TABLE OF CONTENTS:
TOC o “1-3” h z u CHAPTER-1 PAGEREF _Toc516348501 h vi1.INTRODUCTION PAGEREF _Toc516348502 h vi1.1General PAGEREF _Toc516348503 h vi1.2Base isolation: PAGEREF _Toc516348504 h vii1.3Principle of Base Isolation PAGEREF _Toc516348505 h viii1.4Types of isolation devices: PAGEREF _Toc516348506 h x1.4.1High damping rubber bearing (HDRB) PAGEREF _Toc516348507 h x1.4.2Friction pendulum: PAGEREF _Toc516348508 h xi1.4.3Lead-Plug Bearings (LPB) PAGEREF _Toc516348509 h xiii1.5ScopeCHAPTER – 2 PAGEREF _Toc516348511 h xiv2.REVIEW OF LITERATURE PAGEREF _Toc516348512 h xiv2.1General PAGEREF _Toc516348513 h xiv2.2Review of Journals PAGEREF _Toc516348514 h xivCHAPTER 3 PAGEREF _Toc516348515 h xxi3.MODELLING PAGEREF _Toc516348516 h xxi3.1Summary of the Structure. PAGEREF _Toc516348517 h xxi3.2Material Properties PAGEREF _Toc516348518 h xxi3.3Model Geometry PAGEREF _Toc516348519 h xxi3.4Plan View of Building PAGEREF _Toc516348520 h xxii3.5Building Elevation View: PAGEREF _Toc516348521 h xxiii3.6Steps by Step Procedure of Analysis: PAGEREF _Toc516348522 h xxivCHAPTER 4 PAGEREF _Toc516348523 h xxx4.RESULTS AND DISCUSSION PAGEREF _Toc516348524 h xxx4.1.Displacement of Building (Equivalent Static Analysis) – EQX PAGEREF _Toc516348525 h xxx4.2.Study of Inter Story Drift (Equivalent Static Analysis)- EQX PAGEREF _Toc516348526 h xxxi4.3.Time Period (Equivalent Static Analysis) PAGEREF _Toc516348527 h xxxiii4.4.Study of Base Shear (Equivalent Static Analysis) PAGEREF _Toc516348528 h xxxiv4.5.Displacement of Building (Equivalent Static Analysis) – EQY PAGEREF _Toc516348529 h xxxiv4.6.Study of Inter Story Drift (Equivalent Static Analysis)- EQY PAGEREF _Toc516348530 h xxxv4.7.Studies on Displacement (Response Spectrum Analysis)-SPECX PAGEREF _Toc516348531 h xxxvii4.8.Study of Inter Story Drift (Response spectrum Analysis) -SPECX PAGEREF _Toc516348532 h xxxviii4.9.Studies on Displacement (Response Spectrum Analysis)-SPECY PAGEREF _Toc516348533 h xxxix4.10.Study of Inter Story Drift (Response spectrum Analysis) -SPECY PAGEREF _Toc516348534 h xl4.11.Cost comparison: PAGEREF _Toc516348535 h xliCHAPTER 5 PAGEREF _Toc516348536 h xlii5.CONCLUSIONS: PAGEREF _Toc516348537 h xlii5.1.General Conclusions: PAGEREF _Toc516348538 h xlii5.2.Scope for Future Study: PAGEREF _Toc516348539 h xliii
LIST OF FIGURES:
TOC h z c “Figure” Figure 1:1Fixed-Base vs. Base-Isolated Building PAGEREF _Toc516348540 h viiFigure 1:2Behaviour and implementation of base isolator PAGEREF _Toc516348541 h ixFigure 1:3Location of base isolator in buildings PAGEREF _Toc516348542 h xFigure 1:4High damping rubber bearing isolator PAGEREF _Toc516348543 h xiFigure 1:5 Motion in a FPS (a) initial condition, (b) displaced condition at maximum displacement, (c) Idealized Hysteresis Loop of a FPS PAGEREF _Toc516348544 h xiiFigure 1:6LPB isolator (a) components, (b) Idealized Hysteresis Loop of an LPB PAGEREF _Toc516348545 h xiiiFigure 3:1Dimension details. PAGEREF _Toc516348546 h xxiiFigure 3:2Plan View of Building PAGEREF _Toc516348547 h xxiiFigure 3:3 3D View PAGEREF _Toc516348548 h xxiiiFigure 3:4 Dimensions of Building PAGEREF _Toc516348549 h xxivFigure 3:5 Storey Data PAGEREF _Toc516348550 h xxivFigure 3:6 Material Properties of a Building PAGEREF _Toc516348551 h xxvFigure 3:7 Basic Section Properties of a Frame PAGEREF _Toc516348552 h xxvFigure 3:8 Basic Dimensions of a Slab PAGEREF _Toc516348553 h xxviFigure 3:9 Details of Load cases. PAGEREF _Toc516348554 h xxviFigure 3:10 Details of Load combination PAGEREF _Toc516348555 h xxviFigure 3:11 Wall Load PAGEREF _Toc516348556 h xxviiFigure 3:12 Model 1 with fixed base PAGEREF _Toc516348557 h xxviiiFigure 3:13 Model 2 with Base isolation PAGEREF _Toc516348558 h xxixFigure 4:1 Comparison of story v/s displacement in X direction PAGEREF _Toc516348559 h xxxiFigure 4:2 Comparison of Storey v/s storey drifts in X direction PAGEREF _Toc516348560 h xxxiiFigure 4:3 Comparison of mode numbers v/s time period PAGEREF _Toc516348561 h xxxiiiFigure 4:4 Comparison of base shear v/s models PAGEREF _Toc516348562 h xxxivFigure 4:5 Comparison of story v/s displacement in Y direction PAGEREF _Toc516348563 h xxxvFigure 4:6 Comparison of Storey v/s Storey drifts in Y direction PAGEREF _Toc516348564 h xxxviFigure 4:7 Comparison of story v/s displacement for different models in X direction PAGEREF _Toc516348565 h xxxviiFigure 4:8Comparison of Storey v/s Storey drifts for different models in X direction PAGEREF _Toc516348566 h xxxviiiFigure 4:9 Comparison of story v/s displacement for different models in Y direction PAGEREF _Toc516348567 h xxxixFigure 4:10Comparison of Storey v/s Storey drifts for different models in Y direction PAGEREF _Toc516348568 h xl
LIST OF TABLES:
TOC h z c “Table:” Table: 4:1 Displacements –EQX PAGEREF _Toc516348569 h xxxTable: 4:2 Inter Story Drift PAGEREF _Toc516348570 h xxxiTable: 4:3 Time Period PAGEREF _Toc516348571 h xxxiiiTable: 4:4 Base Shear PAGEREF _Toc516348572 h xxxivTable: 4:5 Displacements –EQY PAGEREF _Toc516348573 h xxxivTable: 4:6 Inter Story Drift -EQY PAGEREF _Toc516348574 h xxxvTable: 4:7 Displacements –SPECX PAGEREF _Toc516348575 h xxxviiTable: 4:8 Inter Story Drift-SPECX PAGEREF _Toc516348576 h xxxviiiTable: 4:9 Displacements –SPECY PAGEREF _Toc516348577 h xxxixTable: 4:10 Inter Story Drift-SPECY PAGEREF _Toc516348578 h xlTable: 4:11 Cost Comparison Table PAGEREF _Toc516348579 h xli
