196,094 research outputs found
Identification of general added mass distribution in nanorods from two-spectra finite data
No full textNanomechanical resonators consisting in one-dimensional vibrating
structures have remarkable performance in detecting small adherent
masses. The mass sensing principle is based on the use of the
resonant frequency shifts caused by unknown attached masses. In
spite of its importance in applications, few studies are available
on this inverse problem. Dilena et al. (2019) presented a
method for reconstructing a small mass distribution by using the
first N resonant frequencies of the free axial vibration of a
nanorod under clamped end conditions. In order to avoid trivial
non-uniqueness when spectral data belonging to a single spectrum
are used, the mass variation was supposed to be supported in half
of the axis interval. In this paper, we remove this a priori
assumption on the mass support, and we show how to extend the
method to reconstruct a general mass distribution by
adding to the input data the first N lower eigenvalues of the
nanorod under clamped-free end conditions. The nanobeam is
modelled using the modified strain gradient theory to account for
the microstructure and size effects. The reconstruction is based
on an iterative procedure which takes advantage of the closed-form
solution available when the mass change is small, and turns out to
be convergent under this assumption. The results of an extended
series of numerical simulations support the theoretical results
Dynamic testing of a damaged bridge
No full textIn this paper the results of a campaign of dynamic tests carried
out on an existing reinforced concrete single-span bridge
subjected to increasing levels of damage are presented. The deck
structure consists of a slab and three simply supported beams. The
damage is represented by a series of notches made on a lateral
beam to simulate the effect of incremental concentrated damage.
The modal parameters of the lower vibration modes were estimated
from frequency response measurements obtained under harmonic
excitation. The variation of natural frequencies shows an
anomalous increase in the transition from one intermediate
configuration to the next damage configurations. Changes in
vibration modes are appreciable from the earliest level of
damage. In particular, changes in modal curvature of lower modes
do provide indication on the damage location
Analytical and numerical results for vibration analysis of multi-stepped and multi-damaged arches
No full textThis paper investigates the in-plane linear dynamic behaviour of multi-stepped and multi-damaged circular arches under different boundary conditions. Cracked cross-sections are modelled as massless elastic rotational hinges. In damaged configuration, cracks can be located both at the interface between two adjacent portions as well as inside the portion itself. For each arch portion bounded by two cracks, the differential equations of motion have been established considering axial extension, transverse shear effects and rotatory inertia. The equilibrium equations of arch portions are combined in the coupled fundamental system in terms of radial displacement, tangential displacement and rotation. Analytical and numerical solutions for multi-stepped arches, in undamaged as well as in damaged configurations, are proposed. The analytical solution is based on the Euler characteristic exponent procedure involving the roots of characteristic polynomials, while the numerical method is focused on the Generalized Differential Quadrature (GDQ) method and the Generalized Differential Quadrature Element (GDQE) technique. Numerical results are shown in terms of the first 10 analytical and numerical frequencies of multi-stepped and multi-damaged arches with different boundary conditions. Finally, convergence and stability characteristics of the GDQE procedure are investigated. The convergence rate of the natural frequencies is shown to be very fast and the stability of the numerical procedure is very good
Vibration and damage detection in undamaged and cracked circular arches: Experimental and analytical results
No full textThis paper presents the experimental results of the dynamic behaviour of a circular arch in undamaged and several damaged configurations, and compares them with those obtained by means of analytical methods. The damage is introduced in the undamaged arch by operating a notch and is then modelled as a torsion spring of suitable stiffness localised in the damaged cross-section. Good agreement between analytical and experimental results is observed. An identification procedure based on frequency measurements is proposed and validated. (C) 2008 Elsevier Ltd. All rights reserved
Analytical and differential quadrature results for vibration analysis of damaged circular arches
No full textThe present paper focuses on in-plane linear free vibrations of circular arches,in undamaged and damaged con figurations.For the model herein utilized,the equations of motion,in terms of displacements and rotation,take into account shearing and axial deformations and rotary inertia.The cracked section of
