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International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ “Effectiveness of Lead Rubber Base Isolators, for seismic resistance of Buildings, supported on different soil stratas,” 1 S.K. Sabu, 2H.S.Chore, 3S.B.Patil 1,2,3 Department Of Civil Engg, Datta Meghe College Of Engg., Airoli/ University of Mumbai Email: [email protected], [email protected], [email protected] Abstract- Seismic Base-isolation of building is an innovative technique used in recent years, for reducing seismic energy transmitted to buildings, in highly seismic prone areas,. The basic principle behind the base-isolation system is to introduce a flexible interface between the base of a structure and the foundation. Laminated Rubber Bearings are the most widely used technology in seismic base isolation, because of their technical and economic effectiveness and reliability. Even though base isolation method is extensively used for over 8000 structures internationally, this method is very rarely used in India in spite of the fact that India has so many highly seismically active zones. A case study comprising of the seismic analysis of a sample building, in seismic zone V of India, with lead rubber bearings at the base is done in this paper. Linear static, and nonlinear dynamic analysis are performed for both isolated and non-isolated buildings under code mandated bi-directional earth quake. A comparative seismic parametric study is also done with conventional design. Parametric comparison is done for the building with three types of founding soil strata. The study reveals that for medium rise building construction, isolation can significantly reduce seismic response. The reduction in seismic responses are more in harder soil founding strata. In seismic vulnerable areas where the main concern is the mitigation of the seismic instability with the support of critical components, the study shows the effectiveness of the base isolation system in terms of reduced structural responses under seismic loading. Index Terms—Lead rubber bearings, Effective stiffness, effective damping ratio, response spectrum. I. INTRODUCTION The Indian subcontinent has a history of devastating earth quakes. The shaking memories of high intensity earthquakes of Bhuj(7.7) (on the anniversary of India’s 51st Republic Day, 2001) and lathur(6.2)(1993) are still live in our minds. Third deadliest earthquake in the history of the world, the Sumatra–Andaman earthquake,2004(9.3), and the tsunami generated thereafter, killed over 2,30,000 people in fourteen countries, including15,000 people in India and inundating coastal communities with waves up to 100 ft hight . Even now there is frequent occurrences of earthquakes in the Kashmir and Himalayan region. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes. A World Bank & United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050. The latest version of seismic zoning map of India given in the earthquake resistant design code of India [IS 1893 (Part 1) 2002] divides India into 4 seismic zones (Zone 2, 3, 4 and 5), with Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level. Each zone indicates the effects of an earthquake at a particular place based on the observations of the affected areas and can also be described using a descriptive scale like Modified Mercalli intensity scale or the Medvedev-Sponheuer-Karnik (MSK) scale. The MSK intensity broadly associated with the various seismic zones is VI (or less), VII, VIII and IX (and above) for Zones 2, 3, 4 and 5, respectively, corresponding to Maximum Considered Earthquake (MCE). Zone 5, which is referred to as the Very High Damage Risk Zone in the IS code, assigns zone factor of 0.36 to it, which is indicative of effective (zero period) peak horizontal ground accelerations of 0.36 g (36% of gravity) that may be generated during MCE level earthquake in this zone. The state of Kashmir, the western and central Himalayas, the North-East Indian region and the Rann of Kutch fall in this zone. Conventional structure design approach to earthquake resistant design of buildings, followed all over world, is by providing building with strength, stiffness and inelastic deformation capacity, which are great enough to withstand a given level of earthquake-generated force of medium intensity. Practically they all proved to be very uneconomical and failing under high seismic activities. _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 53 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ Seismic Base-isolation of building , developed in recent years, is an efficient method for reducing seismic demand to buildings, in highly seismic prone areas. This innovative design approach aims mainly at the isolation of a structure from the supporting ground, during earthquake excitation. This system can separate the superstructure and component from the earthquake ground motion, thus reducing the structural earthquake response. Base isolation( B I) keeps buildings, bridges, and other structures completely operational after a major earthquake. Seismic Isolation effectively reduces a substantial amount of the seismic energy and