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Study on Seismic Safety Performance for Continuous Girder Bridge based on Near-fault Strong Ground Motions
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ProcediaEngineering Engineering 00 Procedia 45(2012) (2012)000–000 916 – 922
2012 International Symposium on Safety Science and Technology
Study on seismic safety performance for continuous girder bridge based on near-fault strong ground motions LI Xinlea, JIANG Huib, SHEN Danc,* a
School of Civil Engineering and Architecture, Dalian Nationalities University, Dalian 116600, China b School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China c China Railway Engineering Consulting Group Co., Beijing 100055, China
Abstract In order to explore the safety performance of continuous structure bridge near fault zone, a typical three spans continuous girder bridge of highway was selected to study the structural seismic response. The finite element model of whole bridge considered the force characteristics of bearing and pile-soil interaction was constructed. Typical near-fault records were selected from the important earthquake events. Several artificial waves characterized with the soil type in bridge site were simulated and used for dynamic analysis. Comparing with the seismic response of bridge under three given acceleration design spectrum, the seismic performance of continuous bridge was studied by nonlinear dynamic time-history method. Research results indicates that, the seismic design spectrum considering near-fault effect, especially large amplitude pulse effect of near-fault records, will significantly enhance the seismic response of continuous girder bridge. The fact that the near-fault effect is not considered in China existing highway bridge seismic design code will increase the destructive risk of structure.
© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Beijing Institute of Technology. Keywords: near-fault ground motions; continuous girder bridge; dynamic time-history method; response spectrum analysis; seismic response
1. Introduction The first research results of near fault earthquake effect was described by Benioff (1955) [1] based on the Kern County earthquake event occurred in Califonia. More and more research results for near-fault earthquake effect has been published[2-3]. The significant damage is often found in near-fault zone (also called as near field, fault distance R 36km) during several earthquake events occurred in 80's of 20st century (1994 US Northridge, 1999 Taiwan Chi-Chi) and China Wenchuan earthquake event. Because of the influence of forward directivity effect and fault mechanism, the remarkable characteristics of near-fault records with long-period pulse and directivity, which is different from far field, will enhance the seismic response (internal force and displacement) of structures. With the development of economy of China, more and more common continuous girder highway bridges will be designed and constructed near fault zone, which are characterized with the feather of wide adaptability and simple technology. The safety performance for this kind of bridge must be deeply studied to reduce the damage of near-fault earthquake. A typical continuous girder bridge with pier-pile foundation near fault was selected and studied in this paper. Based on the mechanical characteristics of bearing and soil around pile, a finite element model (FEM) for whole bridge considered the effect of pier-pile interaction was constructed to calculate the seismic response by using of the method of nonlinear dynamic time-history and response spectrum. All actual near-fault records are characterized with near-fault earthquake
* Corresponding author. Tel.: +86-411-87656208; fax: +86-411-87656208. E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.259
