Effect of Base Isolation on Building - IJMTER · International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4 , [April 2016] Special Issue of ICRTET

International Journal of Modern Trends in Engineering and Research

www.ijmter.com e-ISSN No.:2349-9745, Date: 28-30 April, 2016

@IJMTER-2016, All rights Reserved

Effect of Base Isolation on Building Prof. V. R. Rathi1, Gadakh Vishal Balkrishna2

1Professor, Civil Engineering Department, PREC, Loni, [email protected] 2PG Scholar, Civil Engineering Department PREC Loni, [email protected]

Abstract — For seismic response of base isolated building under recorded earthquake ground motion, the analytical studies are presented. In comparison with base isolation system, superstructure is relatively rigid. This can lead to idealization of the superstructure as a rigid body, modelling the base-isolated structure as a single-degree-of-freedom system. Such analysis significantly reduces the computational efforts required to design the base-isolated structure under seismic loading.

A comparison is made of the seismic response of Building with and without base isolators to investigate the seismic response of isolated building and compare it with corresponding response of non isolated building, to identify the effectiveness of isolation system, in this study. Various types of bearings is considered as base isolation systems. Keywords — Elastomeric bearing, sliding system, lead core, Base isolator

I. INTRODUCTION To improve the seismic performance of strategically important buildings such as

commercial buildings, industrial structures, elevated water tanks, residential houses etc. in addition to places where sensitive equipments are intended to protect from the hazardous effects during earthquake, base –isolation is recognized as a mature and efficient technology can be adopted. The choice of the isolation system varies according to the extent of control to be achieved over the seismic response and there upon its design is done to suite the requirement of use of the structure. In the seismic design of base-isolated structures, the superstructure is decoupled from earthquake ground motion by introducing flexible interface between the foundation and the base of the structure. Due to these isolation systems shift the fundamental time-period of the structure to a large value and dissipate the energy in damping, limiting the amount of force that can be transferred to the superstructure such that inter-story drift and floor accelerations are reduced drastically. In the range of 0.1 – 1.0 sec is the dominant time-period of typical earthquake accelerations and maximum acceleration usefully occurs in the range of 0.2 – 0.6 sec. Therefore, when the vibration time-period of the structure is increased beyond these limits the matching of fundamental frequencies of base-isolated structures and the predominant frequency contents of earthquake is avoided thereby the preventing the near-resonance response, resulting in dramatic reduction in structural response. The superstructure is decoupled from the earthquake ground motion in seismically base-isolated systems by introducing a flexible interface between the foundation and the base of structure. Thereby, the isolation system shifts the fundamental time period of the structure to a large value and/or dissipates the energy in damping, limiting the amount of force that can be transferred to the superstructure such that inter-story drift and floor accelerations are reduced drastically. The matching of fundamental frequencies of base-isolated structures and the predominant frequency contents of earthquakes is also consequently

International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4, [April 2016] Special Issue of ICRTET’2016

@IJMTER-2016, All rights Reserved 582

avoided, leading to a flexible structural system more suitable from earthquake resistance viewpoint.

II. ASSUMPTIONS The main purpose of this project is to compare the dynamic analysis of base isolated multi-

storey building by using different base isolator system for different zones of earthquake. Time history analysis is carried out with reference to Isolated multi-storey building reinforced

cement concrete frame structure) for each floor and time history analysis is carried out by using above three systems.

Assumption of study: A. By avoiding SSI effect time history analysis is carried out. B. Near fault motion effect and restoring force (pounding effect), displacement effect on

building is ignored. C. Harmonic ground motion effect, liquid structure interaction effect, after applying Base

isolator base shear and bearing displacement effect on building. Displacement effect on building is neglected.

III. NEED OF STUDY Such studies are indispensible and carry more importance in mitigation of the response

occurring during earthquake. During severe earthquake conditions, it will be essential to study the dynamic behaviour of base-isolated buildings. The modelling of base isolated buildings for the analytical and numerical solution purpose for earthquake response and study the behaviour of base-isolated buildings under earthquake shaking is need for the following reasons:

A. During earthquake, characterizing base-isolation structures and the study for the parameters influencing its behaviour.

