In the framework of developing a non-destructive damage identification technique, vibration monitoring is a useful evaluation tool that relies on the fact that occurrence of damage in a structural system leads to changes in its dynamic properties. In this paper, a technique will be presented to derive from experimentally determined modal characteristics of a reinforced concrete structure its dynamic bending stiffness. The degradation of stiffness, due to the cracking of the reinforced concrete, gives information on the position and intensity of the occurred damage. From the dynamic stiffnesses in each section, one obtains directly an idea of the extension of the cracked zones in the structure. The technique is validated on a progressively damaged prestressed concrete bridge in Switzerland, on which a series of full modal surveys are carried out before and after applying a number of damage scenarios.
No abstract is provided for this article.
A new efficient global optimisation technique, named Coupled Local Minimisers (CLM), will be investigated in the paper. In CLM, a cooperative search mechanism is set up using an ensemble of local optimisers, each starting from a different point in the search space. The local optimisation processes are coupled ’on-line’ by interaction and information exchange. The combination of a fast convergence, due to the derivative information used in the local algorithms, with the capability of finding the global minimum, resulting from the parallel strategy, guarantees an efficient global optimisation algorithm. The principal idea of CLM is worked out in the paper. A test function with several local minima is minimised successfully. Next, CLM is applied to Finite Element Model (FEM) Updating using experimental modal data and illustrated with a simulated example of a damaged beam. The damage pattern can be identified accurately using the CLM-method.
No abstract is provided for this article.
Resonant metamaterials can achieve unprecedented vibroacoustic attenuation in lightweight partition panels through small resonators attached on a subwavelength scale. Conventional metamaterial treatments are effective only in a narrow band and typically cover the entire panel surface, posing important challenges for industrialization due to manufacturing and cost constraints. To address these issues, in this study we propose graded and partial metamaterial treatments, which are selectively distributed where they are the most effective, avoiding unnecessary coverage, and include a spatially varying resonance frequency for broadband effectiveness. The related design challenges are tackled by developing a numerical methodology that simultaneously optimizes the distribution and the grading of the treatment. The optimization objective is to maximize the broadband diffuse sound transmission loss (STL) of the metamaterial panel, while constraining the maximum fraction of treated area. Leveraging effective medium modelling, the approach efficiently incorporates local changes in the effective mass density of the metamaterial panel to represent partial treatments. Different design cases are considered for different target bands and panel dimensions. Results demonstrate that graded partial metamaterial treatments can achieve broadband improvements with a limited treated area, effectively suppressing the STL dips due to the modes of the host panel and to coincidence effects.
The development of a methodology for accurate and reliable condition assessment of civil structures has become very important. The finite element (FE) model updating method provides an efficient, non-destructive, global damage identification technique, which is based on the fact that the modal parameters (eigenfrequencies and mode shapes) of the structure are affected by structural damage. In the FE model the damage is represented by a reduction of the stiffness properties of the elements and can be identified by tuning the FE model to the measured modal parameters. This paper describes an iterative sensitivity based FE model updating method in which the discrepancies in both the eigenfrequencies and unscaled mode shape data obtained from ambient tests are minimized. Furthermore, the paper proposes the use of damage functions to approximate the stiffness distribution, as an efficient approach to reduce the number of unknowns. Additionally the optimization process is made more robust by using the trust region strategy in the implementation of the Gauss–Newton method, which is another original contribution of this work. The combination of the damage function approach with the trust region strategy is a practical alternative to the pure mathematical regularization techniques such as Tikhonov approach. Afterwards the updating procedure is validated with a real application to a prestressed concrete bridge. The damage in the highway bridge is identified by updating the Young's and the shear modulus, whose distribution over the FE model are approximated by piecewise linear functions.
Options for increasing the sound absorption in an existing room may be highly limited, especially when it concerns architectural heritage. In order to expand the number of possible options for large spaces, sound absorbing chandeliers with integrated lighting have been developed and their performance for reverberation control and for the reduction of background multitalker speech noise has been investigated. Prototypes of several chandelier designs have been constructed by assembling existing absorbing elements into relatively dense arrangements. Reverberation room tests have confirmed that the equivalent absorption area of such a chandelier can be approximately predicted from relevant sound absorption data of its constituting elements. When sound absorbing chandeliers are suspended from the ceiling in a room such that the ceiling is the main absorptive surface, the inhomogeneous absorption distribution over the room results in a directional sound field and this leads to a loss of efficiency for reverberation time control. The loss has been quantified, both numerically for a set of rooms, and experimentally for a large historic room. For the same rooms, the effectiveness of the sound absorbing chandeliers for the reduction of background noise during social gatherings has been assessed with a prediction model that accounts for the Lombard effect. Noise measurements have also been conducted in the historic room, during busy receptions before and after applying chandeliers. They are in good agreement with the predictions, at least when the number of persons in the room is increasing. When it is decreasing, the vocal effort is seen to remain at the maximum level, such that the background noise decreases less than expected. After confirmation with experimental data from a different room, the prediction model has been extended to account for this effect.
