混凝土拱壩之系統識別與結構動力分析

Abstract

本研究旨在建立混凝土拱壩結構之系統識別分析方法，利用強震儀實測之地表加速度及壩體振動反應歷時資料，識別拱壩之振動頻率與阻尼比等結構動力特性，並探討蓄水深度及水庫淤泥深度對於壩體頻率之影響，作為壩體結構安全診斷之依據。國內目前僅翡翠水庫及德基水庫兩座混凝土拱壩，其中以翡翠水庫之地震記錄資料較為完整。本文根據翡翠水庫歷年強震資料之系統識別結果顯示，拱壩之基本自然頻率與原設計之理論預測值相近，且振動頻率有隨著蓄水位高度及淤泥深度增加而降低。本研究之分析結果與羅等人根據1994年地震資料所得之識別分析結果差異不大(經水位影響修正後誤差小於10%)，顯示壩體之振動特性於歷次地震事件—特別是九二一地震後，並無顯著的變化，因此研判翡翠水庫壩體目前應無結構性損傷。此外，本研究亦利用有限元素軟體ANSYS進行翡翠水庫之靜力與動力分析，並與實測之大壩動力特性比較以確認分析模型之合理性，進而檢核壩體在常態下﹙蓄水壓及淤泥土壓﹚及地震時之應力分佈情形。

The objective of this study is to develop a system identification scheme for reinforced concrete arch dams based on site measurements of seismic ground acceleration and dynamic structural responses of the dam. Dynamic characteristics such as natural frequencies and damping ratios are to be identified, and the effects of water storage depth and the accumulated mud depth in the reservoir on natural frequencies of the arch dam investigated. The information will be served as the basis for diagnosis of structural safety of the dam. There are two reinforced concrete arch dams in Taiwan, namely the Fei-Tsui and Techi dams, while only the Fei-Tsui dam is well instrumented with complete sets of seismic records available for system identification analysis. Based on the seismic data of the Fei-Tsui dam over the years, results indicate that the identified natural frequencies and modal shapes of the dam agree very well with the analytical III predictions of the original design. Moreover, the vibration frequency becomes lower as the water storage level or the accumulated mud depth increases. The results, with compensation of storage level effect, are also consistent with those obtained by Loh using the seismic records in 1994, implying no damage of the dam after the earthquakes in the past years, in particular the 1999 Ji-Ji earthquake. In addition, this study also conducts static and dynamic analyses of the Fei-Tsui dam using ANSYS. Dynamic characteristics by the analytical model are compared with those identified from the measured data to verify adequacy of the analytical modeling by ANSYS. The stress distributions in the dam under both the working loads (hydraulic pressure and soil pressure) and earthquake are then investigated.