台灣長跨懸臂節塊施工預力混凝土橋梁長期變形研究

Research on Long-Term Deflection of Large-Span Prestressed Concrete Segmental Bridges Built by Cantilever Method in Taiwan

王詠寬(Yung-Kuan Wang)

指導教授 ： 陳振川

國立臺灣大學/工學院/土木工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802868

摘要

近年世界各地老舊橋梁，特別是懸臂節塊施工的預力混凝土箱型橋梁，皆面臨一嚴重的問題，橋梁中間鉸接處長期監測所得的垂直變位遠超過初始設計所預估的範圍；對於新型橋梁以剛性相接(rigid connection)造成的靜不定結構系統，也因為無法明確掌握混凝土潛變與收縮的影響，而無法掌握其各處變位與內應力的傳遞。
透過監測資料發現，橋梁長期變位發展的趨勢並不會逐漸趨緩達到穩定，而是在對數時間座標下朝著一固定的斜率不斷發展。超量的變位會產生接合鉸接處垂直變位太大，鉸接處轉角斜率差大，造成沈陷處對車輛的撞擊，因而導致橋樑服務性、安全性的疑慮，最終可能造成使用年限縮短，需提早進行補強或拆除重建，對政府及社會都會造成重大的負擔。
造成橋梁長期變位的原因，主要可歸因於混凝土的潛變、箱型梁斷面上的差異收縮與差異潛變、以及因潛變與收縮所引致之預力損失。目前各國混凝土潛變與收縮規範的預測公式，在預估數十年的混凝土變形趨勢上普遍皆有低估，而台灣本土的混凝土材料特性亦與國外有所不同，不宜直接採用國外的預測公式。此外，工程界多採用二維梁元素模型分析，無法真實反映斷面因上下版與腹版厚度變化而造成乾燥潛變與乾燥收縮速率不同的現象。
故本研究套用國內外不同的混凝土材料潛變與收縮預測公式，包含ACI 209R、CEB MC10、GL2000、Model B4與Model B4-TW公式，並以國道一號圓山橋案例，考量斷面上因幾何尺度差異所造成收縮與潛變速率不同的現象，建立三維有限元素模型分析，評估其鉸接處之垂直變位以及預力損失的狀況。
透過分析結果與監測資料的比對，於材料面探討各預測公式的差異性以及於結構面討論其推廣至全尺度結構分析可能造成的影響，發展出一套適合台灣的混凝土橋梁長期變形分析模式。除了對現有老舊橋梁安全性的評估，亦可提供未來補強、除役或替代方案規劃之參考；對於新建橋樑設計而言，可以建立一套完整分析混凝土橋梁長期變形與評估使用年限的方法，使工程界能更明確掌握台灣混凝土橋梁長期變位趨勢以及其預力損失，並利於安全監測與評估，適時進行對應的補強措施以避免災難發生，在設計與維護階段方能延長混凝土結構物的生命週期，達到永續性的目的。

關鍵字

混凝土 ； 長期變形 ； 潛變 ； 收縮 ； 預力損失 ； 有限元素法

並列摘要

In recent years, the old bridges around the world, especially the prestressed concrete box bridges constructed by the cantilever method, are facing a serious problem. The vertical displacement obtained by long-term monitoring of the central hinge of the bridge far exceeds the range estimated by the initial design. The indeterminate structural system caused by the rigid connection of the new type of bridge is also unable to grasp the influence of concrete creep and shrinkage, and cannot grasp the deflection and internal stress in the bridge.
Through monitoring data, it is found that the trend of long-term displacement of bridges will not gradually slow down and stabilize, but will continue to develop toward a fixed slope under logarithmic time coordinates. Excessive deflection will cause the vertical displacement of the central hinge and the difference in the slope of the hinge angle to be too large, causing the impact of the vehicle due to the subsidence, thus causing concerns about the serviceability and safety of the bridge, which may eventually shorten the service life. The requirements for earlier retrofit or eventual demolition of bridge will impose a major burden on the government and society.
The reasons for the long-term deflection of the bridge are mainly attributable to the creep of the concrete, the differential shrinkage and differential creep of the box girder section, and the loss of prestress caused by creep and shrinkage. At present, the prediction formulas for concrete creep and shrinkage specifications in various countries are generally underestimated in the trend of concrete deformation for several decades. The characteristics of concrete materials in Taiwan are also different from those in foreign countries. It is not appropriate to directly adopt foreign prediction formulas. In addition, the local engineering community mostly uses the two-dimensional beam element model analysis, which cannot truly reflect the phenomenon that the drying creep and the drying shrinkage rates are different due to the thickness variation of the top and bottom plates and the webs.
Therefore, this study applies different creep and shrinkage prediction formulas for concrete, including ACI 209R, CEB MC10, GL2000, Model B4 and locally developed Model B4-TW formulas, and Yuan-Shan Bridge of the national highway No. 1 is used for study. A three-dimensional finite element model, which can take account of the phenomenon of different shrinkage and creep rate caused by geometrical scale differences, was developed to evaluate the vertical displacement of the location at central hinge and the loss of prestressed force.
Through the comparison between the analysis results and the monitoring data, the differences between the prediction formulas and the influence of the extension from the materials to the real structure are discussed, a long-term deformation analysis model for concrete bridges suitable for Taiwan is developed. In addition to the assessment of the safety of existing old bridges, it can also provide reference for future retrofit, decommissioning or alternative planning for bridges. For new bridge design, a complete method for analyzing the long-term deformation and assessment of the useful service life of concrete bridges was established. The method will enable the engineering community to more clearly grasp the long-term displacement trend of Taiwan's concrete bridges and its prestressed losses, and to facilitate safety monitoring and evaluation, and timely corresponding strengthening measures to avoid disasters, and to extend the service life of concrete structure during the design and maintenance phase to achieve the sustainability.

並列關鍵字

concrete ； long-term deflection ； creep ； shrinkage ； prestress loss ； finite element method

參考文獻

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