27 research outputs found
Vortex Induced Vibration in Submerged Floating Tunnels: DVL a Distributed Vortex Layer Model
No full textThe paper deals with the simulation of the dynamic nonlinear behaviour of seabed anchored submerged floating tunnels (SFTs) under environmental excitation, by addressing the problem of vortex induced excitation.
A model based on the ‘fluid oscillator’ approach is proposed, tailored to be coupled to a geometrically non linear FE developed, for the anchor bars, in a previous work. The approach allows for representing the entire anchoring bar, thus reducing the computational effort required for global modelling of the SFT. In this light, mainly based on a lumped fluid oscillator model, a computationally more efficient approach is here sought in the form of a continuous model (Distributed Vortex Layer - DVL). This consists of a distributed ‘aerodynamic’ mass which is connected to the structural element and to a fixed reference system by distributed non linear spring-dashpot elements. Governing equations of motion are obtained substituting Galerkin-type decomposition of both the structure and the aerodynamic mass displacements; global equations of motion are integrated by a step-by-step procedure based on the Newmark method.
Validations of the numerical model are finally given, regarding experimental results obtained for two basic structures: an elastically suspended rigid cylinder and a long slender cylinder
Effects of the anchoring systems on the dynamic behaviour of the submerged floating systems (SFT)
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The dynamic behavior of submerged floating tunnels under seismic and hydrodynamic excitation
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3D dynamic response of submerged floating tunnels under seismic and hydrodynamic excitation
No full textSubmerged Floating Tunnels (SFTs) have been proposed, in the last decades, for crossing sea straits or, more generally, waterways. Their dynamic behaviour under environmental loading due to earthquakes and wind waves deserves attention in light of both safety and serviceability issues. In this light, a numerical procedure is here described for the dynamic analysis of SFTs, with particular reference to the case of deep water crossings; attention is devoted to the design solution encompassing slender bars as anchor elements. A geometrically nonlinear finite element, developed in previous work, is here refined extending its capabilities to full 3-D analysis and to nonlinear modelling of hydrodynamic loads due to steady current and wind waves. The element is implemented in a numerical procedure for the dynamic time domain step-by-step analysis of nonlinear discretized systems; consistently, hydrodynamic and seismic loading are introduced by generating artificial time-histories of spatially variable seismic motion and wind waves.
An example of on application is shown regarding the behavior of the dynamic model of a submerged tunnel proposed for the Messina Strait crossing. The model is subjected to an extreme multiple-support seismic loading having a PGA equal to 0.64g and to an extreme wave loading with significant wave height of 16 m. The dynamic behaviour in the two loading situations is illustrated and compared, showing interesting facets, especially in terms of interaction between the tunnel and anchoring bars oscillations
Numerical models for the dynamic response of submerged floating tunnels under seismic loading
No full textSubmerged Floating Tunnels are a promising alternative to bridges and underground tunnels for crossing sea straits or, in general, waterways. The dynamic behavior of SFTs involves complex design and analysis issues, mainly related to the response to environmental actions. Within this context, the modeling of tethering elements of seabed anchored floating structures is here addressed, with particular reference to the crossing of deep waters; attention is devoted to the design solution encompassing slender bars as anchor elements. Two numerical tools are proposed: first, a geometrically non-linear finite element (NWB model), developed in previous work, has been refined in order to capture the effect of higher flexural modes of anchor bars. Secondly, a 3D beam element, based on the classical corotational formulation (CR), has been developed and coded. Both elements are implemented in a numerical procedure for the dynamic time
domain step-by-step analysis of non-linear discretized systems; seismic loading is introduced by generating artificial time-histories of spatially variable seismic motion.
An example of application of the NWB element is shown regarding the behavior of the dynamic model of a submerged tunnel proposed for the Messina Strait crossing. The model was subjected to extreme multiple-support seismic loading. The behavior under seismic loading is here illustrated and commented, especially in light of the effect of higher local vibration modes of the anchor bars
Corotational cable elements for the study of fluid-structure interaction
No full textThe present work deals with the development of a numeric procedure adapted to the study of the nonlinear
motion of a flexible structure (i.e. a suspended cable) interacting with a fluid flow. The large
displacements and rotations related to the behaviour of flexible systems require a particular Finite
Element formulation to describe the exact position and orientation of each part of the system during the
motion. The numerical procedure here presented reposes on a step-by-step response calculation method
and on cable finite elements capable to compute the interaction forces induced by a steady or random fluid
flow (wind). Two different Finite Elements are superimposed to model this kind of system: an
isoparametric element (“aerodynamic element”) is adopted to evaluate the aerodynamic forces at the
nodes in the varied dynamic configuration occupied by the structure in motion; in addition, a corotational
beam element (“mechanical element”) is used to compute the mechanical response of the cable itself,
taking into account its flexural and torsional properties and considering large nodal displacements and
rotations. The elements and integration procedure are here applied to some numerical examples; the
results are compared to those of a different approach
The dynamic behaviour of Archimede’s Bridges: numerical simulation and design implications
No full textIn the paper the activity of the research group in the field of the dynamic behaviour of Archimede’s Bridges is reviewed. The
scope is twofold; on one hand the implications, in terms of structural analysis and design, of the results of this activity are
summarized and discussed. On the other hand the needs for future research are defined.
The general criteria adopted in the simulation of the dynamic response of the Archimede’s Bridge are first summarized.
Attention is focused on “slow” dynamic actions; loading conditions due to impacts and internal or external explosions are thus
not considered.
The problem of seismic response is subsequently addressed; the problem of transverse response is discussed, especially in light
of the anchoring system typology. It is then commented how another critical issue can be represented by the longitudinal motion
of very long tunnels and, on design grounds, by the way of providing adequate restraint. Finally the aspects related to
hydrodynamic excitation due to seabed motion (tsunamis and “seaquake”) are briefly presented especially in light of research
development.
The dynamic behaviour under wave and current excitation is addressed in the second part; the adopted wave models are briefly
described along with the criteria for defining hydrodynamic forces; some results are shown illustrating the response to
exceptional wind waves, with particular reference to the motion of the anchoring elements.
The issue of vortex induced vibration is then treated, showing how long anchoring elements can be dangerously prone to
activation of large oscillations, with the additional problem of high Reynolds numbers characterizing the fluid-structure
interaction. The numerical procedure which has been developed for simulating the phenomenon is illustrated along with some
example of application to simple problems.
Finally, some general considerations relating dynamic performance and basic design choices are proposed
