International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), INTERNATIONAL JOURNAL OF CIVIL ENGINEERING ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME AND TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME: www.iaeme.com/Ijciet.asp Journal Impact Factor (2014): 7.9290 (Calculated by GISI) www.jifactor.com

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NONLINEAR FINITE ELEMENT ANALYSIS OF HIGH DAMPING RUBBER BEARINGS FOR BASE ISOLATION G. Arya1, 1, 2

Dr. Alice Mathai2

(Department of Civil Engineering, Mar Athanasius College of Engineering, Kothamangalam, Kochi, India)

ABSTRACT Base isolation is a mechanism that provides earthquake resistance to the new structure. The base isolation system decouples the building from the horizontal ground motion induced by earthquake, and offers very stiff vertical components to the base level of the superstructure in connection to substructure (foundation). In this study High damping rubber bearings are analysed for their behaviour during earthquakes as they are considered better than natural rubber bearings due to its damping action coupled with base isolation. . This work involves modelling and finite element analysis of the bearing. A nonlinear static analysis is done and a parametric study is also conducted to study the effect of replacing steel by CFRP plates. Keywords: Base Isolation, CFRP, High Damping Rubber, Nonlinear Analysis. 1. INTRODUCTION The use of seismic isolation for structures has been gaining worldwide acceptance as an approach to aseismic design. Many experimental and numerical studies are required on isolation pads to substantiate the adequacy of design and service conditions so that they can be used for isolation of structures. This study tries to clarify the advantage of the base isolation technique with respect to buildings using laminated rubber bearings since only few researches were done into this area [1]. In this paper the numerical modelling of high damping rubber bearings (HDRB) is implemented using ANSYS 12.0. A three dimensional finite element model of the isolator is created and a non-linear static analysis is done. Based on the behaviour of the isolator, a parametric study is conducted by varying the material property.

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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

2. ANALYSIS MODEL The isolator is modelled through ANSYS 12.0, finite element software. Half of the isolator is modelled in its real dimension. The dimensions of the model are given in the TABLE 1. Table 1: Dimensions of the Model Diameter 1000 mm No. of rubber layers 10 Thickness of rubber layer 10 mm Thickness of steel plate 4 mm Total height of isolator 136 mm 2.1 Material Property Rubber is a hyperelastic material and the material property is defined by strain energy functions. The Polynomial 2-P function is used here and the material parameters were obtained as follows [2]: C10 = 0.797 C01 = -0.05910 C20 = 0.01609 C02 = 1.103 x 10-3 Steel is modeled as linearly elastic material with E=2x105 MPa and v = 0.3. 2.2 Modeling The elements used are, SOLID185 for the rubber layer and SHELL63 for the steel layer. The isolator is totally constraint at its base and only half of the isolator is modelled as it exhibits symmetric behaviour. The nodes at the top surface are coupled in Y direction and X direction and vertical and horizontal loads are applied to the first node on the top surface. The model along with the boundary conditions is shown in Fig. 1.

Figure 1: Model of the isolator with boundary conditions 253

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

3. ANALYSIS Rubber undergoes large deformations and hence the behaviour is nonlinear. Therefore, a nonlinear static analysis is conducted to understand its behaviour under compressive load as well as combined compression and shear. Horizontal displacement values are applied incrementally. Initially, displacement corresponding to 100% shear strain is applied along with the design vertical load. Displacement values are increased until maximum shear strain is reached. It has been found that the material remains stable upto 350% shear strain. The different analysis cases are shown in TABLE 2. Table 2: Analysis Cases ANALYSIS CASE APPLIED LOAD (Fy)(kN) APPLIED DISPLACEMENT (Ux)(mm) 1 8000 2 8000 100 3 8000 200 4 8000 300 5 8000 390 The isolator was subject to 100% vertical load and a horizontal displacement corresponding to 400% shear strain. But it was found that the material remained stable until 392mm horizontal displacement after which the elements were found to be highly distorted. Hence this is the maximum horizontal displacement that the isolator with the given dimensions can take. The results are shown from Fig.2 to 4. The tensile stresses reach a value close to 5 MPa. The permissible tensile stress in rubber is 4.2MPa [3]. Hence the value exceeds the permissible limit, which will lead to cavitation in the rubber layers and the bearing will lose its stability. The shear stress distribution is uniform with the maximum values at the interface of the steel and rubber layers. The variation of stresses along the isolator is shown in Fig.5 and Fig.6. The horizontal shear force Vs displacement plot shows the typical hyperelastic nature of the material (Fig.7).

