Output-only modal identification of a cable-stayed bridge using wireless monitoring systems

Abstract

The objective of this paper is to present two modal identification methods that extract dynamic characteristics from output-only data sets collected by a low-cost and rapid-to-deploy wireless structural monitoring system installed upon a long-span cable-stayed bridge. Specifically, an extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of the 240 m Gi-Lu cable-stayed bridge located in Nantou County, Taiwan. Two different output-only identification methods are used to analyze the set of ambient vibration data: the stochastic subspace identification method (SSI) and the frequency domain decomposition method (FDD). A total of 10 modal frequencies and their associated mode shapes are identified from the dynamic interaction between the bridge’s cables and deck vibrations within the frequency range of 0–7 Hz. The majority of the modal frequencies observed from recording cable vibrations are also found to be associated with the deck vibrations, implying considerable interaction between the deck and cables.

Introduction

A major engineering challenge associated with cable-supported bridges is complete characterization of the dynamic response of the bridge when loaded by traffic, wind and earthquakes. Accurate analysis of both the aerodynamic stability and the earthquake response of cable-stayed bridges often requires knowledge of the structure’s dynamic characteristics, including modal frequencies, mode shapes and modal damping ratios. Conducting full-scale dynamic testing is regarded as one of the most reliable experimental methods available for assessing actual dynamic properties of these complex bridge structures [1]. Such tests serve to complement and enhance the development of analytical techniques and models that are integral to analysis of the structure over its operational life. During the past two decades, many researchers have conducted full-scale dynamic tests on suspension bridges including forced-vibration testing; however, there is comparatively less information available on full-scale dynamic testing of cable-stayed bridges. Typical examples of full-scale dynamic tests on bridges are provided in the References [1], [2], [3], [4].

A simpler method for the determination of the dynamic characteristics of structures is through the use of ambient vibration measurements. In output-only characterization, the ambient response of a structure is recorded during ambient influence (i.e. without artificial excitation) by means of highly-sensitive velocity or acceleration sensing transducers. The concurrent development of novel sensing technologies (e.g., MEMS sensors, wireless sensors) and high-speed computing and communication technologies currently allow the engineering community to measure and evaluate ambient structural vibrations quickly and accurately. For example, wireless sensors represent an integration of novel sensing transducers with computational and wireless communication elements. Officials responsible for ensuring the long-term performance and safety of bridges depend upon empirically derived vibration characteristics to update analytical bridge models so that the chronological change of bridge load-bearing capacity can be tracked. As such, bridge officials direly need an economical means of rapidly deploying sensors on a bridge to collect ambient response data from which modal information can be extracted; wireless sensors represent a transformative technology that uniquely meets these needs.

The use of wireless communications in lieu of wires within a structural monitoring system was initially proposed by Straser and Kiremidjian [5] as a means of reducing installation costs in large-scale civil structures. In addition, their work illustrated the freedom a wireless system infrastructure provides including rapid and reconfigurable installations. Recently, Lynch et al. has extended their work to include computational microcontrollers in the hardware design of wireless sensors so that various system identification and damage detection algorithms can be embedded for local execution by the sensor [6], [7], [8]. To date, a handful of bridges and buildings have been instrumented with wireless monitoring systems including the Alamosa Canyon Bridge (New Mexico), Geumdang Bridge (Korea), WuYuan Bridge (China), Voigt Bridge (California) and a historic theater in Detroit, Michigan [9]. These extensive field studies attest to the accuracy and reliability of wireless sensors in traditional structural monitoring applications.

The purpose of this study is to employ a rapid-to-deploy wireless structural monitoring system prototyped by Wang, et al. [10] for monitoring long-span bridges during ambient excitation conditions. Towards this end, this study will focus on the experimental determination of the dynamic properties of the newly retrofitted Gi-Lu cable-stayed bridge (Nantou County, Taiwan) using ambient vibration responses recorded by a wireless structural monitoring system. The wireless monitoring system consists of a distributed network of wireless sensors in direct communication with a high-performance data repository where data is stored and analyzed. To extract the bridge modal characteristics, both the frequency domain decomposition (FDD) and stochastic subspace identification (SSI) methods were embedded in the central repository to autonomously identify the dynamic properties of the bridge. The paper concludes with a discussion on the results obtained using the wireless monitoring system, including observation of the interaction between cable and deck vibrations.