Available online at www.sciencedirect.com
Procedia Engineering 14 (2011) 3245–3251
The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction
Seismic Response of Base-Isolated Structures with LRB and FPS under near Fault Ground Motions M.K. SHARBATDAR1, S.R. HOSEINI VAEZ2ab, G. GHODRATI AMIRI3 and H. NADERPOUR4 Department of Civil Engineering, Semnan University, Iran Department of Civil Engineering, Semnan University, Iran 3 College of Civil Engineering, Iran University of Science and Technology 4 Department of Civil Engineering, Semnan University, Iran 2
Abstract Seismic response of structures in the vicinity of causative earthquake faults can be significantly different than those observed further away from the seismic source. In the near fault zone, ground motions are significantly influenced by the rupture mechanism and slip direction relative to the site and by the permanent ground displacement at the site resulting from tectonic movement. Forward directivity and fling effects have been identified by the seismologists as the primary characteristics of near fault ground motions. Because of the unique characteristics of the near-fault ground motions and their potential to cause severe damage to structures designed to comply with the criteria mostly based on far-field earthquakes, the estimation of seismic response of base-isolated structures for a project site close to an active fault should account for these special aspects of near fault ground motions. This paper investigates the seismic response of base-isolated structures with LRB and FPS isolators under near fault ground motions. A seismic evaluation of the building, isolated with the LRB and FPS, is performed using a nonlinear three-dimensional analytical model. The parametric study is concentrated on base shear, accelerations and displacements of isolated models. Large displacement and velocity pulses in records of near fault ground motions can significantly change the results of seismic response of base-isolated structures. Keywords: Near fault ground motions; Lead rubber bearing; Friction pendulum system; Seismic Response; Base-Isolation.
a b
Corresponding author: Email:
[email protected] Presenter: Email:
[email protected]
1877–7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.07.410
3246
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
1. INTRODUCTION During past decades, increasing database of recorded ground motions demonstrated that the ground motions close to a ruptured fault can be significantly different than those observed further away from the seismic source. Forward directivity and fling effects have been identified by the seismologists as the primary characteristics of near fault ground motions (Mavroeidis and Papageorgiou, 2003). These characteristics of ground motion near the fault of major earthquakes contain large displacement and velocity pulses. The estimation of seismic response of base-isolated structures for a project site close to an active fault should account for these special aspects of near fault ground motions. This study investigates seismic response of base-isolated structures under near fault ground motions. 2. Modeling Structural models prepared for analysis include 15-story buildings. The models consist of FPS isolators (Friction Pendulum System) and LRB (Lead Rubber Bearing) (Figure 1). Nonlinear analytical modeling techniques (Nagarajaiah et al. 1991, Tsopelas et al. 2005) were used for dynamic analysis of structural models. The structural models were analyzed under 5 records of near fault ground motions. Two Californian earthquake events selected as near-source ground motions: the 1966 Parkfield and the 1979 Imperial Valley earthquakes (Figures 2 and 3). The 1966 Parkfield event provided the now famous Station 2 (C02) record at a distance of only 80m from the fault break (Housner and Trifunac, 1967). This record contains strong velocity and displacement pulses of relatively long periods which distinguish them from typical far-field earthquakes (Mavroeidis and Papageorgiou, 2003). Modern quantitative analysis of strong ground motion observations started with this record. The characteristics of both earthquakes and the convergence procedures of modal parameters are presented in Tables 1 and 2 respectively.
Figure 1: Typical plan of structural model.
3247
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
Table 1: Characteristics of earthquakes used for analysis Closest to Fault
PGA
PGV
(g)
(cm/sec)
(cm)
SN*
0.476
75.005
22.407
4.2
SN
0.360
76.550
59.056
1.0
SN
0.379
90.535
63.086
E06
1.0
SN
0.439
109.820
65.833
E07
0.6
SN
0.463
109.261
44.472
Location
Date
Mw
Station
Parkfield, CA, USA
27-Jun-66
6.19
C02
0.1
E04 E05
Imperial Valley, CA, USA
15-Oct-79
6.53
Rupture (km)
Component
PGD
*SN= Strike-Normal Table 2: Convergence Procedures of Modal Parameters TD
DD
Keff
WD
Q
K2
K1
Dy
Q
2
0.24874
9749.04
378.81
380.73
8218.44
82184.4
0.00463
387.95
2
0.24874
9749.04
378.81
387.95
8189.4
81894
0.00474
388.12
2
0.24874
9749.04
378.81
388.12
8188.73
81887.3
0.00474
388.12
Figure 2: Time history components; Station 2 (C02) record obtained from the 1966 Parkfield, California, earthquake.
3248
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
(a)
(b)
(c)
(d)
Figure 3: Time history components: (a) E04; (b) E05; (c) E06; (d) E07; Records obtained from the 1979 Imperial Valley, California, earthquake.
