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LRB曲线桥震致碰撞效应的非线性分析方法
作者: 柳国环,李宏男,陆新征
 

 

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LRB曲线桥震致碰撞效应的非线性分析方法
第34卷第5 期《土木建筑与环境工程》Vol.34 No.5
2012 年10 月

摘要:发展了一种适用于体元模拟联间轴向正面碰撞效应的束缚面型碰撞单元(Constraint-surface Impact Element,CIE);基于柔度法给出了不同以往文献中碰撞单元刚度的计算取值,该取值不仅理论合理且在物理角度可解释其合理性。将该方法应用于广州潮汕机场航站楼高架桥的震致碰撞分析。箱梁采用壳单元模拟,伸缩缝位置设有CIE,桥墩与箱梁间设置隔震单元模拟铅芯橡胶隔振支座(Lead-rubber Bearing,LRB),采用通过试验得到的双线性刚度模型模拟其物理行为。采用非线性直接积分法进行时程分析,主要考察联间的碰撞力、LRB滞回耗能、LRB位移量、基底弯矩、剪力以及碰撞力分别对这些因素的影响。
关键词:桥梁;地震;碰撞;束缚面碰撞单元;隔振支座
中图分类号:P315.0; U443.31    文献标识码:A     文章编号:1674-4764(2012)05-0017-08

Abstract:A nonlinear constraint-surface impact element(CIE)was developed for simulating the pounding in axial direction between 3Dshell-element-mesh bridge girders based on the engineering background ofChaoShan airport.Then,a method,different from the previous literatures,for calculating the stiffness ofimpact element was proposed based on the flexibility method.The methods proposed were adopted andused to analyze the elevated bridge of ChaoShan airport in GuangZhou for the purpose of investigating comprehensively the earthquake-resistance capability.The bridge girders were modeled by shell elementsand CIEs were assigned to the positions of the corresponding expansion joints.Isolating elements were usedto simulate LRBs,and typical bilinear LRB hysteresis stiffness model obtained from experiment wereadopted to describe the dynamic hysteresis behavior.Pounding forces,dissipation capability of LRBs,lateral displacements of LRBs,pier base shear forces and moments as well as the pounding effect on thesecases were investigated,respectively.The proposed methods and analysis results aim atimproving theearthquake resistance of the elevated bridge,which provides reference for the similar project.
Key words:bridges;earthquakes;pounding;constraint-surface impact element;bearing


引言
由于伸缩缝的存在,地震激励下相邻联间相对位移一旦大于伸缩缝宽度,则会产生相互碰撞[1-3]。碰撞不仅可能会致使梁体移位过大而塌落,严重的还会导致桥梁发生连续性倒塌。通常可以通过增大伸缩缝来降低碰撞发生的概率,然而过大的伸缩缝会影响路面对平整度的要求。在满足规范的前提下,一种常被工程界采纳的措施为:在桥台与梁体间设置隔震支座,这样可减小地震对梁体输入的能量。2008年10月1日中国新颁布实施的《桥梁抗震设计细则》[4]增加了减隔震桥梁的有关内容。不应否认:隔震后,输入到梁体的地震能量可以得到有效控制。但是,由于隔震支座的引入会使得梁体“变”柔而周期增大,这时梁体更易产生滑移,从而增加了联间的碰撞概率。为此,在概念设计阶段需要考虑伸缩缝处相邻联的自振特性要尽量相近,以充分保证它们在地震作用下的反应具有同步性,从而减小碰撞发生的概率。对于梁高相等且无纵坡的直线型桥而言,在初步设计阶段采用该方法把握结构概念会取得明显效果。但对于曲率不同且有坡道的多联曲线高架桥,尤其是在考虑多维地震动[5]作用下,问题则相对复杂。因此,对这类结构难以用某种指标和
(或)概念加以把握,需要具体问题具体分析。建立精细的有限元模型,设置合理的碰撞参数进行数值计算,不失为分析碰撞问题的一种经济且有效之途径。
新建潮汕机场航站楼高架桥全长约940m,其中752m的桥梁结构由曲率不同共9联C50混凝土箱型梁组成,相邻两联桥梁结构交接处和桥台位置设置一道BEJ型浅埋式伸缩缝;工程位于8度地震区,以该工程为背景,考虑设置铅芯橡胶支座(Leader Rubber Bearing,LRB)。采用SAP2000建立全桥空间有限元模型,由25 294个壳单元、140梁单元以及56个接触非线性单元组成。文献[3]在桥梁空间非线性碰撞方面作了重要的推进性工作,但是将其方法通过大型有限元程序实现并应用于实际桥梁工程目前还存在较大困难。
为了便于工程应用,首先发展了一种可适用于体元(例如:壳单元和实体单元)模拟联间碰撞效应的束缚面型碰撞单元(Constraint-surface Impact
Element,CIE),给出了不同以往文献中碰撞单元刚度的合理计算取值,并将该方法应用于工程实际。主要考察了联间的碰撞力、LRB滞回耗能、LRB位移量、基底弯矩、基底弯矩剪力以及碰撞力分别对这些因素的影响。但应该说明,文中只考虑了相邻联桥的轴向正面碰撞,未涉及到非轴向相对运动产生的摩擦等因素。

结论
以广州潮汕机场航站楼为工程背景,考虑概念设计,满足新颁布的《桥梁抗震设计细则》相关要求,兼顾数值模型的有效性,主要作了如下工作:
1)发展了一种多跨桥联间正面碰撞的束缚面型碰撞单元,该单元是对只适用于梁单元间碰撞模拟的点点式碰撞单元的发展与一般化。其优点在于:可以考虑采用体元(壳单元、实体单元)模拟联间碰撞效应的同时,避免了传统方法中采用刚臂连接切向多桥墩的简化方法,同时可将切向各桥墩上的非线性单元(例如:LRB)直接与梁体和桥台相连接,从而使得数值模拟更能够接近物理模型。
2)给出了不同以往文献的碰撞单元刚度计算取值公式。与以往取值方法或建议相比,不仅可满足物理条件且更具有全面性。
3)将CIE单元模拟方法通过SAP2000实现并应用于机场航站楼抗震分析,通过碰撞力反应曲线分析,方法是可行有效的。
4)与考虑震致碰撞效应因素相比,不考虑碰撞效应时LRB的滞回耗能会被低估。同时,通过反复数值计算而最终选用的LRB低于试验得到的极限位移250mm,检验了耗能元件的安全性。
5)从总体来看,震致碰撞对结构整体和各联桥墩基底弯矩和剪力会产生不利影响且不容忽视。但同时还应看到,碰撞效应对不同联桥桥墩影响程度不同,甚至不考虑碰撞会更加保守,这与结构以及各联桥的几何位置有关。

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