Ambient Vibration Study of the Gi-Lu Cable-Stay Bridge: Application of Wireless Sensing Units Kung-Chun Lu 1 , Yang Wang 2 , J. P. Lynch 3 , C. H. Loh 1 Yen-Jiun Chen 1 , P. Y. Lin 4 , Z. K. Lee 4 1 Department of Civil Engineering, National Taiwan University, Taipei, Taiwan 2 Dept. of Civil & Environmental Engineering, Stanford University, Stanford, USA 3 Dept. of Civil & Environmental Engineering, University of Michigan, Ann Arbor, USA 4 National Center for Research on Earthquake Engineering, Taipei, Taiwan ABSTRACT An extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of a 240 meter cable-stayed bridge – Gi-Lu Bridge in Nan-Tou County, Taiwan. A MEMS-based wireless sensor system and a traditional microcomputer-based system were used to collect and analyze ambient vibration data. A total of four bridge modal frequencies and associated mode shapes were identified for cables and the deck structure within the frequency range of 0~2Hz. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies. Most of the deck modes were found to be associated with the cable modes, implying a considerable interaction between the deck and cables. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using three-dimensional finite element modeling of the bridge. For most modes, the analytical and the experimental modal frequencies and mode shapes compare quite well. Based on the findings of this study, a linear elastic finite element model for deck structures and beam element with P-Delta effect for the cables appear to be capable of capturing much of the complex dynamic behavior of the bridge with good accuracy. Keywords: Input unknown identification, Stochastic system identification, Ambient vibration, Wireless sensors 1. INTRODUCTION One of the engineering challenges of cable-supported bridges is in understanding and allowing for the dynamic response to effects of traffic, wind and earthquakes. Investigation of both aerodynamic stability and earthquake response of cable-stayed bridges are dependent on the knowledge of the structure’s dynamic characteristics, such as modal frequencies, mode shapes and modal damping values, as well as a description of the dynamic loading. Conducting full- scale dynamic tests is one of the most reliable ways of assessing the actual dynamic properties of these structures. Such tests serve to complement and enhance the development of analytical techniques and models that may be applied to dynamic analysis. During the past two decades, many researchers have conducted full-scale dynamic tests on suspension bridges, however, there is less information available on full-scale dynamic testing of cable-stayed bridges. Some typical examples of full-scale dynamic tests on bridges are provided in the references [1~4]. A simpler method for the determination of dynamic characteristics of structures is through ambient vibration measurements. The ambient vibration behavior of a structure is recorded, evaluated and interpreted under ambient influence, i.e. without artificial excitation, by means of highly sensitive velocity or acceleration sensors. The rapid development of measuring technology, on the one hand, and computer technology including software, on the other, enables us to carry out dynamic measurement of ambient structural vibrations and evaluation quickly. The dynamic characteristics which were extracted from the vibration signals are not only used for a single check of calculation models, but also for statements on the chronological development of the load-bearing capacity. The use of wireless communication for structural health monitoring (SHM) data acquisition was illustrated by Straser and Kiremidjian [5]. Recently, Lynch et al. extended the work by embedding damage identification algorithm into wireless sensing units [6, 7]. With the rapid advancement of sensing, microprocessor, and wireless technologies, it is possible to assess the benefits from the application of such technologies in the structural engineering field. The