近断层混凝土结构连续性地震倒塌机理研究

Near-fault ground motions contain often large velocity and displacement pulses, which result significantly difference in the elastic-plastic behaviors and failure mechanism of the seismic structures from those located in the far field. In this project, the damage and progressive collapse behaviors of reinforced concrete structures located in the near fault region are investigated in order to satisfy the needs that the structures should not collapse during a severe earthquake. First the characteristics of the near-fault ground motions such as forward directivity and fling effect are researched by analysis of the instrumented ground motion data and numerical simulation of near-fault ground motions. Then a plastic hinge model considering the cumulative damage effect of the structure is developed on the basis of the quasi-static cyclic test, and the dynamic behavior of the reinforced conrete structure subjected to the near-fault ground motion is simulated numerically by finite element method, so that the process of the structural response including damage, failure and collapse is evaluated. The influence of the near-fault ground motion to the response of single-degree-of-freedom system is also checked compared with those due to far-fault ground motion. Finally, two progressive collapse mechanisms of the reinforced concrete structures located in the near fault area are investigated, one of them is that the progressive collapse during an earthquake is caused by failures of local members because of the lack of structural integrity and/or redundancy, another by impacts of the falling objects separated from the structures due to the earthquake induced severe vibrations of the structures.

近断层地震动含有明显的速度和位移脉冲成分，对结构的弹塑性行为和失效倒塌破坏性态具有重大影响。本项目紧密围绕"大震不倒"的工程抗震设防目标，针对位于近断层区域的钢筋混凝土结构，系统研究结构发生倒塌及连续倒塌破坏的灾变过程及其机理。首先，通过近断层强震观测数据分析和地震动数值模拟两条途径，认识近断层地面运动特有的速度脉冲等特性；其次，通过反映近断层地震动特性的伪静力实验和钢筋混凝土结构精细化弹塑性地震反应分析，发展基于累积损伤效应的塑性铰模型，建立钢筋混凝土结构动态行为分析模型；再次，通过结构弹塑性地震需求谱分析和结构动态反应数值模拟，研究近断层地震特性对结构非线性反应的影响，认识钢筋混凝土结构在近断层地面运动作用下的损伤、破坏直至倒塌的灾变演化发展全过程；最后，通过结构转换模式分析，研究钢筋混凝土结构两类连续倒塌发生机理，即由于局部倒塌引起的连续倒塌机制和由于坠落物体撞击引起的连续倒塌机制。

震害表明，近断层区域内的混凝土结构往往遭受严重的地震损伤甚至发生倒塌。“大震不倒”是世界各国普遍施行的抗震设防目标之一，而实现这一目标就必须在设计时准确把握并合理控制结构临近倒塌时的行为和性态，其前提是正确认识结构地震倒塌的机理。本项目通过研究近断层地面运动特性及其对混凝土结构弹塑性地震反应、动力损伤累积效应、失效破坏行为的影响，试图揭示近断层混凝土结构连续性倒塌的机理。本项目取得的主要研究进展有：. (1) 提出了一种高精度的结构动力时程分析方法。利用结构动力反应的Duhamel积分解析解，逆用微分求积原理（Differential Quadrature，DQ）构造一种求解卷积的数值格式。该方法保持了DQ法精度高、计算工作量小的优点，并且经证明具有无条件稳定特性。这种方法还被进一步推广于时空域解法方面，建立了一套时空DC分析方法。. (2) 发展了一种改进拆除构件法用于结构连续性地震倒塌机制分析。该方法在结构损伤分析基础上实施构件拆除作业，分析不同成分地震作用对存在损伤的剩余结构体系动力响应的影响，可以清晰地辨识结构体系内力重分布过程及其连续倒塌机制。. (3) 发展了一种基于尺寸效应的混凝土墩柱损伤评估方法。在混凝土墩柱损伤评估公式中引入墩柱高宽比指标，考虑墩柱轴压比和约束条件影响。通过88组混凝土墩柱伪静力试验数据和数值模拟结果验证了该修正后的损伤评估方法具有更高的准确性和通用性。. (4) 选择汶川地震中位于近断层区域的2个实际震害开展详细的案例研究。其中江油市新华书店综合楼案例主要研究混凝土框架与填充墙相互作用影响，百花大桥案例主要探讨结构倒塌破坏机理。这两个案例分别体现了混凝土结构连续倒塌的两种不同机理，前一个案例为损伤体系内力重分布引起的失效破坏，而后一个案例则为关键承载构件受到外力冲击引起的倒塌破坏。