B. Evaluation of seismic response and related affecting parameters for aspect ratio and consequently to determine the loss of effectiveness of isolation.

C. To propose strategies for mitigation of such sudden failure of building and verify the performance of the newly modified improved base isolated buildings.

D. Examining the different isolator response and compare it to find out effective isolation system for the consider cases.

IV. METHODOLOGY 4.1 Devices:

A design for base isolation should ensure that the isolators can provide the dual function of period shift and energy dissipation to the isolated structure during earthquakes and the isolators can support the maximum gravity loads of the structure throughout its life. In accordance with these the following design steps should be undertaken.

A. Determine the minimum plan size and locations of isolators under the maximum gravity loads.

B. Compute the dimensions of the isolators. C. Determine the damping ratio of the isolator so that the displacement of the structure can be

within the design limit. 4.2 Lead Rubber Bearing (LRB):

The large displacement problem can be alleviated if substantial damping can be introduced into the bearings or the isolation system. The reason for this is that the laminated rubber bearing with inclusion of a central lead plug has been devised, as shown in Figure.2. Hydraulic dampers, steel



coils, and viscous dampers, are other forms of supplemental dampers, also serve to increase the damping of the isolated structure.

Fig. 1 Lead Rubber Bearing

4.3 Friction Pendulum System (FPS):

The flexibility can also be increased in a structure by providing a Sliding or friction surface between the foundation and the base of the structure. The shear force which equals the product of the coefficient of friction and the weight of the superstructure, transmitted to the superstructure across the isolation interface is limited by the static friction force,.

Fig. 2 Friction Pendulum System 4.4 Elastomeric Rubber Bearing (EMB):

The rubber layer materials used to made high damping rubber bearing are highly nonlinear in terms of shear strains. Effective damping in the range of 0.10~0.20 of critical can easily be exhibited by the HDR, which is achieved through addition of special chemical compounds that can change the material properties of the rubber. The stiffness and damping of the HDR are required to be large enough to resist wind and minor earthquakes. For the purpose of preliminary design, the HDR is assumed to be linear elastic and isotropic in this chapter.

Fig. 3 Elastomeric Bearing

4.5 Mathematical Model of Building:

The idealized mathematical model of the 3-storey base-isolated building is considered as shown in FIGURE 4.

Fig. 4. Mathematical Model of 3-storey Building

4.6 Governing Equations of Motion:



The governing equations of motion are obtained by considering the equilibrium of forces at the location of each degrees of- freedom, for the system under consideration. For the superstructure under earthquake ground acceleration, the equations of motion are expressed in the matrix form as:

[4]

where [Ms], [Cs] and [Ks] are the mass, damping and stiffness matrices of the superstructure, respectively; ( x )= ( x 1 , x2 , …… x N ), x. s , x.. s the unknown relative floor displacement, velocity and acceleration vectors, respectively; ,

and are the and earthquake ground acceleration relative acceleration of base mass, respectively; and {r} is the vector of influence coefficients.

The corresponding equation of motion for the base mass under earthquake ground acceleration is expressed by

+ F [5]

Where and are the base mass and restoring force developed in the isolation system, respectively; is the story stiffness of first floor; and is the first story damping. The restoring force ( ) developed in the isolation system, depends upon the type of isolation system considered and approximate numerical models shall be used.

1) The non-linear force-deformation characteristic of the isolator can be replaced by an equivalent linear model through effective elastic stiffness and effective viscous damping, as per Uniform Building Code and International Building Code. The linear force developed in the isolation system is expressed as

[4]

Where is the effective viscous damping constant; k eff is the effective stiffness; is the effective viscous damping ratio;; and is the effective isolation period: and is the effective isolation frequency[1].

