C Valente

An interval-based technique for FE model updating

Model updating techniques are largely used in civil and mechanical engineering to obtain reliable FE models. The model parameters are iteratively adjusted until the model response matches the measured structural response within a given tolerance. The task is made difficult by the uncertainty that affects both the structural response and the model parameters. In this work, the uncertainty is taken into account by a proper formulation of the problem in the framework of interval analysis. Uncertainty is replaced by interval numbers, functions are replaced by their thin or thick natural extension and the inclusion theorem is exploited to find the problem solution. Sample applications are illustrated using a modal representation of the structure. A numerical example is used to discuss the potential of the proposed method. A case study is solved to demonstrate the advantages of the method with respect to conventional updating techniques.

An Interval Uncertainty Based Method for Damage Identification

The problem of damage identification in presence of uncertainties is faced up in the framework of interval analysis. A method previously developed by the authors in the context of model updating and global minimization for dynamic problems is applied to identify the damage in framed structures. The inclusion property of the interval analysis is exploited to find the bounds of the physical solutions. Model parameters, experimental measures and modelling errors are considered as possible sources of uncertainty. The advantages of the interval approach are discussed through numerical simulations involving the different kind of uncertainties.

Design and testing of corrosion damaged prestressed concrete joists: the Pescara Benchmark

An experimental campaign named the Pescara benchmark and devoted to study the dynamic behaviour of corroded p.c. joists has been conducted. The steel corrosion reduces the area of the reinforcement and causes cracking of concrete so that r/c members are subjected to loss of strength and stiffness. It is of interest to evaluate the corrosion level at which the damage can be detected through signal processing procedures and how close such level is to the r/c member safety limits. Joists of current industrial production having different steel to concrete ratios are tested in different laboratory conditions. Dynamic tests involve either free vibrations and forced vibrations due to a moving mass simulating actual traffic loads in railway bridges. The paper discusses the rationale of the tests including the set up of the artificial corrosion, the static characterization of the joist and the dynamic tests in the different stages of corrosion experienced.

Effects of modal density in system identification using the Hilbert transform

The paper aims to review and deepen the effects of the modal density in the dynamical identification with the Complex Plane Representation (CPR) method that is based on the Hilbert transform of the motion response of mechanical systems. It is demonstrated that the strong accuracy in the identification of the modal parameters with the CPR method can be somewhat corrupted for high modal densities. Theoretical and numerical solutions are used to analyze the problem and validate the CPR results.

Experimental Modal Analysis for Damage Detection of Buildings via Gabor Transform Technique

The paper presents the revised formulation of a signal processing methodology aimed at identifying the dynamics of buildings hit by earthquakes. The methodology is based on the Gabor joint time-frequency representation and allows to identify an evolutionary modal model whose time changes are used to evaluate the damage presence and severity. Dynamic tests are performed in advance to provide reference values and their confidence ranges used for the assessment of the results. The methodology has been extensively applied to the buildings belonging to the Italian national network of seismic structural monitoring. A sample application referred to a representative reinforced concrete buildings is shown.

Railway Bridges Identification Techniques

Output-only methodologies are nowadays well established to extract modal parameters in many areas of engineering, such as civil, mechanical and aeronautical. In the past, civil engineering tests have been mainly developed for road bridges, with the vehicle passage over the bridge deck representing the main source of excitation with some contribution given by the ambient noise. In the road bridge cases, the excitation is considered to be a function of the road surface roughness, the vehicles speed, the weight and suspension vehicles characteristics, and also the random access of the vehicles over the bridge, whilst for the railway case, not all these issues are correctly addressed, and other characteristics rise-up, possibly advantageous for a correct identification process; to demonstrate this statement, we can bear in mind how the random access of the vehicles becomes meaningless for railway bridges, the single train being a quasi deterministic source; furthermore, the influence of the train weight should be considered if compared to usual road vehicles. Since output-only techniques are conceived for random excitation noise, their use in these conditions is considerably stressed and special care, or alternative techniques, has to be considered to avoid errors. In this sense, the bridge reference model becomes more important and some special techniques have to be developed.Copyright

Sensitivity of modal parameters for damage detection in corroded beam elements of the Pescara benchmark

Sensitivity and identifiability problems of the modal parameters in the presence of corrosion damage are studied. The first concerns the rate of change of the modal parameters against the damage increase. The second concerns the uncertainty intervals overlapping of the modal parameters between the sound and damaged states. To this end, different testing methods, different identification methods (time, frequency and time-frequency methods), different corrosion levels and different thermal condition (summer-winter) are considered. Prestressed concrete beams of identical geometry endowed with low to high reinforcement ratios are dynamically tested. The free decaying vibrations are used to identify the modal parameters: frequency, modal shapes and damping. The prestressing force is found to be not a variable of the problem. The damage levels range from very low to moderate. It is found that reliable damage identification is possible only for moderate damage. In spite of higher scatter, damping reveals a more stable and sensitive indicator than frequency. Modal shapes shows regular changes, but within the range of the uncertainty intervals.

MODAL ANALYSIS OF A FRAME MODEL UNDER UNMEASURED SEISMIC INPUT

The paper reports the first results of an experimental campaign aimed at the validation of a procedure, proposed in previous theoretical works by one of the writers, for the structural identification in frequency domain of linear structural systems subjected to base seismic excitation. Differently from the classical identification techniques based on the transfer-function processing, that require to measure the excitation (input-output problem), such procedure allows the solution of the identification problem also in case of unknown input (output-only problem) if the responses of at least three degrees-of-freedom are available for the system. An experimental campaign has been carried out on a three storeys shear-type frame, carefully designed in order to minimize the uncertainties and errors. After validating the model setup and the measuring chain by means of a classical input-output procedure, the paper discusses the first results of the output-only identification. The outcomes show a fair agreement with the results obtained by including the input knowledge.