Ashutosh Bagchi

Performance of Vibration-based Techniques for the Identification of Structural Damage

Early detection of damage is of special concern for civil engineering structures. If not identified in time, damage may have serious consequences, both safety related and economic. The traditional methods of damage detection include visual inspection or instrumental evaluation. A comparatively recent development in the health monitoring of civil engineering structures is vibration-based damage detection. Vibration characteristics of a structure, that is, its frequencies, mode shapes, and damping are directly affected by the physical characteristics of the structure including its mass and stiffness. Damage reduces the stiffness of the structure and alters its vibration characteristics. Therefore, measurement and monitoring of vibration characteristics should theoretically permit the detection of both the location and severity of damage. However, in practice, a number of difficulties persist in vibration-based damage identification. As a result, most of the damage identification algorithms fail when applied to practical civil engineering structures. This article presents a survey of some of the more commonly used algorithms and describes the conditions under which they may or may not work. The success of individual algorithms is measured through computer simulation studies. It may, however, be noted that additional practical difficulties that cannot entirely be reproduced through computer simulation exist, which makes vibration-based damage identification a challenging field with many unanswered questions.

Review of post-earthquake fire hazard to building structures

Fire following an earthquake is an important factor causing damage to buildings and life-line structures. Therefore, besides satisfying structural design requirements for normal loads, such as dead and live loads including the seismic hazard, buildings should also be designed to withstand the fire following earthquakes for a certain minimum duration as required for a desired level of performance. This period of time will allow occupants to evacuate the building safely and the emergency crews to cope with the fire. Also, it is essential to reduce the post-earthquake fire (PEF) ignitions and minimize the damage to active fire protection systems as much as possible to prevent the spread of fire. This paper presents a state-of-the-art review on the PEF hazard and discusses the causes, mitigation measures, and performance of building structures under this hazard. Mitigation measures that could be developed based on the experience from the structural engineering field are identified. Both local and global appro...

Model-Based Damage Identification in a Continuous Bridge Using Vibration Data

Damage often causes changes in the dynamic characteristics of a structure such as frequencies and mode shapes. Vibration-based damage identification techniques utilize the changes in the dynamic characteristics of a structure to determine the location and extent of damage in the structure. Such techniques are applied in this study to the Crowchild Bridge, a steel-free deck continuous bridge located in western Canada. While the numerical models of the bridge are correlated with the measured dynamic characteristics, computer simulation is used to study the identification of a number of different damage patterns, and the effects of measurement errors and incomplete mode shapes on the quality of results are evaluated. The effectiveness of some selected damage identification techniques is examined; the potential difficulties in identifying the damage are outlined; and areas of further research are suggested. A three-dimensional finite-element model and a simple two-dimensional girder model of the bridge have been constructed to study the usefulness of the selected damage identification methods. Another promising damage detection method proposed here is based on the application of neural networks that combines a vibration-based method.

Frequency domain analysis of soil-structure interaction

Abstract In practical soil-structure analysis a number of basic issues need to be addressed. First, the soil-structure interaction problems are generally of a very large size. A suitable transformation should therefore be used to reduce the size of the problem. Second, analysis in the frequency domain, which is the preferred method for problems involving wave propagation in an infinite soil medium, usually requires the application of discrete Fourier transformation. The use of such transformation often leads to unacceptable errors caused by aliasing or overlapping. Analytical techniques that address these issues are presented here. A set of Ritz vectors based on the concept of component mode synthesis is developed for the transformation of the interaction problem. The method of artificial damping, developed by two of the authors, and reported earlier, is applied in the solution to avoid the problem of aliasing. The effectiveness of these techniques is illustrated by means of some examples of soil-structure interaction.

System-Level Deterioration Model for Reinforced Concrete Bridge Decks

AbstractGenerally, in existing bridge management systems, the deterioration is modeled based on visual inspections where the corresponding condition states are assigned to individual elements. In this case, limited attention is given to the correlation between bridge elements from a structural perspective. In this process, the impact of the history of deterioration on the reliability of a structure is disregarded, as it may lead to inappropriate conclusions. The improved estimate of service life of a bridge deck may help decision makers enhance intervention planning and optimize life-cycle costs. The objective of this research is to evaluate the system reliability of conventional bridges that were designed based on existing codes. According to the methodology developed in this study, the predicted element-level structural conditions for different time intervals are applied in the nonlinear finite-element model of a bridge superstructure, and the system reliability indexes are estimated for different time ...