CHAPTER-1INTRODUCTIONGeneralEarthquake occurrence is still remaining unpredictable to the engineers. Our Indian subcontinent has experienced some of the most severe earthquakes in the world and researches have shown that the tectonic activities are still continuing which may result into severe earthquakes in future. Recently, Nepal earthquake (April 2015) killed and injured lot of people. Many residential and public structures have collapsed in this earthquake. Therefore it is essential to protect existing structures from future earthquakes. The existing building found inadequate for resisting future probable earthquake. The old method of providing earthquake resistance to a structure is by increasing its strength, stiffness and capacity. Earthquake resistant design of reinforced concrete structure is a continuing area of research since earthquake engineering has started not only in India but also in other countries. The basic need to the protect structure from earthquake ground motions and keep it to minimum hazard level. The base isolation techniques and energy dissipation techniques such as a damper reduces response of structure hence there is a necessity to investigate the effect of base isolators and dampers on the response of structure. Seismic isolation separates the superstructure from substructure and it prevents transmission of ground motion from foundations to superstructure. The system which separates superstructure from substructure is known as isolators. Isolated structure has fundamental frequency is much lower than that of fundamental frequency of fixed base structure and predominant frequencies of a typical earthquake. Energy dissipation dampers are of different types i.e. viscous damper, friction damper, tuned mass damper, Viscos-elastic damper etc. Base isolator and dampers substantially reduces response of structure.
A large proportion of world’s population lives in regions of seismic hazards, at risk from earthquakes of varying severity and frequency of occurrence. Earthquake causes significant loss of life and damage of property every year. So, to mitigate the effect of earthquake on building the base isolation technique one of the best solutions.. The seismic isolation system is mounted beneath the superstructure and is referred as ‘Base Isolation’. The main purpose of the base isolation device is to minimize the horizontal acceleration transmitted to the superstructure. Base isolation is very promising technology to protect different structures like building, bridges, airport terminals and nuclear power plants etc. from seismic excitation.
Base isolation technology works by separating or greatly reducing the lateral movement of a building’s superstructure from the movement of the ground/foundation during a seismic event. To allow for this difference in lateral movement while still supporting the weight of the superstructure, base isolation bearings are designed to be very flexible laterally while being stiff vertically. This base condition is in contrast to a typical fixed-base structure, in which the connections between the superstructure and its base/foundation are rigid and translation of the superstructure is resisted in all directions. The difference between these two base conditions is illustrated in Figure A above. The ultimate purpose of a base isolation system is to reduce the seismic forces exerted onto a building’s superstructure. This reduction in seismic forces is achieved in part by reducing the superstructure’s spectral accelerations.
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 1Fixed-Base vs. Base-Isolated BuildingBase isolation:
Base isolation device reduces the stiffness and increases the flexibility in the structure. The basic concept of base isolation technique is “to increase time period and reduce acceleration of fixed base structure” from this, reduction of the seismic effect on structure is seen. Base isolation, otherwise called seismic base disconnection or base detachment framework, is a standout amongst the most famous method for securing a structure against earthquake forces. In the base isolation building, the base isolation device or dampers is introduced between foundation and super structure of the building. Idea behind this technique is to detach the building from ground and providing dampers, by doing so the energy induced by earthquake is not transmitted up through building. In numerous, yet not all, applications the seismic isolation device is mounted underneath the structure hence it is called as “base isolation”. The base isolation device provided in between the superstructure and foundation of a building should have lower horizontal stiffness and higher damping characteristics. By using base isolation device in building will reduces base shear, acceleration and increases the time period. It avoids seismic damage of the building. Hence base isolator is used in this project to study the effect of base isolator on building on sloping ground as these buildings have higher base shear. Also these buildings are more susceptible to earthquake forces.
Principle of Base Isolation
The basic principle of base isolation is to modify the response of the structure so that the ground can move below the structure without transmitting these motions into the superstructure. A structure that is perfectly rigid will have a zero period. Base isolators are needed to be placed between superstructure and foundation. When the ground moves, the acceleration induced in the structure will be equal to the ground acceleration and there will be zero relative displacement between the superstructure and the ground. Both structure and ground move with the same amount. Perfectly flexible structure will have an infinite period. For this type of structure, when the ground beneath the structure moves, there will be zero acceleration induced in the structure and the relative displacement between the structure and ground will be equal to the ground displacement.
