Beytollah.Taromi1, Kiarash.Nasserasadi2, Asghar.VataniOskouei3
1- Ms Student, Zanjan University, Zanjan, Iran
Email:beity_4042@yahoo.com
Abstract
Recently the experience of recent near fault earthquakes such as Chi-Chi, Kobe and Tabas earthquakes have shown that the structures are more vulnerable in the near source earthquakes. This has also shown in many studies. The main reason for that is due to a strong and long period velocity pulse which generates in the near-field (NF) earthquakes. Base isolation systems are one of the methods for reducing the damage and vulnerable in the near source earthquakes, seismic isolation system increase period vibration of structures which reduce seismic force and the amount of energy transferred into the structure. In this paper, a parametric study has been carried out to study the behavior of based isolated concrete buildings in the near source earthquakes. One base isolated concrete building with 7 stories is considered. The variation of top floor absolute acceleration, base shear and bearing displacement of the isolated building is plotted under different isolator parameters such as bearing yield strength and post yield stiffness ratio. The comparison of results indicated that for low values of the bearing yield strength and post yield stiffness ratio there is significant displacement in the lead- rubber bearing(LRB) under near-fault motions. In addition the increase of isolation period causes the base shear and floor acceleration reduced, but bearing displacement increases significantly.
Keywords: Lead-rubber bearing, Near-source earthquakes, post yield stiffness ratio, Bearing yield strength.
1. Introduction
Strong ground motion due to an earthquake excitation often calamitous disturbance that severely affects structures and their contents. The importance of the near-fault (NF) earthquakes characteristic has been noted by several researchers, Naeim, Bertero and chopra are researcher worked about NF. Large amplitude, long period and pulse in velocity records are the manually characteristics of NF earthquakes [1], [2].
Seismic isolation decouples a structure, part of it or equipment placed in the structure from the damaging effects of ground accelerations. This devices shift the fundamental frequency of the structure away from the domain frequencies of seismic excitations and the fundamental frequency of the fixed structure [3], [4]. In addition, it’s also provides an energy dissipation mechanism at the level of isolation, reducing the relatively large relative displacements between the superstructure and the supporting ground. Finally, the seismic isolation system provides either rigidity under minor lateral loads such as wind loads.
In recent years some new research has been done on the base isolated buildings under NF. One of these studies has been made by Jangid-R.S. in [5], [6] seismic response of the multi-story buildings isolated by the LRB is investigated under NF motion. It was shown that the LRB with appropriate properties is quiet effective for seismic isolation of structures under NF motions. Also Chan. Win, A. in [7] shows that the story acceleration are reduced significantly in the base isolated building compared to the original building.
In order to demonstrate the effect of the yield strength of the LRB base isolation and post yield stiffness ratioon the dynamic responses of the isolated structures, in this paper a parametric study was made on the Seven-story isolated structure with the LRB in the longitudinal direction X. The mechanism of yield strength is based on the area and yield stress of lead core of base isolation with an aim of controlling the deformation of the isolation bearing and consequently displacement and acceleration of superstructure. This system of isolation has various percentages of yield strength from 0.3% to 1.1% of total weight (F0=Fy/W) of the building and post yield stiffness ratio from 0.06 t0 0.16.
2. modeling and assumptions
2.1. Building Model
In order to study, a seven-story building is considered for analysis. Typical floor plan and elevation of the base-isolated seven story reinforced concrete building, which is used as the subject structure in this study, are shown in Fig 1 and Fig 2, respectively. The building has the regular plane with four longitudinal bays by four transverse bays. The length of the longitudinal and transverse bay is 5 m each. The height of floors is 3m. The dimensions of columns and beams for building have shown in table.1. The dead load is DL=600kg/m2 for all the floors including the wall loaded. The live load is LL=200kg/m2 for all floors of building. The total weight of the building is 3078 tons. For the nonlinear time history analysis, the computer program SAP2000 [8] is used to predict the building responses. Building is idealized by a 3D model consisting of columns, beams as well as floor diaphragms. Each floor diaphragm is assumed rigid in its own plane and the mass is supposed to be lumped at each floor level. The system is subjected to six component of the near fault earthquake ground motion. The elements of superstructure designed according to the conventional building with ACI318-89ASD code.
The reduction factor for fixed-base design allowed by the code is 7.0 and for moment-resisting frame is used for the superstructure is 2.0 according to the UBC97 code.
