Abstract: Damage identification problem involves detection, localization and assessment of the extent of damage in a structure so that the remaining life could be predicted. Visual or nondestructive experimental damage detection methods such as ultrasonic and acoustic emission ones are based on a local evaluation in easily accessible areas, and therefore, they require a certain prior knowledge of the damage distribution. With the purpose of providing global damage detection methods applicable to complex structures, techniques based on modal testing and signal processing constitute a promising approach for damage identification. These methods examine changes in the dynamic characteristics of structure, such as natural frequencies and mode shapes to detect the structural damage. Modal parameters including natural frequencies, mode shapes and damping ratios are known as essential parameters for analyzing the dynamic behavior of a structure. This paper deals with identification of modal parameters of structures using a two-step algorithm. In the proposed method, free vibration response of structure is decomposed using wavelet packet transform. Then, decomposed signal, which has the same energy with the main signal, is used for modal parameter identification using peak picking method. The performance of the proposed method is verified against the results of an experimental benchmark problem.

Abstract: The Canton Tower is a high-rise slender structure with a height of 610 m. A structural health monitoring system has been instrumented on the structure, by which data is continuously monitored. This paper presents an investigation on the identified modal properties of the Canton Tower using ambient vibration data collected during a whole day (24 hours). A recently developed Fast Bayesian FFT method is utilized for operational modal analysis on the basis of the measured acceleration data. The approach views modal identification as an inference problem where probability is used as a measure for the relative plausibility of outcomes given a model of the structure and measured data. Focusing on the first several modes, the modal properties of this supertall slender structure are identified on non-overlapping time windows during the whole day under normal wind speed. With the identified modal parameters and the associated posterior uncertainty, the distribution of the modal parameters in the future is predicted and assessed. By defining the modal root-mean-square value in terms of the power spectral density of modal force identified, the identified natural frequencies and damping ratios versus the vibration amplitude are investigated with the associated posterior uncertainty considered. Meanwhile, the correlations between modal parameters and temperature, modal parameters and wind speed are studied. For comparison purpose, the frequency domain decomposition (FDD) method is also utilized to identify the modal parameters. The identified results obtained by the Bayesian method, the FDD method and a finite element model are compared and discussed.

Abstract: A low-cost vibration-monitoring system was developed and installed on an urban steel-plated stress-ribbon footbridge. The system continuously measures the acceleration [using 18 triaxial microelectromechanical system (MEMS) accelerometers distributed along the structure), the ambient temperature, and the wind velocity and direction. Automated output-only modal parameter estimation based on the stochastic subspace identification (SSI) was carried out to extract the modal parameters (i.e., the natural frequencies, damping ratios, and modal shapes). Thus, this study analyzed the time evolution of the modal parameters over data monitoring for 1 year. First, for similar environmental/operational factors, the uncertainties associated with the SSI-based techniques used and to the acceleration records used were studied and quantified. Second, a methodology for tracking the vibration modes was established, because several of them with closely spaced natural frequencies were identified. Third, the modal par...

Abstract: Many spectacular pedestrian bridges were designed and constructed recently. Owing to their special shapes, it is expected that various types and a wide range of vibration frequency components will be induced by pedestrians. To avoid accidents and reduce risk, the vibration characteristics of pedestrian bridges during their service life must be carefully assessed. The most direct and reliable way to study the vibration characteristics of a structural system is through field vibration tests. In this paper, a series of full-scale field vibration tests (including ambient, forced, and free vibration tests) were carried out on a pedestrian bridge at City University of Hong Kong (CityU). The difficulties encountered in the field tests are reported. The recently developed Bayesian methods were utilized to determine the modal parameters of the bridge based on measurements from all three kinds of tests. In addition to the most probable values (MPVs) of modal parameters, the associated posterior uncertainties were also analytically calculated. Four modes were identified, including three vertical bending modes and one torsional mode. The accuracy of the identified modal parameters was assessed through the posterior uncertainty. Because the amplitudes of the vibration in the three kinds of tests were different, the modal parameters determined from these kinds of tests were compared and discussed. Suggestions related to the proper use and potential vibration problems during the lifecycle of pedestrian bridges were provided based on the analysis results.