The structure will not move, the ground will move. In buildings, High damping rubber bearing isolator protects the structural components from earthquake loads in two ways:
by deflecting seismic energy
by absorbing seismic energy.
The seismic energy is deflected by allowing base of the structure to displace in lateral directions. These will results into increasing the natural time period of the structure. The structure which having longer natural time period attracts less seismic force, the base isolation system deflects the seismic energy. There are different types of base isolator’s i.e. laminated rubber bearing, Lead rubber bearing, High damping rubber bearing and Friction pendulum system. Fig-1 shows behavior of structure with and without base isolated structure.
The fundamental focus with seismic isolation is to present horizontally flexible but vertically stiff material (base isolators) at the base of a structure to substantially uncouple the superstructure of the building from high-frequency earthquake shaking. The main concept of base isolation system is increasing the natural period of the fixed base building. The advantages of adding a base isolation system at the foundation are as follows
(a) Base isolation Lengthens the period of the structure reduces the spectral acceleration for earthquake shaking.
(b) By providing the damping to the isolation systems reduce displacements in the seismic isolators.
Basic requirements of Seismic Isolation Systems: Following are three requirements of seismic isolation system.
1. Except for very soft soil sites, sufficient horizontal flexibility should provide for all buildings to increase the structural period and spectral demands.
2. For the isolators should provide sufficient energy dissipation capacity or damping to reduce the displacement.
3. Isolated building should not different from a fixed-base building under service loading for that provides sufficient rigidity.
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 2 Behaviour and implementation of base isolator
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 3Location of base isolator in buildingsTypes of isolation devices:
The various types of isolation devices are
High damping rubber bearing.
Friction pendulum bearing
Lead core rubber isolator.
Laminated rubber bearing.
High damping rubber bearing (HDRB)
HDRB is one type of elastomeric bearing. This type of bearing consists of thin layers of high damping rubber and steel plates built in alternate layers. The vertical stiffness of the bearing is several hundred times the horizontal stiffness due to the presence of internal steel plates. Horizontal stiffness of the bearing is controlled by the low shear modulus of elastomeric while steel plates provides high vertical stiffness as well as prevent bulging of rubber. High vertical stiffness of the bearing has no effect on the horizontal stiffness. Rubber reinforced with steel plates provides stable support for structures. Multilayer construction rather than single layer rubber pads provides better vertical rigidity for supporting a building. With the help of HDRB earthquake vibration is converted to low speed motion. As horizontal stiffness of the multi- layer rubber bearing is low, strong earthquake vibration is lightened and the oscillation period of the building is increased. Horizontal elasticity of HDRB returns the building to its original position. In a HDRB, elasticity mainly comes from restoring force of the rubber layers. After an earthquake this restoring force returns the building to the original position.
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 4 High damping rubber bearing isolatorFriction pendulum:
This is known as sliding isolator the concept is developed based on the pendulum effect. In this system a articulated slider moves to and pro on a concave surface when a seismic moment occurs uplift of superstructure takes place. A FPS is comprised of a stainless steel concave surface, an articulated sliding element, and cover plate.
The slider is finished with a self-lubricating composite liner (e.g. Teflon). During an earthquake, the articulated slider within the bearing, travels along the concave surface, causing the supported structure to move with gentle pendulum motions as illustrated in Figure 1(a) and 1(b). Movement of the slider generates a dynamic frictional force that provides the required damping to absorb the earthquake energy. Friction at the interface is dependent on the contact between the Teflon-coated slider and the stainless steel surface, which increases with pressure. Values of the friction coefficient ranging between 3% to 10% are considered reasonable for an FPS to be effective. The isolator period is a function of the radius of curvature (R) of the concave surface. The natural period is independent of the mass of the supported structure, and is determined from the pendulum equation:
where g is the acceleration due to gravity.
The horizontal stiffness (KH) of the system, which provides the restoring capability, is provided by:
Where W is the weight of the structure.
The movement of the slider generates a dynamic friction force that provides the required damping for Absorbing earthquake energy. The base shear V, transmitted to the structure as the bearing slides to a distance (D), away from the neutral position, includes the restoring forces and the friction forces as can be
seen on the following equation, where µ is the friction coefficient:
The characterized constant (Q) of the isolation system is the maximum frictional force, which is defined as:
The effective stiffness (keff) of the isolation system is a function of the estimated largest bearing displacement (D), for a given value of µ and R, and is determined by:
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 5 Motion in a FPS (a) initial condition, (b) displaced condition at maximum displacement, (c) Idealized Hysteresis Loop of a FPSLead-Plug Bearings (LPB)The elastomeric LPB which are generally used for base isolation of structures consist of two steel fixing plates located at the top and bottom of the bearing, several alternating layers of rubber and steel shims, and a central lead core as shown in Figure 2(a). The elastomeric material provides the isolation component with lateral flexibility; the lead core provides energy dissipation (or damping), while the internal steel shims enhance the vertical load capacity whilst minimizing bulging. All elements contribute to the lateral stiffness. The steel shims, together with the top and bottom steel fixing plates, also confine plastic deformation of the central lead core. The rubber layers deform laterally during seismic excitation of the structure, allowing the structure to translate horizontally, and the bearing to absorb energy when the lead core yields.
The nonlinear behavior of a LPB isolator can be effectively idealized in terms of a bilinear force-deflection curve, with constant values throughout multiple cycles of loading as shown on Figure 2(b).
