單柱式圓形鋼管橋墩之耐震徑厚比限制探討

Abstract

橋墩為橋梁之韌性構件，當橋梁遭逢極端地震時，可藉塑鉸消能降低橋梁整體結構之損壞。因此，使其展現足夠強度對於橋梁之耐震設計極為重要。鋼材具有高強度與高延展性，為橋墩材料之優良選擇，然而我國相關耐震規範中卻對鋼橋墩設計著墨甚少，使實務設計上無明確可遵循之依據，僅能參考他國耐震設計流程進行設計，惟各國耐震設計規範之精神及性能要求不盡相同，應不可直接引用與參考。此外，過往鋼橋墩之相關研究多以其非線性行為作為主要方向，探討斷面性質、加載方式等因素對其韌性容量或極限強度之影響。然而，對於真實地震下之鋼橋墩，地震強度與其引致之結構反應均具不確定性，除利用擬靜態或非線性歷時分析了解其耐震性能外，亦應利用機率式分析，掌握鋼橋墩於震後之損傷程度與風險，以完整評估及量化其耐震能力。 有鑑於此，本研究以圓形空心斷面單柱式鋼橋墩為研究對象，探討不同徑厚比、長細比、軸壓比等關鍵參數對韌性容量及破壞風險之影響。首先以擬靜態分析求取鋼橋墩之韌性容量，並以此定義後續非線性歷時分析中之破壞基準。而鋼橋墩之破壞行為分兩部分進行討論，其一，假設鋼橋墩工址位於臺北二區，以易損性分析結果配合該區之地震危害度，分析圓形單柱式鋼橋墩於臺北二區之50年破壞風險；其二，透過假設臺北山腳斷層之地震情境，利用符合該情境之近斷層地震歷時進行非線性歷時分析，決定鋼橋墩於此情境下之破壞機率。最後，依據韌性容量與破壞行為分析之結果，對圓形斷面單柱式鋼橋墩之斷面性質提出耐震設計建議。

Bridge piers, which are ductile components of bridges, serve as the primary structural members and need to exhibit sufficient energy dissipation under earthquakes. Due to the high ductility and extraordinary strength-to-weight ratio, steel is an excellent material for bridge piers. However, there is little emphasis on the compactness requirements for steel bridge piers in Taiwan seismic design codes. Structural engineers can only refer to seismic design guidelines from other countries, but the difference in the design objectives makes them potentially inappropriate for Taiwan. In addition, most of the previous studies on steel bridge piers focus on their nonlinear behavior, investigating the effects of sectional properties and loading protocols on their ductility and ultimate strength. Given the uncertainties in the intensity of ground motions and the structural response, probabilistic analysis should be utilized to evaluate the structural damages and seismic performance. To address this shortcoming, this study investigates single-column steel bridge piers with varied seismic compactness and axial load levels. The ductility capacity of each pier is determined by pseudo-static analysis and considered as the failure criteria in the subsequent nonlinear time-history analysis. The failure behavior of steel bridge piers is discussed in two parts: first, using the results of fragility analysis with seismic hazard in the Taipei basin zone II to calculate the 50-year failure risk of steel bridge piers; secondly, the failure probability under near-fault ground motions is evaluated based on the seismic scenario of the Taipei Shanchiao fault. Based on the results of ductility capacity and failure behavior, the seismic design requirements of single-column steel bridge pier compactness are proposed.