考虑SSI效应的近断层区桥梁抗震性能研究
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摘要
摘要：随着近断层强震记录的不断增多,使得全面揭示近断层强地震动的特点成为可能。近断层地震动显著地不同于远离断层的地震动,其上盘效应,破裂方向效应和"Fling step"效应以及由后两种效应引起的不同长周期脉冲以及由破裂传播和辐射图导致的垂直断层走向的地震动大于平行断层走向的地震动是近断层地震动的重要特点。梁式桥作为我国公路桥梁中普遍采用的结构形式,其抗震性能是
一个值得关注的课题。
参照本文所研究的近断层区桥梁所在桥址的《安评报告》,选出符合近场特征的三条美国地震记录、三条中国汶川记录,以及根据《安评报告》拟合出的3条人工波,首先采用精细的非线性时程分析方法,考虑地基柔性效应,对公路桥的抗震性能进行研究。
土—结相互作用问题一直是桥梁结构抗震研究的难点,FEMA440中考虑土结构相互作用的能力谱法为此问题的解决提供了一条有效的途径,可以考虑运动学效应和地基阻尼效应的简化计算方法以及改进的等效线性化方法。本文中,首先建立了考虑地基柔性效应的公路梁式桥模型,对其作推倒分析得到结构的基底剪力—墩顶位移曲线,将其转化为结构的能力谱曲线；然后分别采用FEMA440所提出的考虑运动学效应和地基阻尼效应的方法对地震需求谱进行折减,通过能力谱曲线和需求谱曲线求解结构在相应地震作用下的抗震性能,并与非线性动力时程分析结果进行了比较。本文主要研究内容如下：
1.地震动记录的收集及选取
广泛收集国内外近断层地震记录资料,尤其是2008年我国汶川地震中靠近活动断层的强震记录；根据近断层的特性从中选取3条美国记录、3条汶川记录、3条人工地震波作为非线性地震分析的需求输入。
2.建立桥梁模型
考虑地震作用下土一结构相互作用的地基柔性效应,采用桩基土弹簧来模拟桩土相互作用,考虑支座动力性能,运用有限元软件SAP2000建立桥梁有限元模型,
3.对已建模型进行抗震性能研究
(1)考虑基础柔性效应非线性时程分析
以所选取的汶川及美国部分地震记录和模拟的人工地震记录作为输入,分为EL1和EL2两个水准(分别对应PGA为0.262g和0.351g)对全桥进行非线性时程分析,对比研究两个水准下全桥的墩顶残余位移、墩底塑性铰、墩梁相对变形、伸缩缝变形等桥梁地震响应指标,并结合《公路桥梁抗震性能评定细则》《公路桥梁抗震设计细则》(JTG/TB02-01--2008)、对所得结果进行两水准下的抗震验算。验算结果表明,在两个水准下,除了个别支座外其余指标均能满足要求
(2)考虑桩—土相互作用三种效应的pushover分析
对所建立的模型作推倒分析得到结构的基底剪力—墩顶位移曲线,将其转化为结构的能力谱曲线；然后,分别采用FEMA440的考虑运动学效应和地基阻尼效应对结构地震需求谱进行折减,将桥梁结构的能力谱曲线和需求谱曲线绘制在同一坐标系下进而研究结构在相应地震作用下的抗震性能,并与非线性动力时程分析结果进行比较,对比结果显示FEMA440考虑桩—土相互作用的三种效应能更有利于桥梁结构的抗震分析。
Along with the increasing of earthquake records near active fault, promulgating the characteristics of strong ground motion near active fault comprehensively becomes possible. Seismic motion near active fault is obviously different from that far away from the fault. Its hanging wall effect, rupture directive effect and Fling step effect are the important characteristics of ground motion near active fault. The girder-bridge is used as the structural style generally in highway bridge of our country and its seismic performance is the topic which is worth paying attention to.
Referencing this article's research of《Safety evaluation Report》where bridge site in near active fault zone, three US earthquake records and three Chinese Wenchuan records conformed to the characteristic of near active fault were selected, three artificial records which according to《Safety Evaluation Report》were selected as well.Use the method of fine nonlinear time history analysis, considering the flexible effect of ground and carry on the research of highway bridge's seismic performance.
