In combined vibration testing, an artificial, measured force is used in operational conditions. This requires the identification of a system model that takes both the measured and the operational excitation into account. Advantages with respect to the classical operational modal analysis approach are the possibility of obtaining mass-normalized mode shapes and the increase of the excitation level and its frequency content. An advantage with respect to the classical experimental modal analysis approach, where the ambient excitation is not modeled, but considered as disturbing noise, is the possibility of using excitation levels that are of the same amplitude, or even smaller, than the ambient excitation levels. In this paper, combined modal testing of footbridges is explored using two case studies: a steel arch footbridge with spans of 75.2 m and 30.3 m and a concrete stress-ribbon footbridge with spans of 30 m and 28 m. The comparison of the modal parameters (eigenfrequencies, damping ratios, mode shapes, and modal scaling factors) obtained from a combined vibration test with the ones obtained from other modal tests and from a finite-element model, demonstrates the feasibility of using small and practical excitation devices for the modal testing of footbridges.
No abstract is provided for this article.
No abstract is provided for this article.
In vibration-based damage detection techniques, the change in modal data is used as an indicator to detect and to identify the damage in the structure. An inverse problem is solved that consists in predicting the location and severity of the damage, given the structural dynamic characteristics before and after the damage.
No abstract is provided for this article.
Tubular structural elements with diameters in the range from 200 mm to 1400 mm are widely used in onshore and off-shore constructions. Typical examples include (drilling) platforms, bridges, and pipelines. In most cases several tubular elements (made of steel) are combined by means of weld joints. The current welding procedures for high strength steel are very time-consuming and thus constitute an important part of the total construction cost. In some applications, the need is inherent for pipes being frequently coupled and uncoupled as in the case of drill pipes and tension-leg platforms. Therefore, the application of threaded connections is considered as a valuable alternative and their design as an important challenge, especially the proof of sufficient fatigue strength. In the case of drill strings, fatigue failure is caused by cyclic load during drilling operations. In other applications, dynamic loads can be caused by environmental and operational conditions, e.g. wind, waves, vortex induced vibrations, internal pressure changes, etc. In this context, fatigue damage identification at an early stage plays an important role on the integrity of the structure. The present work aims at assessing progressive damage in threaded connections subjected to cyclically varying loads by means of vibration measurements. For this purpose, a four-point bending fatigue test with standard connection has been setup [1, 2] and a damage assessment approach has been developed based on dynamic system identification. As damage in a threaded connector is a local phenomenon, which may not significantly influence the lower frequencies or the global response, it is essential to gather accurate information about natural frequencies and mode shapes of many vibration modes [3]. The progressing damage will be compared with the forward predictions of a physical model that is able to predict the stiffness degradation as a function of an arbitrary, but known, load history [4].
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
For slender footbridges, the dynamic response to human-induced loading needs to be verified in a vibration serviceability assessment. The assessment is commonly based on a simplified load model and relying on the nominal values of the modal parameters of the structure as predicted by a finite element model. Often, resonance with a single mode is considered and a reduction factor rates the risk for resonance with that mode. For a reliable response prediction, uncertainties in the modal parameters of the structure must be dealt with, however. If the vibration levels exceed a predefined comfort criterion, a Tuned Mass Damper (TMD) can be installed. In this contribution, a robust vibration serviceability assessment is proposed. Different levels of uncertainty are introduced to account for the effect of uncertainties in the modal parameters resulting in a multi-interval assessment. Realistic loading scenarios are implemented using detailed simulations considering variability of the pedestrian walking characteristics. The contribution of multiple modes is considered and the instantaneous peak acceleration is retained and tested to criteria for vibration comfort. The procedure is illustrated for a slender footbridge. First, the response is evaluated for different levels of uncertainty. Second, the procedure is applied in the design of a TMD by tuning its mass, stiffness and damping constants such that an effective reduction of the vibration levels is ensured. The advantage of the robust interval-based approach is that it allows understanding how the uncertainties in the modal parameters of the structure affect the response prediction. Besides, the method investigates the trade-off between the TMD parameters and its robustness against uncertainties in the modal parameters. The level of uncertainty can be determined based on a monitored or assumed range of variations of the modal parameters of the structure.
This work is the outcome of the interest that the Board of Executives of the lASS showed on the papers presented at the lASS-Symposium in Osaka (1986)
The modal parameters of a structure that are estimated from ambient vibration measurements are always subject to bias and variance errors. In this paper, it is discussed how part of the bias errors can be removed and how the variance errors can be estimated from a single ambient vibration test. The bias removal procedure makes use of a stabilization diagram. The variance estimation procedure uses the first-order sensitivity of the modal parameter estimates to perturbations of the measured output-only data. This methodology, that is generally applicable, is illustrated here for the reference-based covariance-driven stochastic subspace identification algorithm. Both simulated and measured vibration data are used to demonstrate the accuracy and practicability of the derived expressions.
No abstract is provided for this article.
When using the analysis of vibration measurements as a tool for health monitoring of bridges, the problem arises of separating abnormal changes from normal changes in the dynamic behaviour. Normal changes are caused by varying environmental conditions such as humidity, wind and most important, temperature. The temperature may have an impact on the boundary conditions (frozen soil) and the Young's modulus of the material of which the structure consists. Abnormal changes on the other hand are caused by a loss of stiffness somewhere along the bridge. It is clear that the normal changes should not raise an alarm in the monitoring system (i.e. a false positive), whereas the abnormal changes may be critical for the structure's safety. This paper tries to give an answer to the question whether it is possible to separate the environmental influences from damage events. In the frame of the European SIMCES-project, the Z24-bridge in Switzerland was monitored during almost one year before it was artificially damaged; what makes it an excellent object to study methods that try to filter out the environmental influences. The paper presents the results of the measurements on the post-tensioned concrete bridge and shows that it is indeed possible to distinguish between abnormal and normal changes of its dynamic characteristics.
