Environmental variability of modal properties

Interest in the ability to monitor a structure and detect damage at the earliest possible stage is pervasive throughout the civil, mechanical and aerospace engineering communities. Significant work has been done in the formulation of vibration-based damage detection algorithms, but unfortunately, investigations studying the variability of dynamic properties caused by changing environmental and operational conditions have been lacking. 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 variability. 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 methods such as acoustic or ultrasonic methods, magnet field methods, radiographs, eddy-current methods and thermal field methods. All of these experimental techniques require that the vicinity of the damage is known a priori and that the portion of the structure being inspected is readily accessible. Subject to these limitations, these experimental methods can detect damage on or near the surface of the structure. The need for additional global damage detection methods that can be applied to complex structures has led to the development of methods that examine changes in the global dynamic characteristics of the structure. The basic concept in linear, vibration-based damage detection is that global modal parameters (notably resonant frequencies, mode shapes, and modal damping) are functions of the