IRIS Pol. Torinohttps://iris.polito.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Sat, 14 Dec 2019 15:13:11 GMT2019-12-14T15:13:11Z10301Enhanced linearized flutter analysis of suspension bridgeshttp://hdl.handle.net/11583/2651521Titolo: Enhanced linearized flutter analysis of suspension bridges
Abstract: We investigate the dynamic stability of a single-span suspension bridge deck, subjected to a uniform transverse wind, through a linearized continuous model. The deck, modelled as an elastic beam of constant cross-section, inextensible and unshearable though deformable in bending and torsion, is connected to the main cables by a continuous system of rigid suspenders; both primary (i.e., Saint Venant) and secondary (i.e., Vlasov) torsional rigidities are taken into account. The integro-differential equations governing the dynamic equilibrium of the deck are derived at first by considering the steady wind component (mean wind). Two separate solutions are obtained, by the Galerkin method, for the antisymmetric and symmetric oscillations, giving the natural frequencies as functions of the aerodynamic loads. Afterwards, dynamic stability (i.e., flutter) is studied by including the unsteady aerodynamic loads in the equations of motion.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26515212016-01-01T00:00:00ZPZT Experimental Detection of Natural Frequencies for Compressed Thin-Walled Beamshttp://hdl.handle.net/11583/2651471Titolo: PZT Experimental Detection of Natural Frequencies for Compressed Thin-Walled Beams
Abstract: By means of PZT pickups, we experimentally measured the natural frequencies of an aluminum thin-walled open profile under centered compression, and investigated the effects of end warping constraints on free vibration and buckling loads. The specimen was mounted on a MTS testing machine, controlling the axial displacement imposed to a beam end by the hydraulic jack. The free vibration frequencies were detected for different values of the compressive force. The experimental results are compared to those provided by an in-house numerical code, which investigates the elastic stability of thin-walled beams in a dynamic setting. Indeed, the code is able to follow the paths of natural frequencies versus the applied load, accounting for the effect of cross-sectional warping on both natural frequencies and buckling loads. The experimental results show that piezoelectric pickups, like the disk adopted, can be efficiently used for the experimental modal analysis of engineering structures.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26514712016-01-01T00:00:00ZOn the Use of Piezoelectric Sensors for Experimental Modal Analysishttp://hdl.handle.net/11583/2651476Titolo: On the Use of Piezoelectric Sensors for Experimental Modal Analysis
Abstract: Piezoelectric disk buzzers are commonly used on stringed musical instruments to acquire the sound in the form of a voltage signal. Aim of the present investigation is to assess the possibility of using these transducers for experimental modal analysis. Piezoelectric disks were therefore used in the laboratory to extract the natural vibration frequencies and mode shapes of an aluminum cantilever beam and of a steel arch. The results are compared with theoretical predictions and with other experimental values obtained using a laser displacement transducer and accelerometers. Due to their high accuracy, small dimensions, low weight, easy usage, and low cost, piezoelectric disks seem to be an attractive tool for experimental modal analysis of engineering structures.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26514762016-01-01T00:00:00ZExperimental modal analysis of straight and curved slender beams by piezoelectric transducershttp://hdl.handle.net/11583/2655511Titolo: Experimental modal analysis of straight and curved slender beams by piezoelectric transducers
Abstract: We present the use of piezoelectric disk buzzers, usual in stringed musical instruments to acquire sound as a voltage signal, for experimental modal analysis. These transducers helped in extracting natural frequencies and mode shapes of an aluminium beam and a steel arch in the laboratory. The results are compared with theoretical predictions and experimental values obtained by accelerometers and a laser displacement transducer. High accuracy, small dimensions, low weight, easy usage, and low cost, make piezoelectric pickups an attractive tool for the experimental modal analysis of engineering structures.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26555112016-01-01T00:00:00ZNatural Frequencies of Long-Span Suspension Bridges Subjected to Aerodynamic Loadshttp://hdl.handle.net/11583/2592713Titolo: Natural Frequencies of Long-Span Suspension Bridges Subjected to Aerodynamic Loads
Abstract: We investigate the effects of steady aerodynamic loads on stability and natural frequencies of long-span suspension bridges through a simplified analytical model. The single (central) span suspension bridge model is considered, and the linearized integro-differential equations describing the flexural-torsional deformations of the bridge deck-girder are adopted as starting point. Thus, taking into account the second-order effects induced by a constant transverse wind in the bridge equations of motion, we derive a generalized eigenvalue problem in which all configurations intermediate between those of pure lateral-torsional buckling, pure torsional divergence, and pure free vibrations can be investigated. We show that the natural frequencies of a suspended deck-girder depend upon the mean (quasi-static) wind loading. As a consequence, the input parameters to the aeroelastic stability analysis result affected by that dependence, suggesting the possibility of modifying the dynamic stability analysis in order to take into account the mentioned influence. Based on this fact, possible implications for the flutter analysis of long-span suspension bridges are discussed.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11583/25927132014-01-01T00:00:00ZNumerical and experimental determination of natural frequencies and critical loads for compressed open thin-walled beamshttp://hdl.handle.net/11583/2651491Titolo: Numerical and experimental determination of natural frequencies and critical loads for compressed open thin-walled beams
