Identification of the nonlinear behaviour of a cracked RC beam through the statistical analysis of the dynamic response

SUMMARY This study investigates a new identification procedure suitable to deal with nonlinear systems. The proposed approach is made up of three main parts: system excitation with a band-limited white noise, solution of the Fokker–Planck equation that describes the motion of the structure in a parametric form and identification of the unknown system parameters by minimizing a suitable functional. The new procedure is able, for instance, to assess the severity of cracking caused by the shrinkage or by the overcoming of the concrete tensile strength in reinforced concrete (RC) structures. Cracked RC elements, in fact, exhibit a nonlinear behaviour due to different values of the flexural stiffness that depends on the opening of the cracks. Some numerical simulations allowed verifying the applicability of the procedure. Copyright # 2008 John Wiley & Sons, Ltd

Active displacement control of a wind-exposed mast

The present paper describes the use of an active mass damper for the reduction of the wind-induced displacements of a full-scale mast built for the purpose at the University of Perugia (Italy). After a brief description of the mock-up structure and the bench tests carried out to characterize the active control device, the results of the tests and the numerical simulations performed to optimize the control gain parameters are displayed. The comparison between the results of the numerical simulations and those from the experimental tests highlighted the effects of the unavoidable imperfections of the physical system, such as the limitation of the available power, the presence of friction, the limited extension of the moveable masses' strokes and the computation time delays, that reduce the effectiveness of the control system. With the best gain parameters, several full-scale tests have been executed to observe the behaviour of the structure under wind loads. The comparison between the performance of the uncontrolled and the controlled structure clearly showed the effectiveness of the proposed technique for the reduction of both displacements and vibrations. Copyright © 2006 John Wiley & Sons, Ltd

On the Evaluation of Prestress Loss in PRC Beams by Means of Dynamic Techniques

Abstract In the last few decades, prestressing techniques have been used to build very important structures and infrastructures. Since the serviceability and the safety of prestressed reinforced concrete (PRC) elements rely on the effective state of prestressing, development of tools and procedures capable of estimating the effective prestress loss would be very useful. Amongst other techniques, dynamic identification has proved to be an economical, quick and reliable method to evaluate structural integrity. However, the influence of prestressing in the dynamic behavior of PRC elements is not completely clear. In fact, while many references in the literature state that the prestressing force does not affect the frequencies of vibration, almost every experimental test carried out on PRC beams shows an increase in the eigenfrequencies for increasing value of the prestressing force. This paper aims to contribute to the debate, investigating the dynamic behaviour of PRC beams taking into account properties, such as nonlinearity, softening, confinement and micro-cracking of concrete subjected to compression and tension stress states, and the variation of the flexural stiffness of the PRC beam along its length according to bending stress distribution. Non-linear discrete modelling, in combination with system identification and optimization was used to define the dynamic properties of PRC beams, taking into account the effect of prestressing level. The proposed model was applied to four PRC beams with known mechanical and dynamic properties in literature. The influence of the prestressing force on the trend of frequencies of vibration was closely captured for each beam, with errors less than 3% of the estimated frequencies. The results of this investigation thus indicate that dynamic identification techniques can potentially be used to identify the prestressing force level, and consequently the prestress loss, provided the complete concrete mechanics is taken into account