Figure STYLEREF 1 s 1: SEQ Figure * ARABIC s 1 6LPB isolator (a) components, (b) Idealized Hysteresis Loop of an LPB1.4 Scope
This study analyzes a fixed-base and a base-isolated 12-story steel office building for an assumed San Diego location. The superstructure of each building has a lateral system composed of special moment frames (SMF) in the longitudinal direction and special concentrically braced frames (SCBF) in the transverse direction. Also, Triple Friction Pendulum (TFP) bearings are used for the isolation system in the base-isolated structure. The seismic performance of each structure is analyzed and compared for both design basis earthquake (DBE) and maximum considered earthquake (MCE) seismic demand levels. 7 ground motions are scaled to each seismic demand level to perform nonlinear time history analyses on both structures. The resulting floor accelerations and interstory drifts are used to determine the levels of structural and non-structural damage inflicted on each building. A cost-benefit analysis is then performed, which takes into account the cost of the isolation system and the damage costs of the structures and their components.
CHAPTER – 2REVIEW OF LITERATUREGeneralEarthquake as one of the most destructive events recorded so far in India in terms of death toll, damage to infrastructure and devastation in the last fifty years. It has been observed that the principal reasons of failure may be accounted to soft stories, majority of the buildings is constructed on the hill slopes with irregular structural configuration having foundations at different levels. Mass irregularities, poor quality of construction material and faulty construction practices etc. Many methods have been proposed for achieving the optimum performance of structures subjected to earthquake excitation. The use of lead rubber bearing isolators for absorbing energy is the best technique. Many papers have been published related with base isolation technique as an earthquake resistant device. Some of them are discussed below.
Review of JournalsRadmila B. Salic et al in this paper the authors have demonstrated the effect of dynamic response of the seven-story residential building under the earthquake ground motions. Mode shapes, natural frequencies and damping ratios of the existing fixed-base building are obtained by ARTeMIS (Ambient Response Testing and Modal Identification Software). The fixed base model represents the dynamic behavior of the structure and seismic isolated model representing the dynamic behavior of the structure isolated by lead rubber bearing seismic isolation system. Dynamic analysis of both models has been performed by ETABS (Nonlinear version 9.0.4). The finite element model was chosen to satisfy the needs of this analysis. The Dynamic responses of fixed base and seismic isolated models have been calculated for four types of real earthquake time histories of different frequency characteristics whose value is determined based on the detailed site response analysis. The authors have showed that increase of natural period of structure increases flexibility of the same structure. In seismic isolated model, base shear force is highly reduced. Increased flexibility of the system led to increase of the total displacements due to the elasticity of the existing isolation. Implementation of the isolation system resulted into the reduction of the understory drifts. Analysis of seismic isolated model has shown significant reduction of the story accelerations.
V. Kilaret al In this paper four-story RCC building is designed according to Euro Code 8 for seismic analysis and dynamic performance evaluation. Different sets of base isolation devices are studied for investigation. First case is the use of simple rubber bearing and second one is the use of lead rubber bearing as a base isolation system. For the investigation of each system a soft, normal and hard rubber stiffness with different damping values were used. Non-linear pushover analysis was performed with the recent version of computer analysis software SAP 2000. From this study it is concluded that the stiffer isolators with higher damping gives smaller target base displacements as compared to softer one with lower damping. It can also be seen that the relative displacement of the superstructure are smaller if the softer isolators are used. The smallest relative displacement can be expected with the use of softer isolators with higher damping. If the used isolators are too stiff it cannot protect the superstructure.
A. B. M. Saiful Islam et al In this paper a soft storey building is analyzed for seismic loading by creating a building model having lot of open spaces. The soft storey creates the major weak point in earthquake which means that during the event when soft storey collapses, it can make the whole building down. It causes very severe structural damage and building becomes unusable. This research includes the placement of two types of isolators, first is lead rubber bearing (LRB) and second one is high damping lead rubber bearing (HDRB). Each storey is provided by isolators and its consequences were studied for different damping values. Finally the study reveals that use of isolators is most beneficial tool to protect soft storey buildings under strong earthquake ground motions. Provision of lead rubber bearing is more efficient than high damping rubber bearing because as time period increases in high damping rubber bearing, acceleration also increases which is undesirable to protect building during earthquake. This condition is exactly reverse in case of lead rubber bearing. It has been also shown the response of base isolated building and arrangement of bearing in the building. As damping provides sufficient resistance to structure against service loading, various damping values are taken into account for this investigation. This study also deals with the main requirement for installation of isolators. Finally, it has concluded that the flexibility, damping and resistance to service loads are the main parameters which affects for practical isolation system to be incorporated in building structures. Additional requirements such as durability, cost, ease of installation and specific project requirements influences device selection; but all practical systems should contain these essential elements.
Di Sarno, et al has presented the structural analysis of the complex irregular multi-storey RC frame used for the hospital building. It was carried out by means of modal analysis with response spectrum. The structure exhibits large mass eccentricity due to its irregularity in shape. The building is provided by circular shape high damping rubber bearing as per required diameter in accordance with Euro Code 8 to act as a base isolation system. The main objective of this study is to improve the earthquake resistant design approach by studying the properties like horizontal flexibility to increase structural period and reduce the transfer of seismic energy to the superstructure; second property is the energy dissipation (due to relatively high viscous damping) to reduce lateral displacements; and last one is the provision of sufficient stiffness at small displacements to provide adequate rigidity to the structure. This case study reveals that base isolation is an effective strategy to improve the seismic performance for relatively flexible framed structures at serviceability point of view. It also concluded that the relatively high horizontal flexibility of the superstructure apparently reduces the beneficial effects of the base isolation system. It is demonstrated that although the base-isolated and the fixed base construction may undergo the same maximum accelerations the structural and non-structural damage are prevented in the frame resting on rubber devices. Savings of about 40% were estimated for percentage of steel reinforcement to be used for the columns and beams of the base isolated frame.