V. NUMERICAL STUDY 5.1 Description of Building:

A three-story RC public building is founded on a rock site is away from active faults. The story heights are 5 m and 4 m for the first, second and third story respectively. The columns, beams, walls, and slab sizes are given as follows: Interior column C1: 0.30 × 0.30 m, Exterior column C2: 0.3 × 0.3 m, Beams B, G: 0.23 × 0.40 m, Equivalent wall W1 thickness: 0.1 m, Slab thickness S: 0.13 m. The story loads on the building are: dead load = 10 KN/m2 and live load = 2.5 KN/m2.The building has a regular plan with three columns spaced at 6 m along the x direction, and also three columns at 4 m apart along the y direction, as shown in Figure 4.2. The total weight WT of the building is 5,150 KN, Density of concrete is 25 KN/m3, modulus of elasticity E=25000 N/mm2 . fck = 25 N/mm2 ,fe=415 N/mm2 . The loads computed for all the columns at their base by a static analysis using the SAP 2000 program, where the bearings are to be installed are shown in Figure 4. For various real earthquake ground motions for linear isolator characteristics seismic response of a base-isolated building is investigated. For the study the earthquake motions selected is N00S component of 1995 Kobe earthquake recorded at JMA. This



implies that the selected ground motions are recorded on stations having firm soil or rocky terrain. The above response quantities are of importance since floor accelerations developed in the superstructure are proportional to the forces exerted due to earthquake ground motion. The bearing displacements are crucial in the design of isolation systems. In this study, the mass matrix of the superstructure [Ms] is a diagonal matrix and characterized by the mass of each floor, which is kept constant (i.e. mj = m for j = 1; 2 . . .N). Further, the base raft of the isolated structure is considered such that the mass ratio, m b/m = 1. The damping matrix of the superstructure, [Cs], is not known explicitly. It is constructed by assuming the modal damping ratio in each mode of vibration for superstructure, which is kept constant. The damping ratio of the superstructure is taken as 0.02 and is constant for all modes of vibration. To achieve a specified fundamental time period of the superstructure Ts, the inter-story stiffness of the superstructure is adjusted. For the three-story building, the inter-story stiffness is taken constant. To provide the fundamental time period of fixed base superstructure as 0.4 sec, the value of k is selected. The natural frequencies of the superstructure with fixed base measured in rad/sec are 15.71. The equivalent linear behaviour is considered by selecting the appropriate values of the effective isolation time period, Teff = 2.5s. The effective stiffness kb and effective viscous damping ratio ξeff of, lead rubber bearing, elastomeric bearing and friction pendulum system are 868 KN/m and 681.59(0.1), 868 KN/m and 1363.702 (0.2), 560KN/m and1363.702( 0.2) respectively and coefficient of friction of friction pendulum system is µ =0.05. The weight of lead rubber bearing elastomeric bearing, and friction pendulum system are 11.922 KN, 0.654 KN and 0.426 KN respectively.

TABLE I. BEARING DETAILS OF BUILDING

Sr. No. Dimension of bearing Lead Isolated Base Elastomeric Isolated Base FPS 1. Diameter of the bearing 700mm 700 mm 450 mm 2. Total height of bearing/depth 552 mm 192 mm 170 mm 3. Numbers of rubber layer 42 12 - 4. Thickness of individual rubber layer 10 mm 10 mm - 5. Numbers of steel plates 41 11 - 6. Thickness of individual plates 2 mm 2 mm - 7. Thickness of bottom and top plates 25 mm 25 mm - 8. Radius of curvature of spherical surface - - 1500 mm 9. Diameter of lead core 130 mm - -

Density of lead, rubber and steel are 11340 kg/ m3 , 960 kg/ m3 and 7800 kg/m3 respectively.



