{Fibre-optic Bragg Grating (FBG) strain sensors hold a great potential for vibration monitoring of civil structures because of their exceptional stability and accuracy; however, the accurate measurement of the very small strains levels occurring during ambient, or operational, excitation has been so far problematic. There are two ways to improve the measurement resolution: employing a strain-enhancing sensor package, and applying an improved wavelength detection algorithm. In this work, the potential of an improved wavelength detection algorithm for the identification of modal characteristics based on sub-microstrain data is investigated. The strategy is illustrated for a steel beam to which a chain of multiplexed FBG sensors has been attached at the top side of the beam. The raw FBG data are processed into strain values with an algorithm that is based on detecting the peak shifts in the wavelength spectrum by correlation analysis rather than by simply tracking the peak values. Subsequently, the strain sequences are used for identification of modal characteristics of the beam (natural frequencies and strain mode shapes) with ``Covariance driven Stochastic Subspace Identification (SSI/cov){''}. Computational results of a Finite Element Model are used to validate the experimental results.}
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The lack of a physically intuitive OMAX approach can be attributed to the difficulty of decomposing the measured joint response in a forced and an ambient part in an accurate way. In this paper, a matrix projection (subspace) approach is developed that achieves this decomposition. It allows to use any experimental and operational modal analysis technique on the forced and ambient parts of the data, respectively, and to combine them for joint modal parameter estimation; here, this is further elaborated for subspace identification. An extensive simulation example illustrates the accuracy and practicability of this approach.
Modal parameter estimation requires a lot of user interaction, especially when parametric system identification methods are used and the modes are selected in a stabilization diagram. In this paper, a fully automated, generally applicable three-stage clustering approach is developed for interpreting such a diagram. It does not require any user-specified parameter or threshold value, and it can be used in an experimental, operational, and combined vibration testing context and with any parametric system identification algorithm. The three stages of the algorithm correspond to the three stages in a manual analysis: setting stabilization thresholds for clearing out the diagram, detecting columns of stable modes, and selecting a representative mode from each column. An extensive validation study illustrates the accuracy and robustness of this automation strategy.
No abstract is provided for this article.
No abstract is provided for this article.
Knowledge of the tensile forces in cables of bridges or in external tendons is required for regular inspection and safety assessments. Force determination by lift-off tests with hydraulic jacks can result in considerable expenditure as well as the danger of damage. However, alternatively conducted vibration measurements have shown that, due to improper identification of natural frequencies or the use of over-simple force–frequency relationships, accuracies achieved up to now are sometimes not achieved. For high cable forces and short cables, errors of up to ±10% could be realized. Fast and accurate methods for assessing cable forces are thus required. The main target of this paper is the presentation of a practical applicable procedure to determine cable forces in civil engineering constructions by means of vibration measurements. By taking into account the identified natural frequencies, the bending stiffness and the boundary conditions, accuracy in the range of ±1% may be achieved.
In Finite Element (FE) model updating the val- ues of the uncertain physical properties of an engineering structure are adapted in the FE model such that the dif- ferences between the numerical and experimental vibration data are minimized. In civil engineering the discrepancies in the modal data, i.e. the eigenfrequencies and mode shapes, are minimized, which are mostly identified from ambient vi- brations. Since the modal data are nonlinear functions of the uncertain properties, an iterative sensitivity-based min- imization method is used to solve this inverse problem. The technique can be used for parameter identification in general; in the paper it is used for damage identification of civil structures, such as dams, bridges, monuments, etc. In particular, the damage pattern existing in the girder of a highway bridge is identified by updating the Young's and the shear modulus in the FE model of the bridge. In order to reduce the number of unknowns, damage functions are used, in the way that the updated stiffness distribution is approximated by a piecewise linear function.
Natural frequencies are probably the most widely used modal characteristics in vibration-based monitoring. However, they can be highly influenced by temperature and this influence can completely mask the effect of even severe damage. This translates into a necessity for time-consuming data-normalization techniques to remove the influence of temperature and identify damage. Displacement mode shapes of homogeneous material structures are less influenced by temperature, but obtaining them in a dense grid, which is required for damage localization, is cumbersome due to the large number of sensors needed. Strain mode shapes on the other hand can be insensitive to temperature variations, while obtaining them in a dense grid is possible when fiber-optic sensors such as fiber-Bragg gratings (FBG) are used. This work presents the results of the continuous monitoring of a steel railway bridge for a period of almost two years, where modal data were collected for a wide temperature range. The bridge is instrumented with eighty FBG strain sensors, multiplexed in four fibers. The natural frequencies and strain mode shapes of ten modes have been automatically identified from operational strain time histories, on an hourly basis. A clear influence of temperature on the natural frequency of most modes is identified, especially during frost periods. On the contrary, the strain mode shapes are mostly insensitive to temperature changes and only these of some higher-order modes are slightly and uniformly influenced when frost occurs. This behavior is confirmed also by a finite element model (FE) of the bridge. Furthermore, the FE model is used to investigate the influence of local stiffness changes on the modal characteristics. A clear and local change of the modal strain amplitude is observed at the location of the reduced stiffness, especially when information from all modes is combined in a sensitive damage index.
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The EC research project ”DETAILS” aims to remove uncertainties on dynamic interaction effects of composite railway bridges. As part of the experimental part of the project, the Sesia viaduct, located on the new Italian high-speed line between Torino and Milano, has been tested under the excitation of Italian ETR500Y high-speed trains. This paper presents the finite element modeling of the Sesia viaduct, as well as the train-bridge interaction model and its implementation. To facilitate the calculation, a user friendly train-bridge interaction toolbox DATIS has been developed in Matlab that communicates with a finite element program. The toolbox is validated by comparing calculated accelerations and strains with measured results.
A very popular floor system consists of precast one-way voided slab elements. A modal analysis of such a floor has been performed using a multivariable AR-model. The eigenfrequencies, the damping ratio's and the mode shapes of the floor have been determined. This test has been repeated at different storeys of a building. The same floors were also tested before and after the installation of the finishing layer. The reinforced hollow core floor has been modeled by means of a Finite Element Analysis (FEA). By comparing the eigenfrequencies and the mode shapes obtained by the ARV-model with the results obtained by FEA the influence of several physical parameters has been investigated: the stiffness of the slab elements in two directions, the transverse connection between two different elements, the connection between the slab elements and the outer walls and the influence of the brick inner walls.
