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Automatic evaluation of the fundamental frequency variations and related damping factor of reinforced concrete framed structures using the Short Time Impulse Response Function (STIRF)
No full textStructural Health Monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures and infrastructures. This paper presents an innovative strategy for automatic evaluation of the variable fundamental frequency and related damping factors of nonlinear structures during strong motion phases. In fact, most of methods for damage detection are based on the assessment of the variations related to dynamic parameters characterizing the monitored structure. In this case, a crucial aspect is a correct estimation of both structural eigenfrequencies and related damping factor also during the nonlinear behavior of a monitored structure. In this paper, nonlinear interferometric analyses combined with the Fourier Transform (FT) are proposed to evaluate the Short-Time Impulse Response Function (STIRF) able to characterize frequencies and damping variations of a monitored structure. Two application of the STIRF are proposed on both numerical and experimental models
Short Time Impulse Response Function (STIRF) for automatic evaluation of the variation of the dynamic parameters for reinforced concrete framed structures during strong earthquakes.
No full textThis study presents an innovative strategy for automatic evaluation of the variable fundamental frequency and related damping factors of nonlinear structures during strong motion phases. Most of methods for damage detection are based on the assessment of the variations of the dynamic parameters characterizing the monitored structure. A crucial aspect of these methods is the automatic and accurate estimation of both structural eigen-frequencies and related damping factors also during the nonlinear behaviour.
A new method based on the nonlinear interferometric analysis combined with the Fourier Transform (FT) (Short-Time Impulse Response Function - STIRF) is here proposed in order to characterize frequencies and damping variations of a monitored structure.
The STIRF approach helps to overcome some limitation derived from the use of techniques based on simple Fourier Transform. These latter techniques provide good results when the response of the monitored system is stationary, but fails when the system exhibits a non-stationary, time-varying behaviour: even non-stationary input, soil-foundation and/or adjacent structures interaction phenomena can show the inadequacy of classic techniques to analysing the nonlinear and/or non-stationary behaviour of structures. In fact, using this kind of approach it is possible to improve some of the existing methods for the automatic damage detection providing stable results also during the strong motion phase. Results are consistent with those expected if compared with other techniques. The main advantage derived from the use of the proposed approach (STIRF) for Structural Health Monitoring is based on the simplicity of the interpretation of the nonlinear variations of the fundamental frequency and the related equivalent viscous damping factor.
The proposed methodology has been tested on both numerical and experimental models also using data retrieved from shaking table tests. Based on the results provided in this study, the methodology seems to be able to evaluate fast variations (over time) of dynamic parameters of a generic reinforced concrete framed structure.
Further analyses are necessary to better calibrate the length of the moving time-window (in order to minimize the spurious frequency within each Interferometric Response Function evaluated on both weak and strong motion phases) and to verify the possibility to use the STIRF to analyse the nonlinear behaviour of general systems
Effetti dell’irregolarità strutturale e dell’interazione dinamica terreno-struttura: tre casi studio
No full textDamage detection on framed structures: modal curvature evaluation using Stockwell Transform under seismic excitation
No full textThe key parameters for damage detection and localization are eigenfrequencies, related equivalent viscous damping factors and mode shapes. The classical approach is based on the evaluation of these structural parameters before and after a seismic event, but by using a modern approach based on time-frequency transformations it is possible to quantify these parameters throughout the ground shaking phase. In particular with the use of the S-Transform, it is possible to follow the temporal evolution of the structural dynamics parameters before, during and after an earthquake. In this paper, a methodology for damage localization on framed structures subjected to strong motion earthquakes is proposed based on monitoring the modal curvature variation in the natural frequency of a structure. Two examples of application are described to illustrate the technique: Computer simulation of the nonlinear response of a model, and several laboratory (shaking table) tests performed at the University of Basilicata (Italy). Damage detected using the proposed approach and damage revealed via visual inspections in the tests are compared
Damage localization on Reinforced Concrete Structures
No full textIt is known that the occurrence of structural damage, for any kind of structure, it is able to changes its dynamic characteristics over time. Generally, the main parameters conditioned by this problem are periods of vibration, damping factors and mode shapes. Several authors showed that also the variations of the modal curvature are strictly related to the damage occurred on a structure. Moreover, using the mode curvature as a control parameter it is also possible to localize where the damage occurred on the structure. In order to analyze the nonlinear
behaviour of a general structure, and localize a possible damage, we propose a band-variable filter based on the Stockwell Transform. This paper shows through many examples as comparing mode shapes and the related curvature variations over time it is possible to easily identify the damage and also localize it on the structure
Non-Stationary vs. Nonlinear Effects Recorded During Earthquakes on Monitored Buildings.
No full textAnalyses of changes in the fundamental frequency of a building is considered the simplest way to detect the onset of structural and non-structural damage. Several authors in the past proposed that the difference in periods that can be observed among ambient noise, and earthquake weak-motion measurements, can also be attributed to transient non-linearity due to reversible modification of the building characteristic (e.g. the degree of coupling between frame and infills in reinforced concrete buildings). Damage to any structure alters its dynamic properties; dynamic monitoring techniques enable the identification of damage by comparing key-parameters before and after the seismic excitation. Short term variations of fundamental frequency observed during a seismic event can be attributed to either non-linearity (i.e. damage) or non-stationary phenomenon (a
particular combination of input and response). This fact may lead to erroneous conclusions attributing variations in frequency due to non-stationary phenomena to the presence of structural damage. Practically, it is possible to confuse apparent fundamental frequency variation, due to non-stationary effects, with real fundamental frequency variations due to nonlinear effects. Then, it is necessary to provide simple criteria to discriminate non-stationary fromnonlinear phenomena.
An innovative approach based on numerical and experimental experiences is proposed: simple thresholds based on the amplitude of the fundamental frequency variation. Further work must be performed in order to fully validate this kind of approach and to completely define these threshold for various structural forms and building typologies
Structural Health Monitoring of Reinforced Concrete Structures using Nonlinear Interferometric Analysis
No full textStructural Health Monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures and infrastructures. Within the Italian research project RELUIS-DPC 2010-2013, funded by the Italian Department of Civil Protection, a specific task deals with the possibility of set up a fast procedure to determine the damage evolution on a large number of structures after seismic events. This paper presents an upgrade of a
method for damage detection based on a statistical approach that uses the most significant data recorded on both the top floor and the bottom of a building, with the purpose of extracting the value of the maximum inter-story drift expected along the building height, adopted as damage indicator. Nonlinear interferometric analyses combined with the S-Transform are used to evaluate frequencies and damping variation of the monitored structure during an earthquake. The method has been tested on numerical reinforced concrete framed structures
Un approccio semplificato per la localizzazione del danno su strutture in cemento armato basato sulla Trasformata di Stockwell
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