可提离式群桩基础抗震性能与损伤控制研究

Pile-group foundations have been the most common type of foundations for bridges in China. However, they usually have lower lateral bearing capacity. The new design code issued after the Wenchuan Earthquake sets a higher fortification criterion for bridge foundations than ever, which introduces great challenge in the seismic design of pile foundations. In this work, a new type pile-group foundation which has separable connections between the pile and the pile cap and therefore allows uplifting of the pile cap during earthquake events is presented. Major concerns include the mechanical behavior, the potential impact effect and subsequently the damage at the connection joint, and the overall effectiveness in seismic control. In addition to numerical method, pseudo-static test and shake table test on scaled models would also be undertaken where the effect of soil would be considered. Based on the testing results, the hysteretic constitution and the model for the impact evaluation at the connection joints would be developed and verified, as well as the major model parameters. In this way, a rational approach for the seismic analysis of the new foundation can be established. Furthermore, sensitivity studies would be conducted for typical bridge models with the new type of foundation, and the resilient ability, the stability against turnover and the structural vulnerability would be investigated and characterized to determine the overall seismic effectiveness of the new foundation.

群桩基础是我国桥梁工程中最常见的基础类型，但其抗震能力相对较弱。汶川地震以后，新修订的桥梁抗震设计规范提高了基础的设防要求，给桩基础的抗震设计带来一定的困难。本项目基于扩大基础考虑提离减震方面的相关成果，提出了可提离式桩基础的解决方案。针对结构受力特点，重点围绕提离减震机理、关键节点碰撞效应与损伤控制、以及结构减震效能三个主题开展研究。结合数值理论分析，重点开展有土体参与条件下的拟静力模型试验和振动台模型试验，研究提离式桩基础的合理滞回本构和连接节点碰撞效应分析模型，并确定模型参数的合理取值，进而建立提离式桩基础的抗震性能合理力学分析模型。在此基础上，结合典型桥式，分析提离式桩基础对结构的自复位性能、动力倾覆稳定性以及地震易损性等方面的影响，评估提离式群桩基础的抗震效能。

目前，结构抗震性能设计正由抗震减震向震后可恢复转变，发展低损伤、自恢复的新型结构抗震体系是未来结构的重要发展方向。群桩基础是桥梁工程中最常见的基础类型，但抗震能力相对较弱，2008汶川地震以后，新修订的桥梁抗震设计规范提高了基础的设防要求，给桩基础的抗震设计带来一定的挑战。为此，本项目提出了可提离式群桩基础的新型结构体系，既可以形成摇摆减震机制，同时也是一种理想的低损伤、自恢复结构体系。开展大比例可提离式群桩基础单墩桥梁结构模型和普通桩基础桥梁模型的拟静力试验和振动台试验，结果表明：可提离式群桩基础桥梁具有明显的旗帜形滞回特性，下部结构地震力降幅达50%以上，上部结构加速度降幅达30%，震后残余位移降幅达80%以上，结构具备低阻尼、低损伤、自恢复的特性。在此基础上，结合数值有限元分析，建立并验证了可提离式群桩基础的合理静动力分析模型。基于Abaqus精细化有限元模型，结合振动台模型试验结构，评估了可提离式桩基础桩头与承台关键节点碰撞效应，提出降低桩头与承台关键节点碰撞损伤的控制措施，改进了桩头与承台关键连接节点构造设计。结果显示：可提离式桩基础在强震作用下的实际提离位移一般不超过6cm，节点碰撞对结构整体损伤无明显影响，局部碰撞损伤可通过增加缓冲垫层加以避免。依托上海嘉闵高架桥标准段桥梁，通过广泛的参数分析，系统评估了不同墩高、不同地震动特性、不同基础布置方案以及多维多向地震作用下的可提离式群桩基础桥梁地震响应特点，结果显示，可提离式群桩基础对高低墩状况都可以显著改善桩基的受力，降低桩基配筋，同时容许结构主体保持弹性工作状态，降低结构震后修复需要。