Mauro Dolce

Mechanical behaviour of shape memory alloys for seismic applications 2. Austenite NiTi wires subjected to tension

Abstract The results of cyclic tensile tests on superelastic NiTi shape memory alloy (SMA) wires are presented and discussed. The tests were carried out within a large experimental test programme for the MANSIDE Project, with the scope of verifying the suitability of SMA superelastic wires as kernel components for seismic protection devices. The mechanical behaviour is described by means of four fundamental quantities, namely: secant stiffness, energy loss per cycle, equivalent damping and residual strain. The sensitivity to temperature and strain rate, as well as the influence of strain amplitude and the effects due to repeated cyclic deformation, are analysed in detail. The experimental results show that the characteristics of the superelastic wires are well suited for seismic applications, as both the recentring and the energy dissipating features of the devices can be easily obtained. Moreover, the influence of the investigated parameters, within their usual range of variation in seismic protection devices, is compatible with the use of superelastic wires for practical applications.

Mechanical behaviour of shape memory alloys for seismic applications 1. Martensite and austenite NiTi bars subjected to torsion

Abstract The mechanical behaviour of several specimens of nickel–titanium shape memory alloy (SMA) has been deeply investigated through a large experimental test program and numerical simulations, in view of their possible use as kernel components for seismic protection devices. The SMA specimens were different in shape (wires and bars with different diameter), physical characteristics (alloy composition, thermomechanical treatment and material phase) and stress mode (tension, torsion, bending and shear). The experimental tests were carried out by applying repeated cyclic deformations. Strain rate, strain amplitude, temperature and number of cycles were considered as test parameters, and their values were selected taking into account the typical range of interest for seismic applications. The aim of the experimental program was to examine the performances of the SMA elements under the working conditions they should be subjected to in a feasible seismic device, under repeated earthquake-like excitations. In this paper, the most important outcomes of the torsion tests are described and analysed in terms of three fundamental mechanical quantities: secant stiffness, energy loss and equivalent damping. The experimental results show that SMA bars subjected to torsion, especially the martensitic ones, have great potential for their use in seismic devices due to their considerable energy dissipation capacity and outstanding fatigue resistance.

Earthquake Damage Scenarios of the Building Stock of Potenza (Southern Italy) Including Site Effects

Damage scenarios relevant to the building stock of the town of Potenza, Southern Italy, are presented. A procedure for the preparation of scenarios has been purposely set up. In the first step, the inventory of the building stock has been made. Location and characteristics of buildings have been obtained from a survey carried out after the 1990 Potenza earthquake and further updated in 1999. In the second step, the absolute vulnerability of the buildings has been evaluated. A hybrid technique has been used, where typological analyses and expert judgement are combined together. Beyond the classes of vulnerability A, B and C of the MSK scale, the class D of EMS98 scale, for the less vulnerable buildings, has been considered. The third step has been the selection of the reference earthquakes by including also local amplification effects. Two events with 50 and 475 years return periods have been chosen as representative, respectively, of a damaging and of a destructive seismic event expected in Potenza. The sites that may exhibit important amplification effects have been identified using the first level method of the TC4 Manual. Damage scenarios of dwelling buildings have been prepared in the fourth step and reported in a GIS. They are relevant to the selected reference earthquakes, taking into account or not site effects. The generally low vulnerability of buildings results in a limited number of damaged buildings for the lower intensity earthquake, and of collapsed buildings, for the higher intensity earthquake. The influence of site effects on the damage distribution is significant.

Analysis of RC Building Dynamic Response and Soil-Building Resonance Based on Data Recorded during a Damaging Earthquake (Molise, Italy, 2002)

During the 2002 seismic sequence in Molise (Italy), the town of Bonefro suffered moderate damage ( I MCS = VII) except for two reinforced concrete (RC) buildings. These buildings are located on soft sediments, close to each other and very similar in design and construction. The main difference is the height: the most damaged one (European Macroseismic Scale damage 4) has four stories, whereas the less damaged (EMS damage 2) has three stories. The M 5.4 shock on 31 October damaged both of them. The second shock on 1 November ( M 5.3) increased the damage on the four-story building substantially, just while a 5-min. seismic recording was taken. We analyzed the recorded data by four different techniques: short-time fourier transform (STFT), wavelet transform (WT), horizontal-to vertical spectral ratio (HVSR), and horizontal-to-vertical moving window ratio (HVMWR). All the results agree upon the estimate of the main building frequency before the second shock and upon the shift of frequency due to damage. All the fundamental frequencies (pre-, during, and postdamage) are in the range 2.5-1.25 Hz. The fundamental frequency of the less damaged building was estimated at about 4 Hz. To test if the soil-building resonance effect could have increased the damage, we also evaluated the soil fundamental frequency by three different techniques: noise HVSR, strong motion HVSR of seven aftershocks, and 1D modeling based on a velocity profile derived from noise analysis of surface waves (NASW) measurements. The results are again in good agreement, showing that resonance frequencies of the soil and of the more damaged building are very close.

Theoretical and Experimental Studies for the Application of Shape Memory Alloys in Civil Engineering

Shape memory alloys (SMAs) have great potential for use in the field of civil engineering. The authors of this paper have been involved, from 1996, in several experimental and theoretical studies of the application of SMAs in civil engineering, for national and international research projects. This paper provides an overview of the main results achieved, consisting of the conceptual design, implementation, and testing of three families of SMA-based devices, namely: (i) special braces for framed structures, (ii) seismic isolation devices for buildings and bridges, and (iii) smart ties for arches and vaults. The main advantage of using SMA-based devices in the seismic protection of structures comes from the double-flag shape of their hysteresis loops, which implies three favorable features, i.e., self-centering capability, good energy dissipation capability, and high stiffness at small displacements. The main advantage of smart ties comes from the thermal behavior of SMA superelastic wires, which is opposite to that of steel rod. This implies a strong reduction of the force changes caused by variations of air temperature.