displacement transferred to the structure from the ground by absorbing energy in base isolators. They carry vertical load yet give lateral flexibility with damping – effectively decoupling the structure from the ground. Many types of structures have been designed and built with base isolation, incorporating different concept of base isolation. Most of these completed buildings and those under construction use rubber base isolation bearings, which are found to be most efficient in base isolation designs. Even though base isolation method is extensively used for various structures internationally to protect civil engineering structures, to withstand heavy earth quakes, this method is very rarely used in India. Base isolators are used in India, only in five government projects and none of buildings in private sector, due to lack of awareness about its benefits and technical feasibility. Technoeconomic feasibility study of base isolators in Indian conditions is the need of the hour, to popularise the benefits of base isolators in India. An attempt is made in this paper to analyse a sample data centre building in seismic zone V of India, employing base isolators. Seismic response parametric comparison is done for the structure analysed with and without base isolators. Parametric comparison is also done for the building, with three types of founding soil strata. The most widely used base isolation method of Lead Rubber Bearings are used in the analysis and design. II. CHOICE OF BASE ISOLATOR: RUBBER BEARINGS VS. SLIDING BEARINGS A variety of isolation devices including elastomeric bearings, frictional/sliding bearings and roller bearings have been developed and employed for aseismic design of buildings. economic effectiveness, inherent stability, and reliability. Developmental studies are in progress regarding sliding bearings on their efficiency, effectiveness and performance. The lead rubber bearings (LRB) invented in the 1970s, are extensively used in New Zealand, Japan and United States. The first building isolated by the LRB was the William Clayton building in Wellington, New Zealand, completed in 1981. Skinner 1975; Robinson 1982; Kelly 1990 did initial developments on LRB. Even though there are some comparative benefits with sliding base isolator such as lengthening the period of vibration of structure and ease to manufacture there are certain major disadvantages as listed below; Due to Mechanical/Friction type action, it is prone to wear and tear and so early replacement is required, which is practically very difficult and is very costly Can settle in the dish after a long period of inactivity and is likely to tear the bowl if an earthquake occurs, with subsequent replacement A small earthquake could create a situation where the pendulum gets stuck half way up the bowl due to sticktion. The reliance on gravity to recentre the device only works if the earthquake is big enough. A rocking effect can occur on the isolated building due to the nature of the bowl Once the outer lip of the bowl is reached they may break Even though there is a disadvantage of large base displacements and excessive structural vibrations, resulting from the increased flexibility of the rubber base isolation system, under severe seismic excitations, they are comparatively having so many advantages compared to sliding base isolators, such as simplicity, economic effectiveness, inherent stability, reliability and re-centring ability. It is proven in so many past earthquakes that it is not necessary to replace the rubber bearings during lifetime of the building and maintenance requirement is also minimal. Even if they are pushed past their design capacity they don’t break, but they still continue to work at a performance level of 85% effectiveness and they won’t fall apart like a pendular system. Feedback in countries exposed to a high seismic risk, confirms the efficiency of these systems Considering all these facts Lead Rubber Base Isolation system is selected as base isolator for this project. The lead rubber bearings and sliding bearings are most commonly employed base isolators for buildings of medium height. Lot of analytical and experimental study III. ANALYSIS AND DESIGN had been done by various researchers for the last three METHODOLOGY decades, as seen in various papers reviewed, mainly on Rubber isolators. Many rubber based base-isolation Analysis and design of Lead rubber base isolated building techniques such as laminated rubber bearings, lead-rubber is done as per the procedure laid down in UBC 1997. All bearings etc. have been implemented to full-scale major structural components have been designed for buildings and bridges because of their simplicity, _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 54 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ appropriate loads and its combinations, as per relevant clauses of IS 456:2000, IS 875 and IS 1893 where ever applicable., for the most unfavourable effects. The structural system is finalised with preliminary sizing of structural members as per loading requirement and material properties selected. The structural members are detailed as per ductile detailing code