The Soil-Structure-Interaction is always the difficult point of seismic study of bridge construction. The capacity spectrum method considering the SSI in FEMA440 provides an effective way to solve this problem, so the simplified calculation method of kinetic effect and foundation damping effect and the improved equivalent linearity method can be considered. In this paper, firstly a highway girder bridge model considering the foundation flexibility effect was built, and through pushover analysis the curve of base shear and displacement at the top of pier was obtained, and turn it to capacity spectrum curve. Applying the method that considering kinetic effect and foundation damping effect in FEMA440, the reduced structure seismic demand spectrum was developed, and through capacity spectrum curve and demand spectrum curve the structure seismic behavior under corresponding action of earthquake can be solved, and also compared with the nonlinear time-history analysis result. The main work of this paper is as follows:
1. Collection and selection of earthquake record
The materials of earthquake record near active fault were collected in worldwide widely, particularly the strong Wenchuan earthquake records near active fault in our country in 2008; according to the characteristic of near active fault, three American records, three Wenchuan records,and three artificial earthquake records as the demanded input of nonlinear seismic analysis were selected.
2. Establishment of bridge model
Considering the ground's flexible effect of soil-structure interaction under earthquake,using the earth springs to simulate the pile-soil interaction,and considering the power performance of supports, the finite element model of bridge was established,by finite element software SAP2000
3. seismic performance research to the model established
(1) Nonlinear time history analysis considering flexible foundation effects
Using Wenchuan earthquake records、the US earthquake records and the atificial earthquake records as the earthquake input, dividing into two standards which are ELI and the EL2 (respectively corresponding PGA of 0.262g and 0.351g), the time history analysis to the entire bridge was carried on. the response characteristic and security of the displacement at the top of pier,the plastic hinge at the bottom of the pier, relative deformation between pier and beam,and deformation of expansion joint was searched, the obtained results unifying《08 Guidelines》under two standards were checked. The checking calculation result indicats that the other targets can satisfy the request except some supports under two standards.
(2)Pushover analysis considering three effects of SSI
Pushover analysis was put to the model established and the curve of base shear and displacement at the top of pier was obtained and transformed as the capacity spectrum curve;Then separately the method that considering kinetic effect and foundation damping effect in FEMA440 was used for the reduction of the structure seismic demand spectrum, and through capacity spectrum curve and demand spectrum curve,the structure seismic behavior under corresponding action of earthquake can be solved, and also compared with the nonlinear time-history analysis result. The contrast result show that FEMA440 is more advantageous to the earthquake resistance analysis.
一个值得关注的课题。
参照本文所研究的近断层区桥梁所在桥址的《安评报告》,选出符合近场特征的三条美国地震记录、三条中国汶川记录,以及根据《安评报告》拟合出的3条人工波,首先采用精细的非线性时程分析方法,考虑地基柔性效应,对公路桥的抗震性能进行研究。
土—结相互作用问题一直是桥梁结构抗震研究的难点,FEMA440中考虑土结构相互作用的能力谱法为此问题的解决提供了一条有效的途径,可以考虑运动学效应和地基阻尼效应的简化计算方法以及改进的等效线性化方法。本文中,首先建立了考虑地基柔性效应的公路梁式桥模型,对其作推倒分析得到结构的基底剪力—墩顶位移曲线,将其转化为结构的能力谱曲线；然后分别采用FEMA440所提出的考虑运动学效应和地基阻尼效应的方法对地震需求谱进行折减,通过能力谱曲线和需求谱曲线求解结构在相应地震作用下的抗震性能,并与非线性动力时程分析结果进行了比较。本文主要研究内容如下：
1.地震动记录的收集及选取
广泛收集国内外近断层地震记录资料,尤其是2008年我国汶川地震中靠近活动断层的强震记录；根据近断层的特性从中选取3条美国记录、3条汶川记录、3条人工地震波作为非线性地震分析的需求输入。
2.建立桥梁模型
考虑地震作用下土一结构相互作用的地基柔性效应,采用桩基土弹簧来模拟桩土相互作用,考虑支座动力性能,运用有限元软件SAP2000建立桥梁有限元模型,
3.对已建模型进行抗震性能研究
(1)考虑基础柔性效应非线性时程分析
以所选取的汶川及美国部分地震记录和模拟的人工地震记录作为输入,分为EL1和EL2两个水准(分别对应PGA为0.262g和0.351g)对全桥进行非线性时程分析,对比研究两个水准下全桥的墩顶残余位移、墩底塑性铰、墩梁相对变形、伸缩缝变形等桥梁地震响应指标,并结合《公路桥梁抗震性能评定细则》《公路桥梁抗震设计细则》(JTG/TB02-01--2008)、对所得结果进行两水准下的抗震验算。验算结果表明,在两个水准下,除了个别支座外其余指标均能满足要求
(2)考虑桩—土相互作用三种效应的pushover分析
对所建立的模型作推倒分析得到结构的基底剪力—墩顶位移曲线,将其转化为结构的能力谱曲线；然后,分别采用FEMA440的考虑运动学效应和地基阻尼效应对结构地震需求谱进行折减,将桥梁结构的能力谱曲线和需求谱曲线绘制在同一坐标系下进而研究结构在相应地震作用下的抗震性能,并与非线性动力时程分析结果进行比较,对比结果显示FEMA440考虑桩—土相互作用的三种效应能更有利于桥梁结构的抗震分析。
Along with the increasing of earthquake records near active fault, promulgating the characteristics of strong ground motion near active fault comprehensively becomes possible. Seismic motion near active fault is obviously different from that far away from the fault. Its hanging wall effect, rupture directive effect and Fling step effect are the important characteristics of ground motion near active fault. The girder-bridge is used as the structural style generally in highway bridge of our country and its seismic performance is the topic which is worth paying attention to.
