EFFECT OF SOFT STOREY LEVEL VARIATION ON TORSIONAL ROTATION OF LEAD RUBBER BASE ISOLATED STRUCTURE

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 3, March 2018, pp , Article ID: IJCIET_09_03_080 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFECT OF SOFT STOREY LEVEL VARIATION ON TORSIONAL ROTATION OF LEAD RUBBER BASE ISOLATED STRUCTURE Rahul N.K. Post-graduate Student, Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India Avinash A.R Assistant Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India Kiran Kamath Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India Krishnamoorthy A Professor, Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India ABSTRACT The effect of variation of soft storey level in the base isolated structure on torsional rotation has been studied in this paper. For the study, thirty storey unsymmetrical reinforced concrete (RC) structure resting on lead rubber bearing (LRB) has been considered. Structure is subjected to bi-directional seismic excitations. The torsional rotation of base isolated structure is studied for each soft story level variation in the structure and compared with fixed base structure. Results indicates that, LRB base isolated structure reduces torsional rotation as compared to fixed base. It is also found that, effectiveness of LRB for torsional responses reduces when soft story are at higher levels of buildings. Keywords: Base Isolation, Soft Storey, Torsional Rotation. Cite this Article: Rahul N.K, Avinash A.R, Kiran Kamath and Krishnamoorthy A, Effect of Soft Storey Level Variation on Torsional Rotation of Lead Rubber Base Isolated Structure, International Journal of Civil Engineering and Technology, 9(3), 2018, pp editor@iaeme.com

2 Effect of Soft Storey Level Variation on Torsional Rotation of Lead Rubber Base Isolated Structure 1. INTRODUCTION An earthquake is vibration of earth surface due to sudden release of energy from earth crust. Vibrations due to seismic activity causes severe damage to buildings, bridges etc. Hence it is necessary to prevent the structure from harmful effects of the earthquake. Base isolation is one of the technique used to protect structures from damaging effects of earthquake ground motion [1]. In base isolated structure, damage is prevented by isolating the structure from vibrations produced during an earthquake. Elastomeric bearing and sliding bearing are two main classification of isolators. Elastomeric bearings are mainly classified as Low damping rubber bearing, lead rubber bearing and high damping natural rubber bearing. These isolators commonly have two thick steel end plates and alternate layers of rubber and steel shims. Steel shims prevent bulging of rubber providing high vertical stiffness. Low damping rubber bearing exhibit low damping which might be effective in all earthquakes. In high damping natural rubber bearing, increase in damping is achieved by adding extra fine carbon blocks, oils, resins to rubber. Lead rubber bearings contains one or more lead plugs in center to improve hysteretic damping [2]. Researchers have shown that, during earthquake the use of LRB as base isolator can reduce dynamic response of structures in terms of roof acceleration, base shear, and roof displacement and as compared to fixed base structures [3-4]. Hence, for the study lead rubber bearing (LRB) is considered. During an earthquake, torsional mode of the building might activate and can cause severe damage to structure. Torsion usually occurs due to non-uniform distribution of mass, stiffness, strength in structures and torsional components of the ground movement etc. In the design of a structure, earthquake loads are considered only along the principal axes. However, earthquake can also act on any other axis of structure which can lead to torsion in structure [5]. In the design of the structure, uni- directional seismic excitation is considered. However, during an earthquake, structure may be subjected to bi-directional excitations as well [6]. Thus, if a structure designed for uni-directional seismic excitation it might not respond well for a bi-directional seismic excitation [7]. Various researches have carried out in order to study the effect of torsional rotation in structure on base isolation and it is observed that base isolation is effective in reducing dynamic response of fixed base structures even in torsionally coupled structures [8-9]. During earthquake resistant design of structures, considering soft storey effect is of great significance. Soft stories are characterized by low, lateral stiffness within structure [10]. As per design codes, soft storey is one, in which lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the average lateral stiffness of three stories above. Various researches have shown that, rubber base isolated structure can reduce the dynamic response of structure due to soft storey effect [11]. Numerous works have been carried out to study the effect of soft storey on torsional response of structures [12]. However limited studies are carried out on the effect of soft storey level variation on torsional response in tall buildings. Thus in this study, effect of soft storey level variation in LRB base isolated tall buildings on torsional response has been investigated. 2. METHODOLOGY Details of the building considered for the study are given in Fig. 1. The height of each story is 3.5 m the grade of concrete considered is M40 for column and M20 (as per IS 456: 2000) for beams and slabs. The unit weight of reinforced concrete cement (RCC) is 25 kn/m 3. Beams and column are of dimensions 0.25 m 0.45 m and 0.85 m 0.85 m respectively. Thickness of slab is 0.15 m. Live loads on all floors is 3 kn/m 2 and on roof are 1.5 kn/m 2. Dead loads on floors is 2 kn/m 2 and on the roof is 1 kn/m 2. A 3D model of the building is developed in editor@iaeme.com

