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2007年8月1日,美国明尼苏达州明尼阿波利斯市跨越密西西比河的I-35W桥突然破坏。根据该桥的原始设计图、美国国家运输安全局提供的调查资料及桥梁上部结构的设计荷载,ACII高级结构工程师S.Hao对桥梁进行了分析,笔者主要介绍他的分析方法和结果。调查和分析的结果是:1)节点板设计厚度和上弦杆边壁厚度与利用平面分析得出的弯矩成正比。这说明原始设计是根据一维模型进行的设计,没有充分考虑斜桁架对主桁内力的影响;2)虽然该桥各桁架杆件都设计合理,但连接底板桁架和主桁架的节点板设计得不合理,不能有效分配并传递荷载。结果,在该处由桁架单元提供的冗余度明显减少;3)基于设计荷载的三维非线性有限元计算显示,桥在承受设计荷载时一些节点板已经达到屈服极限。由于节点板为韧性材料,其极限强度提供给该桥一定的安全余度。
On August 1, 2007, the I-35W Bridge across the Mississippi River in Minneapolis, Minnesota, suddenly collapsed. According to the original design of the bridge, the survey data provided by the National Transportation Safety Administration and the design load of the superstructure of the bridge, S. Haao, ACII senior structural engineer, conducted an analysis of the bridge. The author mainly introduces his analysis methods and results. The results of the investigation and analysis are as follows: 1) The design thickness of the nodal plate and the thickness of the winding wall are directly proportional to the bending moment obtained by the plane analysis. This shows that the original design is based on the design of a one-dimensional model and does not fully consider the influence of inclined truss on the internal force of the main truss. 2) Although the truss members of the bridge are designed rationally, the joint plate connecting the floor truss and the main truss is designed Unreasonable, can not effectively distribute and transmit the load. As a result, the redundancy provided by the truss element is significantly reduced here; 3) Three-dimensional nonlinear finite element calculations based on design loads show that some bridge plates have reached the yield limit when subjected to design loads. Because the nodal board is a tough material, its ultimate strength gives the bridge a certain degree of safety margin.