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effect, and artificial waves in the site condition of bridge are generated. Three acceleration response spectrums for II type site are made in article. Through analyzing the results of response, the safety performance of continuous girder bridge exacted by near-fault strong ground motions is evaluated to enhance the seismic design level of further similar bridge. 2. Typical continuous girder bridge and its FEM The typical prestressed concrete hollow slab continuous bridge is a three-span medium span bridge with equal spans of 16m each near Yishu fault zone in China [4]. The double-column pier-pile foundation is adopted in pier design. The diameter of each pier is 1.3m. The heights of pier No.1 and No.2 are 3.17m and 2.03m, respectively. The diameter of bored pile foundation is 1.5m. The lengths of for No.1 and No.2 bored pile are 8m and 10m respectively. The design peak ground acceleration of earthquake ground motion is 0.15-0.20g, and the seismic defense intensity is equal to 8. The girder and pier are joined by some laminated rubber bearings. The tetrafluoroethylene GYZF4 type and common GYZ type laminated rubber bearing are placed on the top of abutment and pier. The ideal elastic-plastic hysteretic model for bearings is assumed in paper, and the horizontal shear stiffness kx, ky (kN m-1) in elastic stage can be calculated by Eq.(1) given in Article 6.3.7 in 2008 highway seismic design code. The coefficients of bearing stiffness can be calculated and listed in Table 1. Gd Ar (1) k t
where, Gd is the dynamic Shear Modulus of the laminated rubber bearing, kN m-3, its general value is 200 kN m-3; Ar is the shear area of the rubber bearing, m2; t is thinness of the rubber layer, m. Table 1. Coefficients of bearing stiffness Bearing type
Dx/(kN•m-1)
Dy/(kN•m-1)
Dz/(kN•m-1)
Rx/(kN•m•rad-1)
Ry/(kN•m•rad-1)
Rz/(kN•m•rad-1)
GYZ250
1429
1429
108
0.01
0.01
0.01
1077
1077
108
0.01
0.01
0.01
GYZF4200
42 37
If table footnotes should be used, place footnotes to tables below the table body and indicate them with superscript lowercase letters. Be sparing in the use of tables and ensure that the data presented in tables do not duplicate results described elsewhere in the article. In this paper, the yield strength of bearing is equal to the critical sliding fraction force, its value can be calculated by Eq.(2) as following: (2) Fmax=µdF where, µd is a coefficient of sliding friction, its value for the common laminated rubber bearing varies from 0.1 to 0.2, the value of coefficient in this paper is 0.15, and its value of the Tetrafluoroethylene laminated rubber bearing is 0.02. F is the load of bearing caused by superstructure weight, kN. The lumped mass model is used to study on the interaction influence between pile and soil. The soil springs in transverse and longitudinal directions along pile height are assumed. The stiffness of soil springs can be made by the m method given in the reference [5]. The calculated results are listed in Table 2. According to the foregoing parameters, the finite element program sap2000 is used to analyze the effect of near-fault earthquake on the response of continuous girder bridge. The beam element is simulated the superstructure, pier and pile foundation. The spring element is simulated the laminated rubber bearing. The finite element model of whole bridge is created and showed in Fig 1. At First, the natural vibration characteristics of whole bridge are obtained by modal analysis. The first three vibration modes are shown in Fig 2. The first 22 modes are calculated and used to the response spectrum analysis for typical continuous girder bridge, which ensure that the mass contribution factors of cumulative mode in various directions are larger than 0.9. 3. Near-fault ground motions and artificial waves According to the II type site for this typical bridge, three representative near-fault strong earthquake waves recorded in US earthquake event, three near-fault waves recorded in Wenchuan earthquake and three artificial waves produced by
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trigonometric series method were selected and used to analyze the seismic response of continuous girder bridge near fault zone. All of waves are used in dynamic time-historical analysis and listed in Table 3. Table 2. Stiffness of soil spring for pile foundation
Soil spring Number
1# pier
2# pier
Soil layer depth /m
Horizontal stiffness in transverse and longitudinal direction /(kN m-1)
Soil layer depth /m
Horizontal stiffness in transverse and longitudinal direction /(kN•m-1)
1
0.48
11610
0.58
47959
2
1.3
193050
1
72000
3
2.2
1899563
1.3
355388
4
2.02
4708620
2.2
2586375
5
2
6471000
2
5652000
6
2
7452000
7
2
9252000
8
1.92
10575360
Fig.1. Finite element model of whole bridge.
(a)
(b)
(c)
Fig.2. Three vibrating modes for continuous girder bridge for (a) First mode: lognitudinal translation (b) Second mode: transverse translation (c) Third mode: rotation along the Z-axis.
