No abstract is provided for this article.
A monitoring campaign has been performed on a filler beam railway bridge for high-speed trains, measuring strains and accelerations due to train passages. For this purpose a very dense grid of measurement sensors was applied to the bottom side of the bridge deck, which allows the calculation of bridge deformations from strain measurements. Within this calculation the location of the cross section's neutral axis plays an important role, since it is required to derive curvatures from strains. The contribution shows how this location can be determined by means of modal analysis, using both acceleration and strain measurement data from decay curves after train passages. After a detailed description of the evaluation method experimental tests on a downscaled bridge deck are presented to prove the reliability of the proposed method. Similar to the situation on the real bridge deck, strain and acceleration sensors were distributed along the longitudinal axis of the test specimen. By measuring not only strains and accelerations but also deformations due to various kinds of excitation the evaluation method could be validated, since the calculated and measured location of the neutral axis and also the calculated and measured deformations were in good agreement. The evaluation method was successfully applied to measurement data from the presented monitoring campaign. The results are of great interest in view of assessing the bridge's dynamic behaviour. They also reveal valuable information about the cross-section properties of the monitored bridge deck which is finally discussed.
A diffuse sound field is conventionally defined as a zero-mean circularly symmetric complex Gaussian random field. A more recent, generalized definition is that of a sound field having mode shapes that are diffuse in the conventional sense, and eigenfrequencies that conform to the Gaussian orthogonal ensemble. Such a generalized diffuse sound field can represent a random ensemble of sound fields that share gross features, such as modal density and total absorption, but otherwise have any possible arrangement of local wave scattering features. The problem of generating realizations or Monte Carlo samples of a conventional diffuse sound field or, equivalently, of the mode shapes of a generalized diffuse sound field, is addressed here. Such realizations can be obtained from an eigenvalue decomposition of the spatial correlation function. A discrete decomposition is numerically expensive when the sound pressures at many locations are of interest, so a fast analytical decomposition based on prolate spheroidal wave functions is developed. The approach is numerically validated by comparison with a detailed room model, where random wave scatterers are explicitly modeled as acoustic point masses with random positions, and good correspondence is observed. Furthermore, applications involving correlated sound sources and sound-structure interaction are presented.
No abstract is provided for this article.
Due to differences in stiffness, mass, damping, internal connections, external boundary conditions between a real structure and a FE model, differences will exist in the vibration characteristics including natural frequencies and mode shapes. To solve this discrepancy, FE updating has been widely used in civil engineering. The uncertain parameters in the FE model are adapted to minimize the differences. The Jalon Viaduct along the high-speed railway line between Madrid and Barcelona is selected as a case study. The viaduct is a continuous box girder supported on high piers. From ambient vibration, mode shapes have been derived in vertical, lateral and longitudinal direction. For the updating the natural frequencies and the three- dimensional mode shapes of the first five lateral and first five vertical modes of the viaduct are used. For this calibration, 14 updating parameters including 12 spring stiffnesses and 2 Young's moduli are selected. The fact that also longitudinal components of the mode shapes are considered helped the updating process. Due to the existence of some local minima of the objective function induced by interfering modes, a Multistart Solver is used to obtain the global solution. It is shown that the relative weight of some residuals has to be adjusted in order to have better results for the longitudinal components of the vertical modes. The updating results in a much better agreement between the measured and the predicted natural frequencies and mode shapes.
In the domain of civil engineering it is often necessary to monitor constructions such as buildings or bridges to maintain safety conditions. Nondestructive test methods outmatch other methods, because they do not influence the constructions performance. One approach is based on the experimental determination of modal parameters (eigenvalues, damping, mode shapes). From the changes in modal data damage can be detected. By updating a reference finite-element model such that its numerical modal data correspond well with the experimental modal data of the damaged structure, the damage pattern can be identified. When applying this technique to prestressed concrete, the problem arises that, because of the prestressing, cracks close again. Therefore a research program was started to investigate the change of modal parameters of a gradually damaged prestressed concrete beam. Static loads with increasing amplitude were applied. In between each load step a dynamic measurement was performed. The results show that the damage detection remains difficult for prestressed concrete in the early damage state. However, a damage pattern could be identified, which corresponds to the locations of the final failure.
