924 publications from this institution
Damage is introduced in reinforced concrete beams, by subjecting them to shortterm static 3-point bending tests. The maximum static load is increased in a stepwise manner, so that different degrees of damage are obtained. In between each static loading cycle, the modal parameters are measured using different techniques. This allows to draw conclusions about the sensitivity of the modal parameters to structural damage and about the suitability of the experimental techniques. This investigation is the initial phase in the development of a technique for monitoring damage in large concrete structures based on dynamic system characteristics (DSC's).
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.
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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.
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Coupled Local Minimizers (CLM) is a new method applicable to global optimization problems. With the CLM method a cooperative search mechanism is set up using a population of local optimizers, each starting from a different point in the search space. The individual local optimizers are coupled during the search process by interaction and information exchange. The combination of a fast convergence, due to the derivative information used in the local algorithms, with the expected capability of finding the global minimum, resulting from the parallel strategy, offers an efficient global optimization algorithm. The principal idea of CLM is worked out in the paper and illustrated on a test function. Next, CLM is used for Finite Element Model (FEM) Updating using experimental modal data. As an example the damage pattern is identified of a reinforced concrete beam, on which a modal test has been carried out before and after it was damaged.
A 2-D four-noded finite element which contains a <TEX>${\lambda}$</TEX> singularity is developed. The new element is compatible with quadratic standard isoparametric elements. The element is tested on two different examples. In the first example, an edge crack problem is analyzed using two different meshes and different integration orders. The second example is a crack perpendicular to the interface problem which is solved for different material properties and in turn different singularity order <TEX>${\lambda}$</TEX>. The results of those examples illustrate the efficiency of the proposed element.
In this work, an efficient and accurate framework for the prediction of the direct impact sound insulation of layered floors is presented. Both floor models of infinite lateral extent and with simply supported boundaries are considered. For the infinite floors, closed-form mechanical admittance expressions are provided which relate directly to the transfer matrix elements of the conventional transfer matrix method (TMM). For floors with simply supported boundaries, an image source argument is employed for underpinning the modal version of the TMM, or mTMM, which approximately accounts for the boundary conditions by considering admissible trace wavenumbers only. It is demonstrated that both for the TMM and the mTMM, the radiated sound power can be evaluated directly in the frequency–wavenumber domain from a simple integral. The accuracy of the mTMM is confirmed in a numerical verification using detailed finite element models. The method is also extensively validated with laboratory measurements for a set of five heavy floors with stiff and resilient layers. A very good agreement between measurements and mTMM predictions is generally observed. The TMM and mTMM approaches yield very similar results above the critical frequency of the base floor, yet at lower frequencies, the inclusion of the finite-size and boundary conditions effects make the mTMM substantially more accurate.
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