Three design spectrum curves were used to calculate the seismic response of continuous bridge. The first design acceleration spectrum is given by the seismic safety assessment report about the bridge site. The second one is obtained from the design acceleration spectrum for II type site in China existing highway bridge design code [6]. The third one is the near-fault design spectrum for II type site proposed by researchers (much information in reference [3-4]). Three design spectrum curves are showed in Fig 3. 4. Results of earthquake response analysis 4.1. Results of nonlinear time history analysis Nine near-fault waves shown in Table 3 are used to analyze the seismic performance of FEM for bridge based on nonlinear dynamic time-history method. The peak values of near-fault waves are adjusted to 0.262g according to the design ground motion corresponding to the first level (EL1) of the safety assessment report for bridge site before the beginning of calculation. Results of time-history analysis for bridge in axial direction are listed in Table 4. By comparison and analysis, we can found the fact that, all indexes of seismic response for bridge under US wave 1 (B-PTS225) and US wave 2 (WPI046) are obviously superior to that of the others near-fault waves because of the large pulse included in the US waves. In table 4, for ease to compare with design values, we gave the mean of seismic responses for 9 waves and 7 waves with the exclusion US
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wave 1 and 2, respectively. The calculated results shows that the bearing, pier top and pier bottom were still in the elastic state, but the relative displacement values between pier and beam are relatively large. All bearings for pier 1# and 2# have been destroyed because the horizontal displacements of all bearings are larger than the rubber layer total thickness of rubber bearing (3.0cm). Table 3. Near-fault earthquake records and artificial waves Items Artificial wave 1 Artificial wave 2 Artificial wave 3 US wave 1 US wave 2 US wave 3
Earthquake events VIII Intensity, site
type
Superstitn Hills(B)
Bridge site
ys1p1g
0.274
64.85
Bridge site
ys1p2g
0.277
71.89
Bridge site
ys1p3g
0.269
73.68
0.455
22.32
0.455
24.96
0.644
79.86
0.433
193.54
0.530
222.10
0.841
159.98
5051 Parachute Test Site
Northridge
90056 Newhall
1994/01/17 12:31
W. Pico Canyon Rd.
1989/10/18 00:05
Earthquake magnitude
Records
1987/11/24 13:16
Loma Prieta
Fault distance/km
Station
B-PTS225 WPI046
57007 Corralitos
CLS000
0.7
6.7
7.1
6.7
5.1
6.9
Wenchuan wave1
Wenchuan2008/05/12
051GYZ
Guangyuan zengjia
36.4
8.0
Wenchuan wave 2
Wenchuan 2008/05/12
051JYH
Jiangyou hanzeng
24.8
8.0
Wenchuan wave 3
Wenchuan 2008/05/12
051MZQ
Mianzhu qingping
0.7
8.0
PGA /g
Duration /s
Fig.3. Selected design response spectrum.
Pier No.1 as an example, the time-history response curves for typical bridge in Axial Direction exacted by nine records are shown in Fig.4-8. Fig 4, Fig 5, Fig 6, Fig 7 and Fig 8 are the time-history curve of top displacement, relative displacement between pier and beam, bearing shear force, bearing hysteretic curve and bottom moment of pier No.1, respectively. 4.2. Results of response spectrum analysis For comparing with the results of dynamic time-history method, the response spectrum method is adopted to analyze the seismic response of bridge. The design spectrum is shown in Fig 3. According to the safety assessment report for bridge site close to Yishu fault zone, the near-fault effect has been considered for the given design spectrum, and the design peak ground acceleration is equal to 0.262g corresponding to EL1. The design acceleration spectrum advised in China existing highway bridge seismic code is mainly used for far field earthquake, and the design peak ground acceleration is 0.2g corresponding to E1 level when the seismic intensity at the bridge site is 8 degree. The near-fault design spectrum proposed
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by authors considered the near-fault factors is constructed based on 338 actual earthquake records. Research results for response spectrum analysis are listed in Table 5. We can draw some conclusions: the responses calculated by the bridge site spectrum, which is close to those values obtained by means of the spectrum proposed by author, are larger than that of the mean for 7 waves and the maximum for 3 artificial waves; The values achieved from the design spectrum according to existing highway bridge seismic design code are similar equal to the mean of 7 waves, but smaller than the maximum value for 3 artificial waves and the mean value for 9 waves because the existing seismic code is not constructed based on the nearfault earthquake. Table 4. Peak value of Seismic response for each index