Masayoshi Nakashima Praveen Chusilp , et al In this paper. In Kobe earthquake, the ground shaking in some regions was significantly larger than that considered in the Japan seismic design code. It leads to substantial evolution in Japanese design and construction practices. Here, two limit-states design (life safety and damage limitation) are demonstrated with the two levels (the structure should remain elastic in small to moderate earthquakes called level 1) and (sustain some yielding and classification in some structural members in a large earthquake called level 2 design) stipulated in the 1981, Japan seismic design code, but are distinguished because of two new features. Finally it has concluded that the Japanese seismic design code is to be revised towards more performance- based engineering. A review of the Japanese seismic design and construction practices adopted after the 1995 Kobe earthquake has been done.
Juan C. Ramallo, et al In this paper authors have investigated the effects of using controllable semiactive dampers, such as magnetorheological fluid dampers, in a base isolation system. A two degree of freedom model of a base isolated building is used. The fundamental concept is to isolate a structure from ground, especially in the frequency range where the building is most affected. The goal is to reduction in interstorey drifts and floor accelerations to limit damage to the structure and its contents in a cost-effective manner. This paper investigates the improvements that may be achieved by replacing supplemental linear viscous damping devices in base isolation with semiactive dampers. A linear, two degree of freedom (2DOF), lumped mass model of a baseisolated building is used as the test bed for this study. The system model is used in this test is a single degree-of freedom model that has mass and fundamental modal frequency and damping ratio. In this study passive linear viscous damper, active damper and semiactive damper have been used. Generalized semiactive as well as magnetorheological fluid dampers was used in seismic protection system. Authors have concluded that the semiactive damper was able to accomplish nearly as much as the fully active damper. With the semiactive damper, the peak base drifts were decreased as compared to the optimal passive linear damper. This study suggests that semiactive dampers, such as magnetorheological fluid dampers, show significant promise for use in base isolation applications with greatly reduced power requirements as compared to the active systems.
J. C. Ramallo1, et al have presented an innovative base isolation strategy and shows how it can effectively protect the structures against extreme earthquakes without sacrificing performance during the more frequent, moderate seismic events. This innovative concept includes base isolation system with semi active or controllable passive dampers for seismic response mitigation. In this method the structure is modeled as a single degree-of-freedom system representing the fundamental mode. When the isolation layer is added, the augmented model is a two degree-of-freedom system. Test has been shown experimentally that the linear behavior of low-damping rubber bearings can extend to shear strains above 100%. Lead-rubber bearings are considered as the baseline against which the smart damping strategies are compared. The rubber isolation in these two systems is identical. The various ground excitations are used for modal excitation. A family of controllers that decreases base drift and absolute accelerations (compared to the LRB) is obtained for a controllable smart damper. The base isolation system, comprised of low-damping elastomeric bearings, and controllable semi active dampers, was shown to have superior performance compared to several passive base isolation designs using lead-rubber bearings.
AbdolrahimJalali, et al 8 has presented comparative study of seismic base-isolation of multi-story buildings and fixed base building. Nonlinear time-history analysis of created models was conducted by using ETABS (8.5.0). Fundamental periods, modal participation factors and base-shears were studied for all of the model buildings. Base-isolation generally falls into the passive category of structural control, while it can be active or passive. In this investigation the effects of superstructure characteristics on isolation performance, nonlinear time-history analyzed by Ritz vectors are conducted using ETABS 8.5.0. Comparing base-isolated building with fixed base building, the efficiency of isolation is examined. It has shown the results for fixed-base and base-isolated building. For first mode period in base-isolated building is 2.3 times longer than that of fixed-base building. Modal participation factor of the fundamental structural mode of base-isolated building is 0.4 times lesser than that of fixed-base building. Finally conclusion was made that superstructure characteristics have a considerable effect on isolation performance. By using proper characteristics of superstructure, base-isolation performance can be improved. The characteristics of superstructure have different effects on isolation performance in low-rise buildings, medium-rise buildings and high-rise buildings. In low-rise buildings, simple base-isolation has good performance and there is no need to modify the superstructure characteristics. Indeed, these modifications cannot have positive effect on isolation performance. In medium-rise buildings, it can reach better isolation by assigning additional base-mass and increasing the damping of superstructure. In high-rise buildings, stiffening superstructure and increasing the damping will cause an effective base-isolation.
Pan Wen et al 9 have presented the design of base isolation structure. Two step design method for base isolation structure was put forward. In the first step, the estimation of isolation layer, and only a few basic data of structure was required. In the second step, detail design along with the step-by-step time history analysis was adopted for determination of superstructure, foundation and base isolation device. Computer software based on above method with user-friendly interface, pre-processor and post-processor was developed for practical engineering design of superstructure and foundation. The main content of base isolation structure design consists of two parts: the first is design of base isolation device, and the second is design of structural elements above the isolation layer. The study reveals that in first stage design method, base isolation device lengthen the natural period of structure, in which the dynamic characteristic of the structure has been changed to achieve the aim of reducing the earthquake effect on superstructure. After the first step, base isolation device has tentatively selected based on the total horizontal stiffness of isolation layer. In the second step design method for seismic isolation structure is used to calculate earthquake resistant capacity of base isolation structure and optimize arrangement and parameters of base isolation device. It has been concluded that the computation result of two step design method is simple and practical, and its concept is clear and easy for further expansion and application. The method is advantageous to enhance design quality and reduce design period. The simplified method is suitable for multi-storey isolation structure. With the increase of the layer number, influence of higher modes increased. The method is not adapted for high-rise isolation structure.
