The category of “extremal materials” has been introduced in the literature to define materials that simultaneously show very soft and very stiff deformation modes (unimode, bimode, trimode, quadramode and pentamode materials, depending on the number of soft modes). This definition applies to a special class of mechanical metamaterials – composite materials, structural foams, cellular materials, etc. – which feature special mechanical properties. Pentamode materials have been proposed for transformation acoustics and elasto-mechanical cloak, but their potential in different engineering fields is still only partially explored. We here present novel versions of pentamode materials: artificial structural crystals showing shear moduli markedly smaller than the bulk modulus. Novel pentamode lattices with tensegrity architecture are designed, through the insertion of actuated struts and/or prestressed cables within basic pentamode lattices. Such systems are proposed as tunable seismic base-isolation devices, profiting from their low and adjustable shear moduli, which can be easily adapted to the dynamic properties of the structure to be isolated.
isolamento sismico
ANALYSIS OF FIBER-REINFORCED ELASTOMERIC ISOLATORS INCLUDING STRETCHING OF REINFORCEMENT AND COMPRESSIBILITY OF ELASTOMER
The mechanics of fiber reinforced multi-layered elastomeric isolation bearings is studied in this paper. Such bearings offer the possibility of light-weight low-cost seismic isolators that can be mass-produced and used for public buildings and housing of highly seismic areas in developing countries.
The fiber reinforcement, in contrast to the steel reinforcement in conventional isolators, assumed to be rigid both in extension and flexure, is flexible in extension, but completely without flexural rigidity. In addition the rubber layers in these bearings tend to be very thin leading to large shape factors and the need to include bulk compressibility in the elastomer. These aspects of the bearings lead to interesting mechanics problems and in this paper the influence of the stretching of the reinforcement and the compressibility of the elastomer on the mechanical response is developed and confirmed by finite element analyses. A surprising result of the analysis of the combined response of the apparently unrelated effects of stretching of the reinforcement and compressibility in the elastomer is that the mathematical structure of the theory is the same for both effects and that they can be combined in the result- ing solution in a simple way. It is shown that it is possible to produce a fiber-reinforced isolator that matches the behavior of a steel-reinforced isolator. The fiber-reinforced isolator will be significantly lighter and could lead to a much less labor intensive manufacturing process.
CONCAVE SLIDING ISOLATOR’S PERFORMANCE UNDER MULTI-DIRECTIONAL EXCITATION
Due to their large displacement capability and stable energy dissipation associated with a compact shape and new highly performing materials, the use of concave sliding isolators have been continuously increasing for application in buildings and bridges. In this paper the results of dynamic tests on full scale devices are presented. Their response was studied in a wide velocity range, for bi-directional patterns under different compressive loads. In this range of loading characteristics, which is typical of design for earthquake excitation, the behavior of these isolators appears significantly affected by the multi-directionality of the motion, and more specifically by the degradation of the coefficient of friction due to heating phenomena at the sliding surface. An analytical model, applicable to the prediction of bi-directional sliding behavior of friction-based isolators has been experimentally validated. Results of this study suggest that these phenomena should be considered in the design of structures equipped with these popular anti-seismic devices.
SEISMIC RETROFIT OF A PRESTRESSED CONCRETE ROAD BRIDGE
The paper deals with the proposal of a procedure for the seismic retrofit of an existing prestressed concrete bridge. First, the seismic vulnerability assessment of the bridge was carried out. With this aim, a Nonlinear Static Procedure based on the Capacity Spectrum Method as well as the Inelastic Demand Response Spectra was applied. According to the Performance-Based Earthquake Engineering principles, this procedure makes it possible to explicitly correlate the different performance levels to the varying intensities of seismic action. A seismic protection strategy based on the use of isolating system located between pier top and deck was subsequently applied. A design process consisting of an appropriate application of capacity-design principles and the Direct Displacement-Based Design approach was implemented. Finally, the seismic response of the bridge, modeled with an “exact” damping matrix, was evaluated through a linear time-history analysis involving a solution of the complete set of equilibrium equations at each time increment. The results obtained highlight the effectiveness of the seismic retrofit strategy.
ANALYSIS OF THE DYNAMIC BEHAVIOR OF BASE ISOLATED STRUCTURES BY STATE-SPACE FORMULATION
The seismic behaviour of base isolated buildings is herein analysed by using an innovative mathematical formulation in the state space, that allows us to describe the dynamic response of structures in the case of non-classical damping. Particularly, the seismic response of base isolation with linear–viscous behaviour is herein investigated by studying the mode shapes, the frequencies and the modal participation factors, that are obtained by the proposed formulation varying the main design parameters. In such a manner, the effect of these parameters on the isolated structure behaviour as a whole is evident.
The Jazz House
For this paper is available an extended abstract after the text in Italian
Recent application of seismic isolation systems in Italy
At present there are over 10,000 structures in the world that are protected by means of seismic isolation (SI), energy dissipation (ED) and other seismic vibration passive control (SVPC) systems and devices. They are located in more than 30 countries. Italy is now fifth at worldwide level (after Japan, the P.R. China, the Russian Federation and the USA) and is first in Western Europe. In the last years, however, there was a large increase of the number of the applications completed and, especially, of those in progress or under design. This was due at first to the new Italian seismic code, enforced in May 2003 by Ordinance Nr. 3274 of the Prime Minister (mostly as a consequence of the San Giuliano di Puglia tragedy during the 2002 Molise and Puglia quake), which freed and simplified the adoption of the SVPC systems; a second cause was the 6.3 magnitude Abruzzo earthquake of April 6, 2009. The new applications in Abruzzo recently made Italy fourth at worldwide level for the number of isolated buildings. Moreover, Italy remains among the worldwide leading countries for the number and importance of bridges and viaducts protected by the SVPC systems (which are over 250) and also for the manufacturing of the latter (especially of the SI ones) for applications to all kinds of structures abroad. This paper summarizes the state-of-the-art of applications of anti-seismic techniques in Italy, with particular attention to the most recent ones of SI to buildings.
For this paper is available an extended abstract after the text in Italian
The new bridge seismically isolated over the river Fella in Dogna (Italy): design validation as a result of structural dynamic tests
An example of structural validation as a result of dynamic identification of a new bridge over the river Fella in Dogna (Italy) follows. The structure, a prestressed concrete continuous beam, bypasses, with its two spans of 37.5 m each, the wide alluvial riverbed. The bridge, which rises in an high level seismic area, has been isolated by the use of elastomeric isolators able to reduce the heavy stresses of seismic origin. The dynamic tests performed on the bridge and the followed structural identification by means of model updating have led to a «zero reading» of the dynamic characteristics, to be used as reference data for subsequent analysis of degradation, especially following a seismic event. It will be shown a parallel between the stresses identified in the structural model (based on which the bridge was built) and the resulting model from dynamic identification, to extract a possible criterion for structural validation after the construction.
For this paper is available an extended abstract after the text in Italian
«Il viadotto di silea in Veneto»: verifica sismica secondo la normativa italiana e l’eurocodice
The design of an important viaduct in Treviso (a town in Venetian area) is here described. This struc- ture, called «Silea» Viaduct, was designed following the philosophy of the new prestational codes. The viaduct, built in 2007, has medium span and composite structure. In this paper the different safety level has been evaluated on the basis of National and European Codes. The structure is provided with viscous dampers and mechanical dampers to bidirectional earthquake excitation and the main parameters defining the seismic behaviour of seismically isolated bridges are investigated.
This paper is available in Italian only.