Analysis cases
Top displacement of pier /cm
Relative displacement between pier and beam /cm
Bearing shear force /kN
Bottom moment of pier /kN m
1#
2#
1#
2#
1#
2#
1#
2#
Artificial wave 1
1.040
0.659
6.509
6.828
45.32
46.57
1008
659.2
Artificial wave 2
1.034
0.682
5.045
5.433
45.67
46.03
1011
678.8
Artificial wave 3
1.091
0.698
4.978
5.331
44.78
45.11
1061
696.4
US wave 1
1.305
0.866
19.49
19.81
47.11
48.37
1257
853.6
US wave 2
1.340
0.721
26.53
26.88
44.44
44.07
1282
710.9
US wave 3
0.879
0.533
3.512
3.607
44.36
43.29
869.2
538.9
Wenchuan wave 1
0.803
0.470
2.315
2.486
43.28
44.38
788.4
481.1
Wenchuan wave 2
0.548
0.731
1.043
1.389
24.24
31.41
517.3
690.6
Wenchuan wave 3
0.923
0.630
2.642
2.926
39.27
43.72
919.0
638.3
Maximum response of artificial wave
1.09
0.70
6.51
6.83
45.32
46.57
1061
696.4
Mean value of 9 records response
0.996
0.666
8.007
8.299
42.05
43.66
968.10
660.86
Mean value of 7 records response
0.903
0.629
3.721
4.000
40.98
42.93
881.98
626.18
(b)
(a)
(c)
Fig. 4. Time history of top displacement of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 5. Time history of relative displacement between pier and beam of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
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(a)
(b)
(c)
Fig. 6. Time history of bearing shear force of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 7. Bearing hysteretic curve of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 8. Time history of bottom moment of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3. Table 5. Peak value of seismic response for EL1 Top displacement of pier /cm
Relative displacement between pier and beam /cm
Bearing shear force /kN
Bottom moment of pier /kN m
1#
2#
1#
2#
1#
2#
1#
2#
Spectrum given by safety assessment (0.262g)
1.70
1.06
6.71
7.28
70.0
75.9
1504
997
Spectrum advised by author (0.2g)
1.66
1.03
6.91
7.51
72.1
78.3
1481
978.7
2008 code Spectrum (0.2g)
1.01
0.63
3.93
4.26
41.0
44.4
896.5
594.9
Maximum response of artificial wave
1.09
0.70
6.51
6.83
45.32
46.57
1061
696.4
Mean value of 9 records response
1.00
0.67
8.01
8.30
42.05
43.66
968.1
660.9
Mean value of 7 records response
0.90
0.63
3.72
4.00
40.99
42.93
882.0
626.2
Analysis cases
5. Conclusions The response characteristics of continuous girder bridge located in near-fault zone are studied. The nonlinear dynamic time history analysis and response spectrum analysis for FEM are used to calculate the seismic response of bridge Through
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comparing with the design parameters given in existing highway bridge design code, the safety performance of continuous girder bridge with the first design level exacted by near-fault strong ground motion is studied, and some conclusions can be drawn as following: (1) The results of dynamic time-history method show that the seismic response values of bridge exacted by near-fault waves with large pulse (B-PTS225 and WPI046) are significantly larger than the ones obtained by records with no pulse. The seismic dynamic response for continuous bridge was significantly enhanced by near-fault pulse effect. (2) The results calculated by response spectrum considered the near-fault effect are larger than the ones of time-history method. On the contrary, the seismic responses obtained by design spectrum according existing highway bridge design code without near-fault effect are smaller than the values of time-history method. Acknowledgements Funding for this project provided by the independent scientific research grant No. DC120101093, is gratefully acknowledged. References [1] Benioff H., 1955. Mechanism and strain characteristics of the White Wolf Fault as indicated by the aftershock sequence, earthquakes in Kern County, California, During 1955. Califronia Division of Mines Bulletin, No.171. [2] WANG Haiyun, XIE Lili, 2006. Characteristics of near-fault strong earthquake ground motions. Journal of Harbin Institute of Technology, 38(12), p. 2070-2072. [3] LI Xinle, 2005. Study on design earthquake and seismic performance of bridge in near-fault area. Beijing Jiaotong University, p. 130-153. [4] SHEN Dan, 2011. Research of Several Key Problems on Seismic Design of RC Girder Bridges Near Active Fault. Beijing Jiaotong University, p. 37-52. [6] Ministry of Communications People’s Republic of China, 2007. JTG D63-2007 Code for design of ground base and foundations of highway bridges and culverts. People's Communications Publishing House. [7] Ministry of Communications People’s Republic of China, 2008. JTG/T B 02-01-2008 Guidelines for seismic design of highway bridges. People's Communications Publishing House.