Abstract: As is well known, thin-walled open profiles have an appreciable bending stiffness at least about one of the principal axes of inertia, but a low torsional stiffness. In addition, boundary effects do not have rapid extinction, and the contribution of warping stiffness may be crucial. Due to the widespread application of open thin-walled beams in engineering, vibration and stability analyses are of prime interest for this kind of elements. Since strain modes of thinwalled beams can be coupled according to the cross-section geometry, the pre-buckling equilibrium path can affect both the critical load and the natural frequencies. We have investigated on the effects of non-trivial equilibrium paths and warping constraints in thinwalled open profiles by a numerical in-house code based on a finite difference procedure, where stability is analyzed in a dynamic setting. This has provided interesting results when coupled with a direct one-dimensional model of standard beam enriched with a coarse descriptor of warping. Thus, we have undertaken an experimental campaign to verify these results, starting from the most simple cases to prepare and measure, i.e., beams with two axes of symmetry. In this contribution, we present some numerical and experimental results in terms of natural frequencies and critical loads for compressed thin-walled beams, having cruciform cross-sections with a vanishing or a remarkable warping stiffness. The specimens were subject to axial loads by means of a universal testing machine, and the natural frequencies were extracted, using piezoelectric pickups, for increasing values of the axial compressive force.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26514912015-01-01T00:00:00ZExperimental and Numerical Elastodynamic Analysis of Compressed Open Thin-walled Beamshttp://hdl.handle.net/11583/2651516Titolo: Experimental and Numerical Elastodynamic Analysis of Compressed Open Thin-walled Beams
Abstract: Compressed thin-walled beams with open section are prone to torsional, or flexural-torsional, buckling. Here we present the results of several studies where, by piezoelectric pickups and a universal testing machine, we experimentally detected the natural frequencies and buckling loads of centrally compressed aluminum thin-walled beams with open cruciform section, exhibiting remarkable warping stiffness. We detected the free vibration frequencies for different values of the compressive force and for both free and (at least partially) restrained warping of the end sections. We compared the behavior of integer elements to that of analogous beams, where we introduced a localized damage, i.e., a sharp variation of a cross-section. For the integer elements, we compared the experimental results with those provided by an in-house numerical code, which investigates the elastic stability of possible non-trivial paths of thin-walled beams in a dynamic setting. The results show that, on the one hand, piezoelectric pickups can be efficiently used to extract modal parameters of structural elements; on the other hand, the numerical code proves to be robust and accurate in the determination of the buckling loads of the integer elements, in all the analyzed configurations.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26515162016-01-01T00:00:00ZOn the dynamics of suspension bridge decks with wind-induced second-order effectshttp://hdl.handle.net/11583/2651513Titolo: On the dynamics of suspension bridge decks with wind-induced second-order effects
Abstract: In numerical modelling of suspension bridges, a preliminary step for the aeroelastic stability analysis is the determination of natural frequencies and mode shapes of the structure.
They are generally obtained by a modal analysis conducted with respect to the deformed configuration under permanent loads. In this talk, a simplified analytical model will be introduced to discuss the dependence of the natural frequencies of suspension bridges upon the steady aerodynamic loads. This will be obtained by including the second-order effects induced by a constant transverse wind in the linearized bridge equations of motion. Numerical results will be presented and implications in flutter analysis will be discussed.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26515132015-01-01T00:00:00ZTerahertz vibrations in compressed solids and biological materialshttp://hdl.handle.net/11583/2646188Titolo: Terahertz vibrations in compressed solids and biological materials
Abstract: Terahertz vibrations at the nanoscale could provide a common basis to study mechanical
instabilities both in crystalline solids and biological materials. As a matter of fact, dynamic
instabilities induced in the crystal lattices of solids during brittle fracture, as well as protein
folding and unfolding might produce THz resonant vibrations. By using the scattered light
spectrum analysis, the authors are performing experimental studies to obtain information
about the molecular interactions in crystalline solids and protein structures. These analyses
can be useful to understand which kind of excitation should be applied to produce resonant
vibrations in the field of THz. Simplified numerical models are also being implemented to
interpret the variations in the crystal lattices natural frequencies of compressed solids, and to
simulate buckling/snap-through nano-instabilities of specific proteins.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26461882016-01-01T00:00:00ZElastodynamic analysis of damaged open thin-walled beams subjected to axial loadhttp://hdl.handle.net/11583/2651509Titolo: Elastodynamic analysis of damaged open thin-walled beams subjected to axial load
Abstract: Thin-walled beams are widely used in many areas of engineering. As it is known, the low torsional stiffness and the eventual non-coincidence of the shear centre and centroid of the cross-section, make compressed thin-walled open profiles vulnerable to torsional and flexural-torsional buckling. Moreover, in real-world applications, critical loads are affected by initial imperfections, which generally have a significant influence.
In previous studies [1-6], we have investigated numerically and experimentally on the effects of non-trivial equilibrium paths and warping constraints in thin-walled open profiles subjected to axial load. In particular, we compared the results obtained by a numerical in-house code to those found in the lab, for some remarkable cases of beams with two axes of symmetry [5,6]. Indeed, the code is based on a finite difference procedure where a direct one-dimensional model of standard beam is enriched with a coarse descriptor of warping, and stability is analyzed in a dynamic setting, including the geometric nonlinearity of the pre-critical equilibrium path [1-3].
In the present contribution, we extend the previous investigations by including the effects of a localized damage, i.e., a sharp variation of a cross-section, which reduces the global stiffness of the beam, therefore affecting the buckling load and the natural frequencies.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26515092015-01-01T00:00:00Z