Experimental Investigation of Exterior RC Beam-Column Joints Retrofitted with FRP Systems

Premature brittle failures because of seismic actions strongly affect the behavior of existing reinforced concrete (RC) structural systems. They commonly involve exterior beam-column joints of structures designed without transverse reinforcement. This paper investigates the behavior of unconfined joints that do not conform to current seismic codes and the effectiveness of externally bonded fiber-reinforced polymers (FRPs) as a strengthening technique. It presents an experimental program carried out on six full-scale RC corner joints under constant axial load and transverse cyclic loading in the as-built and FRP-strengthened configuration. After describing the specimen design strategy and test setup, seismic performance is compared. Particular attention is paid to comparing the experimental capacity of as-built joints and the capacity, which can be predicted on the basis of models available in the literature. Finally, a discussion on the effectiveness of different FRP-strengthening layouts is reported.

SMA recentering devices for seismic isolation of civil structures

Two similar full scale seismic isolation SMA prototype devices, with 600 KN maximum force and 200 KN supplemental recentering force were the final product of the MANSIDE (Memory Alloys for New Seismic Isolation DEvices) project funded by the European Commission. Exploiting the superelastic behaviour of NiTi wires, they have full recentering and some energy dissipation capabilities, as well as high resistance to large strain cycle fatigue and great durability. They can be used for both bridges and building structures. Their applicability is demonstrated by an experimental application made on a small building in Italy. The building was subjected to a release test, by moving it 150 mm and then suddenly releasing it to measure free oscillations. A description of the devices, their applicability and the relevant experimental tests is provided in the paper.

Transport infrastructure surveillance and monitoring by electromagnetic sensing: the ISTIMES project

The ISTIMES project, funded by the European Commission in the frame of a joint Call “ICT and Security” of the Seventh Framework Programme, is presented and preliminary research results are discussed. The main objective of the ISTIMES project is to design, assess and promote an Information and Communication Technologies (ICT)-based system, exploiting distributed and local sensors, for non-destructive electromagnetic monitoring of critical transport infrastructures. The integration of electromagnetic technologies with new ICT information and telecommunications systems enables remotely controlled monitoring and surveillance and real time data imaging of the critical transport infrastructures. The project exploits different non-invasive imaging technologies based on electromagnetic sensing (optic fiber sensors, Synthetic Aperture Radar satellite platform based, hyperspectral spectroscopy, Infrared thermography, Ground Penetrating Radar-, low-frequency geophysical techniques, Ground based systems for displacement monitoring). In this paper, we show the preliminary results arising from the GPR and infrared thermographic measurements carried out on the Musmeci bridge in Potenza, located in a highly seismic area of the Apennine chain (Southern Italy) and representing one of the test beds of the project.

Jet-Pacs Project: Dynamic Experimental Tests and Numerical Results Obtained for a Steel Frame Equipped with Hysteretic Damped Chevron Braces

The experimental and numerical results obtained by Research Units of the University of Basilicata and University of Calabria for a steel frame, bare or equipped with metallic yielding hysteretic dampers (HYDs), are compared. The shaking table tests were performed at the Structural Laboratory of the University of Basilicata within a wide research program, named JETPACS (“Joint Experimental Testing on Passive and semiActive Control Systems”), which involved many Research Units working for the Research Line 7 of the ReLUIS (Italian Network of University Laboratories of Earthquake Engineering) 2005–2008 project. The project was entirely founded by the Italian Department of Civil Protection. The test structure is a 1/1.5 scaled two-story, single-bay, three-dimensional steel frame. Four HYDs, two for each story, are inserted at the top of chevron braces installed within the bays of two parallel plane frames along the test direction. The HYDs, constituted of a low-carbon U-shaped steel plate, were designed with the performance objective of limiting the inter-story drifts so that the frame yielding is prevented. Two design solutions are considered, assuming the same stiffness of the chevron braces with HYDs, but different values of both ductility demand and yield strength of the HYDs. Seven recorded accelerograms matching on average the response spectrum of Eurocode 8 for a high-risk seismic region and a medium subsoil class are considered as seismic input. The experimental results are compared with the numerical ones obtained considering an elastic-linear law for the chevron braces (in tension and compression), providing that the buckling be prevented, and the Bouc-Wen model to simulate the response of HYDs.

EXPERIMENTAL BEHAVIOUR OF R/C FRAMES RETROFITTED WITH DISSIPATING AND RE-CENTRING BRACES

An extensive program of shaking table tests on 1/4-scale three-dimensional R/C frames was jointly carried out by the Department of Structure, Soil Mechanics and Engineering Geology (DiSGG) of the University of Basilicata, Italy, and the National Laboratory of Civil Engineering (LNEC), Portugal. It was aimed at evaluating the effectiveness of passive control bracing systems for the seismic retrofit of R/C frames designed for gravity loads only. Two different types of braces were considered, one based on the hysteretic behaviour of steel elements, the other on the superelastic properties of Shape Memory Alloys (SMA). Different protection strategies were pursued, in order to fully exploit the high energy dissipation capacity of steel-based devices, on one hand, and the supple-mental re-centring capacity of SMA-based devices, on the other hand. The experimental results confirmed the great potentials of both strategies and of the associated devices in limiting structural damage. The retrofitted model was subjected to table accelerations as high as three times the acceleration leading the unprotected model to collapse, with no significant damage to structural elements. Moreover, the re-centring capability of the SMA-based bracing system was able to recover the undeformed shape of the frame, when it was in a near-collapse condition. In this paper the experimental behaviour of the non protected and of the protected structural models are described and compared.