1,721,049 research outputs found
Free vibration of flexible cables
No full textThe free undamped vibrations of cables undergoing stretching, bending and twisting are investigated. To this end, a geometrically exact model of elastic cables accounting for bending and torsional stiffness is employed. The cable kinematics retain the full geometric nonlinearities. Starting from a prestressed catenary configuration, the nonlinear equations of motion are linearized about the initial equilibrium. In particular, two initial equilibrium states (shallow and taut) are considered while varying the cable elastic axial stiffness. The influence of the bending flexibility on the cable frequencies is assessed by direct comparisons with the frequencies predicted by classical cable theories of purely extensible cables
Aeroelastic stability analysis of the Bridge of Sounds, City of Rome
No full textReport for the General Contractor Mattioli SpA
Crosstalk between oxidative stress and inflammation in Alzheimer-like dementia
Full text linkTrisomy of Chr 21, commonly known as Down syndrome
(DS), is the most prevailing genetic cause of intellectual
disability with an incidence of 1:700 births. DS individuals,
after the age of 40, develop a type of dementia that closely
resembles that of Alzheimer’s disease (AD) with deposition
of senile plaques containing Aβ and neurofibrillary tangles
(NFTs) composed of hyperphoshorylated tau. Inflammation
and oxidative stress (OS) are known to occur in DS and AD
brains in response to Aβ plaques and NFTs. Furthermore,
several studies demonstrated also the involvement of brain
insulin resistance (BIR) in the progression of AD-like
pathology. Therefore, we believe that OS, inflammation and
BIR could contribute to the severity of AD and DS
pathology and act as potential accelerators of AD pathology.
Within this scenario, we focused our attention on the
dysfunction of molecular pathways that are closely related
with increased OS and that might be involved in the
development of Alzheimer-like dementia including
autophagy (the major cellular pathway responsible for the
removal of Aβ and tau aggregates), insulin signaling and
inflammation. Recent studies from our laboratory reported,
in human DS samples, an early accumulation of protein
oxidative damage concomitant with the alteration of mTOR/
autophagy signaling. However, the relationship between OS
and mTOR/autophagy signaling is intricate and needs to be
clarified (Project 1). Furthermore, increasing evidence
supports the involvement of inflammation-related miRNAs
in neurodegenerative diseases and because miR146a and
miR155 are key regulators of the microglia-mediated
inflammatory response we hypothesized that a dysregulation
of these miRNAs may occur in DS and AD pathology
(Project 2). Finally, BVR-A being a direct target of IR
kinase activity and once IR-phosphorylated is able to
phosphorylate IRS1 on Ser inhibitory, and because we
previously demonstrated that OS induces impairment of
BVR-A in human AD brain, we investigate if BVR-A
dysregulation could be associated with the onset of BIR in
AD (Project 3).
Aim of this
work
In order to clarify the intricate relationship between
increased OS and alteration of mTOR/autophagy signaling
we analyzed the integrity of the mTOR axis in DS mouse
model (Ts65Dn) at different ages and we evaluated the
effects of autophagy inhibition (by rapamycin treatment) on
protein oxidative damage in SH-SY5Y cell line (Project 1).
To elucidate the potential link between deregulation of
inflammation-related miRNAs, neurobehavioral deficits and
AD pathology; we investigated the expression and cellspecific
distribution of both miR146a and miR155 in the
developing hippocampus from controls, patients with DS
and adults with DS-AD pathology. In addition, we evaluated
the levels of these miRNAs, their putative targets as well as
the levels of IL-1β in human hippocampus from sporadic
AD (sAD) at different stages of the disease and in Ts65Dn
mice and in experimental models AD mouse (APP/PS1)
(Project 2). To make clear the potential involvement of
BVR-A dysregulation in the onset of BIR we analyzed the
age dependent changes of (i) BVR-A protein levels and
activation, (ii) total OS markers levels (PC, HNE, 3-NT) as
well as (iii) IR/IRS1 levels and activation in triple transgenic
mouse model of AD (3xTg-AD). Furthermore, ad hoc invitro
experiments have been performed to clarify the
contribution of oxidative/nitrosative stress on insulin
resistance. Finally, because mTOR is able to phosphorylate
IRS1 we investigated also if the insulin resistance could be
associated with mTOR hyper-activation (Project 3).
