924 publications from this institution
No abstract is provided for this article.
No abstract is provided for this article.
The vibrational response of railway bridges is an issue of main concern, especially since the advent of High-Speed traffic. In the case of short-to-medium lengths and simply-supported spans excessive transverse acceleration levels may be induced at the platform, with detrimental consequences for passengers and infrastructures. The orthotropic plate has proven to be an appropriate model for the prediction of the response of certain typologies in the aforementioned cases such as multiple girder decks, solid or voided slabs or filler-beam multiple-track decks. In this contribution, the vibrational response of orthotropic plates, simply and elastically supported, circulated by vertical moving loads is investigated. First, maximum free vibration and cancellation conditions are derived analytically. From these, bridge span length-characteristic distance ratios leading to maximum and minimum resonances under series of equidistant loads are depicted. Second, the applicability of these ratios in oblique decks is analysed for the most common first three mode shapes: first longitudinal bending, first torsion and first transverse bending modes, and the errors in relation to the straight reference case are bounded. To this end, an extensive bridge catalogue of girder bridges is designed in the range of lengths of interest, covering flexural stiffnesses typical from both conventional and High-Speed railway lines. Finally, the applicability of the previous theoretical results is exemplified with experimental measurements performed on a bridge from the Spanish railway network.
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.
Identified modal characteristics are often used as a basis for the calibration and validation of dynamic structural models, for structural control, for structural health monitoring, etc. It is therefore important to know their accuracy. In this article, a method for estimating the (co)variance of modal characteristics that are identified with the stochastic subspace identification method is validated for two civil engineering structures. The first structure is a damaged prestressed concrete bridge for which acceleration and dynamic strain data were measured in 36 different setups. The second structure is a mid-rise building for which acceleration data were measured in 10 different setups. There is a good quantitative agreement between the predicted levels of uncertainty and the observed variability of the eigenfrequencies and damping ratios between the different setups. The method can therefore be used with confidence for quantifying the uncertainty of the identified modal characteristics, also when some or all of them are estimated from a single batch of vibration data. Furthermore, the method is seen to yield valuable insight in the variability of the estimation accuracy from mode to mode and from setup to setup: the more informative a setup is regarding an estimated modal characteristic, the smaller is the estimated variance.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
paper introduces a methodology for shape and size optimization of shell structures with variable thickness. A model is defined that reduces the number of variables without losing freedom. Several optimization methods are compared. The method of the Coupled Local Minimizers (CLM) offers the certainty of the identification of the global minimum. This methodology is implemented by using MATLAB and ANSYS. It is used successfully for two instructive examples.
No abstract is provided for this article.
No abstract is provided for this article.
Vibration monitoring from strain data is a promising alternative to acceleration-based monitoring because a dense measurement grid can be achieved at a relatively low cost and because strain mode shapes are more sensitive to local stiffness changes than displacement mode shapes. However, the feasibility of monitoring strain mode shapes of full-scale civil structures, where the operational dynamic strain levels are of very low amplitude and temperature changes can influence the modal characteristics, has remained an open question. The present work provides a proof of concept in which the deck of a steel bowstring railway bridge is instrumented with 80 Fiber-optic Bragg Grating strain sensors, multiplexed in four fibers, that are interrogated with a technique that achieves high accuracy and precision. For more than a year, the natural frequencies and strain mode shapes of 10 modes have been automatically identified from operational strain time histories, with typical root- mean-square values of 0.01 microstrain, on an hourly basis. Furthermore, using these modal data, the influence of temperature fluctuations and that of a retrofitting of the hangers connecting the bridge deck and the bow, which took place during the monitoring period, are extensively investigated. Both have an influence on the overall stiffness of the bridge and therefore they result in clear changes in the natural frequencies. They do not have an influence on the local stiffness and therefore they do not influence the strain mode shapes, except when the retrofitting induces an interaction between previously well-separated modes.</p>