ProcediaEngineering Engineering 00 Procedia 45(2012) (2012)000–000 916 – 922
2012 International Symposium on Safety Science and Technology
Study on seismic safety performance for continuous girder bridge based on near-fault strong ground motions LI Xinlea, JIANG Huib, SHEN Danc,* a
School of Civil Engineering and Architecture, Dalian Nationalities University, Dalian 116600, China b School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China c China Railway Engineering Consulting Group Co., Beijing 100055, China
Abstract In order to explore the safety performance of continuous structure bridge near fault zone, a typical three spans continuous girder bridge of highway was selected to study the structural seismic response. The finite element model of whole bridge considered the force characteristics of bearing and pile-soil interaction was constructed. Typical near-fault records were selected from the important earthquake events. Several artificial waves characterized with the soil type in bridge site were simulated and used for dynamic analysis. Comparing with the seismic response of bridge under three given acceleration design spectrum, the seismic performance of continuous bridge was studied by nonlinear dynamic time-history method. Research results indicates that, the seismic design spectrum considering near-fault effect, especially large amplitude pulse effect of near-fault records, will significantly enhance the seismic response of continuous girder bridge. The fact that the near-fault effect is not considered in China existing highway bridge seismic design code will increase the destructive risk of structure.
© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Beijing Institute of Technology. Keywords: near-fault ground motions; continuous girder bridge; dynamic time-history method; response spectrum analysis; seismic response
1. Introduction The first research results of near fault earthquake effect was described by Benioff (1955) [1] based on the Kern County earthquake event occurred in Califonia. More and more research results for near-fault earthquake effect has been published[2-3]. The significant damage is often found in near-fault zone (also called as near field, fault distance R 36km) during several earthquake events occurred in 80's of 20st century (1994 US Northridge, 1999 Taiwan Chi-Chi) and China Wenchuan earthquake event. Because of the influence of forward directivity effect and fault mechanism, the remarkable characteristics of near-fault records with long-period pulse and directivity, which is different from far field, will enhance the seismic response (internal force and displacement) of structures. With the development of economy of China, more and more common continuous girder highway bridges will be designed and constructed near fault zone, which are characterized with the feather of wide adaptability and simple technology. The safety performance for this kind of bridge must be deeply studied to reduce the damage of near-fault earthquake. A typical continuous girder bridge with pier-pile foundation near fault was selected and studied in this paper. Based on the mechanical characteristics of bearing and soil around pile, a finite element model (FEM) for whole bridge considered the effect of pier-pile interaction was constructed to calculate the seismic response by using of the method of nonlinear dynamic time-history and response spectrum. All actual near-fault records are characterized with near-fault earthquake
* Corresponding author. Tel.: +86-411-87656208; fax: +86-411-87656208. E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.259
917
LI LI Xinle et al. / Procedia Engineering – 922 Xinle et al. / Procedia Engineering45 00(2012) (2012) 916 000–000
effect, and artificial waves in the site condition of bridge are generated. Three acceleration response spectrums for II type site are made in article. Through analyzing the results of response, the safety performance of continuous girder bridge exacted by near-fault strong ground motions is evaluated to enhance the seismic design level of further similar bridge. 