Vibration-based methods are among the most widely studied in structural health monitoring (SHM). It is well known, however, that the low-order modes, characterizing the global dynamic behaviour of structures, are relatively insensitive to local damage. Such local damage may be easier to detect by methods based on wave propagation which involve local high frequency behaviour. The present work considers the numerical analysis of wave propagation in cables. A two-step approach is proposed which allows taking into account the cable sag and the distribution of the axial forces in the wave propagation analysis. In the first step, the static deformation and internal forces are obtained by the finite element method (FEM), taking into account geometric nonlinear effects. In the second step, the results from the static analysis are used to define the initial state of the dynamic analysis which is performed by means of the spectral element method (SEM). The use of the SEM in the second step of the analysis allows for a significant reduction in computational costs as compared to a FE analysis. This methodology is first verified by means of a full FE analysis for a single stretched cable. Next, simulations are made to study the effects of damage in a single stretched cable and a cable-supported truss. The results of the simulations show how damage significantly affects the high frequency response, confirming the potential of wave propagation based methods for SHM.
No abstract is provided for this article.
No abstract is provided for this article.
Interval-based uncertainty models have proven to be well-suited for structural safety engineering with few data at hand. The practical use of interval analysis is hindered by the dependency problem, which leads to an overestimation of the uncertainty on the results. Affine arithmetic is a generalization of interval arithmetic that accounts for the relation between variables. By circumventing the dependency problem, it yields more accurate results. This paper presents a novel method to solve affine systems of linear equations, which allows for the application of affine arithmetic in finite element analysis. The proposed procedure is illustrated with three applications.
Headed studs are widely used in steel–concrete composite bridges to resist longitudinal shear forces at the interface of steel girder and concrete slab. These studs are subjected to high-cycle fatigue loading due to the growth of traffic and increase in train speed. Within the frame of this paper, the dynamic structural behavior of the shear studs during train passages is studied. Different fatigue endurance models are employed for fatigue life estimation. A parametric study is performed to investigate the effects of different parameters that influence the fatigue life of shear studs. Finally a fatigue-life-cycle design procedure based on the train–bridge interaction analysis and the fatigue endurance model is proposed.
The measurement of sound absorption in reverberation rooms following the ISO 354:2003 standard relies on Sabine's equation to derive absorption coefficients from reverberation times. This equation assumes perfect diffusivity, i.e. the sound field is composed of many statistically independent plane waves with uniformly distributed spatial phases, themselves uncorrelated to the corresponding amplitudes. In this work, both existing and fictitious reverberation rooms are numerically modelled using the finite element method. Finite porous absorbers are introduced in the rooms as equivalent fluid models. Standardized sound absorption measurement are simulated in the rooms through the determination of reverberation times. The respective effects of the sample size, sample placement, source positioning, and presence of finite panel diffusers are investigated. The resulting absorption coefficients are then confronted to the theoretical values in a perfectly diffuse sound field, that interacts with a baffled, finite-sized absorber, as obtained with a hybrid deterministic-statistical energy analysis model. The process notably underlines the strong, yet often disregarded, beneficial effect of panel diffusers at low frequencies in highly regularly-shaped rooms. Another conclusion of this work is that reverberation room design represents a crucial factor that can influence sound absorption measurements at low frequencies.
Impact sound insulation of floors is one of the main concerns when designing multi-story buildings. While conventional floor systems use flat or regularly ribbed configurations, interesting opportunities to improve performance are given by enhancing the design of the load-bearing slab. In this work, we propose a methodology to maximize broadband impact sound insulation by designing slabs with a non-uniform material thickness distribution, which is numerically optimized. In the design process, efficient and accurate sound insulation predictions are employed through a detailed finite element floor model coupled with a diffuse room model. In particular, we minimize a Single Number Quantity (SNQ) that represents the aggregate broadband sound pressure level in the receiving room under point-load excitation. The design of single slab floors is considered first. Results indicate that, even if the optimization effectively minimizes the sound pressure level in the different third-octave bands, it does not lead to noteworthy decreases in the SNQ. The design of floating floors is then considered. In this case, a better overall performance is obtained, corresponding to significant improvements at low frequencies and a slight degradation at high frequencies, where, nevertheless, floating floors already perform well. This slight high-frequency decline is linked with the resonances of the thin sub-plate regions present between the optimized ribs, akin to what is observed in waffle ribbed floors. Finally, the robustness of the optimized results against changes in the initial guess is assessed by showing that no SNQ improvements are obtained when transitioning from flat to waffle ribbed initial design.
No abstract is provided for this article.
A vibration serviceability assessment of footbridges is required in design stage to evaluate the response to human-induced excitation. If the calculated response does not meet the desired criteria for vibration comfort, a Tuned Mass Damper (TMD) can be installed as a vibration mitigation device. The TMD mass, stiffness and damping constant must be tuned to the modal parameters of the structure to obtain the desired response reduction. The response prediction and TMD design rely on a good knowledge of the modal parameters of the footbridge and usually assume that the response is governed by a perfect harmonic loading causing resonance. Differences between the predicted and actual modal parameters may lead to both an unreliable response prediction and a suboptimal TMD performance. Therefore, a robust design of the TMD is proposed accounting for uncertainties in the modal parameters. Realistic walking scenarios of continuous pedestrian traffic are simulated to obtain an effective response reduction by the TMD. In this contribution, the robust design of a TMD is demonstrated for a slender footbridge accounting for reasonable levels of uncertainties in the modal parameters. Numerical optimisation is therefore applied and vibration serviceability requirements are imposed as design constraints. The obtained TMD parameters are compared to those obtained from the formulae proposed by Asami, determining the stiffness and damping constant as a function of its mass. It is found that the TMD mass can be further reduced when the TMD parameters are tuned independently from each other compared to the classical design according to Asami but a higher computation cost is needed. Furthermore, an increasing degree of robustness against variations in the modal parameters is obtained by increasing the TMD mass and damping.
Footbridges are often designed as slender structures, sensitive to human-induced excitation. In the case where the prevailing vibration serviceability requirements are not met, vibration reduction measures such as tuned mass dampers (TMDs) are needed. Both the prediction of the structural response and the design of the TMD rely on the modal parameters of the footbridge. Measurements after construction of footbridges have shown that it is difficult to accurately predict these modal parameters even with detailed finite element models based on structural drawings. Moreover, these parameters evolve in time due to changing environmental conditions and degradation. It is important to take into account these uncertainties in the vibration serviceability assessment as well as in the design of vibration reduction measures. The present paper proposes a robust optimisation approach for the design of a TMD which accounts for uncertainties in the modal parameters. The aim is to minimise the mass of the TMD, assumed as a measure for the cost, by tuning the mass, stiffness and damping values, while guaranteeing that vibration serviceability is satisfied for a range of possible values of the natural frequency and modal damping. In order to investigate the trade-off between the mass of the TMD (cost) and the level of uncertainty, a multi-interval approach is adopted. The optimal parameters are found to change significantly with the level of uncertainty. The TMD mass and damping increase for a higher level of uncertainty to satisfy the vibration serviceability requirements in all possible cases.
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.
