeric bearing for ElCentro, Kobe and Northridge Earthquakes is represented in Figure7. It is seen that the bilinear behavior assumption made in the design stage according to UBC97 is appropriate. A close inspection of the curve shows that the characteristics force, yield displacement, initial stiffness and post yield stiffness for elastomeric bearing appear as designed and modeled. As per table3 the peak displacement is P1=11.3 Cm. in case of Northridge earthquake, which is found much smaller than the predicted total design displacement of 55.9 Cm. However this prediction includes the building to be subjected to near fault earthquake and corresponding near source factor was included in the design displacement calculations. The peak base displacement in case of Kobe earthquake jumps to P1=49.13 Cm. which is 91% of the total design displacement calculate according to UBC97.

5. Conclusions

The analysis of fixed base and base isolated 3D four storey building is performed in this thesis. An exhaustive study has been performed on the performance of base isolated structures. The behavior of building structure resting on elastomeric bearing is compared with fixed base structure under maximum capable earthquake. A complete list of performance criteria is presented in Table3. Seismic base isolation can reduce the seismic effects and therefore floor accelerations, interstory drifts, and base shear by lengthening the natural period of vibration of a structure via use of rubber isolation pads between the columns and the foundation. However, in case the deformation capacity of the isolators exceeded, isolators may rupture or buckle. Therefore, it is vitally important to accurately estimate the peak base displacements in case of major earthquakes, particularly if the base isolated building is likely to be struck by nearfault earthquakes. Nearfault earthquakes may contain longperiod velocity pulses which may coincide with the period of the base isolated structures. In such a case, the isolators may deform excessively. Uniform Building Code97, is widely used in design of base isolation systems which contains provisions accounting for nearfault earthquake effects. In order to investigate



289

the performance of base isolation systems designed according to UBC97, for bidirectional nonlinear time history analyses of a 4story base isolated building, located to the specific distance from an active fault, are carried out. Based on the simulations carried out, it is concluded that seismic base isolation is a successful technique that can be used in earthquake resistant design. To check the effectiveness of the isolation system, performance criteria have been carried out for fixed base isolated structure. According to analysis study, following conclusions are as follows:

1. The peak base shear (P4) of the baseisolated building is only 39% of its fixedbase counterpart in case of El Centro, 55% for Kobe and 25% for Northridge Earthquakes.

2. The peak top floor acceleration is reduced from P3=5.986 m/s 2 to P3=2.60 m/s 2 in case of El Centro, P3=5.69 m/s 2 toP3=2.02 m/s 2 for Kobe and P3=11.31 m/s 2 toP3=4.172 m/s 2 for Northridge Earthquake.

3. The roof drift ratio is reduced 56% in case of ElCentro, 63% for Kobe and drastically reduced (nearly 90%) in case of Northridge Earthquake respectively showing the success of BaseIsolation System.

4. According to the area covered by the hysteresis loop of the isolator more energy dissipated in case of Northridge Earthquake and less energy dissipated in case of Kobe Earthquake.

5. From the forcedeformation loop it is found that force carried by isolator F=12 kN and displacement D=12.9 Cm. for ElCentro, for Kobe F=12 kN and displacement D=12.9 Cm. and F=12 kN and displacement D=12.9 Cm. for Northridge Earthquake.

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