The technique of Finite Strip method has been extended for zoned strips. One strip can have several different materials in horizontal direction including dummy materials. The new developed zoned Finite Strip (Z.F.S.) method has many engineering applications, particularly in geomechanical problems. The formulation is presented and three different geotechnical problems have been solved using the new Z.F.S. method, which could not be solved by the original Finite Strip method. The results are compared with the solution for the same problems obtained by analytical and/or numerical (Finite Element) methods. The reduced input and output for the Z.F.S. method compared to the Finite Element method makes this method attractive particularly for practising engineers. However the results can be as good as the well known Finite Element method.
Propelled by its small carbon footprint, low weight and high stiffness, cross-laminated timber (CLT) has experienced a significant commercial growth in the last decade. However, these latter two features potentially contribute to poor acoustic performance. A critical factor is flanking sound, in which vibrational energy is transmitted between two elements connected through a common junction, driving the need to include vibration isolation solutions such as resilient interlayers in the junction design. In this work, the vibration reduction index across CLT junctions is predicted with an analytical wave approach for semi-infinite thin plates. Each CLT panel is modelled as an orthotropic or isotropic equivalent single layer (ESL). Multiple options to determine the ESL properties are compared, including the law of mixtures and the modified gamma method. The interlayer is considered to be a thick, flexible waveguide, whose out-of-plane motion is governed by shear. The prediction model is validated experimentally with on-site measurements for a vertical T-junction with a polyurethane foam interlayer. The predictions are moderately accurate in the entire frequency range, with deviations below 10 dB across all 1/3 octave bands. Depending on the layer configuration, the orthotropy of the ESL can significantly influence the predictions.
This paper investigates under which conditions dynamic train–bridge interaction should be considered for the dynamic analysis of a bridge during a train passage. The results of a moving load model are compared to those of an analysis of dynamic train–bridge interaction considering different vehicle models with a varying degree of sophistication. The effect of several parameters related to the train and the bridge is studied. The ratio of the mass of the vehicle and the bridge and the ratio of the natural frequency of the vehicle and the bridge, the train speed and the damping ratio of the bridge are identified as significant factors that determine the effect of dynamic train–bridge interaction on the bridge response.
Algorithms for structural identification and damage detection of steel-concrete composite bridges
A new optimisation method to find the global minimum of a cost function, named Coupled Local Minimisers (CLM), is presented. In CLM, several local optimisation processes are coupled, i.e. they interact and exchange information, resulting in better solutions than multi-start local optimisation with independent runs. The principal idea of CLM is worked out and illustrated with a mathematical test function, on which successful optimisation runs are carried out. Next, CLM is applied to finite element (FE) model updating. FE models of civil engineering structures are updated by minimising differences between experimental and numerical modal data, i.e. natural frequencies and mode shapes. The former reflect the global dynamic behaviour and can be measured quite accurately, the latter contain local information but the measured values are more noisy.
No abstract is provided for this article.
The efficiency of any damage detection and localization technique depends on the care with which the damage is modeled. Usually, to simulate damage, we either reduce the Young modulus or cross sectional area of the elements housing the damage. Certain authors suggest more realistic damage models that help locate more precisely the damage in the structure. These models range from very simple to more complex damage models. Here is particularly considered the flexibility variation method. The objective of this work is to compare different damage models concerning their efficiencies in detecting and localizing not just one damage but a few, whether they be very distinct or close.
No abstract is provided for this article.
The authors would like to express their gratitude for the Fundacao para a Ciencia e Tecnologia, from Portugal, for providing a doctoral scholarship to the first Author, Contract SFRH/BD/24688/2005.
