Federico Perotti

Iterative system solvers for the frequency analysis of linear mechanical systems

Abstract The paper deals with the numerical treatment of the direct frequency domain (DFD) analysis of linear mechanical systems. Attention is mainly focused on the solution of the complex system of equations needed for each analyzed frequency. Strategies for transforming the system into the “shifted” form ( T −σ I ) z = d are proposed and discussed, where σ is related to the frequency. For two formulations of the shifted system the performance of some Krylov sub-space iterative solvers is tested and compared to that of a multi-frontal direct method. Advantage is taken of the shifted form in solving simultaneously a large number of systems resulting from different values of the shift (frequency). Numerical experiments on some prototype structural dynamics problems are reported; the results shown demonstrate how the devised strategies for the iterative solution can outperform, in many cases, the direct solver.

Seismic analysis of the Asinelli Tower and earthquakes in Bologna

Abstract The existence of a valuable historical heritage of the Medieval age in Bologna has motivated a deeper study on the seismic behaviour of the Asinelli Tower, which, beyond being the town symbol, has a structural configuration prone to seismic damage. Accordingly, in the present paper, the seismicity of Bologna is reviewed and a synthesis of the damage observed in the urban area, based on an accurate historical research, is given, with the objective of evaluating the behavior of the Asinelli Tower within the framework of the effects localized in the municipality. An assessment of the towers stability with respect to compatible seismic events, at least qualitatively, with the region seismicity is then carried out by means of a non-linear dynamic analysis on a simplified model.

The dynamic response of seabed anchored floating tunnels under seismic excitation

In this paper a procedure for analysing the seismic response of seabed anchored floating tunnels is presented. The first step of the research was the development of an ‘ad hoc’ finite element for modelling the behaviour of anchor elements, with particular reference to the problem of transverse oscillations under time varying axial loads. The element was subsequently inserted in a step-by-step procedure for the numerical analysis of non-linear response to multiple-support seismic input; the procedure encompasses simplified modelling of fluid–structure and soil–structure interaction effects. An example of an application is given concerning two 4680 m long floating tunnels with different seabed profiles. Copyright

Numerical analysis of the non-linear dynamic behaviour of suspended cables under turbulent wind excitation

In this paper, a numerical procedure is presented for the dynamic analysis of elastic cables subjected to turbulent wind excitation in quasi-steady conditions. The proposed methodology, which takes geometrical and aerodynamic non-linearities into account, is based on artificial simulation of turbulence, on finite-element modeling of the cables and on a step-by-step implicit procedure for the integration of the dynamic equilibrium equations. As a first application, the dynamic behaviour of a cable in 1 : 2 internal resonance conditions is studied, focusing on some aspects of the influence of wind turbulence on galloping oscillations.

Structural Analyses of the Submerged Floating Tunnel Prototype in Qiandao Lake (PR of China)

In this paper, after a general presentation of the Submerged Floating Tunnel (SFT) as an attractive technical solution for waterway crossings, the SFT prototype to be built in Qiandao Lake (Peoples Republic of China) is introduced. The main peculiarities of such an innovative system are briefly summarized and the importance of realizing a SFT prototype is underlined. The types of action that the SFTs can be generally subjected to are illustrated and the specific load conditions for the SFT prototype in Qiandao Lake are presented. The numerical analyses carried out for investigating the system behaviour in presence of the environmental loads, such as waves, currents and earthquakes, are focused. Three cables configurations are considered for the hydrodynamic analyses, in order to select the most performing one, which is subsequently analyzed also under seismic loads. The model assumptions and the results of the performed analyses are shown and critically discussed. Finally, the displacement and strength safety checks are shown, which allow to state that the designed SFT prototype is able to withstand the environmental design actions in Qiandao Lake.

Cyclic behaviour of axially loaded members: Numerical simulation and experimental verification

Abstract This paper deals with the simulation of the cyclic behaviour of axially loaded members required in order to assess the performance of bracing systems under strong seismic actions. A numerical model is first presented; the experimental cyclic tests performed by the authors following the procedure developed by the ECCS are then described. Finally the results obtained by the two approaches are compared and discussed.