Results Our results show that: i) defects of mTOR signaling
contribute to the buildup of protein oxidative damage which
characterizes AD and DS neuropathology; ii) the
involvement of both miR146a and miR155 in the
hippocampus during brain development and their
dysregulation in DS and AD; iii) the OS-induced
inactivation of BVR-A promote BIR possibly trhough
hyper-activation of mTOR.
In conclusion, this study provides the evidence of a central
role of mTOR in the neurodegenerative process. Indeed,
hyperactivation of mTOR signaling impairs autophagy,
insulin signaling and, indirectly, the inflammation-related
miRNAs function
Three-dimensional model of suspension bridges via a fully nonlinear continuum formulation
No full textA fully nonlinear three-dimensional dynamic model of a suspension bridge under fairly general loading conditions is proposed. The nonlinear balance equations are obtained via a direct total Lagrangian formulation and the kinematics, for the deck and the two cables, include the finite displacements of the centroidal lines and the flexural and torsional finite rotations of the deck cross-sections (otherwise rigid in their own planes). The deformation parameters are nonlinear functions of the displacement gradients. The proposed model takes into account the fully nonlinear extensional-flexural-torsional coupling and examines the aeroelastic phenomena induced by static wind actions. With reference to the Hu Men Suspension Bridge, the critical wind velocities are calculated and compared with the results obtained via a linear analysis. The proposed applications are numerically performed via a finite element analysis
Aeroelasticity of suspension bridges using nonlinear aerodynamics and geometrically exact structural models
Full text linkLe vibrazioni indotte dall’azione del vento su ponti sospesi di grandissima luce rappresentano uno
dei maggiori problemi per i progettisti di tali opere. Vi `e pertanto la necessit`a di migliorare le attuali
tecnologie progettuali, facendo uso delle pi`u avanzate tecniche computazionali, poich`e ci`o `e essenziale per
una migliore comprensione della fluido dinamica che governa il problema e che `e la causa dell’accopiamento
fluido-strutturale in ponti cos`ı flessibili e dunque delle vibrazioni che ne conseguono. La progettazione
di ponti aventi luci significativamente pi`u lunghe di quelle tutt’ora esistenti rappresenta oggi una grande
sfida. Per poter effettivamente migliorare gli strumenti computazionali necessari per la progettazione di
ponti di grandissima luce, `e proposto un contributo di ricerca multi disciplinare mirato alla modellazione
avanzata di ponti sospesi di grande luce. Tali strutture hanno un comportamento aeroelastico alquanto
differente da quello dei ponti convenzionali.
Nel presente lavoro di ricerca, `e proposto un modello completamente nonlineare di ponte sospeso
parametrizzato attraverso una singola coordinata spaziale al fine di descrivere la dinamica tri-dimensionale
globale del sistema. Le equazioni del moto non lineari sono ottenute mediante una formulazione Lagrangiana
diretta e la cinematica, per l’impalcato e per i cavi di sospensione, `e basata sull’ipotesi di
spostamenti finiti e di rotazioni flessionali e torsionali finite delle sezioni trasversali dell’impalcato. Le relazioni
di congruenza interna, deformazione-spostamento, dei parametri generalizzati della deformazione
- l’elogazione dei cavi, quella dell’impalcato e le tre curvature - conservano le non linearit`a geometriche
complete, ovvero nessuno sviluppo in serie `e stato condotto al fine di semplificare le loro espressioni.
