INTERNATIONAL JOURNAL OF APPLIED ENGINEERING … · 5. James M.Kelly, Farzad Naeim, (1999),...

INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 2, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

REVIEW ARTICLE ISSN 09764259

290

Effects of seismic isolation in the reduction of the seismic response of the structure

Athamnia Brahim, Ounis Abdelhafid Department of Civil Engineer and Hydraulic, University of Biskra BP145, Algeria

[email protected]

ABSTRACT

During the attacks of seismic, the movement of ground is transmitted to the constructions via their foundations; if the ground could move freely under the works; this movement could not be transmitted. In this context emerged, these last years, the idea to introduce between the ground and the buildings a filter, allowing dissipating the maximum of energy, and located large deformations on this filter. This strategy called seismic isolation. Among the goal of the effectiveness of these devices of seismic isolation in the dissipation of energy induced by the seismic, a comparative study was carries out in this article on the answers of structure of 8 story with system of seismic isolation (LRB) and other structure at fixed base.

Keywords: seismic isolation, dissipation of energy, isolation system of LRB, seismic response.

1. Introduction

A seismic corresponds to a fracturing of indepth rocks along a generally preexistent fault. This rupture is accompanied by a sudden release of a great quantity of energy which is translated on the surface by vibrations of the ground. The effect of seismic energy on the seismic response is governed by the equation of energy as follows:

K S H V E E E E E = + + + (1)

WhichE is the energy induced by the seismic excitation, K E is the kinetic energy, S E is the elastic energy of tension, H E is the energy of attenuation by hysteresis, V E and is the energy of viscous damper. The seismic isolation system (SIS) is employed in order to decrease the energy of earthquake acting on the structure. The seismic isolation is a passive system structural control because its operation does not depend on the external energy sources. It is based only on the properties of its materials constitutive (horizontal stiffness, modulus of stiffness, modulus of elasticity….i.e.). The goal above is of present the effects of the dissipation of energy, as well as the response of displacement and the shear force at the base in the structure base isolated per rapport the structure at base fixed.

2. Modeling and Analysis

2.1. Structural Model

The structure used in this study is a building on eight (8) story concrete floors of form (L) irregular in plan, comprising three spans in the longitudinal direction and three spans in the transverse direction with a six (6) m length each one. The beams are of section 30×50 cm ²,




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the columns are of section 50×50 cm² in the first four (4) story and the other stories of section40×40 cm ², with the height of story is of three (3) m and the full slap twenty (20) cm thickness. This structure, models without and with the isolation system, is represented respectively on Figure 1 and Figure 2. We consider in this comparative study the damping of the superstructure equal to 5% for structure at base fixed, and 5% for the structure at base isolated.

Figure 1: 3D structure base fixed Figure 1: 3D structure base isolated

2.2. Parameter of the seismic isolator

For the seismic analysis, we consider the seismic isolation device of rubber with lead plug (LRB) (Figure 3) of nonlinear behavior, whose properties are the following (Table 1, Figure 4);

Table 1: Parametric of seismic isolation system of LRB

Isolation type

u K ( / ) KN m % eff β α eff K ( / ) KN m d Q

( ) KN v K ( / ) KN m

LRB 5742.385 20 0.1 837.277 67.296 2566968.71

Figure 4: Hysteretic Loop (the relation between displacement and the force, model

bilinear equivalent

Figure3: Dimension of seismic isolation LRB according to

IBC2000




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With:

u K : Elastic stiffness

eff β : Effective damping

α : Ratio of postdeformation (relationship between respectively elastic stiffness and plastic stiffness)

eff K : Effective stiffness

d Q : Post yielding force

v K : Vertical stiffness

2.3. Time History Analysis

A nonlinear analysis by Time history is carried out by software ETABS.V9.7.0 would show the behavior of the structure at base isolated and structure at base fixed under the seismic excitations respectively of El Centro, Loma Prieta, and Northridge which are brackets simultaneously in the two directions. The three seismic excitations are watches in Figure 5, Figure 6, and Figure 7.and Table2.

Table 2: Peaks of the time history use in the dynamic analysis.