Pradeep Kumar T. V. et al 10 has shown force deformation behavior of isolation bearings. In this paper the isolation bearing consists of an isolator which increases the natural period of the structure away from the high-energy periods of the earthquake and a damper to absorb energy in order to reduce the seismic force. The most common isolation bearing used was the lead– rubber bearing. It has been observed that lead–rubber bearings have little strain-rate dependence for a wide frequency range which contains typical earthquake frequencies. Authors have designed the isolated bearing. The isolation bearings are modeled by a bilinear model based on the three parameters: initial stiffness, lower stiffness, and characteristic strength. It has given new relationship to find out the yield displacement and yield force for an equivalent bilinear isolation bearing system. Compared to the conventional method the newly derived equations give accurate results and are less time consuming. The graphical representation of the new relationship shown in the paper is useful for bearing design.
Shirule .P.A et al. , studied the effect of base isolator on 18 storeys fixed base RCC building model by using time history method in zone IV with Lead rubber bearing (LRB) and friction base bearing. Objective of this study was to compare performances of fixed building at base (without isolator) with building with isolator (LRB and FB). Another objective was comparative study of performance of building with different past time history like BHUJ, KOYANA and LACC T.H. Finally base shear, acceleration and displacements were compared for all building models. Results shows base shear was reduced by 46% for building with lead rubber bearing and 35% for friction base bearing. Displacement was increased for isolated building. It shows the reduction of base shear for both in X-direction and Y- direction was more in KOYANA time history than BHUJ and LACC
H P Santhosh et al. , Conducted comparative study on low rise and medium rise base isolated and fixed base building. a 6 storey building model is prepared analyzed by using response spectrum method, considering zone III in soil type II (medium).From this study they concluded that base shear reduces for base isolation building as compare to the fixed base. Also time period increases for base isolation building up to 3.3 sec as compare to the fixed base building that is 1.4 sec. Acceleration is about 1.7 m/sec2 for base isolated and 2.7 m/sec2 for fixed base building. Storey shear for fixed base building is 1800 kN and for base isolated building 800 kN. Acceleration and storey shear were reduced for base isolated building.
S. M. Dhawade et al. , studied the behavior of base isolated building and compared results with fixed base building.A G+14 storey building is analyzed by using ETABS software. Static equivalent method is used according to UBC 97and design is done according to IS1893-2002 (part-1). High damping rubber bearing (HDRB) is used as isolator. Top storey displacement for base isolated building is found to be 0.0020 and 0.000 in X and Y direction respectively and for fixed base building the top storey displacement is about 0.2782 and 0.1133 in X and Y direction respectively. For base isolated building storey drift also less.
Bakre S.V et al , Conducted study on elastomeric base isolation building of four storey by considering different time history data (El cento, Kobe, Northridge earthquakes). All design parameters were taken according UBC97. Results were compared with fixed base building. From this study they concluded that base shear of base isolated building was 39% of fixed base building in case of El Centro, 55% for Kobe and 25% for Northridge Earthquakes. The roof drift ratio was reduced 56% in case of El-Centro, 63% for Kobe and 90% in case of Northridge Earthquake.
Dr. Rajesh Prasanna P et al, Study is conducted on base isolated four storied MRF (moment resisting frame) with elastomeric seismic isolation bearing by using SAP2000 software. Dynamic analysis was carried out for the moment resisting frame with base isolation building and the results were compared with, fixed base building. Building was located in zone V and the frame taken for study was moment resisting fame with shear wall. This study concludes that the displacement for fixed base building is 0.06 m and 0.0031 m for fixed base building. And reduction in base shear was observed up to 88.38% for base isolated building as compare to fixed base building.
CHAPTER 3MODELLINGSummary of the Structure.In this chapter, the common step by step procedure of modeling the basic regular structure is explained. The related modeling procedure is used to model the other model with base isolation to study the seismic behaviour of both structure.
The Models are given no’s to identify it further:
Type 1- Fixed base model
Type 2- Base isolated model
The material considered for analysis RC is M-35 grade concrete and Fe-500 grade reinforcing steel:
Young’s- Modulus – steel, Es = 2, 10,000 MPa
Young’s – Modulus – concrete, EC =29,580 MPa
Characteristic strength of concrete, fck = 35 MPa
Yield stress for steel, fy = 500 MPa
Ultimate strain in bending, ?cu =0.0035
The Building is a 10-storied, RC frame with properties as specified below. The floors are modeled as solid slab section. The details of the model are given as follows:
Number of stories = G+10
Number of bays along X Dir. = 8 Bay, Y-Dir. = 7 Bay
Storey height = 3.0 meters at Ground Floor, Remaining Floors.
Bay width along X Dir.= Varies, Y Dir. = Varies.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 1Dimension details.Plan View of BuildingThe plan view of building plan is shown in the Fig The bay width, columns and beams positions can be seen.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 2Plan View of BuildingBuilding Elevation View:
The Fig. shows the 3D view of the structure.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 3 3D View
Steps by Step Procedure of Analysis:
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 4 Dimensions of BuildingThe above preview, provides dimensions of the building.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 5 Storey Data
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 6 Material Properties of a BuildingInput the grade of concrete, steel and other properties.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 7 Basic Section Properties of a FrameThe Image shows the beam and column sections.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 8 Basic Dimensions of a Slab
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 9 Details of Load cases.To define static load case, Define and Static Load case, all load cases self-weights and its multiplier shall be zero other than Dead load.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 10 Details of Load combinationOnce inputting the dead & live load on the structure, now we apply the earthquake load. In this the first step, to select add new load combination choice & then give all the parameters, by using IS1893 (part 1):2002.
The Loads are taken on the structure as specified in IS875: Part 2. For live load.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 11 Wall LoadThe above figure 3.10 & 3.11 shows the Loading details assigned to the Slab and the Wall.