Le caratteristiche aerodinamiche non lineari della sezione scatolare del Great Belt Bridge in Danimarca
sono state investigate mediante l’uso di due metodi di analisi fluidodinamica computazionale tradizionali,
il modello di turbolenza k-ε, implementato nel codice di calcolo FLUENT-ANSYS, che utilizza le equazioni
Reynolds Averaged Navier Stokes (RANS) e il metodo discrete vortex per la soluzione delle equazioni di
Navier Stokes (NS), implementato nel codice DVMFLOW-COWI. Tali strumenti di calcolo fluidodinamico
sono stati utilizzati per la formulazione di una aerodinamica instazionaria che tenga in conto degli effetti
viscosi, come la separazione del filetto fluido e l’accrescimento in spessore dello strato limite, attraverso
modelli aerodinamici di ordine ridotto (ROMs). Le rappresentazioni nel dominio delle frequenze delle
forzanti aerodinamiche in termini delle derivate aeroelastiche sono state ottenute per determinati valori
di angolo d’attacco iniziale del vento. Di conseguenza, funzioni indiciali non lineari sono state derivate
per tali angoli e incorporate nei ROMs proposti.
Infine, un modello completamente non lineare di accoppiamento fluido strutturale per ponti sospesi `e
stato messo a punto per analizzare il comportamento statico e dinamico di tali strutture e mirato allo
studio di stabilit`a aeroelastica statica, tipo divergenza torsionale, e di instabilit`a dinamiche, tipo flutter,
nonch`e all’analisi della risposta dinamica in regime di post-flutter.
La formulazione geometricamente esatta sviluppata in questo lavoro si presta per natura a studi parametrici
di sensitivit`a degli stati limite dinamici e statici dei ponti rispetto alle variazioni dei parametri
strutturali caratteristici. Ulteriori studi sono stati rivolti all’analisi della risposta dei ponti sotto l’azione
di carichi aerodinamici con distribuzioni spaziali e temporali del vento non uniformi, come quelli indotti
da raffiche, e alla valutazione degli effetti di distribuzioni spaziali non uniformi del vento sulla condizione
critica di flutter. Infine, `e stato studiato il comportamento in post-flutter mediante l’uso di metodi di
continuazione con lo scopo di evidenziare gli scenari biforcativi post-critici ed enfatizzare la complessa
risposta non lineare di strutture snelle soggette a carichi dinamici autoeccitanti.Wind-induced vibration in super-long-span bridges is a major concern for the designers. There is a
need to enhance the structural design technology, through improved computational capabilities, a critical
step for a better understanding of fluid-flow physics that induce vibration and fluid-structure dynamics
of flexible bridges. The design of bridges with spans significantly longer than those existing today is quite
challenging. To refine the computational tools required for such bridges, a multi-disciplinary research
effort devoted to the advanced modeling of flexible long-span suspension bridges is proposed. These
structures exhibit an aeroelastic behavior quite different from conventional bridges.
In the present work, a fully nonlinear model of suspension bridges parameterized by one single space
coordinate is proposed to describe the overall three-dimensional motion. The nonlinear equations of motion
are obtained via a direct Lagrangian formulation and the kinematics, for the deck-girder and the
suspension cables, feature the finite displacements of the associated base lines and the flexural and torsional
finite rotations of the deck cross sections. The strain-displacement relationships for the generalized
strain parameters - the cable elongations, the deck elongation, and the three curvatures - retain the full
geometric nonlinearities.
The nonlinear aerodynamic characteristics of the boxed sharp-edge cross section of the Danish Great
Belt Bridge are investigated by using two state-of-the-art computational methods, the k-ε turbulence
model implemented in FLUENT-ANSYS to solve the Reynolds Averaged Navier Stokes (RANS) equations
and the Navier Stokes (NS) discrete-vortex method implemented in DVMFLOW-COWI. The computational
fluid dynamics tools have been used to develop computationally efficient unsteady aerodynamic
models taking into account viscous effects, including flow separation and boundary layer thickening,
treated using Reduced-Order Models (ROMs). Frequency-domain representations of the aerodynamic
loads in terms of flutter derivatives are obtained for selected values of the wind initial angle of attack.
Consequently, nonlinear indicial functions are derived for these angles and incorporated into the proposed
ROMs.
As a result, a fully nonlinear coupled fluid-structure model for suspension bridges is assembled to study
the nonlinear static and dynamic behavior thus addressing problems of static aeroelastic stability, such
as torsional divergence, and dynamic aeroelastic instabilities, such as flutter and post-flutter.