VI. RESULT AND DISCUSSION 6.1 Displacement at Top of Building:

TABLE II. DISPLACEMENT OF BUILDING W/O BASE ISOLATOR AND WITH BASE ISOLATOR

Time (sec)

w/o base isolator (mm)

Deflection in isolator (mm)

Figure 5: Displacement in m (x-axis ) –Time in s (y- axis) graph for w/o base isolator and Elastomeric bearing at top of building

EMB LRB FPS

0 0 0 0 0 5 0.00406 0.00303 0.00338 0.0034

10 -0.0033 -0.00266 -0.0025 -0.00277 15 -0.00343 -0.0013 -0.00131 -0.00155 20

-0.00306 -3.36E-

04 -3.96E-

05 -4.23E-

04 25

-0.00238 -1.24E-

05 5.59E-04 1.45E-04

From displacement-time graph (Figure: 5), it is observed that displacement of the building is more in case of without isolator as compare to elastomeric bearing. The maximum displacement of building is nearly 5 mm and 2 mm for without isolator and for elastomeric bearing between 10 to 15 second respectively.

From displacement-time graph (Figure: 6), it is observed that displacement of the building is more in case of without isolator as compare to lead rubber bearing. The maximum displacement of building is nearly 5 mm and 3 mm for without isolator and for lead rubber bearing between 10 to 15 second, respectively.

From displacement-time graph (Figure: 7), it is observed that displacement of the building is more in case of without isolator as compare to friction pendulum system. The maximum displacement of building is nearly 5 mm and 3 mm, for without isolator and for friction pendulum system between 10 to 15 second respectively.

Figure 6: Displacement in m (x-axis) –Time in s (y- axis) graph for w/o base isolator and Lead rubber bearing at top of building

Figure 7: Displacement in m (x-axis)–Time in s (y- axis) graph for w/o base isolator and Friction pendulum system at top of building



Figure 8: Displacement in m (x-axis ) –Time in s ( y- axis) graph for w/o base isolator Elastomeric Bearing,

Lead Rubber Bearing, Friction pendulum Displacements at top of Building It is obvious that building displacement is more in without base isolator(Figure: 8). The

maximum displacement of building is 5 mm and 3 mm for without isolator and for all types of isolator between 10 to 15 second, respectively. The displacement is reduces with time and elastomeric bearing shows better performance compare to LRB and FPS.

Storey displacement of building without base isolator is increase according to the building height and difference in storey drift is more (Figure: 9). Storey displacement of building with base isolator is decrease according to the building height and difference in displacement of storey drift is less. Due to the bearing displacement, the building top level displacement is influenced. Difference between displacement at base and at top level is more in w/o base isolator as compare to with base isolator.

TABLE III. MODE SHAPE OF BUILDING WITH AND W/O BASE ISOLATOR

Storey height (m)


Deflection in base isolator (mm)

Figure 9: Mode shape of building for w/o base isolator and with base isolator

EMB LRB FPS

0 0 1.171 1.248 1.106 0.5 5.439 1.213 1.284 1.168 5.5 3.478 2.818 2.681 2.704 9.5 4.696 4.406 4.414 4.455 13.5 6.020 4.932 5.384 5.169

6.2 Acceleration at Top of Building:

TABLE VI. ACCELERATION OF BUILDING WITH TYPES OF BASE ISOLATOR AND W/O BASE ISOLATOR

Time (sec)


Deflection in isolator (mm)

Figure 10: Acceleration in m/ s2 (x-axis) –Time in s (y-axis)Graph for w/o base isolator and Elastomeric bearing at top of building

EMB LRB FPS

0 0 0 0 0 5 -0.14518 -0.12234 -0.12505 -0.12655

10 0.03221 0.03359 0.02059 0.03476 15 0.00596 0.00176 -0.00109 0.00159 20 0.01056 -5.78E-05 -0.00216 -2.03E-04 25 0.01348 2.33E-04 1.21E-05 8.12E-05

From acceleration-time graph (Figure 10), it is observed that acceleration of the building

is more in case of without isolator as compare to elastomeric bearing. The maximum acceleration of building is 0.4822 m/s2 and 0. 46942 m/ s2 for without isolator and for elastomeric bearing at 3.55 and 3.6 second, respectively



Figure 11: Acceleration in m/ s2 (x-axis) –Time in s (y-axis) Graph for w/o base isolator and Lead rubber bearing at top of building

