924 publications from this institution
The application of the multi-level substructuring technique to the analysis of elasto-plastic and local nonlinear problems is studied. After a review of principles and formulations a practical implementation and a typical flow chart of the procedure for the technique are presented. Two numerical examples are provided to demonstrate the efficiency that can be reached by using a multi-level substructuring algorithm.
The Palmas Altas footbridge is a recently built bridge in Sevilla (Spain) designed by architect Richard Rogers.It is a 78.6m long steel-concrete box girder.The bridge has three spans, with two pairs of intermediate oblique piers.The cross section of the box girder and the piers are variable along the bridge length.The footbridge has a cylindrical roof with a complex structural geometry, made of elliptical crossing bars and a cylindrical shell.Before being taken in use, the structural response of the footbridge was experimentally identified by an operational modal analysis.Mode shapes and frequencies have been clearly identified.In addition, the static response of the structure was obtained from a static load test.A detailed finite element model has been built for analyzing both the static as well as dynamic behavior of the bridge.A model updating process has been performed to improve static and dynamic response predictions.The present analysis is mainly focused on the in plane behavior of the bridge, for which both static and dynamic experimental information is available.Special attention is paid to the role of the bearings and foundations in the structural response.It is shown that the measured static response provides useful information about the response of the bridge and its foundation, which is not available from a dynamic analysis only.
Dynamic testing of civil infrastructure is required for reasons ranging from, for example, checking performance of new or retrofitted structures, evaluating and predicting vibration serviceability, calibrating structural models and establishing baseline performance for structural health monitoring. There are also, in limited cases, possibilities to identify, characterise and diagnose structural defects. For the above reasons and with increased interests in performance of constructed facilities, challenging new applications for dynamic testing are found and met with improving technology. Together with advances in instrumentation capabilities, there have been in the last decade great advances in techniques for system identification, allowing for effective dynamic testing of practically any kind of structure. Among dynamic testing technologies, forced or artificial excitation of civil structures and system identification procedures borrowed from mechanical/aerospace disciplines have proven ability to identify true mode shapes in the most demanding conditions (strong noise, large scale and having closely spaced, complex and even nonlinear modes). At the other end of the spectrum, operational modal analysis (OMA) now has proven capabilities for structures where forces cannot be applied or used effectively, to the extent that OMA now appears to be the preferred choice. We find that in some cases both procedures may be required for full identification, and a hybrid form that uses a mix of artificial and ambient excitation has been developed in order to extract the missing mode scaling (mass) parameter for OMA. The paper presents a sample of techniques in different applications to bridges, illustrating strengths and weaknesses, to provide engineers with information to understand the process of dynamic testing and appreciate the validity of the results.
The ISO 354:2003 standard relating to sound absorption measurements is currently under revision to improve the reproducibility of the procedure it describes. Round robin tests conducted across various reverberation rooms indeed revealed significant disparities between sound absorption measurements of the same sample. One of the reasons is that, at low frequencies, the sound field in a single laboratory cannot be considered fully diffuse. However, the average sound field across different laboratories may be considered diffuse if the interaction between the finite sample and the diffuse field is duly accounted for and the direct field close to the absorber is disregarded. In this work, a method is developed for optimizing reverberation room design such that measured absorption values are as close as possible to ensemble average diffuse values. The reverberation room is modelled using the finite element method and standardized measurements of an absorptive sample are simulated. The distance between resulting absorption coefficients and diffuse target values is minimized in an optimization procedure having the geometrical characteristics of the model as input parameters. The results are anticipated to participate to the revised ISO 354 as guidelines for the construction of new reverberation rooms or the improvement of existing ones.
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An exact stiffness formulation for harmonic and transient wave propagation in multilayered dry, saturated and unsaturated poroelastic media is used to study the effect of full and partial saturation on harmonic wave propagation in an axisymmetric poroelastic layered halfspace. The layering is due to a water table at a depth H below the free surface.
In the present paper, a comparison is made between the Coupled Local Minimizers (CLM) method and the Differential Evolution (DE) algorithm to perform FE model updating for the damage detection in a cracked beam. CLM method is a gradient-based method with multiple local optimization runs. DE algorithm is a direct search approach which uses a population of solution vectors collecting the design parameters. Two benchmark examples of damage assessment are considered, i.e., beams under flexural vibrations with one crack and two cracks, with unknown position and depth. The effectiveness of the two methods to obtain the set of unknown parameters has been verified by performing a number of optimization processes starting from initial values of parameters selected randomly. Both exact and pseudo-experimental input data are used. A statistical analysis of the optimization results is presented. Both methods give results much better than the classical gradient optimization method. Better performances in term of speed rate and precision have been obtained by CLM when the number of identified parameters is limited. On the other hand, DE shows good efficiency when the number of parameters increases or in the case of pseudo-experimental input data.
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In this paper, two stochastic system identification methods are compared. Stochastic means that the method has to cope with output-only data (stochastic, unknown input). The first method is a stochastic subspace algorithm that finds the system matrices of a stochastic state space model. It is a linear method but said to be suboptimal: it is not based on the minimization of a criterion. The second method is a prediction error method that finds the parameters of a multivariable ARMA-model in a nonlinear, iterative way. Both methods are discussed and applied to experimental data from a dynamic test on a concrete beam. The uncertainties of the estimated modal parameters are compared. The determination of these uncertainties is very relevant for structural monitoring based on dynamic measurements. In this way, changes of the dynamic characteristics due to structural modifications (e.g. damage) can be separated from random changes which fall within the uncertainties of the estimated modal parameters.
No abstract is provided for this article.
Three expressions for the strain energy release rate for three-dimensional singular and non-singular finite elements are derived based on Irwin's virtual crack-closure method. The strain energy release rate for the three modes of fracture mechanics can be expressed in terms of the nodal forces ahead of the crack front and the opening displacements behind it. The material is assumed to be isotropic. The formulae are derived for three different element types, namely, 20-node singular element, eight-node and 20-node non-singular elements. The validity of the derived formulae is checked by comparing their results to those from the literature for mode I and mixed mode I and II problems.
No abstract is provided for this article.
The main limitations in the finite element (FE) model updating technique lie in the ability of the FE model to represent the true behavior of the structure (modelling problem), and in the ability to identify enough modal parameters with sufficient accuracy, especially for large structures that are tested in operational conditions (identification problem). In this paper, the identification problem is solved with an OMAX approach, where an artificial force is used in operational conditions and a structural model is identified that takes both the forced and the ambient excitation into account. From an extensive case study on a real three-span bridge, it is observed that, while updating the FE model using the experimental output-only data yields a good fit, discrepancies show up when the more extensive set of OMAX data is used for validation, or even for updating. It can be concluded that an OMAX approach not only increases the well-posedness of the updating problem, it also allows to detect potential inaccuracies in the FE model.