The seismic response of concentrically braced moment-resisting steel frames

The paper deals with the dynamic non-linear behaviour of concentrically braced steel moment-resisting frames under severe earthquake conditions. The first part of the paper is devoted to the description of a non-linear beam element which has been developed starting from the classical ‘one-component’ formulation, having concentrated inelastic hinges at the ends. The hysteretic behaviour of the end hinges is calibrated on the basis of a set of experimental tests previously performed at Politecnico di Milano. The non-linear brace element used to model the diagonal bars and the procedure for step-by-step dynamic analysis are then briefly summarized. In the second part of the paper two examples of application are presented; the first one concerns a six-storey MRF designed in accordance with Eurocode 8, in which concentric braces are introduced only to satisfy the serviceability limit state requirements (interstory drifts) but are not taken into account in the ultimate conditions. The numerically computed seismic behaviour of the system is analysed and compared to that of the unbraced frame. The second example is a six-storey CBF designed according to the Eurocode 8 specifications; the non-linear behaviour under seismic loading is studied especially to the aim of analysing the effect of the columns flexural stiffness and resistance, which was not considered in the design, on the CBF response.

SEISMIC ISOLATION OF THE IRIS NUCLEAR PLANT

The safety-by-design™ approach adopted for the design of the International Reactor Innovative and Secure (IRIS) resulted in the elimination by design of some of the main accident scenarios classically applicable to Pressurized Water Reactors (PWR) and to the reduction of either consequences or frequency of the remaining classical at-power accident initiators. As a result of such strategy the Core Damage Frequency (CDF) from at-power internal initiating events was reduced to the 10−8 /ry order of magnitude, thus elevating CDF from external events (seismic above all) to an even more significant contributor than what currently experienced in the existing PWR fleet. The same safety-by-design™ approach was then exported from the design of the IRIS reactor and of its safety systems to the design of the IRIS Nuclear Steam Supply System (NSSS) building, with the goal of reducing the impact of seismically induced scenarios. The small footprint of the IRIS NSSS building, which includes all Engineered Safety Features (ESF), all the emergency heat sink and all the required support systems makes the idea of seismic isolation of the entire nuclear island a relatively easy and economically competitive solution. The seismically isolated IRIS NSSS building dramatically reduces the seismic excitation perceived by the reactor vessel, the containment structure and all the main IRIS ESF components, thus virtually eliminating the seismic-induced CDF. This solution is also contributing to the standardization of the IRIS plant, with a single design compatible with a variety of sites covering a wide spectrum of seismic conditions. The conceptual IRIS seismic isolation system is herein presented, along with a selection of the preliminary seismic analyses confirming the drastic reduction of the seismic excitation to the IRIS NSSS building. Along with the adoption of the seismic isolation system, a more refined approach to the computation of the fragility analysis of the components is also being developed, in order to reduce the undue conservatism historically affecting seismic analysis. The new fragility analysis methodology will be particularly focused on the analysis of the isolators themselves, which will now be the limiting components in the evaluation of the overall seismic induced CDF.Copyright

The seismic behaviour of steel moment-resisting frames with stiffening braces

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Three prototype frames are studied within the context of Eurocode 8 (EC8) provisions; these are MRFs which are traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frames, which are not considered in the collapse resistance of the structural system. The non-linear behaviour of these systems under ultimate seismic environment is studied and compared to the one of the unbraced MRFs. The influence of second order effects is also investigated.

ON THE MODELING OF SELF-DAMPING IN STRANDED CABLES

A new formulation is presented to model the hysteretic bending behaviour of metallic strands. The interaction between the wires of the strand is modeled through a frictional contact model based on the Amontons-Coulomb law and accounts for the effects of the tangential compliance mechanism. The new model is presented with reference to a single-layered mono-metallic strand and is applied to the study of the energy dissipation in cyclic bending.