2. Typical continuous girder bridge and its FEM The typical prestressed concrete hollow slab continuous bridge is a three-span medium span bridge with equal spans of 16m each near Yishu fault zone in China [4]. The double-column pier-pile foundation is adopted in pier design. The diameter of each pier is 1.3m. The heights of pier No.1 and No.2 are 3.17m and 2.03m, respectively. The diameter of bored pile foundation is 1.5m. The lengths of for No.1 and No.2 bored pile are 8m and 10m respectively. The design peak ground acceleration of earthquake ground motion is 0.15-0.20g, and the seismic defense intensity is equal to 8. The girder and pier are joined by some laminated rubber bearings. The tetrafluoroethylene GYZF4 type and common GYZ type laminated rubber bearing are placed on the top of abutment and pier. The ideal elastic-plastic hysteretic model for bearings is assumed in paper, and the horizontal shear stiffness kx, ky (kN m-1) in elastic stage can be calculated by Eq.(1) given in Article 6.3.7 in 2008 highway seismic design code. The coefficients of bearing stiffness can be calculated and listed in Table 1. Gd Ar (1) k t
where, Gd is the dynamic Shear Modulus of the laminated rubber bearing, kN m-3, its general value is 200 kN m-3; Ar is the shear area of the rubber bearing, m2; t is thinness of the rubber layer, m. Table 1. Coefficients of bearing stiffness Bearing type
Dx/(kN•m-1)
Dy/(kN•m-1)
Dz/(kN•m-1)
Rx/(kN•m•rad-1)
Ry/(kN•m•rad-1)
Rz/(kN•m•rad-1)
GYZ250
1429
1429
108
0.01
0.01
0.01
1077
1077
108
0.01
0.01
0.01
GYZF4200
42 37
If table footnotes should be used, place footnotes to tables below the table body and indicate them with superscript lowercase letters. Be sparing in the use of tables and ensure that the data presented in tables do not duplicate results described elsewhere in the article. In this paper, the yield strength of bearing is equal to the critical sliding fraction force, its value can be calculated by Eq.(2) as following: (2) Fmax=µdF where, µd is a coefficient of sliding friction, its value for the common laminated rubber bearing varies from 0.1 to 0.2, the value of coefficient in this paper is 0.15, and its value of the Tetrafluoroethylene laminated rubber bearing is 0.02. F is the load of bearing caused by superstructure weight, kN. The lumped mass model is used to study on the interaction influence between pile and soil. The soil springs in transverse and longitudinal directions along pile height are assumed. The stiffness of soil springs can be made by the m method given in the reference [5]. The calculated results are listed in Table 2. According to the foregoing parameters, the finite element program sap2000 is used to analyze the effect of near-fault earthquake on the response of continuous girder bridge. The beam element is simulated the superstructure, pier and pile foundation. The spring element is simulated the laminated rubber bearing. The finite element model of whole bridge is created and showed in Fig 1. At First, the natural vibration characteristics of whole bridge are obtained by modal analysis. The first three vibration modes are shown in Fig 2. The first 22 modes are calculated and used to the response spectrum analysis for typical continuous girder bridge, which ensure that the mass contribution factors of cumulative mode in various directions are larger than 0.9. 3. Near-fault ground motions and artificial waves According to the II type site for this typical bridge, three representative near-fault strong earthquake waves recorded in US earthquake event, three near-fault waves recorded in Wenchuan earthquake and three artificial waves produced by
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LI Xinle et /al. / Procedia Engineering 45 (2012) 916 – 922 LI Xinle et al. Procedia Engineering 00 (2012) 000–000
trigonometric series method were selected and used to analyze the seismic response of continuous girder bridge near fault zone. All of waves are used in dynamic time-historical analysis and listed in Table 3. Table 2. Stiffness of soil spring for pile foundation
Soil spring Number
1# pier
2# pier
Soil layer depth /m
Horizontal stiffness in transverse and longitudinal direction /(kN m-1)
Soil layer depth /m
Horizontal stiffness in transverse and longitudinal direction /(kN•m-1)
1
0.48
11610
0.58
47959
2
1.3
193050
1
72000
3
2.2
1899563
1.3
355388
4
2.02
4708620
2.2
2586375
5
2
6471000
2
5652000
6
2
7452000
7
2
9252000
8
1.92
10575360
Fig.1. Finite element model of whole bridge.
(a)
(b)
(c)
Fig.2. Three vibrating modes for continuous girder bridge for (a) First mode: lognitudinal translation (b) Second mode: transverse translation (c) Third mode: rotation along the Z-axis.
