Abstract

Vibration-based damage detection is a nondestructive structural health monitoring approach that focuses on changes in the dynamic characteristics of a structure, such as its natural frequencies and mode shapes, as indicators of damage. Because vibration-based damage detection techniques require data with a high signal to noise ratio for analysis, the choice of sensors used to measure the vibrations is an important consideration. The objective of this paper is to demonstrate the use of inexpensive geophones for determining the modal parameters of civil engineering structures, particularly bridges, for use with vibration-based damage detection techniques. A geophone is a velocity

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transducer commonly used by seismologists for subsurface exploration, and their use for bridge vibration measurement provides some advantages over accelerometers. In most cases, engineers employ accelerometers for the purpose of measuring bridge vibrations.

However, compared to geophones, accelerometers are relatively expensive, have lower sensitivity, and require a power supply and amplifier in addition to the signal recorder. In comparison geophones are passive sensors, they connect directly to the recording system, and provide an economical way to acquire vibration data simultaneously at a large number of measurement points. In order to validate the use of geophones for modal parameter identification, a simple beam experiment was conducted, and the results compared with theoretical values and a finite element model. Finally, modal parameters extracted from vibration data acquired using geophones during testing of a full-scale reinforced concrete bridge located in Knoxville, TN, are presented. The results show that modal parameters obtained using geophones are reliable and could be used with vibration-based damage detection techniques.

3.2 Introduction

Researchers have been using vibration testing as a means of assessing the structural health of buildings and bridges for several years. Results obtained through vibration testing have proved useful for structural health monitoring (Zhao and DeWolf 2002, He et al. 2009), finite element model updating and calibration (Bell et al. 2007, Catbas et al. 2007), condition assessment of structures (Halling et al. 2001, Ren et al.

2004), and structural damage detection (Kim and Stubbs 2003, Huth et al. 2005). The

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focus of this paper is nondestructive evaluation of bridges by means of a new approach for obtaining vibration records using highly sensitive geophones that are passive in nature and relatively inexpensive for large scale implementation in practice.

The nondestructive evaluation methods that have been developed over the past several years for the purpose of assessing structural health fall into two major categories:

local and global. Local damage detection methods used for assessing bridge health include techniques such as impact-echo, ground-penetrating radar, ultrasonic pulse velocity, spectral analysis of surface waves, infrared thermography, and mechanical sounding (Gassman and Tawhed 2004). One of the drawbacks associated with local damage detection techniques is that the location of the damage must be known or guessed before a test is conducted, and the area to be tested on the bridge must be accessible.

Because of such drawbacks, global damage detection methods have been developed that instead rely on changes in the overall response of a bridge as an indication of damage.

Vibration-based methods are one class of global damage detection that has received much attention in the literature.

Vibration-based damage detection focuses on changes in the dynamic characteristics of a structure, such as natural frequency and mode shape, as indicators of damage. Several different global evaluation techniques and associated quantitative damage indices have been published in recent years. Detailed reviews of these techniques as applied to bridges and other structures can be found in Doebling et al.

(1996, 1998) and Sohn et al. (2004). In summary, the methods discussed monitor shifts in natural frequency, absolute changes in mode shapes, changes in mode shape curvature

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or strain energy, and variations in stiffness and flexibility matrices. Numerical methods that involve experimental data and the updated response of boundary value problems using the finite element method (FEM) or artificial neural networks (Xu and Humar 2006 ) are also in the literature.

The premise for the implementation of all vibration-based damage detection techniques is that vibration data is available for analysis. In most cases, engineers employ accelerometers for the purpose of measuring bridge vibrations. Other instruments and techniques that appear in the literature include the use of anemometers, temperature sensors, strain gauges, displacement transducers, global positioning systems, weigh-in-motion systems, corrosion sensors, elasto-magnetic sensors, optic fiber sensors, tiltmeters, level sensors, total stations, seismometers, barometers, hygrometers, pluviometers, and video cameras (Ko and Ni 2005).

Several factors influence the selection of sensors, but often times cost and ease of installation play significant roles in the process. Accelerometers, for example, are often relatively expensive. Thus, the number of accelerometers available to a single research group for use in vibration testing is often limited. It has been shown by researchers that the effectiveness of vibration-based damage detection techniques decreases with an increase in sensor spacing (Zhou et al. 2007). Therefore, the number of sensors used and their placement are important considerations when acquiring vibration data for implementation with vibration-based damage detection techniques. Strain gauges, on the other hand, are relatively inexpensive. However, their installation process is time consuming, and their location is fixed. This leads to the fact that the sensors used for

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bridge vibration testing should be easy to move to facilitate vibration measurements at various locations on the bridge and optimal sensor placement. Considering these factors, the purpose of the current research is to demonstrate the use of inexpensive and highly sensitive passive sensors, geophones, for determining modal parameters of full-scale bridges for use with vibration-based damage detection techniques.