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
Figure 3a to 3d show the time history components of 4 records of Imperial Valley earthquake with forward directivity effect. Large displacement and velocity pulses could be seen in these records. The difference between parameters of peak ground values of the records in the vicinity of causative earthquake faults can be seen from Table 1 and the mentioned Figures. So, the seismic response of base isolated structures under these records can be different compared with those cause of far field ground motions. 3. Analytical Results Time variation of base displacement at center of mass for each model is illustrated for investigating the effect of large displacement pulses in the records of near fault ground motions. Since reduction of acceleration in superstructure of system is a substantial parameter in isolation systems, top floor acceleration against time are shown. Figure 4 shows seismic responses of isolated model under E04 record of Imperial Valley earthquake. Figure 4a shows that LRB isolator exceeds from the allowable displacement while FPS isolator is in the allowable zone (Horizontal lines in Figures 4a to 8a show the limitation of displacement of isolators for these models.). From Figures 5 to 7 it can be declared that because of large displacement pulses in the records, both isolation systems exceed from the displacement limitation for E05 to E07 records of Imperial Valley earthquake. The displacements of isolators are in the allowable zone for record C02 of Parkfield earthquake (Figure 8).
Figure 4: Dynamic responses under E04 record of Imperial Valley earthquake; (a): Time variation of base displacement; (b): Time variation of top floor acceleration
Figure 5: Dynamic responses under E05 record of Imperial Valley earthquake; (a): Time variation of base displacement; (b): Time variation of top floor acceleration
3249
3250
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
Figure 6: Dynamic responses under E06 record of Imperial Valley earthquake; (a): Time variation of base displacement; (b): Time variation of top floor acceleration
Figure 7: Dynamic responses under E07 record of Imperial Valley earthquake; (a): Time variation of base displacement; (b): Time variation of top floor acceleration
Figure 8: Dynamic responses under C02 record of Parkfield earthquake; (a): Time variation of base displacement; (b): Time variation of top floor acceleration
Maximum amounts of response for different models is summarized in Table 3; these responses include maximum base shear to weight of superstructure, maximum base displacement at center of mass and maximum acceleration. According to the table, it can be declared that the value of maximum base displacement can be different up to 66% for 4 records of Imperial Valley earthquake in a zone restricted within a distance of about 4km from the ruptured fault. Also in this zone maximum top floor acceleration can be differed up to 35% for the records of Imperial Valley. 4. CONCLUSIONS In the vicinity of causative earthquake faults, ground motions at a particular site are significantly influenced by the rupture mechanism and slip direction relative to the site and by the permanent ground
3251
M.K. SHARBATDAR et al. / Procedia Engineering 14 (2011) 3245–3251
displacement at the site resulting from tectonic movement. These effects of ground motion near the fault of major earthquakes contain large displacement and velocity pulses. Large displacement and velocity pulses in records of near fault ground motions can significantly change the results of seismic response of base-isolated structures. Numerical results from the models under 4 records of Imperial Valley earthquake show that the value of maximum base displacement can be differed up to 66% in a zone restricted within a distance of about 4km from the ruptured fault. Also in this zone, maximum top floor acceleration can be differed up to 35% for the records of Imperial Valley. Table 3: Maximum responses of different structural models
Record
Maximum
Maximum
Maximum
Isolator
Top Floor Acceleration
Base Displacement (cm)
Base Shear/ Weight
FPS
0.470
10.99
LRB
0.467
12.16
0.142
FPS
0.491
24.90
0.292
LRB
0.453
30.01
0.298
FPS
0.662
31.62
0.359
LRB
0.615
38.01
0.366
FPS
0.595
41.97
0.463
LRB
0.607
49.96
0.470
FPS
0.575
33.75
0.381
LRB
0.521
40.01
0.384
(g) Parkfield, CA, USA – C02
Imperial Valley, CA, USA – E04
Imperial Valley, CA, USA – E05
Imperial Valley, CA, USA – E06
Imperial Valley, CA, USA – E07
0.151
REFERENCES [1]
Ghodrati Amiri, G., Naderpour, H., and Hoseini Vaez, S.R., (2010), “Numerical Evaluation of Base-Isolated Structures with Optimized Distribution of LRB and FPS Isolators”, 9th US National & 10th Canadian Conference on Earthquake Engineering, July 25-29, 2010, Toronto, Ontario, Canada.
[2]
Halldorsson, B., Mavroeidis, G. P., & Papageorgiou, A. S. (2010). “Near-Fault and Far-Field Strong Ground Motion
[3]
Housner, G. W., and M. D. Trifunac (1967). Analysis of accelerograms: Parkfield earthquake, Bull. Seism. Soc. Am. 57,
[4]
Matsagar, VA. and Jangid, RS.,: (2004) , “Influence of isolator characteristics on the response of base-isolated structures” ,
[5]
Mavroeidis, G. P., & Papageorgiou, A. S. (2003). "A mathematical representation of near-fault ground motions". Bulletin of
[6]
Nagarajaiah, S., Reinhornm A.M., and Constantinou, M.C., (1991), “Nonlinear Dynamic Analysis of 3D Base-Isolated
[7]
Tsopelas, P.C., Roussis, P.C., Constantinou, M.C., Buchanan, R., Reinhorn, A.M., (2005), “3D-BASIS-ME-MB”, Computer
Simulation for Earthquake Engineering Applications Using the Specific Barrier Model” ACSE, Accepted Manuscript. 1193–1220. Engineering Structures, 26, 1735-1749 the Seismological Society of America, 93(3), 1099-1131. Structures”, Journal of Structural Engineering, ASCE, Vol. 117, No. 7, pp. 2035-2054. Program for Nonlinear Dynamic Analysis of Seismically Isolated Structures, Manual.
