利用振動頻率判別橋墩基礎裸露之可行性研究

Abstract

利用量測橋體之振動頻率判別橋墩基礎裸露或沖刷之想法常被提及；本研究之目的即為利用現地實驗及數值模擬探討該想法之可行性。利用廢棄之宜蘭牛鬥橋進行現地實驗，考慮一橋墩基礎裸露四米對橋梁振動頻率之影響。透過ARV(Auto-Regressive Vector)模式及ARX(Auto regressive model)模式，分別分析微動及自由振動量測反應，識別整個橋體及橋墩局部自然振動頻率。如所預期，基礎裸露對橋墩局部自然振動頻率有較明顯之影響，但對整體橋梁之自然振動頻率無明顯之影響。為進一步探討橋樑型式(單跨、兩跨及三跨連續) 、土壤性質及裸露程度對該局部自然振動頻率之影響，本研究利用SAP2000進行模擬分析。

It is a common approach to identify the exposure of a bridge pier foundation via the measured vibration frequencies. The main purpose of the work is to investigate the feasibility of this approach in a real application through situ tests and numerical simulations. The situ tests including ambient vibrations tests and impulse tests are carried out at Nioudou Bridge in Ilan County. An excavation up to four-meter deep is performed around a pier foundation of the bridge to study the effect of the scour depth around a pier foundation on the vibration frequencies of the bridge. The ARV(Auto-Regressive Vector) and ARX (Auto-Regressive with Exogenous Input) models are employed to process the measured velocity responses from ambient vibration tests and impulse tests, respectively, and to estimate the dynamic characteristics of the bridge. It is found that the exposure of a pier foundation can yield a significant decrease in vibration frequencies of local modes, but not for the global modes of the bridge. Finite element analyses are further carried out using SAP2000 to investigate the effects of the scour depth around a pier foundation on vibration frequencies of two-span continuous and three-span continuous bridges with different soil properties.