Three design spectrum curves were used to calculate the seismic response of continuous bridge. The first design acceleration spectrum is given by the seismic safety assessment report about the bridge site. The second one is obtained from the design acceleration spectrum for II type site in China existing highway bridge design code [6]. The third one is the near-fault design spectrum for II type site proposed by researchers (much information in reference [3-4]). Three design spectrum curves are showed in Fig 3. 4. Results of earthquake response analysis 4.1. Results of nonlinear time history analysis Nine near-fault waves shown in Table 3 are used to analyze the seismic performance of FEM for bridge based on nonlinear dynamic time-history method. The peak values of near-fault waves are adjusted to 0.262g according to the design ground motion corresponding to the first level (EL1) of the safety assessment report for bridge site before the beginning of calculation. Results of time-history analysis for bridge in axial direction are listed in Table 4. By comparison and analysis, we can found the fact that, all indexes of seismic response for bridge under US wave 1 (B-PTS225) and US wave 2 (WPI046) are obviously superior to that of the others near-fault waves because of the large pulse included in the US waves. In table 4, for ease to compare with design values, we gave the mean of seismic responses for 9 waves and 7 waves with the exclusion US
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wave 1 and 2, respectively. The calculated results shows that the bearing, pier top and pier bottom were still in the elastic state, but the relative displacement values between pier and beam are relatively large. All bearings for pier 1# and 2# have been destroyed because the horizontal displacements of all bearings are larger than the rubber layer total thickness of rubber bearing (3.0cm). Table 3. Near-fault earthquake records and artificial waves Items Artificial wave 1 Artificial wave 2 Artificial wave 3 US wave 1 US wave 2 US wave 3
Earthquake events VIII Intensity, site
type
Superstitn Hills(B)
Bridge site
ys1p1g
0.274
64.85
Bridge site
ys1p2g
0.277
71.89
Bridge site
ys1p3g
0.269
73.68
0.455
22.32
0.455
24.96
0.644
79.86
0.433
193.54
0.530
222.10
0.841
159.98
5051 Parachute Test Site
Northridge
90056 Newhall
1994/01/17 12:31
W. Pico Canyon Rd.
1989/10/18 00:05
Earthquake magnitude
Records
1987/11/24 13:16
Loma Prieta
Fault distance/km
Station
B-PTS225 WPI046
57007 Corralitos
CLS000
0.7
6.7
7.1
6.7
5.1
6.9
Wenchuan wave1
Wenchuan2008/05/12
051GYZ
Guangyuan zengjia
36.4
8.0
Wenchuan wave 2
Wenchuan 2008/05/12
051JYH
Jiangyou hanzeng
24.8
8.0
Wenchuan wave 3
Wenchuan 2008/05/12
051MZQ
Mianzhu qingping
0.7
8.0
PGA /g
Duration /s
Fig.3. Selected design response spectrum.
Pier No.1 as an example, the time-history response curves for typical bridge in Axial Direction exacted by nine records are shown in Fig.4-8. Fig 4, Fig 5, Fig 6, Fig 7 and Fig 8 are the time-history curve of top displacement, relative displacement between pier and beam, bearing shear force, bearing hysteretic curve and bottom moment of pier No.1, respectively. 4.2. Results of response spectrum analysis For comparing with the results of dynamic time-history method, the response spectrum method is adopted to analyze the seismic response of bridge. The design spectrum is shown in Fig 3. According to the safety assessment report for bridge site close to Yishu fault zone, the near-fault effect has been considered for the given design spectrum, and the design peak ground acceleration is equal to 0.262g corresponding to EL1. The design acceleration spectrum advised in China existing highway bridge seismic code is mainly used for far field earthquake, and the design peak ground acceleration is 0.2g corresponding to E1 level when the seismic intensity at the bridge site is 8 degree. The near-fault design spectrum proposed
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by authors considered the near-fault factors is constructed based on 338 actual earthquake records. Research results for response spectrum analysis are listed in Table 5. We can draw some conclusions: the responses calculated by the bridge site spectrum, which is close to those values obtained by means of the spectrum proposed by author, are larger than that of the mean for 7 waves and the maximum for 3 artificial waves; The values achieved from the design spectrum according to existing highway bridge seismic design code are similar equal to the mean of 7 waves, but smaller than the maximum value for 3 artificial waves and the mean value for 9 waves because the existing seismic code is not constructed based on the nearfault earthquake. Table 4. Peak value of Seismic response for each index
Analysis cases
Top displacement of pier /cm
Relative displacement between pier and beam /cm
Bearing shear force /kN
Bottom moment of pier /kN m
1#
2#
1#
2#
1#
2#
1#
2#
Artificial wave 1
1.040
0.659
6.509
6.828
45.32
46.57
1008
659.2
Artificial wave 2
1.034
0.682
5.045
5.433
45.67
46.03
1011
678.8
Artificial wave 3
1.091
0.698
4.978
5.331
44.78
45.11
1061
696.4
US wave 1
1.305
0.866
19.49
19.81
47.11
48.37
1257
853.6
US wave 2
1.340
0.721
26.53
26.88
44.44
44.07
1282
710.9
US wave 3
0.879
0.533
3.512
3.607
44.36
43.29
869.2
538.9
Wenchuan wave 1
0.803
0.470
2.315
2.486
43.28
44.38
788.4
481.1
Wenchuan wave 2
0.548
0.731
1.043
1.389
24.24
31.41
517.3
690.6
Wenchuan wave 3
0.923
0.630
2.642
2.926
39.27
43.72
919.0
638.3
Maximum response of artificial wave
1.09
0.70
6.51
6.83
45.32
46.57
1061
696.4
Mean value of 9 records response
0.996
0.666
8.007
8.299
42.05
43.66
968.10
660.86
Mean value of 7 records response
0.903
0.629
3.721
4.000
40.98
42.93
881.98
626.18
(b)
(a)
(c)
Fig. 4. Time history of top displacement of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 5. Time history of relative displacement between pier and beam of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
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(a)
(b)
(c)
Fig. 6. Time history of bearing shear force of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 7. Bearing hysteretic curve of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3.