3 Rahul N.K, Avinash A.R, Kiran Kamath and Krishnamoorthy A ETABS software package. Beam and column elements are modelled as frame elements and slab as shell element. Rigid diaphragm property has been assigned to all the floors. LRB is modelled as link/support element in the software. Various parameter of LRB like vertical stiffness, effective horizontal stiffness, yield force, post yield stiffness ratio are calculated based on the methods given in [13-14]. Table 1 lists typical FPS isolator property values for thirty story building. Figure 1 Plan of the building Table 1 LRB Isolators properties Properties Values Effective horizontal stiffness 1942 kn/m Vertical stiffness kn/m Yield force 175 kn Post-yield stiffness ratio 0.1 Effective damping 0.1 Linear time history analysis is carried out for the study by considering three different earthquake records. As per guidelines of ASCE , minimum three different previously recorded earthquake data should be considered for the design. Out of three earthquake records considered (Fig.2, 3 and 4), Chi-Chi and Kobe are far field earthquakes and El-Centro is near fault earthquake. Details of the earthquake records are given in Table 2. Table 2 Details of earthquake records Earthquake Recording station Magnitude Peak ground acceleration (PGA) in g Chi-Chi Hualian, Taiwan Kobe Kobe university, Japan El-Centro El Centro Array # Figure 2 Accelerograms of Chi-Chi earthquake Figure 3 Accelerograms of Kobe earthquake editor@iaeme.com

4 Effect of Soft Storey Level Variation on Torsional Rotation of Lead Rubber Base Isolated Structure Figure 4 Accelerograms of El-Centro earthquake For the study bi-directional seismic excitations have been considered by varying angle of incidence of earthquake from 0 o to 180 o at an increment of 10 o. This incidence angle variation considered in order to capture effect of torsion on structures. The torsional rotations are obtained by the rotation of diaphragms along vertical axis in terms of radians. In this study, soft storey considered has a height of 5.5 m. Soft storey level are varied according to relative positions ratio H/h with in thirty storey building considered for the study. Where, H is the total height of building in meters and h is height of upper edge of the soft storey in meters. The torsional rotation of base isolated structure is studied for each soft story level variation in the structure and compared with fixed base structure. 3. RESULTS AND DISCUSSION The results obtained after carrying out linear time history analysis are discussed here. Tables 3, 4 and 5 represent the percentage reduction in torsional rotation when fixed base structure is compared with base isolated structure for different H/h ratios of various earthquake records. It can be seen that, in all three earthquake, torsional rotation in fixed base structure increases as H/h ratio reduces. It is also seen that LRB base isolator has effectively reduced the torsional rotation for all H/h ratios of soft storey in all three earthquakes. As observed from tables 3, 4 and 5, in all three earthquakes, for two particular soft storey positions in the structure, reduction in torsional rotation is minimum. Maximum reduction in torsional rotation by LRB base isolator is observed for higher H/h ratios. Maximum reduction in torsional rotation for Chi-Chi earthquake is % for H/h ratio of and minimum reduction in torsional rotation are % and % at H/h ratios of 4.04 and 1.56 respectively. Similarly, for Kobe maximum reduction in torsional rotation is % for H/h ratio of 19.45, minimum reduction in torsional rotation are % and % for ratios of 2.43 and 1.30 respectively. For El-Centro earthquake Maximum reduction in torsional rotation is % for H/h ratio and minimum reduction are % and % at ratios of 3.47 and 1.24 respectively editor@iaeme.com

5 % Reduction in torsional rotation Rahul N.K, Avinash A.R, Kiran Kamath and Krishnamoorthy A 90 Chi-Chi Kobe El-Centro H/h ratios Figure 5 Reduction in torsional rotation by LRB base isolator Reduction in torsional rotation by LRB base isolator for various earthquake can be seen in Fig. 5. It can be seen that, for El-Centro earthquake, which is a near fault earthquake with peak ground acceleration (PGA) of 0.386g, reduction in torsional rotation by LRB isolator is least for all H/h ratios. For Chi-Chi and Kobe earthquakes which is a far field earthquakes with PGA of 0.152g and 0.284g respectively, reduction in torsional rotation by LRB isolator is less for Kobe with higher PGA then Chi-Chi earthquake. Table 3 Torsional response of fixed and base isolated structure for Chi-Chi earthquake H/h ratios Maximum torsional rotation (Radians) for fixed base structure Torsional rotation (Radians) in LRB base isolated structure corresponding to maximum rotation in fixed structure % Reduction in torsional rotation editor@iaeme.com

