A vibration-based damage detection algorithm for bridge type structures

During the past decades, the structural health monitoring SHM for civil infrastructure has received a considerable attention due to its capability to monitor the status of the structure and provide early warning if any damage introduced to the structure or any development of inherent faults. The civil structures due to the age, operating conditions, function adjustment, overloading and environment effects their components may become more susceptible to damage or/ and failure. Therefore, there is a necessity to assess the condition and foresee probable failures of these structures. The rapid expansion of the technology of the sensors and the statistical methods besides the improvement in the computational techniques and signal processing techniques made the SHM an efficient and economical ways to assess the civil structural systems. The core of the SHM is identifying of physical properties of the structural systems using the dynamic response of those systems. One of the important applications of the SHM area is the damaged detection; damage detection techniques use dynamic responses of civil structural systems to detect the existence, location, and magnitude of the damage. There are several methods to detect the fault in the structural system; these methods diagnosis the damage based on the changes in dynamic response for structural system. The strategy that used in each method to process the captured data is variety. Furthermore, the capability of each method to assess the characteristics of the damage changing from method to other; these methods have advantages and disadvantages. The disadvantages may due to cost, timing, accuracy, or/ and usage. The thesis is included theoretical parts and an experimental part. The theoretical side of this thesis has clarified the types of main damage detection techniques, the procedures that used to process the vibration data that collected in the field. In addition, a new technique named Two Points Condensation Technique (TPC) is clarified. The experimental part shows the main steps to apply the TPC, besides presenting the main elements of applying this technique practically. Moreover, an application of TPC techniques is presented. In this study, an algorithm is developed to detect the damage in bridge based on vibration where the thesis proposes a new damage identification technique that identifies the damage location. A methodology Two Points Condensation Technique (TPC) is presented. It utilizes the identification of stiffness matrix terms to assess damage, based on the incomplete measurement of captured vibration test information. The free vibration test data is used in this study to identify damage in the time domain. Most other techniques used at present are based on data in the frequency domain. The TPC method is condensed the structural system to a two degrees-of-freedom system and then compares the identified coefficients of the stiffness matrices with the coefficients of the reduced theoretical stiffness matrices. The damage location is found by detecting the change in the value of the stiffness coefficients of the two degrees-of-freedom systems. TPC method only requires a few sensors to locating the damage, therefore it may use in the cases which consider the number of sensors is an issue. The concept of model reduction plays an important role in this view of point. By condensation, it is possible to compare large analytical set of DOF to the relatively small set of experimental DOF. Consequently, the set of tested DOF requires the large model to be reduced that much smaller size without missing any information that is dealing. Dealing with 2 points, the programming becomes easier and in terms of programming, it becomes multicore or GPU (Graphical Processing unit) programmable subject, which will help to overcome time-consuming calculation. For the calculation, an optimization uses a program written in MATLAB code. The code can be executed under both the MATLAB and Octave environments. In this study, a multi-objective optimization technique used to optimize the equation of motion used in the TPC technique. Multi-objective optimization is a mathematical optimization technique used to optimize more than one objective function simultaneously. The TPC technique applied to experimental data collected from a two types of structures, first a simply supported steel beam and second a steel truss bridge model. The TPC technique applied to experimental data obtained from a steel beam model structure after introducing a thickness change in one element. Two case studies are considered. In both cases, the method accurately detects the damage and determines its location. The experimental results obtained from a specimen model structures demonstrate the usefulness of the TPC technique. This method has several advantages: - The concept is simple and easy to apply without using complex calculations; - Using the TPC technique, the structure reduced to a 2-DOF structure; this decreases the volume of data to be captured and the volume of data to be dealt with at a time. Using more tests will help to obtain more precise values for the change in stiffness coefficients and improve damage location; - Accuracy is achieved in detecting the location of the damage where the TPC technique results have demonstrated good agreement with actual results. The results promote that the TPC technique is capable of indicating the damage location for different sizes of damage. According to the sensitivity of damage size tests, the technique can show stiffness degradation. Moreover, the TPC technique shows the location of damage regardless of the place of the acceleration sensors or the size of the damage. The technique finds the location of damage when the acceleration sensors are near to the damage as well as when they are far from the damage. Additionally, experimental results demonstrate that the SEREP reduction method is suitable for use with the TPC technique. The damping effect has to be accounted for to ensure accurate results when applying this solution technique. Furthermore, the method is applied to the experimental data of a steel truss bridge model structure after inducing the damage by removing an element from the specimen. Four damage cases are analyzed. The results show that the method detects the damage location area. The results illustrate that observing changes in the stiffness matrix coefficients can be a useful tool for structural damage detection; the Guyan condensation has supported the method as long as the structure is simple. Although the frequencies error existed during condensation, the error of evaluation of the coefficients of stiffness matrix does not exceed 10% compare with real change. The results illustrate that the SEPER condensation method supports the TPC technique with more stable data and the difference between the theoretical and experimental result less than 5% which reflect the advantage of SEREP over Guyan. Finally, the results illustrate that observing changes in the stiffness matrix coefficients can be a useful tool for structural health monitoring. As the method proposed here is in a time-domain, to eliminate the time-consuming calculations this procedure suitable for the structures that are monitoring within scheduled periods.