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.
seismic isolation
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.
A sustainable seismic input reduction system for monuments, for existing and new structures by creating large, stiff and strong foundations – practical applications
A system is described, aiming at reducing the seismic excitation of structures, based on pure struc- tural solutions. The basic idea is to exploit the phase lag of the incident seismic waves along the foundation and, accordingly, to design it in order to possess the adequate stiffness and strength. The longer the foundation is, the larger the phase lag becomes. It is, therefore, well understandable that under this requirement, foundations of the maximum possible length must be designed. The presented methodology might be proved quite valuable for exist- ing structures and especially for monuments, where, in most cases, it is not possible to proceed to the necessary strengthening interventions in the structure above its foundation. As a technical support of the present investigation, the size of the foundation as a two dimensional elastic beam and the velocity of the propagation of the ground motion are examined as basic parameters. Two strong ground motions have been used, each one with quite differ- ent characteristics compared to the other one: an artificial time history of rather high frequency, fitting to EC8, Type 1, ground class A and a natural ground motion of the Edessa, Greece 1990, M = 5.9, earthquake. The Edessa earthquake is characterized by much longer predominant periods of vibration compared to the artificial one. Vari- ous lengths of the foundation beam have been examined in combination with the velocity of the propagation of the ground motion along the longitudinal direction of the beam. The achieved motions at the center of gravity of the beam as well as the pertinent response spectra are calculated. These spectra are compared to the free field ones. At the beginning of the paper, it is tried to explain the inconsistency between macroseismic observations and earth- quake code requirements concerning the effects of the size of the building foundation. At the end of the paper, the results of the described methodology are demonstrated in several practical case studies.
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
Analysis of the seismic risk of major-hazard industrial plants and applicability of innovative seismic protection systems Part II: Steel liquid storage tanks with floating roof
In this paper the main results obtained within different research projects, developed about the applicability of seismic passive control of major-hazard industrial installations are shown. The work has been subdivided into two papers; in this second part the results relating to big steel tanks equipped with floating roof are presented. After a brief discussion about the seismic vulnerability of tanks and the study of the equations of motion of fixed and isolated base tanks, the main results of a series of shaking table tests, carried out on a steel tank with diameter of 4 m and filled with water up to 1m, are presented. The tank has been tested both in fixed base configuration with floating roof and seismically protected with elastomeric and elasto-plastic isolation devices. The results have confirmed the reliability of the simplified models and the high effectiveness of the base isolation systems in reducing the pressure on the tank wall and the relative stresses, also with floating roof. On the contrary, a low increasing of the oscillation amplitude of the liquid surface has been observed, partially compensated by an increasing of damping, which drastically reduces the number of the free oscillations of the floating roof, in the post-earthquake phase.
For this paper is available an extended abstract after the text in Italian