Figure 12: Acceleration in m/ s2 (along x-axis) –

Time in s (along y-axis)Graph for w/o base isolator and Friction pendulum system at top of building

From acceleration-time graph (Figure 11), it is observed that acceleration of the building

is more in case of without isolator as compare to lead rubber bearing. The maximum acceleration of building is 0.4822 m/s2 and 0.44711 m/ s2 for without isolator and for to lead rubber bearing at 3.55 and 3.6 second, respectively. From acceleration-time graph (Figure 12), it is observed that acceleration of the building is more in case of without isolator as compare to Friction pendulum system. The maximum acceleration of building is 0.4822 m/s2 and 0. 47631 m/ s2 for without isolator and for friction pendulum system at 3.55 and 3.6 second, respectively.

Figure: 13: Acceleration in m/ s2 (x-axis) –Time in s (y-axis) Graph for w/o base isolator Elastomeric Bearing, Lead Rubber Bearing and friction pendulum at top Building

The expected result i.e. the structure without base isolator accelerate more compare to with bearing are shown from Time acceleration graph (Figure: 13), same as deflection case acceleration in case of elastomeric bearing is less.

TABLE V. MODE SHAPE OF BUILDING FOR BASE W/O AND WITH ISOLATOR

Storey height

(m)

w/o base isolator ( mm/s2)

Deflection in base isolator (mm/s2)

Figure 14: Mode shape of building for w/o base isolator and with base isolator

EMB LRB FPS 0 0 4.207 4.357 4.215

0.5 1.261 4.397 4.546 4.467

5.5 4.988 5.125 4.989 5.161 9.5 4.755 4.935 5.054 4.737

13.5 4.823 4.695 4.472 4.765

Storey acceleration of building (Figure 14), without base isolator is increase according to the

building height and difference in storey drift is more. Storey acceleration of building with base



isolator is decrease according to the building height and difference in acceleration of storey drift is less, The building top level displacement is influenced by the bearing displacement. Difference between acceleration at base and at top level is less in with base isolator as compare to w/o base isolator.

VII. CONCLUSION Under real earthquake and pulse type motion, analytical seismic response of multi-story building supported on base isolation system is investigated. Numerically by step-by-method, the response of the isolated building is obtained and compared with the different values of the initial stiffness of the isolation system. To investigate the effects of the isolator, the variation of top floor acceleration, mode shape and bearing displacement of the isolated building is studied under different system parameters. From the results of the present study, the following conclusions are as under:

A. The performance of base isolated structure is good compare with non base isolated structure.

B. Because of the stiffness of elastomeric bearing is enough to sustain vertical load, elastomeric bearing giving less displacement.

C. From above discussion it concludes that elastomeric bearing is more suitable for building as compare to other type of bearings.

REFERENCES

[1] Jangid RS, Datta TK. Seismic behaviour of base isolated building}A state-of-the-art-review. Structures and

Buildings 1995; 110:186–203. [2] Jangid, R. S. 'Seismic response of torsionally coupled base isolated structure to random ground motion', PhD

thesis, Indian Institute of Technology, New Delhi, 1993. [3] Kelly, J.M., 1986. A seismic base isolation: a review and bibliography. Soil Dynamics and Earthquake

Engineering 5, 202–216. [4] Matsagar, V. A. and Jangid, R. S., 2004, “Influence of isolator characteristics on the response of base-

isolated structures,” Engineering Structures 26(12), 1735–1749. [5] Matsagar, V. A. and Jangid, R. S., 2003, “Seismic response of base-isolated structures during impact with

adjacent structures,” Engineering Structures 25(10), 1311–1323. [6] Matsagar, V. A. and Jangid, R. S., 2005, “Base-isolated building with asymmetries due to the isolator

parameters,” Advances in Structural Engineering 8(6), 603–622.

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Effect of Base Isolation on Building - IJMTER · International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4 , [April 2016] Special Issue of ICRTET