汶川地震庙子坪特大桥主桥箱梁开裂震害分析

2008年汶川地震中庙子坪特大桥主桥箱梁开裂严重，并在两个端部（过渡墩处）有较大的横向残余位移。为深入了解主梁开裂震害原因，选取桥址附近台站实测的八条强震动记录及常用El Centro等波作为输入，考虑箱梁施工过程及初始应力影响，通过时程分析进行数值模拟并与实际震害比较。结果表明:不同地震波输入下得到的箱梁高应力区与实际开裂区吻合较好，预示箱梁开裂震害不属于偶然现象。箱梁在主墩处、边跨和中跨合龙段区域处的顶板，边跨1/6~1/2及中跨1/4~3/4区域处的腹板，边跨和中跨合龙段及其相邻2~3个节段处的底板，在地震中易出现高拉或压应力区。地震过程中估计纵向预应力筋应力增加接近100MPa，横桥向挡块破坏降低了跨中箱梁的拉和压应力，对箱梁开裂震害有一定减轻作用。

Seismic damage analysis of box girder cracking of the Miaoziping bridge in Wenchuan earthquake

The box girder of Miaoziping bridge cracked seriously and had large lateral residual displacements at the two ends of box girder (the transition pier) in 2008 Wenchuan earthquake. In order to further understand the damage causes of main girder cracking, eight strong motions recorded near the bridge site and the general used El Centro record are selected as the inputs for time history analysis of the bridge. The construction process and initial stress of the box girder are considered for seismic numerical simulation. The results of numerical simulation are compared with the actual damage in the earthquake. The results show that the high stress zones of the box girder are agreement with seismic damage under various inputs of seismic records. These results indicate that the cracking damage of the box girder is not an individual phenomenon. In strong earthquakes, it is prone to reach high tensile stress state and high compressive stress state in the zones, such as the roof of the box girder near the main pier and the closure section of the side span and the middle span, the web near the side span 1/6 to 1/2 and the middle span 1/4 to 3/4, the bottom of the box girder near the closure section and the adjacent 2-3 sections of the side span and the middle span. During the earthquake, the estimated stress of the longitudinal prestressed tendons increases to nearly 100 MPa and the damage of the lateral shear keys can reduce the tensile and compressive stress of the girder’ sections among the center spans of the bridge, which has some benefits for reducing the girder cracking damage during strong earthquake.