Referencing this article's research of《Safety evaluation Report》where bridge site in near active fault zone, three US earthquake records and three Chinese Wenchuan records conformed to the characteristic of near active fault were selected, three artificial records which according to《Safety Evaluation Report》were selected as well.Use the method of fine nonlinear time history analysis, considering the flexible effect of ground and carry on the research of highway bridge's seismic performance.
The Soil-Structure-Interaction is always the difficult point of seismic study of bridge construction. The capacity spectrum method considering the SSI in FEMA440 provides an effective way to solve this problem, so the simplified calculation method of kinetic effect and foundation damping effect and the improved equivalent linearity method can be considered. In this paper, firstly a highway girder bridge model considering the foundation flexibility effect was built, and through pushover analysis the curve of base shear and displacement at the top of pier was obtained, and turn it to capacity spectrum curve. Applying the method that considering kinetic effect and foundation damping effect in FEMA440, the reduced structure seismic demand spectrum was developed, and through capacity spectrum curve and demand spectrum curve the structure seismic behavior under corresponding action of earthquake can be solved, and also compared with the nonlinear time-history analysis result. The main work of this paper is as follows:
1. Collection and selection of earthquake record
The materials of earthquake record near active fault were collected in worldwide widely, particularly the strong Wenchuan earthquake records near active fault in our country in 2008; according to the characteristic of near active fault, three American records, three Wenchuan records,and three artificial earthquake records as the demanded input of nonlinear seismic analysis were selected.
2. Establishment of bridge model
Considering the ground's flexible effect of soil-structure interaction under earthquake,using the earth springs to simulate the pile-soil interaction,and considering the power performance of supports, the finite element model of bridge was established,by finite element software SAP2000
3. seismic performance research to the model established
(1) Nonlinear time history analysis considering flexible foundation effects
Using Wenchuan earthquake records、the US earthquake records and the atificial earthquake records as the earthquake input, dividing into two standards which are ELI and the EL2 (respectively corresponding PGA of 0.262g and 0.351g), the time history analysis to the entire bridge was carried on. the response characteristic and security of the displacement at the top of pier,the plastic hinge at the bottom of the pier, relative deformation between pier and beam,and deformation of expansion joint was searched, the obtained results unifying《08 Guidelines》under two standards were checked. The checking calculation result indicats that the other targets can satisfy the request except some supports under two standards.
(2)Pushover analysis considering three effects of SSI
Pushover analysis was put to the model established and the curve of base shear and displacement at the top of pier was obtained and transformed as the capacity spectrum curve;Then separately the method that considering kinetic effect and foundation damping effect in FEMA440 was used for the reduction of the structure seismic demand spectrum, and through capacity spectrum curve and demand spectrum curve,the structure seismic behavior under corresponding action of earthquake can be solved, and also compared with the nonlinear time-history analysis result. The contrast result show that FEMA440 is more advantageous to the earthquake resistance analysis.
引文
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[5]王云海,谢礼立.近断层强地震动的特点[J].哈尔滨工业大学学报,2006,38(12)：2071～2074.
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[2]周国良,崔成臣.汶川8.0级地震中儿座近断层桥梁失效模式的探讨[J].震灾防御技术,2008,3(4)：370～371.
[3]张鹏,陈新民.近断层地震动与汶川地震灾区滑坡破坏特征分析[J].南京工业大学学报,2009,31(1)：55～57.
[4]雷涛,李碧雄.汶川地震近断层附近桥梁震害浅析[J].重庆交通大学学报,2010,29(3)：358～359.
[5]王云海,谢礼立.近断层强地震动的特点[J].哈尔滨工业大学学报,2006,38(12)：2071～2074.
[6]胡人礼.桥梁抗震设计[M].北京：中国铁道出版社,1984.
[7]范立础.桥梁抗震[M].上海：同济大学出版社,1997.
[8]刘启方,袁一凡,金星.近断层地震动的基本特征[J].地震工程与工程振动,2009,26(1)：1～3.
[9]胡进军.近断层地震动方向性效应及超剪切破裂研究[J].国际地震动态,2009,373(1)：40～42.
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