6 Effect of Soft Storey Level Variation on Torsional Rotation of Lead Rubber Base Isolated Structure Table 4 Torsional response of fixed and base isolated structure for Kobe earthquake H/h ratios Maximum torsional rotation (Radians) for fixed base structure Torsional rotation (Radians) in LRB base isolated structure corresponding to maximum rotation in fixed structure % Reduction in torsional rotation editor@iaeme.com

7 Rahul N.K, Avinash A.R, Kiran Kamath and Krishnamoorthy A Table 5 Torsional response of fixed and base isolated structure for El-Centro earthquake H/h ratios Maximum torsional rotation (Radians) for fixed base structure Torsional rotation (Radians) in LRB base isolated structure corresponding to maximum rotation in fixed structure % Reduction in torsional rotation CONCLUSIONS Effect of soft storey level variation on torsional rotation of LRB base isolated structures is studied. Effectiveness of LRB base isolator in reducing torsional rotation is investigated by considering soft storey level variation within structure, subjected to bi-directional seismic excitations. For the study, Chi-Chi and Kobe as far field and El-Centro as near fault earthquakes were considered. Based on the study it can be concluded that, LRB base isolator can effectively reduce torsional rotation in tall structures soft storey level variation. LRB reduces torsional rotation up to % for thirty storey buildings with soft storey level variation. The effectiveness of LRB base isolator in controlling torsional rotation was found to reduce with increase in PGA of earthquake. ACKNOWLEDGEMENTS Authors would like to acknowledge, the help and facility provided by the Manipal Institute of Technology, Manipal Academy of Higher Education towards this research editor@iaeme.com

8 Effect of Soft Storey Level Variation on Torsional Rotation of Lead Rubber Base Isolated Structure REFERENCES [1] Talikoti, R.S. and. Thorat V.R. Base isolation in seismic structural design. International Journal of Engineering Research & Technology. 3, 2014, pp [2] Naeim, F. and Kelly, J.M. Design of seismic isolated structure: from theory to practice. California, John Wiley and Sons, [3] Rao, P.B and Jangid, R.S. Experimental study of base isolated structure. ISET Journal of Earthquake Technology. 38, 2001, pp [4] Tolani, S. and Sharma, A. Effectiveness of base isolation technique and influence of isolator characteristics on response of a base isolated building. American Journal of Engineering Research. 05, 2016, pp [5] Hosseini M., and Salemi A. Studying the effect of earthquake excitation angle on the internal forces of steel building s elements by using nonlinear time history analyses. 14th World Conference on Earthquake Engineering, China, [6] Khoshnoudian F. and Poursha M. Responses of three dimensional buildings under bidirectional and unidirectional seismic excitations. 13th World Conference on Earthquake Engineering. 55, Canada, [7] Jangid R.S. Seismic response of sliding structure to bidirectional earthquake excitation. Earthquake Engineering and Structural Dynamics. 25, 1996, pp [8] Jangid R.S. and Datta T.K. Seismic response of torsionally coupled structure with elastoplastic base isolation. Journal of Structural Engineering. 16, 1993, pp , [9] Jangid R.S., Eeri M., and Kelly J.M. Torsional displacement in base isolated building. Earthquake Engineering Spectra. 16, 2000, pp [10] Kheni H.L. and Chandiwala A.K. Seismic response of RC building with soft stories. International Journal of Engineering Trends and Technology. 10 (12), 2014, pp [11] Komur M.A. Soft-story effects on the behavior of fixed-base and LRB base-isolated reinforced concrete buildings. Arab J Sci Eng [12] Goh K.S. and Pan T.C. Torsional response of non-ductile structures with soft-first-storey. Earthquake engineering & structural dynamics [13] Cardone D., Gesualdi G. and Bran Cato P. Restoring capability of friction pendulum seismic isolation systems. Bull Earthquake Engineering [14] Constantinou M.C., Kalpakidis I., Filiatrault A. and. Ecker Lay R.A, LRFD-based analysis and design procedures for bridge bearings and seismic isolators. University at Buffalo. Department of Civil, Structural and Environmental Engineering editor@iaeme.com