No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
Two expressions for the strain energy release rate for threedimensional singularity finite elements are derived based on the Irwin's virtual crack closure method. The strain energy release rate for the three modes of fracture mechanics can be separately calculated from the nodal forces ahead of the crack front and the opening displacements behind it. The material is assumed to be isotropic. The stress distribution is written in terms of the eight nodal forces of the crack front element, and the crack opening displacement function is expressed in terms of the nodal opening displacement behind the crack front. The derived formula is a bit complicated because it involves eight nodal forces and five opening displacements. Therefore, a modification is made to get a simpler formula by eliminating some of the nodal forces from the original one. Comparisons with some problems from the literature indicate that the derived formulae are accurate for the computation of the strain energy release rate from 3-D singularity finite elements.
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.
This paper presents the in situ dynamic measurements and the experimental validation of the numerical model for the prediction of the response of a steel–concrete composite railway bridge during the passage of a high-speed train. The dynamic response of the bridge during the passage of high-speed trains is predicted and compared with the experimental data. This study provides a better understanding of the structural behavior of a composite railway bridge under the excitation of high-speed trains.
Quasi-periodic structures, as found in multi-span bridges, multi-bay and multi-storey buildings, are often characterized by a high modal density, even at low frequencies. This clustering of modes poses challenges in operational modal analysis, as well as in finite element model updating, where a pairing of experimental and numerically predicted modes is required. To overcome these difficulties, an alternative method for model updating based on the so-called free wave characteristics has recently been proposed. In this work, this method is applied to calibrate a finite element model of the K032 viaduct of the A11 highway in Bruges, Belgium. Vibration measurements are conducted to experimentally determine the free wave characteristics of the viaduct. In the model updating, the discrepancy between the calculated and identified free wave characteristics is minimized. The results are validated by comparing measured frequency response functions with those obtained from the updated finite element model.
This paper discusses two very relevant practical issues in the application of vibration-based health monitoring to civil engineering structures: the excitation source and the effect of temperature. The idea of vibration-based damage detection is to measure dynamic characteristics such as eigenfrequencies, damping ratios and mode shapes on a regular basis. The state, and eventually degradation, of the structure is reflected in the evolution of these characteristics. Unfortunately, it is not only the health of a structure that influences its measurable dynamics, but also the applied excitation and the changing temperature are important factors and may erode the damage detection potential. In the first part, the results of different excitation types are compared: band-limited noise generated by shakers, an impact from a drop weight and ambient sources such as wind and traffic. In the second part, the undeniable effect of temperature on measured eigenfrequencies is demonstrated and a methodology is proposed to distinguish these temperature effects from real damage events. The method could be validated on a unique data set from a bridge that was artificially damaged after a one-year monitoring period.
No abstract is provided for this article.
No abstract is provided for this article.
Vibration-based damage identification can constitute a successful approach for Structural Health Monitoring (SHM) of civil structures. It is a non-destructive condition assessment method, dependent on the identification of changes in the modal characteristics of a structure that are related to damage. However, the damage identification from the modal characteristics of existing structures currently suffers from a low sensitivity of eigenfrequencies and mode shapes to certain types of damage. Furthermore, the sensitivity of eigenfrequencies to environmental influences may be sufficiently high to completely mask the effect even of severe damage. Modal strains and curvatures are more sensitive to local damage, but the direct monitoring of these quantities is challenging when the strain level is very low. In the present work, the identification of the modal strains of a pre-stressed concrete beam, subjected to a progressive damage test, is performed. Dynamic measurements are conducted on the beam at the beginning of each cycle and its response is recorded with multiplexed Fiber-optic Bragg Grating (FBG) strain sensors. Bending, lateral and torsional modes are accurately identified from dynamic strains with a typical measured root mean square (RMS) strain value of about 0.35 micro-strains. The evolution of the modal characteristics of the beam after each loading cycle is investigated. Changes of the eigenfrequency values, the amplitude and the curvature of the strain mode shapes are observed. The changes in the strain mode shapes appear at the locations where the damage is induced, and are already identified from an early damaged state.
No abstract is provided for this article.
No abstract is provided for this article.
<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel method for the estimation of a finite part of the impulse response is presented. The method is consistent in case the measured output data are prone to colored noise. The combination of Kung's realization algorithm yields a consistent system identification algorithm that is particularly useful for the identification of systems with uncoupled deterministic and stochastic dynamics. This algorithm is called IREAR. A detailed linear sensitivity analysis of the impulse response estimation and Kung's realization algorithms yields novel error and covariance formulae for the estimated impulse response and system matrices and the poles and frequency-response functions that result from the identified system description. </para>
This paper presents the in situ dynamic measurements and the experimental validation of the numerical model for the prediction of high-speed train-induced vibration. The Sesia viaduct is considered, which is a composite railway bridge consisted by seven spans. The responses of the bridge are measured both under ambient vibration and under the excitation of Italian ETR500Y high-speed trains. From the modal analysis of the ambient vibration data, two types of mode shapes are identified by operational modal analysis. Based on the dynamic behavior of the adjacent spans, the modes of the bridge can be distinguished as symmetrical and anti-symmetrical patterns, which indicates that although each span is statically decoupled, the ballast and the rails realize a connection between the adjacent spans of the bridge. To predict the bridge response due to the passage of high-speed trains, two numerical models are considered. First, a train–bridge interaction model for a vehicle system with 15 degrees of freedom is implemented. Second, the train is modeled as a series of moving load. The numerical simulation is validated by comparing the predicted accelerations and strains with measured results, Both models give a good correspondence between the predicted and the measured response. This study provides a better understanding of the structural behavior of a composite railway bridge under the excitation of high-speed trains.