Figure 2: Deformed shape 254

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

Figure 3: Axial stress, σy

Figure 4: Shear stress, τxz 255

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

Figure 5: Variation of σy along the isolator

Figure 6: Variation of τxy along the isolator 256

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

Figure 7: Horizontal force Vs Displacement 4. STUDIES BY REPLACING STEEL WITH FRP The development of light-weight low cost isolator is crucial if this method of seismic protection is to be applied for a wide range of buildings. To make the base isolation a viable method in such buildings, it is necessary to reduce the cost of the isolators. Conventionally, steel plates are used as reinforcing material. Bearings using steel as reinforcing material are known as steelreinforced elastomeric bearings (SREI). Steel is heavy and makes up for most of the weight of the isolator. Further, thick end-plates are needed on both ends of the isolators which adds to the total cost. The process of bonding steel with the rubber involves placing steel plates between rubber layers and heating them under pressure for several hours. The entire process is complicated and expensive. Many fibre materials whose stiffness is comparable to steel are now available. Seismic isolators can be designed using layers of rubber, bonded with thin layers of bidirectional fibre fabric [4]. Replacing steel with fibre, isolators of much lesser weight can be manufactured. Bearings with fibre reinforcement and elastomeric damping material are called fibre-reinforced elastomeric isolator (FREI) bearings. CFRP fibres have orthotropic material property and are modelled as linearly elastic with the following material constants [5]: Ex = 44000 MPa; Ey = 44000 MPa; Ez = 10000 MPa vxy = 0.3; vyz = 0.25 ; vzx = 0.25 Gxy = 10000 MPa; Gyz = 5000 MPa; Gzx = 5000 MPa 4.1 Ply Orientation Two types of ply orientations were studied: 1. Alternate layers oriented at 0/90◦ 2. Layers oriented at 0/45/90◦ symmetrically from the middle layer 257

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

Nonlinear static analysis was done for FREI bearings as in SREI bearings and TABLE 3 shows the comparison of results for both the ply orientations at a horizontal displacement corresponding to 100% shear strain. From these results we can conclude that the 0/90◦ orientation is the most efficient since the stresses are less.

Orientation 0/45/90 0/90

Table 3: Comparison of Ply Orientations σy (MPa) τxz (MPa) 1.63 13.4 0.544 7.03

τxy (MPa) 1.8 0.59

The model with 0/90◦ was further studied and analysis was done with increments in horizontal displacement. The fibre-reinforced isolator was found to perform well and remained stable over 350% shear strain in the same way as the steel-reinforced elastomeric bearings. The Fig.8 shows the comparison of axial stress and shear stress values for both the isolators at 390% shear strain. From the results it can be concluded that the performance of FREI is comparable to SREI and in fact more efficient than the other since the vertical and shear stress values are lesser for FREI when compared to SREI. Hence CFRP can be used as an effective replacement to steel in elastomeric bearings.

Figure 8: Comparison of SREI and FREI 258

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 252-259 © IAEME

5. CONCLUSIONS The high damping rubber bearing was modeled and nonlinear finite element analysis was done using ANSYS 12.0. The main conclusions were:
The maximum displacement that the isolator can undergo without losing its stability corresponds to 350% of shear strain or 350% of the thickness of rubber layer.
However the isolator can take upto 400% shear strain but the tensile stress exceeds the permissible limit which can damage the bearing.
By replacing steel with CFRP in the multi-layer isolator it was found to behave in a similar manner and more efficient than steel plates.
Fibre-reinforced plates can be used as an alternative to steel there by reducing the weight of the bearing and the use of isolators can be made more widespread. REFERENCES [1] S.B. Bhoje, P. Chellapandi, S. Chetal, R. Muralikrishna and T. Salvaraj, Comparison of computer simulated and observed force deformation characteristics of anti-seismic devices and isolated structures, Indira Gandhi Centre for Atomic Research, India, 1998. [2] Federico Perotti, Giorgio Bianchi, Davide C.M, Limit State Domain of High Damping Rubber Bearings In Seismic Isolated Nuclear Power Plants, Politecnico Di Milano, 2011. [3] C. Constantinou, A. S. Whittaker, Y. Kalpakidis, D. M. Fenz and G. P. Warn, Performance of Seismic Isolation Hardware under Service and Seismic Loading, Technical Report MCEER07, State University of New York, Buffalo, 2007. [4] Kelly, J.M and Takhirov, S.M, Analytical and Experimental Study of Fibre-Reinforced Elastomeric Isolator, PEERR Report, 2001. [5] Animesh Das, Anjan Dutta and S.K Deb, Modeling of Fiber-Reinforced Elastomeric Base Isolators, The 15th World Conference on Earthquake Engineering, Lisboa, 2012. [6] N.Ganesan, Bharati Raj, A.P.Shashikala and Nandini S.Nair, “Effect of Steel Fibres on the Strength and Behaviour of Self Compacting Rubberised Concrete”, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 94 - 107, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [7] Dr. Salim T. Yousif, “New Model of Cfrp-Confined Circular Concrete Columns: ANN Approach”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 2013, pp. 98 - 110, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. [8] Ming Narto Wijaya, Takuro Katayama and Toshitaka Yamao, “Dynamic Analysis of Folded Cantilever Shear Structure and Base Isolated Structure”, International Journal of Civil Engineering & Technology (IJCIET), Volume 5, Issue 2, 2014, pp. 9 - 19, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.

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