The geometrically exact formulation developed in this study lends itself naturally to parametric studies
about the sensitivity of the static and dynamic limit states of the bridges with respect to variations of
the characteristic structural parameters. In addition, the study addresses the dynamic response of the
bridges under time- and space-dependent loading conditions due to time- and space-wise distributed gust
excitations as well as the study of the effects of spatial nonuniform wind distributions on the critical flutter
condition. Finally, the post-flutter behavior is studied by using a continuation method to highlight the
post-critical bifurcation scenarios and emphasize the complex nonlinear response of slender self-excited
suspended structures
Nonlinear parametric modeling of suspension bridges under aeroelastic forces: torsional divergence and flutter
No full textA fully nonlinear model of suspension bridges parameterized by one single space coordinate is proposed to describe overall three-dimensional motions. The nonlinear equations of motion are obtained via a direct total Lagrangian formulation and the kinematics, for the deck-girder and the suspension cables, feature the finite displacements of the associated base lines and the flexural and torsional rotations of the deck cross-sections assumed rigid in their own planes. The strain-displacement relationships for the generalized strain parameters, the elongations in the cables, the deck elongation, and the three curvatures, retain the full geometric nonlinearities. The proposed nonlinear model with its full extensional-flexural-torsional coupling is employed to study the torsional divergence caused by the static part of the wind-induced forces. Two suspension bridges are considered as case studies: the Runyang bridge (main span 1,490 m) and the Hu Men bridge (main span 888 m) in China. The evaluation of the onset of the static instability and the post-critical behavior takes into account the prestressed condition of the bridge subject to dead loads. The dynamic bifurcation that occurs at the onset of flutter is also studied accounting for the prestressed equilibrium state about which the equations of motion are obtained via an updated Lagrangian formulation. Such a bifurcation is investigated in the context of the parametric nonlinear model considering the model parameters of the Runyang Suspension Bridge together with its aeroelastic derivatives. The calculated critical wind speeds for the onset of the static and dynamic bifurcations are compared with the results obtained via linear analysis and the main differences are highlighted. Parametric sensitivity studies are carried out to assess the influence of the design parameters on the instabilities associated with the bridge aeroelastic response
Aerostatic torsional divergence of suspension bridges via a fully nonlinear continuum formulation
No full textA fully nonlinear three-dimensional dynamic model of a suspension bridge under fairly general loading conditions is proposed. The nonlinear balance equations are obtained via a direct total Lagrangian formulation and the kinematics, for the deck and the two cables, include the finite displacements of the centroidal lines and the flexural and torsional finite rotations of the deck cross-sections (otherwise rigid in their own planes). The deformation parameters, namely, the elongations in the cables, the deck elongation and the three curvatures, are nonlinear functions of the displacement gradients. The proposed model takes into account the fully nonlinear extensional-flexural-torsional coupling and examines both torsional divergence and lateral buckling phenomena induced by static wind actions. An application of the three-dimensional nonlinear model, for the study of aeroelastic stability, is illustrated considering the Hu Men Suspension Bridge on the Zhu Jiang river in China. For this structure, the calculated critical wind velocities are compared with the results obtained via a linear analysis. Moreover, the proposed nonlinear model allowed to carry out effective parametric studies so as to assess the influence, on the critical wind velocity, of the global parameters governing the structural bridge response
Static and Aeroelastic Limit States of Ponte della Musica via a Fully Nonlinear Continuum Model
No full text
Nonlinear response of elastic cables with flexural-torsional stiffness
No full textA geometrically exact mechanical formulation is proposed to describe three-dimensional motions of flexible cables without any restriction on the amplitude of such motions. The nonlinear equations of motion are formulated via an updated Lagrangian formulation taking the prestressed catenary equilibrium under gravity as the start configuration. By employing a single space coordinate parametrization, the kinematics feature finite displacement and rotation of the cable cross sections, assumed rigid in their own planes. The ensuing generalized strain parameters and curvatures retain the full geometric nonlinearities. By considering several case-study cables within the groups of taut and shallow cables, nonlinear equilibrium analyses are performed to investigate the effects of the cable bending and torsional stiffness for nontrivial boundary conditions. The full nonlinear formulation allows to estimate the size of the boundary layers as well as the stress states within them otherwise unknown adopting classical theories of purely stretchable cables
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