Figure1. Plan of structure Figure 2. Section of base isolated structure
Table1- Dimensions of columns and beams
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Stories
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1-2
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3-4-5
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6-7
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Column
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C55*55
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C50*50
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C45*45
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Beam
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B45*45
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B40*45
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B40*40
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2.2. LRB Characteristics
The LRB are the most common base isolators used for isolating bridge and buildings. This base isolation invented in 1975 in New Zealand and used extensively in New Zealand and the United States. Lead rubber bearings include a central lead plug that is used to increas the initial sttifness of the bearing and increase the energy dissipation capacity of the bearing.Fig 3-a show typical bi-linear behavior of isolator. In this paper the isolators are modeled by bi-linear hysteretic behaviors. The introduction of LRB isolators in the present nonlinear dynamic analysis was achieved by considering the RUBEER ISOLATOR nonlinear link element capability in SAP2000 modeled. The characters of base isolators calculated based of method described by Naeim [9]. In order to conducting parameter study, the yield strength of lead and Post-yield stiffness ratio of base isolation is considered variant and others parameter are considered constant. The building is assumed to be located in a high-seismic region, zone 4, and assigned a seismic zone factor Z=0.4 according to table 16-1 of UBC97. The closest distance to a known fault that is capable of producing large magnitude events and that has high rate of seismic activity (Class A seismic source according to Table 16-U of UBC97 is assumed to be 5 km. In result the amount of seismic coefficient is. The design results of lead rubber bearing are shown in table 2 and cross section are shown in Figure 3-b.
3-a) Bi-linear modeling of LRB 3-b) Cross section of LRB
Figure3. LRB base isolation and modeling parameter [7, 10]
Design of LRB base isolation with assuming that are following:
Table2- Dimension of LRB at design target period,(TD=2.5sec)
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Name
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h(cm)
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h(cm)
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N(Nos)
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t(cm)
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dp(cm)
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ts(cm)
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Ns(Nos)
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Ttp(cm)
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LRB
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55
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35
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20
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1
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6
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0.3
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19
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2.5
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2.3. Earthquakes Record
To compare the influence of yield strength on the dynamic response of the isolated structure with the various percentages of yield strength and under various seismic excitations, an analysis of the response by accelerograms is carried out and the considered seismic loadings are the following [11]:
1. The component of EI Centro of the earthquake of Imperial Valley (1979).
2. The component of Otakatori of the earthquake of Kobe (1995).
3. The component of TCU068 of the earthquake of Chi-Chi (1999).
4. The component of 24Lucerne of the earthquake of Landerz (1992).
5. The component of 16Lgpc of the earthquake of Loma Prieta (1989).
6. The component of 9101Tabas of the earthquake of Tabas (1978).
With PGA (Peak Ground Acceleration) of 0.41 g, 0.41 g, 0.56 g, 0.72 g, 0.96 g and of 0.83 g respectively. The velocity records of these excitations are represented respectively on the figures 4.
Figure 4. Velocity records of selected earthquakes in this paper.
3. Results and discussions
According to the results of displacements which are represented graphically by the figure 5, we notice that under all the seismic loadings used in this parametric study, the displacements of the superstructure or of the seismic isolation system are decreased with the increase of the yield strength of lead and post yield stiffness ratio. This shows the influence of the yield strength and post yield stiffness ratio of the seismic isolation system which reduces the displacements in the superstructure and the displacements at the basis for an isolated structure.
The optimum range of values of yield strength and post yield stiffness ratio that due to minimizing the story displacement in the structure can be obtained from Figure 6, respectively in range of 0.8% - 1% of the total weight of the building and in range of 0.08 – 0.12 of the ratio of plastic stiffness to elastic stiffness of base isolation under near fault motions.
Figure 5- Variations of maximum displacement of the isolator and roof story with the different percentage of yield strength
Figure 6- Variations of maximum displacement of the isolator and roof story with the different ratio ofpost yield stiffness
3.2 Acceleration
The figure 7 respectively from left to right had indicated that due to increase in the yield strength and post yield stiffness ratio of the LRB there is an increase in the roof acceleration under near-fault motions. In fact, there exists a particular value of the yield strength and post yield stiffness ratio, for which the top floor acceleration is minimum. Further it is also observed that in the vicinity of the particular yield strength, the top floor acceleration is not much affected with the variation of the bearing yield strength. One of the advantages of the above behavior of the isolated structure can be considered in designing of the optimum LRB that the yield strength can be kept slightly higher, than the corresponding particular value of minimum acceleration to achieve maximum and bearing displacement minimized. Thus the optimum yield strength of the LRB can be obtained by minimization of a force quantity which is function of both the peak top floor acceleration and bearing displacement. The optimum range of values of yield strength and post yield stiffness ratio that due to minimizing the story acceleration in the structure can be obtained from Figure 7, respectively in range of 0.8% - 1% of the total weight of the building and in range of 0.08 – 0.12 of the ratio of plastic stiffness to elastic stiffness of base isolation under near fault motions.