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 12 Model 1 with fixed baseMODEL-2
Figure STYLEREF 1 s 3: SEQ Figure * ARABIC s 1 13 Model 2 with Base isolationCHAPTER 4RESULTS AND DISCUSSIONThe Models are given no’s to identify it further:
Type 1- Fixed base model
Type 2- Base isolated model
After the analysis is completed, results are extracted. The response of different structures will be tabulated from ETAB software. The regular model and base isolated model were also studied for different load cases and combinations. The response spectrum analysis is carried out to compare with equivalent static analysis.
COMPARISION OF DIFFERENT MODELS OF BUILDING:
Displacement of Building (Equivalent Static Analysis) – EQXThe displacement in mm for earthquake in X direction for different storey are resulted as below
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 1 Displacements –EQXSTOREY MODEL 1 MODEL 2
12 40.1 272.1
11 39.2 272.3
10 37.7 273.8
9 35.5 275.7
8 32.8 278.0
7 29.7 280.5
6 26.2 283.2
5 22.4 285.9
4 18.5 288.6
3 14.4 291.0
2 10.3 293.1
1 6.3 294.6
0 0.0 0.0
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 1 Comparison of story v/s displacement in X directionThe displacement in the model 1 increases parabolically due to higher seismic forces in upper floors. However, the displacement in the model 2 is more and all most same in all floors. This shows the whole building is displaced because of base isolation.
Study of Inter Story Drift (Equivalent Static Analysis)- EQXTable: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 2 Inter Story DriftSTOREY MODEL 1 MODEL 2
12 0.00033 0.00033
11 0.00051 0.00049
10 0.00072 0.00064
9 0.00090 0.00076
8 0.00105 0.00085
7 0.00118 0.00090
6 0.00127 0.00091
5 0.00133 0.00088
4 0.00137 0.00081
3 0.00139 0.00069
2 0.00138 0.00052
1 0.00131 0.00034
0 0.00000 0.00000
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 2 Comparison of Storey v/s storey drifts in X directionThe drift values in the model 2 is very less compared to model 1. This is because of base isolator, which tends to reduce the difference in the displacement between successive stories of model 2.
4.3 Time Period (Equivalent Static Analysis)The table below shows that the different time periods for various models of earthquake in X direction
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 3 Time PeriodMODE NUMBERS MODEL 1 MODEL 2
1 2.38 0.00
2 2.16 0.00
3 2.13 0.00
4 0.73 1.35
5 0.69 1.08
6 0.67 1.04
7 0.40 0.50
8 0.39 0.47
9 0.38 0.46
10 0.27 0.31
11 0.26 0.30
12 0.26 0.29
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 3 Comparison of mode numbers v/s time periodThe natural time period of vibration for model 1 is maximum at the first 3 modes, later it will reduce due to internal friction and material damping. But However, the whole building moves, when the seismic force hits the building, the time period of vibration starts at 4th mode, this is due to inertial force later to the building movement.
Study of Base Shear (Equivalent Static Analysis)The table below shows different base shear values for different models
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 4 Base ShearBASE SHEAR
MODEL 1 MODEL 2
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 4 Comparison of base shear v/s modelsThe base shear in the building is observed in the model with fixed support. Whereas the base shear value is almost nullified in the model 2. This is due to the presence of base isolator. When seismic force hits the building, the forces will be transferred to the base in the model with fixed support. Whereas it will tend to oscillate the building without transfer of internal forces.4.4 Displacement of Building (Equivalent Static Analysis) – EQYThe displacement in mm for earthquake in Y direction for different storey are resulted as below
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 5 Displacements –EQYSTOREY MODEL 1 MODEL 2
12 50.6 261.2
11 48.7 263.2
10 46.0 266.0
9 42.5 269.2
8 38.2 272.8
7 33.4 276.7
6 28.2 280.7
5 23.3 284.8
4 18.4 288.7
3 13.6 292.3
2 9.2 295.5
1 5.1 298.1
0 0.0 0.0
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 5 Comparison of story v/s displacement in Y directionThe displacement in the model 1 increases parabolically due to higher seismic forces in upper floors. However, the displacement in the model 2 is more and all most same in all floors. This shows the whole building is displaced because of base isolation.
Study of Inter Story Drift (Equivalent Static Analysis)- EQYTable: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 6 Inter Story Drift -EQYSTOREY MODEL 1 MODEL 2
12 0.00079 0.00084
11 0.00094 0.00094
10 0.00121 0.00110
9 0.00144 0.00123
8 0.00163 0.00132
7 0.00176 0.00136
6 0.00162 0.00137
5 0.00165 0.00132
4 0.00162 0.00123
3 0.00147 0.00109
2 0.00137 0.00088
1 0.00115 0.00075
0 0.00000 0.00000
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 6 Comparison of Storey v/s Storey drifts in Y directionThe drift values in the model 2 is very less compared to model 1. This is because of base isolator, which tends to reduce the difference in the displacement between successive stories of model 2.
4.5 Studies on Displacement (Response Spectrum Analysis)-SPECXThe displacement in mm for earthquake in X direction for different storey are resulted as below
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 7 Displacements –SPECX STOREY MODEL 1 MODEL 2
12 32.1 113.1
11 31.5 111.7
10 30.3 111.8
9 28.5 112.1
8 26.2 112.4
7 23.5 112.8
6 20.5 113.2
5 17.7 113.7
4 14.6 114.2
3 11.3 114.6
2 8.2 115.1
1 4.9 115.4
0 0.0 0.0
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 7 Comparison of story v/s displacement for different models in X directionThe displacement in the model 1 increases parabolically due to higher seismic forces in upper floors. However, the displacement in the model 2 is more and all most same in all floors. This shows the whole building is displaced because of base isolation.
