The dynamic response characteristic of an unreinforced base-isolated (BI) masonry building subjected to different intensities of incoming earthquakes is compared with the response of the same building without a base isolation system. 141 Table 6.5 Peak acceleration and displacement at different levels of the model subjected to the 30% intensity level of four earthquakes along the X-axis.
G ENERAL
A N O VERVIEW OF S TRUCTURAL C ONTROL S YSTEM
- Passive Control System
- Active Controlled System
- Hybrid Control System
- Semi Active Controlled System
Some of the hybrid control systems are (a) base isolation - actuator/ATDM (Active Tuned Mass Damper) and (b) visco-elastic dampers - ATDM. Some of the semi-active control systems are (a) stiffness clamping system (b) fluid viscous damper systems (c) Electromagnetic (EM) / Magneto-rheological (MR) dampers.
B ASE I SOLATION
Advantages of Base Isolation
Isolated building undergoes rigid body motion and isolators undergo large displacement or deformation and absorb the seismic energy through hysteresis. Typical deflected pattern of non-base insulated and base insulated building is shown in fig.
Leading Base Isolation System
The dominant feature of the system is the parallel operation of the springs and the armature flap as shown in the following figure. The schematic diagram of the R-FBI bearing system is shown in Figure 1.7. Schematic diagram of the R-FBI bearing system. e) Electricite de France (EDF) system.
B ASE I SOLATED B UILDING IN I NDIA
The steel springs are completely undamped and the system is always used in conjunction with the visco-damper to provide damping to the system. 1.5(a) also represents the schematic of GERB base isolation system, which consists of spring and viscodamper.
P ROBLEM I DENTIFICATION
In this study, preliminary design of square and circular FREIs is carried out using the available formulation reported in previous literature. FE analyzes of FREIs are performed to find out the insulator mechanical properties and detailed stress/strain profile at different insulator layers.
O BJECTIVES
Shaking table studies of a two-storey 1/5 scale masonry building supported by four U-FREIs are being carried out at the Structural Engineering Laboratory of the Indian Institute of Technology, Guwahati. Developing a numerical model for the isolated basic building, so that the expensive experimental research using the shaking table can only be carried out selectively.
S COPE OF S TUDY
Experimental Study
Conduct laboratory experiments to determine the relationship between FREI force displacement under cyclic horizontal displacement, mechanical and dynamic characteristics of the isolator. Direction of loading to be changed to assess its impact on the performance parameters of FREIs.
Analytical Study
O UTLINE OF THE T HESIS
Various dynamic response parameters along the height of a fundamentally isolated masonry building are evaluated from a vibration table. Numerical analysis is performed for different intensities of the input seismic excitation along different directions of the building model.
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I NTRODUCTION
B ASE I SOLATION B EARING
Effective damping of the bearing increases with the increase in axial load on the bearings. Substituting the values for geometric properties from Table 1 and material properties from Table 2 into Eq. 3.1), the horizontal stiffness of the circular insulator is calculated to be 57.16 N/mm. The compressive stress is almost constant throughout the height of the insulator as shown in fig.
Details of the test model and model FREIs are presented in the next subsection. Details of FREI bearing used for seismic isolation of the test model are shown in Chapter 3.
E XPERIMANTAL AND A NALYTICAL S TUDY OF E LASTOMERIC I SOLATORS
FE A NALYSIS OF E LASTOMERIC B EARING
A constant vertical load of 12 kN is applied to the top surface of the isolator and horizontal displacement is applied to evaluate the horizontal stiffness of both bonded and unbonded isolator at different displacement magnitudes. -axial force balanced accelerometers were fixed at each floor level of the test model as shown in Fig. Peak acceleration and displacement at different floor levels of the model subjected to 30% intensity of all four input earthquakes are shown in Table 6.5.
T ESTING OF B ASE I SOLATED BUILDING
C ONCLUDING R EMARKS
The feasibility of FREI as a viable alternative to the conventional SREI is reviewed in previous sections. Very limited number of studies have been conducted in FE modeling and analysis of the FREI to date. The validation of these results through numerical modeling will further confirm the suitability of these bearings and also enable analysis of bearings when it is not possible to do so experimentally.
I NTRODUCTION
FREI can be classified into bonded and unbonded bearings based on the insulator end fixing conditions. In the case of unbonded insulators, there are no thick end plates, the insulator is simply placed between the superstructure and the substructure. Thus, the closed-form solutions that are available to estimate the horizontal stiffness of bonded isolators are not directly applicable to unbonded isolators.
D ESIGN OF I SOLATORS
- Bonded Fibre Reinforced Elastomeric Isolator
- Bonded Square FREI
- Bonded Circular FREI
- Un-bonded Fibre Reinforced Elastomeric Isolator
To achieve high vertical stiffness of the bearings, 19 layers of elastomer, each 5 mm thick, are selected. To achieve the desired stiffness of the bearings, 19 layers of elastomer, each 5 mm thick, are selected. The buckling load of the model bearing estimated by different formulas proposed by different investigators is listed in Table 3.4.