IS 13920 as Special Moment Resisting Frame members and shear walls. A three dimensional structural model is developed using the most popularly used structural analysis software called ETABS developed by “ Computers and Structures Incorporated ”. The structural models of the buildings were composed of normal shell finite elements for the shear walls. the columns were simulated by beam elements. The isolators were modelled as spring elements. The shear stiffness of the isolators was taken into account as the estimated value corresponding to a 100% strain, imposed under the action of the vertical loads. This assumption allowed for a linear analysis of the structural response. The critical damping factor of the isolators normally ranges from 0.20 to 0.30. The response acceleration spectrum method was selected for the linear dynamic analysis; The IS code defined response spectrum is selected and assigned for the model. It is checked for lateral static wind load as well as static seismic loads as per IS codes. The required data for the seismic analysis of the sample building is identified, including the floor layouts, loading parameters, preliminary sizing of members along with material properties. . Technical parameters of Base isolator also is required to be identified. The base isolator is idealised in the mathematical model, and the seismic analysis of the 3 D model fitted with LRB is done to find the seismic parameters of the structure. The seismic design parameters of analysis files with and without base isolator are compared. Parametric seismic analysis comparison of the structure with base isolators is also done for various foundation soil strata to find the performance of base isolators in various types of founding soils. The proposed data centre Building consists of Basement + Ground + 1st to 6th floors + Terrace floor. The appropriate sized base-isolators using LRB is to be designed for the worst combination of all possible vertical and lateral loads. The structure is to be analysed for all loads with and without LRBs fitted to all columns and shear walls as per codal requirements. Parametric comparison is to be done for all seismic parameters for the building analysed with and without base-isolators. Fig. 1 Typical floor structural plan V. DETAILS OF STRUCTURAL SYSTEM OF DATA CENTRE BUILDING The proposed Building is designed for Basement + Ground + 1st to 6th floors + Terrace floor. Ground floor houses some equipments such as chillers, first to third floors are for computer racks whereas fourth to sixth are office floors. Floor height of Basement floor is 4.5m and typical floors 5.0m. The structural system supports the vertical loads consisting of Dead and Super-Imposed Loads as well as lateral forces of Earthquake and Wind. The sub-structure mainly consists of raft slab foundation and retaining walls. Columns and shear walls carry all the gravity loads and lateral forces to the foundations. The Structural system of typical floor is of beam slab system. IV. CASE STUDY To study the effect of using Lead rubber bearing on the buildings to carry the seismic forces in highly seismic prone areas, in Indian conditions with different soil founding strata, Lead Rubber Bearings are designed for a proposed medium height data centre building near Delhi region of India. Data centre is a heavily loaded building, which is handling very sensitive electronic information of various corporate groups. The data stored need to be protected and the function of data centre need to proceed uninterrupted, during and after the occurrence of a heavy earthquake. Even though the building is located in zone IV, as per client’s request, it is designed for one zone higher i.e. zone V. The structure is analysed with Soil Structure Interaction, for Various Founding Strata Of Hard, Medium And Soft Soils for parametric comparison Fig 2. Section Showing Base Isolator Below Column. The required data for the seismic analysis such as loading parameters, preliminary sizing of members along with material properties are worked out. Technical parameters of Base isolator such as compression stiffness, effective secant stiffness, equivalent damping ratio etc. are identified from vendor specifications of Robinson Seismic Bearings and as per UBC 97. A three dimensional model of the structure is developed. The base isolator is also idealised in the mathematical model, and the seismic _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 55 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ analysis is done to find the seismic parameters of the structure. A. Design Loads All partitions are of lightweight aerated blocks of 6.7 KN/m3. Live load is considered as per IS: 875 (Part-2)-1987 and as per the requirement of DATA CENTER Loading requirements. Live load in ground floor is 5 KN/m2 and. Office areas of fourth to sixth floors is 5 KN/m2 . In computer rack floors of first to third floors live load in rack areas is 18.5 KN/m2 and other areas it is 8 KN/m2. Superimposed dead load consists of Tiled Floor finish 1.0 KN/m2, false ceiling 0.50 KN/m2 and light weight partition of 1.0 KN/m2. Water proofing load of 3 KN/m2 is applied in terrace. Load