Study of Inter Story Drift (Response spectrum Analysis) -SPECXThe results of Different model with different storey is compared as follows
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 8 Inter Story Drift-SPECXSTOREY MODEL 1 MODEL 2
12 0.00030 0.00005
11 0.00052 0.00007
10 0.00076 0.00009
9 0.00094 0.00011
8 0.00107 0.00013
7 0.00117 0.00015
6 0.00102 0.00016
5 0.00109 0.00017
4 0.00115 0.00017
3 0.00108 0.00015
2 0.00111 0.00012
1 0.00105 0.00007
0 0.00000 0.00000
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 8Comparison of Storey v/s Storey drifts for different models in X directionThe drift values in the model 2 is very less compared to model 1. This is because of base isolator, which tends to reduce the difference in the displacement between successive stories of model 2.
Studies on Displacement (Response Spectrum Analysis)-SPECYThe displacement in mm for earthquake in X direction for different storey are resulted as below
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 9 Displacements –SPECYSTOREY MODEL 1 MODEL 2
12 40.6 120.7
11 39.2 121.0
10 37.2 121.4
9 34.6 122.0
8 31.5 122.5
7 28.0 123.2
6 24.1 123.9
5 20.4 124.7
4 16.6 125.4
3 12.6 126.2
2 8.7 126.9
1 4.9 127.4
0 0.0 0.0
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 9 Comparison of story v/s displacement for different models in Y directionThe displacement in the model 1 increases parabolically due to higher seismic forces in upper floors. However, the displacement in the model 2 is more and all most same in all floors. This shows the whole building is displaced because of base isolation.4.6 Study of Inter Story Drift (Response spectrum Analysis) -SPECYThe results of Different model with different storey is compared as follow
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 10 Inter Story Drift-SPECYSTOREY MODEL 1 MODEL 2
12 0.00068 0.00014
11 0.00084 0.00016
10 0.00108 0.00019
9 0.00125 0.00021
8 0.00138 0.00024
7 0.00146 0.00026
6 0.00133 0.00027
5 0.00137 0.00028
4 0.00139 0.00027
3 0.00130 0.00025
2 0.00127 0.00021
1 0.00110 0.00016
0 0.00000 0.00000
Figure STYLEREF 1 s 4: SEQ Figure * ARABIC s 1 10Comparison of Storey v/s Storey drifts for different models in Y directionThe drift values in the model 2 is very less compared to model 1. This is because of base isolator, which tends to reduce the difference in the displacement between successive stories of model 2.
Cost comparison:The member sizes and quantity are extracted from Etabs software and are tabulated as below:
Table: STYLEREF 1 s 4: SEQ Table: * ARABIC s 1 11 Cost Comparison TableSL. NO. MEMBERS MODEL WITH FIXED SUPPORT (m3) MODEL WITH BASE ISOLATION (m3) REDUCTION IN CONCRETE (m3) REDUCTION IN STEEL (KG) REDUCED COST [email protected] [email protected] REDUCED COST @ 65 PER KG % REDUCTION IN CONCRETE
1 SLAB – M25 286.4 286.35 0.0 0.0 0 0 0.0%
2 BEAM – M25 201.2 198.25 2.9 368.7 16225 23968.75 1.5%
3 COLUMN – M35 144.3 96.6 47.7 14307.0 286140 929955 33.1%
4 FOOTING – M35 192.7 186.47 6.2 467.2 37380 30371.25 3.2%
339745 984295 The model is checked for fixed base and with base isolation. We can observe from the table that, there is a comparable reduction in the case of only column quantity. The base isolator presence will not vary concrete quantity for slabs and beams comparatively. About 33% of reduction in case of column concrete quantity and also 3% reduction in case of footing quantity. This is due to reduction of self-weight of super structure.
CHAPTER 5CONCLUSIONS:General Conclusions:The displacement of the building with base isolator is very high compared with fixed base building. However, the displacement is almost same in all floors in base isolated model but it will decrease from top to bottom storey in case of fixed base.
It is clear from the displacement values; the building will move to and fro instead of bending. This will reduce the drift values due to inter storey displacement.
From the results it can be concluded that, presence of the base isolator has an advantage in reducing the drift limits to a maximum extent.
The displacement in the Y direction is more compared to X direction, since length of the building is more in X direction.
The displacement of the building for equivalent static analysis is more compared with Response spectrum analysis and the reduction is about 20 % for fixed base model. However, there is a 50% reduction in base isolated model.
The results of static and dynamic analysis conclude that the interstory drift parameter is not much of the concern, since it is very negligible in case of Base isolation.
The time period of the building is same for static and dynamic analysis. The graphs show first three modes are dominating while the time period is less after the third mode of vibration. However, the base isolated model shows starting three modes as zero value, this indicates the movement of the building rather than oscillation. However, there will be an inertial force which effects the building to sway, can be noticed from fourth mode.
The base shear value can be completely neglected in case of base isolated model, since the supporting condition acts as a roller compared with fixed support. This again proves that the roller / base isolated model will not carry horizontal force. The fixed base model is having base shear, in which base shear is transferred up to base level due to fixed support condition.
The cost comparison is also arrived from the project based on only structural members. The comparison shows a considerable reduction of 33% only in the columns. However, there is no major reduction in other structural members like beam, slab and footings.
This reduction in the column quantity is mainly due to reduction in base shear value. We can observe, the columns are governing for only axial load as major area of concerns. This will greatly reduce in the cost of construction, thereby reducing the self-weight, shuttering cost, Labor cost, construction speed and finally sizes of column, which are crucial from architect point of view.
Scope for Future Study:
It can further have studied with Effect of infill walls.
Since the costing is majorly depends on column, the composite type of column and beams can be adopted.
The different irregularity can be considered for further area for study.
And also, ultra-high-rise buildings are analyzed to carry out the study.
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