往复轴力对钢筋混凝土桥墩抗震性能及其损伤机制影响的试验研究

Bridge piers are subjected not only to horizontal loading effects but also to variable axial forces during earthquakes. Variable axial force can affect the seismic behavior and lead to an unintended failure mode. Due to the limitation of test loading system, little has been done experimentally to investigate variable axial loading effects at present. This research project is focused on the seismic behavior of reinforced concrete bridge pier with a circular section under a cyclic axial force. At first, the axial force variation law of bridge pier due to both vertical and horizontal earthquake ground motions is revealed by structural nonlinear response numerical simulation and correlation analysis. And the irregular variable axial force loading pattern of quasistatic test is established. Then, the reinforced concrete model bridge pier quasistatic experimental test is conducted under a variable axial load and a lateral cyclic load to investigate the mechanical properties of bridge pier. The effects of amplitude, frequency and phase changing of variable axial force loading pattern on strength, stiffness, hysteretic energy absorption and plastic deformation are studied. The earthquake-induced damage evolution law is revealed. Finally, the damage index based deformation and accumulative dissipated energy is developed considering cyclic axial loading effects. By introducing damage degration function expressed in terms of damage index, the relationship between strength, stiffness degradation and damage index of component are determined according to test data. Based on above, the accumulative damage constitutive model of bridge pier under cyclic axial force is established.

地震中桥墩除受到水平作用外，其轴力也会发生变化，轴力的变动对桥墩抗震性能会产生较大的影响，导致非预期的破坏模式。目前，由于试验加载系统等条件的限制，考虑动轴力影响的研究还比较欠缺。本项目以往复轴力作用下的钢筋混凝土圆柱形桥墩为研究对象，首先，通过结构非线性反应数值模拟和相关性分析，揭示竖向和水平地震共同作用下桥墩轴力的变化规律，建立拟静力试验中考虑竖向地震作用的非规则动轴力加载制度；其次，开展往复轴力下桥墩拟静力试验，考察动轴力和水平荷载共同作用下桥墩的力学性能，研究动轴力幅值、频率及相位变化对构件承载力、刚度、滞回耗能及塑性变形的影响，揭示往复轴力下桥墩的地震损伤演化规律；最后，建立往复轴力下考虑变形和累积耗能影响的损伤指标，通过引入以损伤指标为参量的损伤退化函数，并基于试验数据，建立构件强度、刚度退化与损伤指标间的函数关系，发展一种可体现往复轴力下钢筋混凝土桥墩累积损伤效应的本构模型。

地震中桥墩除受到水平作用外，其轴力也会发生变化，轴力的变动对桥墩抗震性能会产生较大的影响，导致非预期的破坏模式。目前，由于试验加载系统等条件的限制，考虑动轴力影响的研究还比较欠缺。本项目以往复轴力作用下的钢筋混凝土桥墩为研究对象，通过结构非线性反应数值模拟和相关性分析，揭示了竖向和水平地震动共同作用下桥墩轴力的变化规律，建立了桥墩拟静力试验中动轴力加载制度；采用大型结构构件地震荷载模拟试验设备NJTech-MUST（Multiple Usage Structures Tester）开展了往复轴力下钢筋混凝土圆柱形桥墩拟静力试验，考察了往复轴力和水平荷载共同作用下桥墩的受力性能，揭示了往复轴力下桥墩的地震损伤演化规律；利用纤维模型对试验构件的全受力过程进行数值模拟，扩充不同参数下构件的滞回曲线结果，研究了轴力变化幅值、变化频率及变化相位等因素对构件承载力、刚度、滞回耗能性能及塑性变形能力等特性和损伤机制的影响；以目前广泛应用的钢筋混凝土构件滞回模型为基础，建立了一种可体现往复轴力下钢筋混凝土构件恢复力特性的本构模型。研究结果表明，在水平地震动作用下，轴力和侧移成比例变化，在竖向地震动作用下，轴力以恒定频率变化，加载过程中不考虑频率变化是合理的。轴力的变动导致构件的力学性能呈现非对称性，变化幅度越大，构件性能的非对称性越显著；变化频率为侧移加载频率偶数倍时，构件性能表现出对称性，变化频率为侧移加载频率奇数倍时，构件性能表现出非对称性；相位变化实质上是轴力与侧移对应关系的变化，相位差增加，构件性能非对称性减弱。往复轴力下构件恢复力曲线可以分别采用两个恒定轴力下恢复力曲线的正、负向替代，即分别采用往复轴力位移循环正、负向位移最大时所对应的轴力作为恒定轴力下的恢复力曲线。本项目的研究结果可以为往复轴力和水平荷载复合作用下钢筋混凝土桥墩的抗震分析与设计奠定理论基础，提供技术支撑。