of Water tank for 84,000 and 80,000 litres over the stairs as per functional requirement is applied. Fig. 3 Response spectra for 5% damping as per IS 1893-2002. E. Structural 3d Models Of Data Centre Building B. Wind Loads The wind pressure as per IS: 875(Part 3)-1987, Basic Wind Speed Vb = 47 m/sec (For Delhi region) Risk Coefficient ,K1 = 1 Terrain Category 2,Structure Class B,Factor K2 As per IS 875 (Part III) Topography factor K3 = 1,Design Wind Speed Vz max = VbK1K2K3, Design Wind Pressure,Pz = 0.6 Vz 2 Fig. 4 Typical Floor drawing C. Earthquake Load Static loading due to earthquake is assessed based on the provisions of IS: 1893(Part-1):2002, for Seismic Zone V ,Zone factor (Z)= 0.36, for Maximum Considered earthquake(MCE) for the service life of the structure. Importance Factor (I) = 1.0 Response Reduction Factor (R) = 5.0, Damping = 5 % . Design horizontal seismic coefficient Ah shall be Dynamic analysis loading due to earthquake on the provisions of IS: 1893(Part-1):2002 & UBC 97 Response Reduction Factor (R) = 2.00 Damping = 25 % with Sa/g multiplying factor of 0.55 for spectral acceleration coefficient. D. Materials Used The following materials are used for the structure. Columns = M60/50/40 , Beams and Slabs = M50/40/30, For raft foundation = M50 The reinforcements considered is of Grade Fe500 Fig. 5 Full Model Structural System VI. PARAMETRIC STUDY A. Comparison Of B.I Structure With Fixed Structure. Seismic parameters are compared, for the sample building, by analyzing it, as a fixed base building and with Lead Rubber Base isolators introduced at the base of the columns and shear walls, considering medium soil founding strata and the results are summarized in the table 1. As seen from the analysis, the time period of the structure is increased to almost 2.5 times the Table 1 Parametric comparison of Fixed base and B.I Parameters Fixed base With B.I. Comparison _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 56 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ Time period 1.1082 2.8612 + 158% of first mode base shear 14700 8130 - 44.69% (KN) Overturning 3.39 x 105 1.54 x 105 - 54.57% moment (KNm) Base 0.0 30.9 displacement (mm) Top 25.7 60.0 - 133% displacement (mm) value of fixed base building, resulting in a substantial reduction of spectral acceleration and hence reduction of other seismic parameters of the building. Base shear and moments are reduced by almost half. There is some base displacement due to the introduction of base isolator but the effective displacement of the top with respect to the base is well within the codal limit of H/500. B. Comparison Of B.I Structure With Different Soil Stratas. The building is analysed with three types of foundation soil strata systems A. Hard soil consisting of rocky strata B. Medium soil consisting of hard murrum and sandy clay C. .Soft soil consisting of soft clayey soil. The following are the results of soil-structure interaction analysis of the base-isolated building. 1. Base shear and moments are increased for structure supported on softer soils,Compared to hard soil. 2. The time period of vibration of the building is not affected with the type of soil. 3. The response spectrum acceleration is less for buildings supported on harder soils 4. The Base displacement and top storey drift are less for buildings supported on harder soils Table 2 Parametric Comparison of structure on different soils Parameter s Time period base shear (KN) BASE moment (KN m) Base displacem ent (mm) Top Storey drift w.r.t base (mm) Soil Syste m S1-har d With B.I. 2.909 4 6060 Soil System S2-mediu m With B.I. 2.8612 8130 Soil Syste m S3-sof t With B.I. 2.909 4 10100 1.15 x 105 1.54 x 105 1.95 x 105 23 30.9 38.3 25.2 29.1 41.7 Comp arison S1-S2 Compari son S2-S3 -1.65 % + +1.68% 34.16 % + 33.91 % + 34.35 % + 15.48 % + 24.23% + 26.62% + 23.95% + 43.30% Fig. 8 Base displacement comparison for different soil 15000 10000 HARD SOIL S1 5000 MED. SOIL S2 0 SOFT SOIL S3 BASE SHEAR Fig. 6 Base shear comparison for different soils 3 2 1 0 Fig. 9 Top storey displacement comparison for different soil VII. HARD SOIL S1 SUMMARY AND CONCLUSIONS As described above the analytical study comprising of the seismic analysis of a sample building, in seismic zone V MED. SOIL S2 of India, with lead rubber bearings at the base, is done with a three dimensional structural model with response SOFT SOIL S3 BASE MOMENT-KNM(x E spectrum analysis as per code mandated bidirectional 05) earthquake spectrum. The parametric comparison of B.I building with fixed base model shows that for a medium Fig. 7 Base moment comparison for different soils rise building there is a substantial reduction in the base shear and moments almost to the tune of 50%. Base lateral _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 57 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ displacements, and top storey displacement were also found to be within reasonable limits. Parametric comparison done with three types of founding soil strata reveals that the reduction in seismic responses are more in harder soil founding strata. And the responses are increased by almost 35% for structure supported on harder to