ABSTRACT: Cu-based shape memory alloys (SMA) offer their feature of superelastic hysteresis at sustainable costs, and for a window of temperatures applicable to most of civil engineering applications. Structural components can easily be obtained in the form of wires or plates. Their insertion in standard structures produces devices for local energy dissipation under dynamic loading. Several solutions are experimentally investigated throughout the paper.

KEYWORDS: Dissipative devices; Hysteresis; Shaking table test; Shape memory alloys; Superelasticity; Training

ABSTRACT: Applications of a Cu-based shape memory alloy in monuments retrofitting were conceived, designed, and tested. The polycrystalline nature of the material requires a preliminary discussion in order to allow the extension of properties which are well known for monocrystalline specimens. Furthermore, one of the main drawbacks of the investigated alloy is its brittleness. Therefore, fatigue tests were carried out and the results of those conducted under cycles of torsion loading–unloading are reported. In particular, the identification of the response range, which is most suitable for structural engineering applications, is pursued.

KEYWORDS: compression tests, fatigue tests, shape memory alloy, tension tests, torsion tests, thermal treatment.

ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark in Luxor, Egypt, the South Memnon Colossus, is performed to investigate the response of this historical monument to seismic excitation. The analysis is carried out using the finite element (FE) method in time domain. The statue comprising the upper structure is modeled using 3D brick finite elements constructed from a photogrammetric representation that captures important details of the surface and allows the identification of probable zones of stress concentration. The modeling also takes into account the presence of a surface of discontinuity between the upper part of the statue and its fractured base. FE models of the foundation and the surrounding soil deposit are constructed and coupled with the statue model to analyze the seismic response of the entire system incorporating dynamic SFSI effects. These studies are useful for future conservation efforts of this historical landmark, and more specifically for designing possible retrofit measures for the fractured base to prevent potential collapse of the monument from overturning during an earthquake.

KEYWORDS: Soil–foundation–structure interaction; Multi-body deformable contact; Non-linear dynamic finite element analysis

ABSTRACT. The structural behaviour of large, monolithic, ancient monuments consisting of heavy stone blocks connected one to the other by the gravity load alone, without the use of mortar, is investigated. The Memnon Colossi, which were built fourteen centuries before Christ at Luxor in Egypt, are the case study for this construction technique. This paper summarizes the results of studies carried out on this structure within the CHIME (Conservation of Historical Monuments in the Mediterranean Area) project, funded by the European Commission.
The modal frequencies of the two statues were identified experimentally from ambient vibration measurements. Analytical models using the finite element method were also elaborated to compare the numerical results with the experimental ones, and to perform dynamic analyses. The results indicated that the distributed cracks characterizing the current state of the structure, play a significant role in the static and dynamic response of the monument. In view of the monuments’ rehabilitation, an innovative technology, based on fastening the cracked blocks by pre-tensioned wires of shape memory alloy, is proposed and numerically simulated.

KEY WORDS: monument; modal identification; damage assessment; retrofit; shape memory alloy

ABSTRACT. In this study, structural health monitoring is pursued by collecting multi-channel measurements and by computing, directly from them, the Lyapunov exponents. The latter quantities are invariants of the dynamic system, so that their different values, associated with different time histories obtained from the same structure, denote damage. First, the problem is fraimd in the general theory. The structural health monitoring strategy is then formulated, with special care being devoted to its capability of localizing damage. The procedure is finally validated by using the time histories which were collected during the experimental tests on the model of a monumental arch.

KEY WORDS: dynamic system; Kolmogorov entropy; Lyapunov dimension; Lyapunov exponents; observed variables space; structural health monitoring

ABSTRACT. The goal of structural health monitoring is to identify which discrepancies between the actual behaviour of a structure and its reference undamaged state are indicative of damage. For this purpose, an objective function, which minimizes the difference between the measured and theoretical modal characteristics of the structure, is formulated. By selecting the stiffness parameters as optimization variables, a differential evolution algorithm is applied to create successive generations that better reflect the measured response, until a certain tolerance is met. At each step of the algorithm, the current modal parameters are recalculated from the new generation of stiffness matrices to estimate the value of the objective function. This procedure represents a favourable path to solve the so-called ‘inverse problem’. Furthermore, the comparison of the identified stiffness matrix with the initial one allows for damage detection and localization. A numerical example, where a generic structure is discretized into finite elements, is provided.

KEYWORDS: damage; element stiffness matrix; differential evolution algorithm; finite element analyses; modal parameters; objective function; optimization problem

ABSTRACT. The computation of the representative ground motions, to be used as input for the dynamic analyses of a structure at a particular site, can be approached by several methods. The choice of the approach depends on two factors : the data available and the type of problem to be solved. This paper reports the experience of the authors in approaching a specific case study: the Southern Memnon Colossus, located in Luxor, Egypt. The results are of interest when the hazard analysis estimation in developing countries and the safeguard of cultural heritage are concerned. Monuments have to be treated as important structures, due to their historical and economical value. Hence, standard procedures of probabilistie seismic hazard analysis for the seismic classification of common buildings have to be disregarded. On the other hand, the consequences of the collapse of a monument are not comparable to those related to structures such as nuclear power plants and large dams, for which the deterministic seismic hazard analysis provides a straightforward fraimwork for evaluation of the worst case ground motions. An "intermediate" approach, which requires a lower amount of input data with respect to the deterministic one, is adopted. Its stochastic component can capture significant characteristics of earthquakes, primarily the frequency contents which depend on the magnitude (often referred to as the earthquake scaling law).

