Damage induced within the deck of a bridge superstructure produces concomitant changes to its vibration characteristics (notably its natural frequencies and mode shapes). Vibration-based damage detection (VBDD) methods exploit these changes to infer information regarding the nature of the damage. This paper focuses on interpreting the spatial patterns of changes produced in the fundamental mode shape with the goal of determining whether the presence of damage can be reliably detected. The study was carried out under the constraint that mode shapes are derived from limited data, available only at a relatively small number of measurement points on the surface of the bridge deck. A detailed finite element (FE) model of a two-span, slab-on-girder, integral abutment bridge was developed and calibrated to match the measured natural frequencies and mode shapes of a structure located in Saskatoon, Canada. This model was used to simulate the dynamic response of the bridge as various states of small-scale damage were induced at different locations on the deck. The variation of the change in the fundamental mode shape along three longitudinally oriented lines was studied to identify patterns that would allow a reliable determination of whether damage is present and in what region of the bridge it might be located. It is shown that normalizing mode shapes along individual lines separately, rather than along all three lines simultaneously, emphasises localized changes caused by damage, but also magnifies the influence of random measurement noise, making it more difficult to recognize the global spatial patterns indicative of damage.
Vibration-based damage detection (VBDD) methods use changes to the dynamic characteristics of a structure (i.e. its natural frequencies, mode shapes, and damping properties) to detect the presence of damage and determine its location. The application of these methods to constructed civil engineering facilities is complicated by a number of factors unique to these structures. Despite the challenges, the development of reliable VBDD methods for constructed facilities has the potential for great benefit and cost savings to infrastructure owners. This paper focuses on the application of VBDD techniques based on changes to mode shapes to a two-span, slab-on-girder, integral abutment bridge in Saskatoon, Canada. The dynamic response of the bridge under ambient traffic loading has been measured periodically using temporarily installed accelerometers over a range of ambient temperatures. A detailed finite element (FE) model has been developed and calibrated to match the first three measured natural frequencies and mode shapes. This model was then used to simulate the dynamic response of the bridge as various states of small-scale damage were induced, and several VBDD techniques were applied to detect and locate the damage. Preliminary results show that the ambient temperature significantly influences measured natural frequencies. In addition, the presence and location of damage may be found using any of VBDD techniques. The performance of the techniques is influenced by the number of sensors used to characterize mode shapes, as well as by the procedures used to normalize the mode shapes.
Vibration-based damage detection (VBDD) methods utilize measured changes in the dynamic characteristics of structural systems (natural frequencies, mode shapes, and damping characteristics) to indicate the presence and location of damage. Previous studies have demonstrated that small-scale damage can be reliably located in simple bridge systems when resonant harmonic loading is used as the excitation source for the VBDD measurements. In full-scale bridge applications, however, random loading due to traffic or wind is often more readily achievable. A numerical study was therefore undertaken to investigate the use of random loading for damage detection in a simple-span, slab-on-girder bridge deck. Transient dynamic analyses of a finite element model of the bridge deck subjected to randomly varying loading were performed for nine different simulated small-scale damage states. To reduce the inherent uncertainty arising from the random loading, averaged results from a large number of repeated random trials were used. Several factors that may influence the probability of successfully locating the damage were investigated, including the number of repeated random trials used, the distance from the damage to the nearest sensor, the proximity of the damage to simple supports, the severity of the damage and the presence of random measurement error. It was found that a large number of repeated random trials was required to achieve reasonable probabilities of successfully locating the damage; even then, reliable detection results were not guaranteed for all of the damage conditions considered. Based on these results, therefore, random excitation appears to be less reliable in VBDD than harmonic loading.