C HARACTERIZATION OF M ATERIAL OF I SOLATOR
- Testing of Elastomer
- Hardness Test
- Tensile Stress-Strain Properties
- Compression Set at Constant Strain
- Accelerated Ageing Test
- Testing of CFRP
Neoprene is a synthetic chloroprene rubber, suitably compounded to achieve the mechanical properties for making a square insulator. The load and elongation are read while the specimen is being stretched when it breaks. The tensile strength is calculated by dividing the breaking load by the initial cross-sectional area of the test piece.
C ONCLUDING R EMARKS
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I NTRODUCTION
F INITE E LEMENT M ODELLING
- Element Type for Finite Element Model
- Element type of fibre reinforcement layer
- Element type of elastomeric layer
- Contact and Target Elements
- Material Model
- Loading History for Vertical Load
- Loading History for Horizontal Load
- Solution Method
However, in the case of U-FREI, the top and bottom plates lose contact when lateral displacement is applied. In the numerical simulation, two rigid horizontal plates at the top and bottom of the insulator are considered to represent the top and bottom structure. The rigid end plates used to represent the top and bottom structure are directly connected to the top and bottom rubber surfaces of the insulator, respectively.
F INITE E LEMENT A NALYSIS OF S QUARE I SOLATOR
- Analysis for Horizontal Stiffness
- Stress and Strain of Square Isolator
- Square isolator with 0 0 loading direction
- Square isolator with 45 0 loading direction
- Force Displacement Hysteresis Behaviour of Square Isolator
- Force-displacement hysteresis behaviour of square isolator
- Force-displacement hysteresis behaviour of square isolator
- Discussion on FE result of Square Isolators
The normal stress contour S33 for a square unbonded and bonded insulator is shown in the figure. The peak pressure in the bonded insulator is 55% higher than that of the unbonded insulator, corresponding to 60 mm of horizontal displacement. a) 60 mm horizontal displacement (b) 40 mm horizontal displacement. The maximum pressure in a bonded insulator is 45% higher than that of an unbonded insulator for 60 mm of horizontal displacement.
F INITE E LEMENT A NALYSIS OF C IRCULAR I SOLATOR
- Analysis for Horizontal Stiffness
- Stress and Strain of Circular Isolator
- Force Displacement Hysteresis Behaviour of Circular Isolator
- Discussion on FE Result of Circular Isolator
The contour of normal stress S33 corresponding to the maximum applied displacement of 60 mm is shown in Figure 4.32 for both bonded and unbonded circular insulators. The peak pressure in the bonded insulator is 94% higher than that of the unbonded insulator, which corresponds to a horizontal displacement of 60 mm. The horizontal stiffness of both bonded and unbonded circular FREIs is quite comparable to that of square FREIs.
E FFECT OF M ESH S IZE ON A NALYSIS R ESULT
The percentage reduction in effective horizontal stiffness of unbonded circular isolator with increase in horizontal displacement is in the same range for unbonded square isolator. Therefore, the seismic performance of both square and circular unbonded isolators are roughly comparable to each other. Maximum compressive stresses developed at the central overlapping part of circular unbonded isolator and tensile stress at the edges are also comparable to compressive and tensile stresses developed in square unbonded isolator for applied shear along the X-axis.
E FFECT OF V ERTICAL L OADING ON S HEAR C APACITY OF FREI
C ONCLUDING R EMARKS
Comparison of the FE analysis results and experimental results are carried out in the next chapter. The horizontal stiffness of bonded FREI appears to correspond well with the existing closed form solution. The horizontal stiffness of U-FREI is significantly smaller than that of bonded FREI at larger displacement and therefore the seismic isolation efficiency is higher.
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I NTRODUCTION
T EST FOR E VALUATION OF V ERTICAL S TIFFNESS
- Test Set-up and Instrumentation
- Test Results and Discussion
The test setup for evaluating the vertical stiffness of the FREI is shown in Fig. The vertical stiffness test result shows a close agreement with the value calculated from the simple formula given by Eqn. As seen from the test result, the vertical stiffness of the U-FREI is about 125 times higher than the maximum horizontal stiffness obtained from the FE analysis of the U-FREI for the 0° loading direction.
T EST FOR EVALUATION OF H ORIZONTAL S TIFFNESS
- Test under Horizontal Load: Arrangement and Instrumentation
- Lateral Cyclic Loading
This greater vertical stiffness of elastomeric bearings ensures that the rocking modes do not participate in the response of the underlying isolated structures (Nezhad et al, 2009). The isolators and loads are placed symmetrically so that each isolator carries the same load of 12kN, which is the design load of the isolator. The resulting hysteresis data obtained from the test are used to evaluate the dynamic properties of the insulators.