Health Monitoring of Structures Using Statistical Pattern Recognition Techniques

The primary objective of structural health monitoring (SHM) is to determine whether a structure is performing as expected or if there is any anomaly in its behavior compared with the normal condition. It is also useful in detecting the existence, location, and severity of damage. Vibration-based damage detection methods are very frequently used in SHM. However, because of complicated features of real-life structures, there are uncertainties involved in the key input parameters (e.g., measured frequencies and mode shape data), which affect the performance of these methods. If vibration-based methods are incorporated with semianalytical methods, such as statistical pattern recognition techniques, better accuracy can result in structural health assessment. This paper explores the statistical pattern recognition techniques for damage detection and/or degradation in structures. A case study, the Portage Creek Bridge in Victoria, British Columbia, Canada, has been used. The following two approaches of the statistical pattern recognition techniques have been used: statistical pattern comparison and statistical model development. After filtering and normalizing the data obtained from the SHM system installed in the bridge, damage sensitive features have been extracted by autoregressive modeling of the time series data. Both idle and excited states of the bridge are considered in this case. From the statistical analysis of the strain and acceleration data, although the bridge is in a good condition, there is a small but steady deterioration in its performance. The study also demonstrates the feasibility of the statistical pattern recognition techniques in assessing the structural condition of a practical structure.

A Study of Image-Based Element Condition Index for Bridge Inspection

This paper presents an innovative computer vision method for condition assessments of bridges with multiple defects in bridge elements using digi tal mages. This work utilizes 3D model of existing bridges and overlays digital images on 3D model to simulate on-site visual inspection. The analysis of element condition index (ECI) of bridges requires i nformation about the severity and extent of defects in elements. In general, ECI is evaluated manually dur ing routine bridge inspection considering the sever ity of dominant defects. The evaluation of ECI with mul tiple defects needs to be addressed with considerat on of dominant defect as well as the interaction among defects. However, Image-based quantification tech niques largely depend on geometry of objects (i.e. shapes). Shape vectors of a given object change as they are translated, rotated, and scaled with different magnitudes. This work considers shape preserving al gorithms such as, affine and projective transformatio n for proper image alignment. Semi-automated approa ch for detection and quantification of concrete distre s such as cracks and spalling are considered for t he defects analysis. The proposed methodology ensures th e consistency in reporting ECI and eliminates the shortcoming of traditional approaches.

Seismic design and performance evaluation of steel-frame buildings designed using the 2005 National building code of Canada

The seismic design provisions in the current edition of the National building code of Canada significantly differ from those in the earlier version of the code . Although the code has moved from the earlier prescriptive provisions towards a more performance-based design, such design principles have not yet been fully implemented. In the present work, four ductile steel moment-resisting frame buildings with heights of 5, 10, 15, and 20 storeys, were designed for Vancouver using the newer code, and their performances were studied to determine the level of seismic protection implied in the code. Synthesized and scaled real ground motion records were used to evaluate the nonlinear dynamic response of these structures. Although the buildings achieved the collapse prevention performance objective of the code, variations in evaluated performance parameters for the different buildings were observed. Also, building performance was found to be affected by the presence of infill walls, as well as by the nature of se...

A simplified methd of evaluating the seismic performance of buildings

This paper presents a simplified method of evaluating the seismic performance of buildings. The proposed method is based on the transformation of a multiple degree of freedom (MDOF) system to an equivalent single degree of freedom (SDOF) system using a simple and intuitive process. The proposed method is intended for evaluating the seismic performance of the buildings at the intermediate stages in design, while a rigorous method would be applied to the final design. The performance of the method is evaluated using a series of buildings which are assumed to be located in Victoria in western Canada, and designed based on the upcoming version of the National Building Code of Canada which is due to be published in 2005. To resist lateral loads, some of these buildings contain reinforced concrete moment resisting frames, while others contain reinforced concrete shear walls. Each building model has been subjected to a set of site-specific seismic spectrum compatible ground motion records, and the response has been determined using the proposed method and the general method for MDOF systems. The results from the study indicate that the proposed method can serve as a useful tool for evaluation of seismic performance of buildings, and carrying out performance based design.

Reliability-Based Condition Assessment of an Externally Restrained Bridge Deck System Considering Uncertainties in Key Design Parameters

AbstractThe objective of this research is to evaluate the reliability of bridge decks that use nonconventional materials or structural forms such as externally restrained deck systems where the available deterioration models developed for conventional systems are not applicable. The method developed here adopts the reliability theory and establishes a deterioration model for such bridge decks based on their failure mechanisms. The externally restrained deck is an innovative structural system introduced about a decade ago to the construction industry and provides corrosion-free replacement for conventional reinforced concrete decks. Because there is no established deterioration model available for these innovative systems, it is difficult to predict the reliability of such bridge decks at different instances of time. The developed method has been applied to an innovative structure with an externally restrained deck system, namely the Crowchild Trail Bridge, in Calgary, Canada, as a case study. A finite-ele...