Seismic excitation Station

Peak acceleration (G) + Angle of excitation

Directorate 1

Peak acceleration (G) + Angle of excitation Directorate 2

EL Centro 1979

Loma Prieta 1989

Northridge 1994

Array #6 Lexington Dam

Sylmar County ,Hospital Parking Lot

0.376(140°) 0.442(0°)

0.604(0°)

0.436(230°) 0.178(90°)

0.662(90°)

Figure.6: The acceleration of ground under the effect of seismic of Loma Prieta (direction 1).

Figure.7: The acceleration of ground under the effect of seism of Northridge (direction 1).

Figure.5: The acceleration of ground under the effect of seismic of the El Centro (direction 1).




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3. Results (Presentations and Comparison):

The results of the dynamic response of the structure at base fixed and isolated in direction X are detailed as follows:

Figure 8: Hysteretic loop (the relation between displacements and the shears efforts of the structure at base isolated).

Figure 9: Diagrams of energies for structure at base fixed.

Figure 10: Diagrams of energies for structure at base isolated.

Figure 11: Displacements between the Storys of the structures at base fixed and isolated




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Figure 12: Comparison displacements of the last level of the structure at base fixed and isolated

Figure 13: Comparison accelerations of the last level of the structure at base fixed and isolated

Figure 14: Comparison the shears forces at the base of the last level of the structure at base fixed and isolated

According to the Figure 11, Figure 12, Figure 13, Figure 14, on results that the increase in the displacement of the structure at base isolated on the level from the base gives a reduction in displacements between the story. This implies that the reduction of accelerations of the last level as well as the shears forces at the base by comparing of the structure at fixed base. This reduction varies according to seismic input energy. For the structures at base fixed the reduction of the seismic excitation depends on the vibration of the structure and the characteristics of the superstructure, frictions between granulated (damping) as well as the stiffness of the vertical elements (columns). This implies that the kinetic and potential energy production hang the duration of the seismic excitation Figure 10, Figure 9. On the other hand, the structure at base isolated concentrates the deformations on the level of the seismic isolation system of (LRB), because of the energy production of dispersion. According to the Figure 8, We observes that under excitation of EL Centro the base isolation system LRB at dissipates a great energy compared to the excitation of Northridge and with that of Loma Prieta. This implies that energy is varied according to seismic excitation.




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4. Conclusion

The results obtained and the comparisons carried out on the two structures at base fixed and isolated with seismic isolation LRB, enable us to deduce that; the lengthening of displacement of the superstructure of the isolated structure decreases the response, of the acceleration and shears forces at the base, from where the seismic dispersion of energy is exerted there. This system of isolation concentrates the deformations on the level of the base of the structure. Finally, the seismic isolation device of LRB decreases the seismic response of the structure to leave its characteristic without resorts to external energy.

Reference

1. Ali SehatTabatabaei, « Energy dissipation systems for seismic resistance», the construction Industry Portal of Iran.

2. Radmila B.Salic, Mihail A.Garevski, Zoran V.Milutinovic, (2008), « Response of lead rubber bearing isolated structure», The14 th World Conference on Earthquake Engineering, Beijing, China.

3. Ikou Shimoda, Masayoshi Ikenaga, Yasuo Takenaka, Atsuhiko Yasaka (1992), « Development of a lead rubber bearing with stepped plug »,Earthquake Engineering, Tenth World Conference Balkema,Rotterdam, ISBN9054100605.

4. Kyu S.Park, Hyung J.Jung, In W. Lee (2002), « A comparative study on aseismic performances of base isolation systems for multi span continuous bridge »Engineering Structures.

5. James M.Kelly, Farzad Naeim, (1999), «Design of Seismic Isolated Structures » From Theory to Practice, by John Wiley & Sons, Inc.

6. J.S. Hwang, J.M chiou, (1996), «An equivalent linear model of lead rubber seismic isolation bearing» engineering structure, 18(7), pp 528536.

7. Bong Yoo, Jae Han Lee and GyeongHoi Koo (2001), « Effects of Lead Plug in Lead Rubber Bearings on Seismic Responses for an Isolated Test Structure »transactions, SMiRT, Washington DC ,Paper1789.

INTERNATIONAL JOURNAL OF APPLIED ENGINEERING … · 5. James M.Kelly, Farzad Naeim, (1999),...

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