3.3 Base shear
Figure 8 Shows changes of the base shear of isolated building against the normalized bearing yield strength, F0 and post yield stiffness ratio under near-fault motions respectively. The response are shown for a seven-story building with, Tf =0.69 sec, Tb=2.5 sec and β=15%. It is observed from the figure 8 that as the bearing yield strength and post yield stiffness ratio, increases the average of base shear for all seismic excitations increases low. In addition, it is observed that as the bearing yield strength increases, the base shear first decreases and then increases with the increase of yield strength for some of the near-fault motions.
Figure8- Maximum base shear of isolated building with the different percentages of yield strength of lead and ratio of post yield stiffness respectively left to right.
4. Optimum parameters of LRB
The optimum value of the yield strength of lead core and post yield stiffness ratio In order to make the minimum values of story acceleration, story displacement and bearing displacement can be obtained from the figures 5 to 7. The optimum yield strength and post yield stiffness of the LRB based on the criterion of minimization of the both top floor acceleration and bearing displacement is found to be respectively in range of 0.8% - 1% of the total weight of the building and in range of 0.08 – 0.12 of the ratio of plastic stiffness to elastic stiffness of base isolation under near fault motions.
The results of the seismic response obtained by the parametric study on the yield strength and post yield stiffness ratio of the seismic isolation system LRB in order to reduce that the displacements of the superstructure and the isolation system with the increase of the yield strength of lead and post yield stiffness ratio under the seismic excitations used, the acceleration transmitted to the superstructure increase with the increase of yield strength and post yield stiffness ratio very small.
According to the results obtained of the present study, the following conclusions may be drawn:
1. For low values of the bearing yield strength there is significant displacement in LRB under near-fault motions.
2. The increase in the yield strength and post yield stiffness ratio can reduce the bearing displacement significantly without causing significant changes in acceleration of superstructure.
3. There exists a particular value of the yield strength and post yield stiffness ratio of the LRB that the top floor acceleration has the lowest value.
NOMENCLATURE
d = Diameter of the bearing, (cm)
h =Total height of the bearing, (cm)
N = Number of rubber layers, (Nos)
t =Thickness of individual layers, (cm)
dp =Diameter of the lead core, (cm)
ts =Thickness of steel plate, (cm)
Ns =Number of steel plates, (Nos)
Ttp =Thickness of top and bottom cover plates, (cm)
Fy =Yield force of bearing,
DD = Design displacement,
Keff = Effective stiffness,
WD = Energy dissipated,
Qd = Characteristic strength,
K1 = Elastic stiffness,
K2 = post elastic stiffness,
TD= Design period.
WDL+LL=Total weight of building.
PDL+LL=Maximum axial force of columns.
Tf= Typical construction period.
TD= Period of isolated buildings.
6. References
1.Hall, JF. Heaton, TH. Halling, MW. Wald, DJ., (1995), “Near-Source Ground Motion and Effects on Flexible Buildings”, Earthquake spectra, 11(4 ),pp 569-605.
2. Providakis, C.P., (2007), “Pushover analysis of base-isolated steel-concrete composite structures under near-fault excitations”, Soil dynamics and earthquake Engineering, (28), pp 293-304.
3.Naeim, F., (1995), “on seismic design implications of the 1994 Northridge earthquake record”, Earthquake Spectra, 11(1), pp 91-109.
4. Buckle, I. G. and Mayes, R. L., (1990), “Seismic isolation history: application and performance- a world review”, Earthquake spectra, 6, pp 161-201.
5.Komodromos, P., (2000), “Seismic isolation for earthquake resistant structures”, WIT Press, Southampton,UK.
6.Jangid, RS. (2007), “Optimum lead-rubber isolation bearings for near-fault motions”, Engineering structures, 29, pp 2503-2513.
7.Chan Win, A., (2008), “Analysis and design of base isolation for multi-storied building”, International Conference on Sustainable, pp 1-8.
8.Computers and Structures Inc., “SAP2000 Static and dynamic finite element analysis of structures”, Vertion 14, Berekley, USA, 2009.
9. Naeim, F., (2001), “The seismic Design Handbook”, 2nd edition, Kluwer Academic Publishers.
10. Symans,MD., “ Seismic protective systems: Seismic isolation”, Advanced Earthquake ,Topic 15-7.
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