Abstract: This work presents a strategy for testing and validating structures connected together with bolted joints, which are the most common components in mechanical structures. Considering the great number of coupled mechanical structures and research studies on this subject, the authors focused this research work on bolted flanges of aircraft engine casings. In fact, the coupling of engine casings is generally obtained by a large number of joints which assure the correct sealing at the flanges’ interfaces. From a finite element (FE) modelling perspective, joints are often modelled by either rigid connections or springs, otherwise incurring a very expensive computational time. This modelling approach is not a problem when dealing with low amplitude levels of vibrations. For higher levels of vibrations, joints and flanges cannot be considered rigidly connected and that exerted flexibility at the joints’ area can determine nonlinear dynamic behaviour. This work aims to study the dynamic behaviour of bolted flanges by using modal testing performed under controlled response amplitude. Two test structures, (1) a simple bolted flange test case and (2) a sector of a Rolls-Royce aero-engine casing, are tested under high level of vibrations. Both test structures are modelled by FE method, and nonlinear elements are used for modelling the flanges’ interfaces so as to perform prediction of nonlinear responses. These predictions are eventually correlated with the measured data.

Abstract: Changes of modal frequencies induced by temperature variation can be more obvious than those caused by structural damage, which will lead to the false damage identification results. Therefore, quantifying the temperature effect on modal frequencies is a critical step to eliminate its interference in damage detection. Due to the nonuniform and time-dependent characteristics of temperature distribution, it is insufficient to obtain the reliable relationships between temperatures and modal frequencies using temperatures in air or at surface. In this paper, correlations between measured temperatures (air temperature, surface temperature, mean temperature, etc.) and modal frequencies for the slab and beam are comparatively analyzed. And the quantitative models are constructed considering nonuniform temperature distribution. Firstly, the reinforced concrete slab and beam were constructed and placed outside the laboratory to be monitored. Secondly, the correlation coefficients between modal frequencies and three kinds of temperatures are calculated, respectively. Thirdly, simple linear regression models between mean temperature and modal frequencies are established for the slab and beam. Finally, five temperature variables are selected to construct the multiple linear regression models. Prediction results reveal that the proposed multiple linear regression models possess favorable accuracy to quantify the temperature effect on modal frequencies considering nonuniform temperature distribution.

Abstract: In this study, the damping properties of carbon fiber reinforced polymer (CFRP) and basalt fiber reinforced polymer (BFRP) cable potentially applied in long-span cable-stayed bridges were simulated and evaluated based on experimental data and theoretical derivations. The modal shapes were first identified according to a previous dynamic test on FRP cables, based on which the modal damping ratios of the in-plane vibration were estimated by the structural damping model of mode-dependence. Meanwhile, the modal damping ratios of the out-of-plane vibration were evaluated by the combined Rayleigh and frequency independent damping (CRFID) model. The results show that (1) the modified equation for the modal damping ratio validated by the results of in-plane vibration of the steel cable can be used for modeling the damping ratios of FRP cables, (2) the coefficients of dynamic strain damping energy of CFRP and BFRP cables were derived by backward calculating and fitting the experimental data, which can represent the damping difference among each material, and (3) the modal damping ratios of the out-of-plane modes of different stayed cables were fitted by the CRFID model and show good agreement with the test results.

Abstract: A two-step method based on modal strain energy is presented for damage identification in thin plates. In the first step, damage is localized with the modal strain energy change ratio approach. A method is proposed to weaken the ‘Vicinity Effect’, thus reduces the false alarms in the localization of damage. In the second step, the damage extents of the suspected damaged elements are obtained iteratively with modal strain energy change sensitivity-based finite element model updating approach. Model reduction is introduced to eliminate the measurement of rotation degrees-of-freedom in the mode shapes. Two numerical examples are studied to demonstrate the effectiveness and robustness of the proposed method. Both single damage and multiple damage cases are studied and good identification results are obtained. The effect of measurement noise on the identification results is investigated.