Structural Testing Series: Part 4 by zyxwvuts P. Cornwell, C.R. Farrar, zyxwv S.W. Doebling and H. Sohn zy ENVIRONMENTAL VARIABILITY OF MODAL PROPERTIES nterest in the ability to monitor a structure and detect damage at the earliest possible stage is pervasive throughout the civil, mechanical and aerospace engi- I neering communities. Significant work has been done in the formulation of vibration-based damage detection algo- rithms, but unfortunately, investigations studying the vari- ability of dynamic properties caused by changing environ- mental and operational conditions have been lacking. zyxwvu A thorough understanding of this variability is necessary so that changes in vibration response resulting from damage can be discriminated from changes resulting from such var- iability. In this paper the variability in modal properties of the Alamosa Canyon Bridge in southern New Mexico will be discussed. INTRODUCTION Current damage-detection methods are either visual' or localized experimental meth- ods such as acoustic or ultra- sonic methods, magnet field methods, radiographs, eddy- current methods and thermal field methods2. All of these experimental techniques re- quire that the vicinity of the damage is known zyxwvutsr a priori and that the portion of the structure being inspected is readily accessible. Subject to these limitations. these ex- zyxwvutsr ' quencies, mode shapes, and modal damping) are functions of ' the physical properties of the structure (mass, damping, and : stiffness). Therefore, changes in the physical properties will . cause changes in the modal properties and the measured . response of the structure. Recent research has focused on . developing methods to measure and analyze changes in + these global dynamic properties in an effort to detect and * locate damage on a local level. * Recent advances in wireless, remotely monitored data ac- ' quisition systems coupled with the development of vibration- . based damage detection algorithms make the possibility of . remotely monitoring a bridge appear to be within the capa- bilities of current or near- future technology. However, before such a system can be relied upon to perform this monitoring, the variability of the dynamic properties that are the basis for the damage detection algorithm must be understood and quantified so that changes in vibration re- sponse resulting from dam- age can be discriminated from changes resulting from such variability. This varia- bility could be the result of both changing environmen- tal and operational condi- tions as well as the testing and data reduction proce- dures. perimental methods can de- Fig. I: Alamosa Canyon Bridge Near Truth or Consequences, tect damage on or near the surface of the structure. The New Mexico need for additional global damage detection methods that can be applied to complex structures has led to the development of methods that ex- amine changes in the global dynamic characteristics of the structure. The basic concept in linear, vibration-based damage detec- tion is that global modal parameters (notably resonant fre- Several field tests of the Ala- mosa Canyon Bridge have been performed to study various aspects of applying vibra- tion-based damage detection methods to a real world zy in situ structure. This bridge is aligned primarily in the north-south direction and is located adjacent to Interstate 25 (1-25) ap- proximately 16 km (10 miles) north of Truth or Conse- quences, New Mexico. The bridge, as seen from the arroyo it crosses, is shown in Figure 1. Editor's Note: This is the fourth of a series of articles on vibration testing tech- niques applied to large structures by researchers and practicing engineers in dif- ferent countries around the world. In this article, Phillip Cornwell ofRose-Hulman Institute zyxwvutsrqponml of Technology, and Charles Farrar, Scott Doebling and Hoon Sohn, ofLos Alamos National Laboratory, present the results of a study on how environmental and operational conditions change the dynamic properties of a bridge. The article includes an interesting discussion on how these changes can be accounted for when condwting vibration studies to detect possible structural damage. As a case study to illustrate these issues, the variability in modal properties of the Alamosa Canyon Bridge in southern New Mexico is presented and discussed. We hope you find this article interesting and informative. -Carlos Ventura, Feature Series Editor P. Cornwell is Associate Professor, Rose-Hulman Institute of lkchnology. C.R. Far- rar (SEM Member) and S. K Doebling are Technical Staff Members, and H. Sohn is Post-Doctoral Research Associate, at Los Alamos National Laboratory. Initially, it was the investigators' intent to introduce various types of damage into this bridge and study several damage detection methods along with the feasibility of continuously monitoring such a structure. However, restrictions that the damage to the Alamosa Canyon Bridge be relatively benign or repairable made it difficult to take the damage identifi- cation portion of the study to completion, as realistic damage scenarios could not be introduced with the equipment on hand during these tests. Subsequently,this study focused on quantifying the variability in identified modal parameters caused by sources other than damage. During various tests of the Alamosa Canyon Bridge, variability caused by envi- ronmental effects, vehicles on the bridge, the excitation Nouernhr/Decernber 1999 EXPERIMENTAL TECHNIQUES 45