(a)
(b)
(c)
Fig. 8. Time history of bottom moment of 1# pier for (a) Artificial wave 3 (b) US wave 2 (c) Wenchuan wave 3. Table 5. Peak value of seismic response for EL1 Top displacement of pier /cm
Relative displacement between pier and beam /cm
Bearing shear force /kN
Bottom moment of pier /kN m
1#
2#
1#
2#
1#
2#
1#
2#
Spectrum given by safety assessment (0.262g)
1.70
1.06
6.71
7.28
70.0
75.9
1504
997
Spectrum advised by author (0.2g)
1.66
1.03
6.91
7.51
72.1
78.3
1481
978.7
2008 code Spectrum (0.2g)
1.01
0.63
3.93
4.26
41.0
44.4
896.5
594.9
Maximum response of artificial wave
1.09
0.70
6.51
6.83
45.32
46.57
1061
696.4
Mean value of 9 records response
1.00
0.67
8.01
8.30
42.05
43.66
968.1
660.9
Mean value of 7 records response
0.90
0.63
3.72
4.00
40.99
42.93
882.0
626.2
Analysis cases
5. Conclusions The response characteristics of continuous girder bridge located in near-fault zone are studied. The nonlinear dynamic time history analysis and response spectrum analysis for FEM are used to calculate the seismic response of bridge Through
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comparing with the design parameters given in existing highway bridge design code, the safety performance of continuous girder bridge with the first design level exacted by near-fault strong ground motion is studied, and some conclusions can be drawn as following: (1) The results of dynamic time-history method show that the seismic response values of bridge exacted by near-fault waves with large pulse (B-PTS225 and WPI046) are significantly larger than the ones obtained by records with no pulse. The seismic dynamic response for continuous bridge was significantly enhanced by near-fault pulse effect. (2) The results calculated by response spectrum considered the near-fault effect are larger than the ones of time-history method. On the contrary, the seismic responses obtained by design spectrum according existing highway bridge design code without near-fault effect are smaller than the values of time-history method. Acknowledgements Funding for this project provided by the independent scientific research grant No. DC120101093, is gratefully acknowledged. References [1] Benioff H., 1955. Mechanism and strain characteristics of the White Wolf Fault as indicated by the aftershock sequence, earthquakes in Kern County, California, During 1955. Califronia Division of Mines Bulletin, No.171. [2] WANG Haiyun, XIE Lili, 2006. Characteristics of near-fault strong earthquake ground motions. Journal of Harbin Institute of Technology, 38(12), p. 2070-2072. [3] LI Xinle, 2005. Study on design earthquake and seismic performance of bridge in near-fault area. Beijing Jiaotong University, p. 130-153. [4] SHEN Dan, 2011. Research of Several Key Problems on Seismic Design of RC Girder Bridges Near Active Fault. Beijing Jiaotong University, p. 37-52. [6] Ministry of Communications People’s Republic of China, 2007. JTG D63-2007 Code for design of ground base and foundations of highway bridges and culverts. People's Communications Publishing House. [7] Ministry of Communications People’s Republic of China, 2008. JTG/T B 02-01-2008 Guidelines for seismic design of highway bridges. People's Communications Publishing House.