medium soil and almost 25% for medium to soft soils. There is additional flexibility of the structure due to reduced structural responses through incorporation of the isolator. For strategic buildings In seismic vulnerable areas, the main concern is the protection of critical components from damage and seismic instability. The study shows the effectiveness of the LRB base isolation system in terms of reduced structural responses under seismic loading. Extensive further study is required both analytical and experimental to boost the confidence of the construction industry. Performance of structures fitted with isolators in high seismic activity need to be observed and documented. Shake table study of prototype models need to be done to arrive at parameters and confirm the effectiveness. As the base isolators are extensively used worldwide in high seismic areas , in near future, we will expect the same in India also. At least in seismic zone 4 and 5 the use of base isolators has to be encouraged, as they are technically very effective and economically feasible. The use of base isolators reduces inter-storey drift and structural damages during earthquake. The building will be ready to occupy with minor repair. [2] [3] [4] J.P. Talbot and H.E.M. Hunt “On the Performance of Base-Isolated Buildings” Buiding acoustics Vol 7, No 3, (2000) pp 163-178 [7] Thomas L. Attard And Kittinan Dhiradhamvit “Application And Design Of Lead-Core Base isolation for reducing structural demands in short stiff and tall steel buildings and highway bridges subjected” Journal of Mechanics of Materials and Structures Vol 4, No 5, pp 799-817 (2009) [8] Kelly JM. A seismic base isolation: review and bibliography. Soil Dynamics and Earthquake Engineering 5:202–216 (1986) [9] Jangid RS, Datta TK. Seismic behaviour of base isolated building—a-state-of-the-art-review. Structures andBuildings; 110:186–203 (1995) [10] Yang, J.N., Wu, J.C., Reinhorn, A.M. and Riley, M. (1996). “Control of Sliding-Isolated Buildings Using Sliding-Mode Control.” J.Sfruct.Engrg., ASCE, 122, 179-186. [11] Skinner RI, Kelly JM, Heine AJ. Hysteretic dampers for earthquake-resistant structures. Earthquake Engineering and Structural Dynamics 1975; 3:287–296. [12] Ghobarah A, Ali HM. Seismic design of base-isolated highway bridges utilizing lead–rubber bearings. Canadian Journal of Civil Engineering 1990; 17:413–422. [13] Skinner RI, Beckand JL, Bycroft GN. A practical system for isolating structures from earthquake attack. Earthquake Engineering and Structural Dynamics ; 3:297–309 (1975) R.S. Jangid, Optimum lead-rubber isolation bearings for near-fault motions, Engi- neering Structures 29 (2007) 2503—2513. [14] Hwang JS, Chiou JM. An equivalent linear model of lead–rubber seismic isolation bearings, Engineering Structures 1996; 18:528–536. Buckle I, Mayes R. Seismic isolation: history, application and performance a world overview. Earthquake Spectra; 6(2):161–202 (1990) [15] T.E. Kelly, W.H. Robinson, R.I. Skinner, Seismic Isolation for Designers and Struc- tural Engineers, Robinson seismic Ltd., Wellington, 2007 R. S. Jangid “Stochastic response of building frames isolated by lead–rubber bearings” Struct. Control Health Monit. Vol 17 pp1-22 (2010) [16] Skinner RI, Beckand JL, Bycroft GN. A practical system for isolating structures from earthquake attack. Earthquake Engineering and Structural Dynamics ; 3:297–309 (1975) Constantinou MC, Tadjbakhsh IG. Hysteretic dampers in base isolation: random approach. Journal of Structural Engineering (ASCE) 1985; 111:705–721. [17] F. Naeim, J. Kelly, Design of Seismic Isolated Structures: From Theory to Practice, Wiley, 1999 REFERENCES [1] [6] [18] E. Wilson, Three-dimensional Static and Dynamic Kelly JM, Hodder SB. Experimental study of lead Analysis of Structures, 3rd ed Computers & and elastomeric dampers for base isolation system Structures Inc, Berkeley, 2002. in laminated neoprene bearings. Bulletin of the New Zealand National Society for Earthquake [19] A.B.M.S. Islam, R.R. Hussain,M. Jameel, M.Z. Engineering Vol. 15 pp 53–67 1982; _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 58 [5] International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ Jumaat, Non linear time domain analysis of base isolated multi-storey building under site specific bi-directional seismic loading., Automation in construction 22 (2012) 554—556. [20] [21] S. Nagarajaiah, A.M. Reinhorn, M.C. Constantinou, Nonlinear dynamic analysis of 3-D base-isolated structures, Journal of Structural Engineering 117 (1991) 2035—2054. A. Sharma, R. Jangid, Behaviour of base-isolated structures with high initial isolator stiffness, International Journal of Applied Science, Engineering and Technology 5 (2009). [22] K. Goda, C.S. Lee, H.P. Hong, Lifecycle cost—benefit analysis of isolated buildings, Structural Safety 32 (2010) 52—63. [23] V.A. Matsagar, R.S. Jangid, Influence of isolator characteristics on the response of base-isolated structures, EngineeringStructures 26 (2004) 1735—1749. [24] IS 456-2000, IS 1893-2002, IS Codes for R.C.C and Earth quake resistant designs respectively. _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-4, 2014 59