Modal Identification And Damage Diagnosis Of Pre-stressed Concrete Bridge Under Ambient Excitation

Along with the traffic increasing, the vehicle loads increasing, as well as the extension of service life, make the bridge traffic capacity, carrying capacity and other functional defects in the growing, the surface and internal defects of bridge structure inevitable exist. With the rapid development of China's bridge construction it is important how to determine the health status for a larger number of old bridges by certain detection means. With computer technology and the theory of bridge design rapid development, making the bridge health monitoring, damage diagnosis of engineering science and technology at home and abroad has gradually become a research hotspot. For the bridge structure the dynamic detection method is practical. Dynamic detection method is based on structural dynamic response. Modal parameters changes in the damage that occurred before and after is used to damage diagnosis. Modal parameter identification and reasonably accurate way to diagnose the damage detection method is the core technology of dynamic detection.For the bridge structure, using the dynamic detection effective external excitation is difficult to impose. Although there are some mechanical, electromagnetic excitation equipment and jumping off the tram, impact and other incentives, but there is the high cost of equipment bulky and a waste of manpower and material resources shortcomings, and the bridge must be closed to interrupt traffic when the test is carried out as to large economic losses are resulted in. To solve these problems, modal parameter identification techniques based on ambient random excitation have emerged. The technology use traffic loads, wind loads, earthquake and other natural excitations as a system input dynamic response obtained by the dynamic testing of structural modal parameters of information identification. This technology is a simple, easy, economical, fast way because it does not affects the structure of normal use, no damage to the structure and greatly reduces the cost of test.Judging from the current modal parameter identification and damage diagnosis of the status quo, although the study on modal parameter identification techniques based on ambient random excitation is more, but most study focused on basic theory research method itself and the analysis is carried through the use of homogeneous material cantilever and simple beam model as the authentication method. Under different ambient excitation there is few studies on feasibility and effectiveness and stability of the existing method in practical engineering application. At the same time higher-order modals of the structure are difficult to excite by using of ambient excitations, while the existing damage detection methods often require structural higher modals in order to effectively damage diagnosis.In this context, the study on modal parameter identification method of the bridge under different ambient excitation and based on the lower-order modes of structural damage diagnosis are performed in this paper. Around these objectives, the thesis from the theoretical analysis, methods, implementation, and numerical simulation is systematic studied. The main work of this paper is as follows:In chapter 1 the existing method of modal parameter identification and damage diagnostic methods are systematically reviewed and summarized especially for modal parameter identification method and damage diagnosis methods of civil engineering structures under ambient excitations. The comprehensive exposition about the theoretical background, the existing results, engineering applications as well as the main problems exist is made.In chapter 2 one of frequency domain identification methods-the power spectrum peak method based ambient excitation is studied regarding its basic theory, the traditional distinction between frequency-domain methods and application of background. An operational process of Power spectral peak method is given. Through the use of white noise as an input the numerical simulations of a concrete Simply-Supported Beam is carried out to verify the effectiveness of peak power spectral method which is based on ambient vibration modal parameter identification.In chapter 3 time-domain modal parameter identification method based on ambient excitation is discussed in detail. First the random decrement technique, ITD method, including the natural excitation technology (NExT), eigensystem realization algorithm (ERA) the basic theory of four kinds of time-domain method which are fitted for modal parameter identification under ambient excitation are discussed; then based on four methods the respective characteristics, the programs are proposed that random decrement technique combine ITD methods and NExT combined ERA to became two kinds of time-domain joint algorithm for structural modal parameter identification. An operational process of two kinds of time-domain joint algorithm is given, through the use of white noise as an input the numerical simulations of a concrete continuous beam is carried out to verify the effectiveness and feasibility of two kinds of time-domain joint algorithm.In chapter 4 a Pre-stressed concrete continuous beam bridge modal parameter identification of the finite element numerical simulation analysis is performed. In the process of numerical simulation power spectrum peak method and the NExT/ERA joint time-domain algorithm is used to further study on the practical application ability under ambient excitation by simulating traffic load.In chapter 5 a Pre-stressed concrete continuous beam bridge modal parameter identification of