무리 말뚝 효과가 터빈-발전기 기초의 동적 응답에 미치는 영향

Abstract

The Turbine-Generator Foundation is critical in the design of power station structures and the design process is very complicated. It needs to consider static analysis on basic fundamentals as well as dynamic analysis regarding mechanical vibration. Many power stations are located in soft ground along coastlines or near medium or large sized rivers; consequently, they are generally supported on pile foundations. Piles are usually installed in groups, and application of an equivalent spring constant and damping ratio is desirable to take into account the pile group effect. However, no firm theoretical grounds are currently established with regard to movement of pile groups during vibration; in addition, the proposed guidelines differ slightly for each code. As a result, the actual application of pile groups in a vibrating foundation often depends on expert judgment. The aim of the present study was to examine the dynamic response of a turbine generator vibrating foundation using single piles or pile groups under the constant load condition. The pile group efficiency is also compared following changes in pile diameter and spacing. The results showed that the natural frequency generally decreased and maximum response displacement increased following application of pile groups, with diminishing maximum response rate. The decrease in natural frequency can be interpreted as a failsafe in design since the frame-form vibrating foundation has structural features of a low-tuned foundation. The vertical maximum response displacement of the foundation on pile groups increased about 45 ~ 60 % over design value of the single pile foundation when the pile spacing was shorter than four times the pile diameter; this should be considered for cases using single pile design. The differences of dynamic response value based on 5D pile spacing dramatically decreased between single piles and pile groups, implying insignificant impacts of pile group effect. The pile group efficiency changes very little in response to the slenderness ratio.