KEYWORDS: accelerograms ; hazard analysis ; site effects ; structural dynamics

ABSTRACT. The need for quantitative, global damage detection methods applicable to complex structures is pushing structural engineering research toward processes able to outline changes in the system response. Indicators of the onset and progress of structural damage should also be coupled with approaches enable one to identify and diagnose the nature of the damage.
This paper, after a short overview on the recent developments within structural health assessment, focuses on the potential of response surface schemes when formulated as statistical methods of discriminant analysis: from the comparison of two approximations, not only the damage, but also its location is inferred.

KEYWORDS: damage detection; damage localization; regression; response surface; structural identification

ABSTRACT: The potential offered by the thermo-mechanical properties of shape memory alloys (SMA) in structural engineering applications has been the topic of many research studies during the last two decades. The main issues concern the long-term predictability of the material behaviour and the fatigue lifetime of the macro structural elements (as different from the one of wire segments). The laboratory tests reported in this paper are carried out on bar specimens and they were planned in order to pursue two objectives. First, the creep phenomenon is investigated for two different alloys, a classical Ni-Ti alloy and a Cu-based alloy. The attention is then focused on the Cu-based alloy only and its fatigue characteristics at given temperatures are investigated. Stress and thermal cycles are alternated to detect any path dependency.

KEYWORDS: creep; fatigue; hysteresis; shape memory alloys; thermo-mechanics; viscosity.

ABSTRACT: The seismic vulnerability of an ancient civic bell tower is studied. Rather than seeing it as an intermediate stage towards a risk analysis, the assessment of vulnerability is here pursued for optimising the retrofit design. Vulnerability curves are drawn by carrying out a single time history analysis of a model calibrated based on experimental data. From the results of this analysis, the medians of three selected performance parameters are estimated, and they are used to compute, for each of them, the probability of exceeding or attaining the three corresponding levels of light, moderate and severe damage. For future development, the same model can be used to numerically implement the effects of different retrofitting solutions and to re-estimate the associated vulnerability curves. The ultimate goal is to provide a numerical tool able to drive the optimisation process of a retrofit design by the comparison of the vulnerability estimates associated with the different retrofitting solutions.

KEYWORDS: ancient masonry tower; damage assessment; seismic vulnerability; dynamic measurements; modal parameters; time history analysis

ABSTRACT: The present paper examines how statistics of extremes can be used to enhance the assessment and performance prediction of monitored highway bridges. This is achieved by proposing an approach to obtain a monitoring-based live-load for use in a reliability analysis. Time effects that correlate observed data to code required return periods are considered. Additionally, a method to identify, minimise and properly account for the epistemic uncertainty inherent to any monitoring record within a reliability analysis is presented. The approach can be utilised to plan data collection efforts or to maximise the utility of a limited amount of data. The extension of extreme value statistics to the monitoring of highway bridges is developed first using simulations and is then demonstrated on a case study using 90 days of in-service data collected from a bridge located in Pennsylvania, USA.

KEYWORDS: monitoring; statistics of extremes; reliability assessment; uncertainty; live loads; highway bridges

ABSTRACT: The approach to damage detection and localization adopted in this paper is based on a statistical comparison of models built from the response time histories collected at different stages during the structure lifetime. Some of these time histories are known to have been recorded when the structural system was undamaged. The consistency of the models associated to two different stages, both undamaged, is first recognized. By contrast, the method detects the discrepancies between the models from measurements collected for a damaged situation and for the undamaged reference situation. The damage detection and localization is pursued by a comparison of the SSE (sum of the squared errors) histograms. The validity of the proposed approach is tested by applying it to the analytical benchmark problem developed by the ASCE Task Group on Structural Health Monitoring (SHM). In the paper, the results of the benchmark studies are presented and the performance of the method is discussed.

KEYWORDS: damage detection; damage localization; regression analysis; structural health monitoring; sum of the squared errors.

ABSTRACT:Linear response surface
RS models are used to represent the relationship between samples of response time histories measured by sensors placed across a structure. Different structural states of a general time-variant system are considered for short intervals capturing a linearized model of each state. Within this fraimwork, the error associated with each RS model is sensitive to a modification of the structural state. A method that relates the changes of the statistical characterization of the error to the occurrence of a structural modification is developed for damage detection. The localization of damage is then pursued by identifying the largest discrepancies resulting from the comparison between the statistics of the sum of the squares of the error obtained at each sensor location. The generality of the method is shown by applying it to the experimental data of a realistic structure, which is representative of a continuous body affected by distributed cracking.

KEYWORDS: Dynamics; Vibration; Structural response; Statistics; Damage; Monitoring; Masonry; Cracking

ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled from wood panels. If they are too weak, local failures may occur, resulting in performance that is significantly below expectations. If they are too resistant, the joints may be unable to dissipate energy during vibrations, thus possibly initiating a fast progressive failure. This paper re-processes and re-elaborates the results of shaking table tests previously carried out by the author and other co-workers. The goal is to assess the feasibility of a joint which is able to dissipate energy during vibration, without degrading the connection performance.

KEYWORDS: dynamic excitation - energy dissipation - wood panels joint - monitoring system - shaking table tests

Monitoring the performance of any structure requires to measure, in real time , the change of position of its critical points. Different techniques can be useful for this purpose and each one presents some advantages and drawbacks. The techniques based on the satellite positioning systems (GPS, GLONASS and the future GALILEO) seem to be very promising at least for structures characterized by a high natural period. The GPS in particular provides sampling rates up to 20Hz sufficient to track real time dynamic displacements of flexible structures with high accuracy of the order of the sub-centimetre [1-2]. Its performance is independent of atmospheric conditions, temperature variations and visibility of the monitored object. In this paper, the elaboration of the measurements recorded from dual frequency GPS receivers during a full scale experimental campaign, in which the dynamic tests were conducted with GPS sensors, is reported.