Bruce F. Sparling

Influence of Excitation on Dynamic System Identification for a Multi-Span Reinforced Concrete Bridge

In vibration-based damage detection, changes to structural modal properties are tracked over time in order to infer the current state of damage or deterioration. As such, the ability to obtain reliable estimates of modal parameters, particularly natural frequencies and mode shapes, is of critical importance. In the present study, the influence of the dynamic excitation source on the accuracy and statistical uncertainty of modal property estimates for a three span reinforced concrete bridge was investigated experimentally and numerically. Comparisons were made between the dynamic responses due to vehicle loading, harmonic and random forcing, impact, and environmental excitation. It was demonstrated that natural frequencies and mode shapes extracted from the free vibration response following vehicle and random loading events were of higher quality than corresponding values determined during the forcing phase of those events. Harmonic excitation at resonant frequencies and impact were also found to produce statistically reliable results.

Structural Health Monitoring of Precast Concrete Box Girders Using Selected Vibration-Based Damage Detection Methods

Precast, prestressed concrete box girders are commonly used as superstructure components for short and medium span bridges. Their configuration and typical side-by-side placement make large portions of these elements inaccessible for visual inspection or the application of nondestructive testing techniques. This paper demonstrates that vibration-based damage detection (VBDD) is an effective alternative for monitoring their structural health. A box girder removed from a dismantled bridge was used to evaluate the ability of five different VBDD algorithms to detect and localize low levels of spalling damage, with a focus on using a small number of sensors and only the fundamental mode of vibration. All methods were capable of detecting and localizing damage to a region within approximately 1.6 times the longitudinal spacing between as few as six uniformly distributed accelerometers. Strain gauges configured to measure curvature were also effective, but tended to be susceptible to large errors in near support damage cases. Finite element analyses demonstrated that increasing the number of sensor locations leads to a proportional increase in localization accuracy, while the use of additional modes provides little advantage and can sometimes lead to a deterioration in the performance of the VBDD techniques.

Application of vibration-based damage detection to an integral abutment bridge

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.

Identifying damage on a bridge deck using vibration-based damage indices derived from limited measurements

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.

Damage detection on a steel-free bridge deck using random vibration

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.