the arch is modeled with an elastic spring. An exact analytical method of solution and an approximate numerical one are presented.The first method solves the fundamental system in closed form,y means of a characteristic polynomial;the second one is based on a simple and ef ficient differential quadrature and
domain decomposition technique.Natural frequencies and mode shapes are computed for some signi ficant cases,showing very good agreement between the two approaches
Archi circolari piani integri e danneggiati: rilevazioni sperimentali e modelli analitici
No full textQuesto lavoro si propone di: a) illustrare i parametri modali rilevati sperimentalmente in un arco circolare, sia integro che danneggiato da un intaglio; b) confrontare questi con i valori ottenuti da modelli analitici. Il danno è stato modellato per mezzo di una molla rotazionale di opportuna rigidezza localizzata nella sezione corrispondente all’intaglio. Si è osservata una buona corrispondenza tra le evidenze sperimentali ed i risultati analitici
Damage localization in bridges via FRF interpolation method
No full textVibration based methods are powerful tools to assess the structural behavior of a bridge. In this paper, the Interpolation Damage Detection Method is used to give an interpretation of frequency response function (FRF) measurements performed on a reinforced concrete single span bridge subject to increasing levels of concentrated damage. The sensitivity of the method is document-ed and discussed with respect to the amount of experimental data available and the severity of the damage. The results are encouraging. They show the ability of the method to correctly detect the damage location in the majority of the scenarios considered and to give quantitative indica-tion of the relative severity of the damage. A comparison with the results obtained by the Modal Curvature Method is also presented
Dynamic identification of a stone masonry building: influence of damage and of CRM retrofitting
No full textThe dynamic identification of a full-scale, two-storey building, made of rubble stone masonry, was achieved. Different configurations were analyzed: unstrengthened masonry and retrofitted masonry, in both undamaged and damaged conditions. Damage was accomplished by testing the building under lateral cyclic loading, to reproduce the seismic effects. The strengthening technique (CRM – Composite Reinforced Mortar) consisted in plastering the outer facades by means of a 30 mm tick mortar coating reinforced with glass-fiber polymer meshes and in introducing transversal connectors injected in the masonry. The mode shapes and natural vibrating frequencies were analyzed and compared, evidencing the effects of damage and of retrofitting. A finite element numerical model was developed, to perform eigenvalue analysis and calibrate the equivalent masonry stiffness for the different configurations
Identification of metallic rods by frequency estimation on the historic vhurch tower in S. Vito al Tagliamento
No full textIn the present paper a dynamic procedure
for the evaluation of the constraining rate
between the ends of metallic rods supported
between masonry elements is presented.
As a first step, an analytical model
for flexural free vibrations of Euler-
Bernoulli beams subjected to axial forces,
is introduced. Accordingly, closed-form
natural frequencies associated to first fundamental
modes of the rod can be obtained.
The reliability of the model is verified
through experimental tests performed on
some tie-rods subjected to different levels
of axial force. Natural frequencies of
lower flexural modes can be assessed for
each configuration and the corresponding
axial force are measured by means of
strain gauges. Once the characteristic dimensions
and boundary conditions are set,
equating the first two experimental frequencies
and the corresponding analytical
frequencies, permits to evaluate the axial
force of the rod. Following this procedure,
the axial forces acting in some metallic
rods of the church tower in San Vito al
Tagliamento near Pordenone, have been
determined
Dynamic identification of a reinforced concrete damaged bridge
No full textThe results of a series of harmonically forced tests carried out
on a reinforced concrete single-span bridge subjected to
increasing levels of damage are interpreted in this paper. The
deck structure of the bridge consists of a slab and three simply
supported beams. The damage is represented by a series of notches
made on a lateral beam to simulate the effect of incremental
concentrated damage. The variation of lower natural frequencies
shows an anomalous increase in the transition from one
intermediate damage configuration to the next ones. Vibration mode shapes show an appreciable asymmetry in the reference
configuration, despite the nominal symmetry of the bridge. A
justification of this unexpected dynamic behavior is presented in this paper. The analysis is based on progressive identification of
an accurate finite element model of the reference configuration
and on reconstruction of damage evolution {}from natural frequency
and vibration mode measurements. Changes in modal curvature of the
first two vibration modes evaluated along the main beams are
successfully used to identify the location of the damage
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