C OMPARISON OF N UMERICAL AND E XPERIMENTAL R ESULTS
- Result of Loading along 0 0 Orientation
- Result of Loading along 45 0 Orientation
Comparison of FE analysis result and experimental result of horizontal effective stiffness (Keffh ) and damping of square U-FREI subjected to loading along 00 for different. Similar to the previous section, the U-FREI square force displacement hysteresis loops subjected to loading along 450 are shown in Fig. stiffness (.. Keff) and damping of square U-FREI subjected to loading along 450 are shown in Table 5.3.
D ISPLACED S HAPE OF I SOLATOR
Comparison of shear force-displacement hysteresis loops obtained from experiments and FE analysis for loading along the 450 to X axis (Fig.5.7) shows that the discrepancy is relatively smaller compared to those corresponding to loading along the X axis. The value of the lateral stiffness for the displacement along the 450 axis in X is slightly higher than those corresponding to the displacement along the X axis.
C ONCLUDING R EMARKS
An approximately 55% reduction in stiffness is observed when the displacement increases from 10 mm to 60 mm for both load directions. The percentage of damping increases with the increase of horizontal displacement, which is in the range of 10-16% for both load directions. The experimentally observed displaced shapes of the isolators corresponding to both loading directions are in very good agreement with those from the numerical analysis.
I NTRODUCTION
Shaking table testing of a model two-story masonry building supported on the U-FREI is conducted to ascertain its effectiveness in controlling the seismic response. The dynamic response characteristic of a masonry isolated (BI) building subjected to different intensities of incoming earthquakes is compared with the response of the same building without a basic isolation system. The objective of this study is to investigate the dynamic response of unreinforced brick masonry construction model supported on U-FREI.
T EST M ODEL AND I SOLATION S YSTEM
- Scaling and Similitude
- Test Model and its Construction Details
- Properties of U-FREI
In the model building, beams are provided at the base level, which are supported by four U-FREIs installed at the four corners of the building. The dimensions of the building elements of the test model are obtained by scaling the corresponding element of the prototype according to the similarity requirement. The total height of the test model is 1.51 m, including the base ring and two flat plates.
S HAKE T ABLE T ESTING S ETUP AND I NSTRUMENTATION D ETAILS
The BI test model supported on four isolators located at the four corners of the base floor is shown in Fig. Concrete blocks as shown in Fig. 6.4 are placed on each floor to calculate the compensatory measure. The location of the LVDTs to measure the displacement at the base beam level and the first floor level of the model during the earthquake excitations are also shown in Figs.
S AMPLE G ROUND M OTIONS FOR S HAKE T ABLE T EST
S HAKE T ABLE T EST R ESULT
- Test Results for Excitation along X-axis of FREI
- Test Results for Excitation along 45 0 to X-axis of FREI
The displacement histories at the base beam level (LVDT 1) and the first floor level (LVDT 2) are compared in Figs. It is also observed that the displacement at different times at the level of the base beam and at the level of the first floor are almost the same. The displacement amplitude recorded at the base beam level and the first floor level is compared in Fig.
C OMPARISON OF R ESPONSES OF BI AND FB T EST M ODEL
Comparison of different dynamic parameters for both BI and FB models subjected to different intensity levels of PGA for all four input earthquakes is presented. 6.20(a), where the accelerations at different floor levels in the BI model are very low compared to the FB model. The peak base displacement in the FB model is much higher than that of the BI model for any equivalent level of input earthquake intensity.
C ONCLUDING R EMARKS
The maximum excitation intensity is limited to a fraction of the PGA of input earthquakes, so that no damage occurs in the FB structure. There is no damage in any part of the BI building model at full PGA intensity of input movements. Therefore, the comparison of seismic responses of BI and FB models is limited to an intensity of 30% of the PGA of input earthquakes.
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- I NTRODUCTION
- M ODELLING OF T EST S TRUCTURE
- M ODELLING OF FREI
- C OMPARISON OF N UMERICAL AND S HAKE T ABLE T EST R ESULTS
- C ONCLUDING R EMARKS
The inter-floor offset at different levels of the test model building is insignificant. A comparison of the acceleration time histories obtained from the shaking table test and the numerical analysis at different levels of construction of the test model corresponding to the Koyna earthquake (full intensity) applied along the X-axis is shown in Fig. Similar exercises are repeated considering another seismic movement with a different one. test and numerical analysis at different levels of the test model corresponding to the full intensity history of the scaled acceleration of the Parkfield earthquake applied along the X-axis are shown in Fig. a) Experimental result at the base beam level (b) Analytical result at the base beam level.
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S UMMARY
M AJOR F INDINGS
S UGGESTIONS FOR F UTURE W ORK
