模板支撐架縮小模型之自然頻率量測及理論分析

Measurement and Analysis of the Natural Frequencies of Small Scale Models of Formwork Supporting Frames

模板工程之倒塌通常會造成嚴重之生命財產損失，因此有必要建立一套倒塌預警系統。本文之目的在於建立一實用簡便之指標，以提供倒塌預警之用。模板工程之倒塌，多半係因為支撐剛架勁度的不足，在承受載重的狀態下，造成桿 件的挫屈，而引起全面性的崩塌。影響支撐剛架的因素包括：幾何缺陷、側位移、半剛性接頭、無支撐長度、邊界條件等。由於上述之支撐架較一般構架之水平勁度為低而且贅度也小。因此在加載後，由於勁度衰減，常引致自然振動頻率有明顯之下降。本文中以實驗量測佐以理論分析，對鷹架縮小模型受載重時，其自然振動頻率之變化來做探討，並以此振動頻率作為監測指標，提供預警訊號。本研究建議，當鷹架加載下之自然頻率平方達未加載自然頻率平方之百分之七十五時，即應發出警告訊號來停止加載。此一結論尚得經由實尺寸鷹架實驗來確定，方可應用到工地之鷹架系統。

The collapse of a formwork usually causes a lot of loss on life and property. Therefore, it is necessary to develop a precaution system against collapse, which is the purpose of this study. The collapse of a formwork is often attributed to the lack of sufficient stiffness of the supporting frame. One typical example for this is that the buckling of some members of a supporting frame under certain loading conditions, which may result in the collapse of the entire framework. The buckling strength of the supporting frame is affected by factors such as the geometrical imperfection, sidesways, semi-rigid joints, unrestrained length, and support conditions. For this kind of supporting frame, its horizontal stiffness is usually small compared with regular frames and also its redundancy is small. Therefore, its natural frequencies decrease significantly under increasing applied loads because of the degradation of its stiffness. In this study, experiments in conjunction with numerical analysis are carried out to trace the change in the natural frequencies of small scale scaffold systems under increasing loads. By monitoring the frequency changes, warning messages can be issued as the collapse conditions are approached. It is suggested from this study that when the square of the natural frequency of the loaded scaffold systems drops to 75 percent of the square of the unloaded natural frequency, warning messages should be issued. The conclusion needs to be confirmed by experiments on full scale scaffold systems before its application to in situ scaffold systems.