Identification of Modal Characteristics of Shinkansen RC Viaducts Using Laser Doppler Vibrometers

In this paper, a new measurement system using Laser Doppler Vibrometers (LDV) for monitoring Shinkansen RC viaducts will be presented. The LDV with its high accuracy, frequency resolution, and scanning capability, is an ideal non-contact tool for monitoring large structures by ambient vibration measurement. However, the large stiffness of the viaducts cause small ambient vibrations compared with surrounding noise, requiring the need for data compensation. This is accomplished by deducting noise measured by a velocimeter attached to the LDV from the measured ambient vibration data. Additionally, in order to determine the local mode shapes of the columns an impact load using a wooden hammer was used to excite the local vibration modes. The processing of the data to determine the modal characteristics of viaducts will also be discussed. The processing includes noise and angle compensation, synchronization of measured vibrations, and the determination of natural frequencies and mode shapes using the peak-picking method. Analysis of ambient vibration data along the transverse direction result in global mode natural frequencies of 2.83 Hz, 2.93 Hz and 3.03 Hz. It was also observed that these global mode shapes derived from modal analysis are not pure torsional and lateral modes but a combination of the two. These results suggest the presence of dynamic interaction between adjoining viaducts because of the continuity of the rail tracks connecting the individual viaducts. On the other hand, column natural frequencies of 62 Hz and 153 Hz were determined for the first and second local column modes, respectively. The local mode shapes of a regular column compared with a retrofitted column will also be presented. INTRODUCTION The Shinkansen RC viaducts, shown in Figure1, are simple frame structures which support the railway and other electrical equipment needed by the Shinkansen during operation. Because the Shinkansen has proven to be a safe and reliable form of transportation for over 40 years now, there are several thousands of these RC viaducts across Japan, connecting major cities and prefectures. Thus, the Shinkansen transport system is very important to the socio-economic life of the country. However, the Shinkansen RC viaducts are deteriorating due to age and are subjected to increased service loads. Also, these viaducts have been repeatedly subjected to strong earthquakes that initiate minor damage or increase existing damage. Dept of Civil Eng, Univ. of Tokyo, jaime@bridge.t.u-tokyo.ac.jp, phouthaphone@geot.t.u-tokyo.ac.jp, fujino@civil.t.u-tokyo.ac.jp; 2 Nagaoka Univ. of Technology, mtakeshi@vos.nagaokaut.ac.jp: 3 Yokogawa Bridge Corp. h.ishii@yokogawa-bridge.co.jp