the finite element numerical simulation analysis is performed. In the process of numerical simulation random decrement technique/ITD and NExT/ERA is used to further study on the practical application ability under ambient excitation by simulating wind load. In particular, NExT/ERA joint time-domain algorithm are more in-depth discussed based on the aforementioned study.In chapter 6 further study on stability and applicability of the NExT/ERA Joint time-domain algorithm in different ambient excitations is carried out. A Pre-stressed concrete continuous beam bridge modal parameter identification of the finite element numerical simulation analysis is performed to in-depth study the effectiveness and feasibility of the NExT/ERA joint time-domain algorithm under seismic excitation. Under the common ambient excitation the stability and applicability and parameter identification capabilities of NExT/ERA Joint time-domain algorithm are validated.It is a great problem that because ambient excitation is difficult to stimulate higher-order mode of structure effective diagnosis can not be performed. In chapter 7 through improvement of the existing modal strain energy indicators, using only low-order modes for the bridge structure, damage diagnostic methods based on ambient excitation is discussed. Through the establishment of concrete simple beam model of three-dimensional solid finite element numerical simulation is performed to verified effectiveness and feasibility of the method.Basing on the work of the former Chapter 7, the major work done are summarized by this article in chapter 8, and reached the following conclusions:1. Through the concrete beams numerical example and numerical simulation by Pre-stressed concrete continuous beam bridge under traffic load effectiveness and feasibility of the peak power spectral method based on ambient excitation to identify concrete structure modal parameter is proved.2. Using power spectral peak parameter identification method to identify high-order modal error is relatively large; damping ratio of the recognition results error is big, the results can not be credible; Less able to resist noise interference, so poor practice, appropriate methods of signal processing must be taken .Otherwise, recognition accuracy will be significantly affected; phenomenon of modal loss may appear.3. Structural natural frequency and lower-order model identified by the power spectrum peak has high accuracy. The method is simple, quick, practical. In practical engineering application, other identification methods can be used as a supplement and confirmed.4. Through the concrete beams numerical example and numerical simulation by Pre-stressed concrete continuous beam bridge under wind load effectiveness and feasibility of random decrement technique/ITD based on ambient excitation to identify concrete structure modal parameter is proved.5. Though random decrement technique / ITD method is based on the white noise excitation, but through the simulation results can be seen that this method identifying the natural frequencies and mode shapes are a better accuracy, damping ratio identification is no good. The results fully show that the method can be practical engineering application.6. Through the concrete beams numerical example and numerical simulation by Pre-stressed concrete continuous beam bridge under three kings of common ambient excitation effectiveness and feasibility of NExT/ERA based on ambient excitation to identify concrete structure modal parameter is proved.7. NExT/ERA joint algorithm has strong anti-noise ability. No signal processing, the method has outstanding practicality.8. The fact under different ambient excitation identification results of NExT/ERA joint algorithm were more accurate demonstrate a strong ability to adapt and the stability of identification. NExT/ERA joint algorithm is an advanced modal parameter identification methods, and fully meet the needs of engineering practice. 9. This paper studies have shown that the appropriate method depends on the specific use, and in engineering practice should be used several ways to work together to complement each other, cross-checked to ensure reliable results.10. The improved the modal strain energy damage diagnosis method improve the original method can only be a simple locate or identify the extent of damage defects. Through numerical simulation by the establishment of concrete simple beam three-dimensional solid model, only with less lower-order modes can be well identified concrete beam structure location and extent of the damage, the method shows identify high precision, And fully reflects the validity and reliability of the method.11. The method initially solves the problem that effective damage diagnosis is difficult due to ambient excitation is difficult to stimulate of higher-order mode of the structure, avoiding the different damage location identification and damage assessment of the extent used in the structural dynamic testing methods. The method is simple, without a large number of terms, has a certain practical engineering application value.Current study on dynamic detection method of bridge structure is still at an exploratory stage, and there are still many more issues about modal parameter identification and damage diagnosis that must be resolved and further explored. In this paper, modal parameter identification and damage diagnosis based on ambient excitation for the bridge structure is studied systematically effective means and methods are put forward. Theoretical basis for the bridge structure health monitoring technology are provided. The study has important theoretical significance and practical engineering value.