924 publications from this institution
A finite element model updating method using experimental modal para- meters is presented. Such a procedure can be regarded as an optimisation problem. The objective function consists of the (weighted) sum of differ- ences between the experimental modal data (eigenfrequencies and mode- shapes) and the corresponding analytical predictions. The function can be completed with additional orthogonality conditions. The updating para- meters are the uncertainties of the model, which are changed locally until the objective function is minimised. A gradient-based algorithm is used to solve the optimisation problem. As a result a physically more correct model is obtained on which further analysis can be made. The presented proce- dure is applied on a railway bridge at Xntoing. Belgium, on which dynamic measurements have been made. Several variants of objective functions are minimised and the results are compared.
Since structural design requirements related to fire safety focus on safe evacuation and the avoidance of structural collapse, post-fire condition assessment has received relatively little attention so far. However, an adequate post-fire assessment methodology is required if one wants to avoid unnecessary structural refurbishment and demolition, while still ensuring structural safety. In this context, reinforced concrete beams have been subjected to a fire exposure of moderate severity, with the aim of investigating its influence on their dynamic and incremental static behavior. The fire was experimentally simulated with a radiant panel, such that a temperature–time profile similar to the Eurocode parametric fire could be induced at the beams’ surface. During fire testing, the beams were also loaded in a four-point bending test setup to mimic the structural loads that occur in practice. Before and after the fire test, the beams were vibration isolated from the environment and subjected to a modal test, using both conventional accelerometers and fiber-optic Bragg grating (FBG) strain sensors. While the fire event did not result in significant changes in the eigenfrequencies and displacement mode shapes, the strain mode shapes were clearly affected. The neutral axis positions under bending deformation, which could be derived from the strain mode shapes, exhibited significant shifts in the zone that had been exposed to fire. The nature of the shift can be explained by thermal and subsequent nonlinear structural finite element modeling of the fire event. Both the finite element model and additional quasi-static experiments confirmed that in post-fire conditions, the neutral axis under bending in the damaged zone may shift significantly with increased structural loading. This effect can therefore be exploited when assessing the capacity of a structural member in post-fire condition.
This chapter focuses on a seismic monitoring experiment conducted on the bell tower of the San Frediano Church in Lucca, Italy. The tower, dating back to the 11th century, has been fitted along its height with four triaxial seismometric stations which were then left active on the tower for four days. Ambient vibration monitoring provides important information on the structural health of ancient masonry constructions, as it is a non-destructive technique able to capture the most important features of their dynamic behaviour, such as natural frequencies, mode shapes and wave propagation velocities. The chapter presents the results of a seismic monitoring experiment conducted on the bell tower of the Basilica of San Frediano in Lucca, Italy. The tower has been instrumented with four highsensitivity triaxial seismometric stations, left active on the tower from 29 May to 3 June, 2015. This sophisticated instrumentation, usually employed in seismic monitoring networks, had already been installed on the Asinelli and Garisenda towers in Bologna.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
This paper presents the processing methods and the analysis results of ambient vibration data recorded during a sixmonth period on a highway bridge. Data analysis was focused on the variations of the modal parameter related to the accuracy of the estimates and to the temperature effects. The first flexural and torsional modes were estimated with their variance from short acceleration time series records using stochastic covariance driven subspace identification techniques. The frequency variation estimates were compared to the variations induced by structural modifications simulated with a finite element model for assessing the detection threshold level. Subsequently the temperature induced variations in the measured frequencies were analysed. Results of a damage detection test based on the computation of a null-space residual derived from the covariance estimates show the response to the temperature effects. Significant increases in the variance of the parameter estimates were also detected by the test. Finally, the effectiveness of the temperature robust version of this promising damage detection method was investigated with the available data records.
This paper presents a technique for offline time synchronization of data acquisition systems for linear structures with proportional damping. The technique can be applied when direct synchronization of data acquisition systems is impossible or not sufficiently accurate. The synchronization is based on the acquired dynamic response of the structure only, and does not require the acquisition of a shared sensor signal or a trigger signal. The time delay is identified from the spurious phase shift of the mode shape components that are obtained from system identification. A demonstration for a laboratory experiment on a cantilever steel beam shows that the proposed methodology can be used for accurate time synchronization, resulting in a significant improvement of the accuracy of the identified mode shapes.
No abstract is provided for this article.
The FE model updating method can be used to identify unknown structural parameters and determine structural damage. The method consists in minimizing the discrepancies between numerical and experimental modal data by adjusting the FE unknowns. The discrepancies in eigenmodes are usually minimized. Damage functions are used to approximate the stiffness distribution, as an efficient approach to reduce the number of unknowns. First, the Gauss-Newton method is used as a standard sensitivity-based method for local optimization of least squares problems and is ameliorated here by implementing it with the trust region strategy. Secondly, the method of Coupled Local Minimizers (CLM) is a new method applicable to global optimization of functions with multiple local minima. The updating method is applied to two concrete beams tested in laboratory and to a highway bridge, artificially damaged.
Contemporary footbridges are often designed as slender structures and tend to be susceptible to human induced vibrations. Codes of practice have been developed enabling the designer to evaluate the vibration serviceability of the structure based on simplified load models to simulate crowd induced loading. This paper evaluates the methodology of the recent European guideline HiVoSS and the French guideline Sétra, which are widely applied in practice. For a selection of eight slender footbridges, the assessment is performed in design stage, based on the available finite element model, and at completion, based on the in situ identified modal characteristics. Comparison of the initially predicted and the in situ identified modal characteristics shows that uncertainty with respect to the predicted dynamic properties of the structure is inevitable. The methodologies are, however, sensitive to small variations in modal parameters, such as the natural frequencies. As a result, the guidelines in their current form could be exploited by designers to tune the dynamic characteristics of the structure in order to pass the vibration serviceability check. The present contribution recommends a modified load model that leads to a more robust vibration serviceability assessment.
In this paper, an overview is presented of recently developed algorithms and techniques that are available to the Operational Modal Analysis community through the recently developed Matlab toolbox with Graphical User Interface, MACEC Enhanced.
