「Final: 最終原因揭曉」美國邁阿密一座剛建成人行橋忽然倒塌國際頭條網

「Final: 最終原因揭曉」美國邁阿密一座剛建成人行橋忽然倒塌

大家先回顧一下之前的新聞：

經過1年7個月的調查，邁阿密人行橋倒塌事件的原因終於有了最後的定論。美國時間2019/10/22事故調查小組(NTSB)開了公眾見面會，公佈了最後的調查結果。最終的報告還需要幾周出來，但是結論已經公佈。

一句話總結就是，設計方計算錯誤導致橋樑倒塌。

公眾見面會新聞稿

具體更多的細節如下：

Bridge design and construction plan errors, and unique bridge characteristics and mechanisms of failure. The uniqueness of designing a concrete truss bridge led to thecircumstances that accounted for the collapse of the pedestrian bridge. The bridge design team made two errors that resulted in the under-design of the nodal area (11/12) that failed, resulting in the collapse. First, the design team underestimated the demand (loads imposed on structural members) that would be acting on the nodal area. The investigation compared postcollapse calculations for the demands on the node with the design calculations. This comparison found that the demand for the node was nearly twice what the design team had calculated. The investigative report discusses how this error was made. Second, the design team also overestimated the capacity of the node to resist shear (horizontal force) where the nodal region (11/12) was connected to the bridge deck. This overestimation was the result of the designer using incorrect loads and load factors in its calculations. These two design errors resulted in a node that lacked the capacity to resist the shear force pushing the node to the end of the bridge. The NTSB recommends improving the discussion of calculating demand loads and capacity resistance in bridge design guides。翻譯/解讀：低估了節點的受力(demand)，只考慮了一半的受力；高估的節點的剪力強度(shear capacity)。這些計算錯誤直接導致了節點的破壞，進而整個橋樑倒塌。
Independent peer review of complex bridge design. Errors in design may occur, but systems should be in place to catch those errors when they do occur. In this case, a firm was hired to independently review the bridge design for errors. However, the review conducted by this firm did not evaluate the nodes of the bridge truss where they connected with the bridge deck and canopy nor did it consider the multiple stages the bridge construction involved. Although the design reviewer recognized that he should have examined the nodes and stages, he indicated that there was not enough budget or time to evaluate those factors. Contributing to this review failure was the reviewing firm’s lack of qualification to do this work. Further, no specific guidelines call for nodes or construction stages to be included in independent bridge design reviews. The NTSB recommends changes to bridge design review procedures to ensure that bridge nodes and construction stages are included in independent design reviews. 翻譯/解讀：第三方獨立審查機構沒有發現設計中的計算錯誤。
Shortcomings in oversight of evaluation of and response to significant observed bridge structure distress prior to collapse. As soon as the bridge had to support its own weight, cracks appeared at the under-designed nodes, particularly node 11/12. Over the next 19 days, the cracks grew until the bridge collapsed. The construction and inspection firms working on the bridge were aware of the cracks and reported the cracks to the design firm, asking for guidance. The engineer of record at the design firm repeatedly indicated that the cracks were of no safety concern. On the day of the collapse, the firms met to discuss a plan by the engineer of record to remediate the cracks. The bridge collapsed as the firms were implementing the remediation plan. In addition, the repair work was conducted without closing the road below the bridge to traffic. The NTSB recommends changes to Florida bridge construction oversight procedures to emphasize the need for bridge closure to protect public safety when structural cracking (beyond what sound engineering judgment considers acceptable) occurs and to increase state oversight of complex bridge construction. 翻譯/解讀：現場應對處理不善，當發現混凝土的裂縫，並嘗試現場維修的時候，沒有關閉交通，致使橋樑倒塌時，很多行駛的車輛被砸在橋底。
Lack of redundancy guidelines in specifications for pedestrian and concrete truss bridges. The design of the pedestrian bridge did not include redundancy in the bridge load path. As a result, when the 11/12 nodal region failed, the bridge collapsed. The design firm incorrectly believed that the bridge had a redundant design. For typical bridge designs, a bridge designer would use a safety factor greater than one to ensure that the bridge was overdesigned to prevent a collapse. The NTSB, recognizing that no design guidance exists discussing redundancy in concrete truss bridges, recommends that bridge design guides include a discussion of redundancy in concrete bridge designs. 翻譯/解讀：設計方錯誤的認為設計中包含了冗餘度(redundancy)，而事實是，橋樑沒有任何冗餘度，一個節點破壞，不能重新形成新的受力機制，直接倒塌。當然現有的規範中對這類橋也沒有硬性的冗餘度要求。

蔡子解讀：