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    Thanasis Sfetsos
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    Thanasis Sfetsos
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    Earthquakes represent a serious threat for several European countries, particularly for Mediterranean bordering countries, where seismic events have been triggering significant destruction and loss over the last decades. Since the early 1970's, when the first studies on seismic vulnerability assessment of buildings at large geographical areas were made, the scientific state-of-the-art of vulnerability assessment has undergone a notable development. However, initiatives for seismic risk assessment applied at a large scale still face significant challenges, such as the inventory of exposure data in very disparate databases and the selection of up-to-date vulnerability models compatible with the information available and with different construction practices. Having the latter challenge in mind, the principal activity described in this paper is to investigate the practices and methodologies for assessing seismic vulnerability of the existing building stock in Europe, in areas of moderate to high seismicity. The study involves an extensive collection and review of analytical fragility curves available in the technical literature and their evaluation according to a set of qualitative criteria in order to select the most appropriate ones for each building class and geographic region. The fragility curves assessed according to the above-mentioned criteria may be used for different seismic risk studies, namely: (i) the assessment of the seismic risk of buildings in Europe for decision-making on building renovation, (ii) the impact estimations after major earthquake disasters for emergency response, (iii) the seismic scenario simulation for emergency planning and disaster risk reduction and (iv) the definition of procedures for seismic risk assessment to be integrated in multi-risk approaches for different natural hazards in Europe. The review revealed a number of issues that are not sufficiently addressed in the current literature, such as the use of more refined numerical models that capture better the response of structures, the effect of non-structural elements, shear failure modes of reinforced concrete elements, out-of-plane response and types of floors for masonry buildings and geometric irregularities.
    Earthquakes continue to represent a serious threat for some European countries, particularly for Mediterranean bordering countries, where these events have been triggering significant destruction and loss over the last decades. Despite the unpredictable nature of earthquakes, seismic risk assessment should be addressed having in mind the main cause of damage, which is related to the inadequate seismic resistance of the existing structures such as residential, industrial or cultural heritage buildings. A literature review on the existing methodologies for deriving fragility curves suitable to the European building stock is herein presented. Even though a brief overview of all the existing approaches for deriving fragility curves is made, this report focuses on analytical fragility curves and discusses the most relevant features inherently associated with the computation of these curves. Additionally, these methodologies are qualitatively evaluated by means of different sets of criteria. With this report it is intended to provide a clear insight about the main differences between existing analytical methodologies for deriving fragility curves, highlighting as well some of their most important advantages and drawbacks. The proposed evaluation criteria can be used further on to help not only on the selection of the most suitable fragility curves for a given geographical location and structural typology, but also for the further comparison and validation of fragility curves.
    Critical infrastructures are the backbone of modern society and provide many essential goods and services to urban areas. Given the risks associated with the impact of natural hazards on critical infrastructures, the move towards safer and more resilient cities requires the development and application of an improved risk assessment framework to address high-impact and low-probability events. In this context, the STREST project developed a stress test framework to determine the risk and resilience of non-nuclear critical infrastructures.
    A novel seismic retrofitting method for an RC column is proposed in this paper. In the proposed retrofit method, both ends of an RC column are actively confined with steel plates and pretensioned high-strength external hoops. Then, a steel brace and the RC column are connected together to increase the lateral strength. The effectiveness of the retrofit technique was verified through cyclic lateral load tests. The experimental results show that the retrofitted specimen exhibits excellent seismic performance in term of strength while the non-retrofitted specimen failed in flexure-shear-critical manner. Numerical studies on the retrofitted and non-retrofitted specimens are carried out using OpenSees. For non-retrofitted specimen, shear spring element and shear limit curve are used to predict its shear-related hysteretic behavior. For retrofitted specimen, the buckling behavior of steel brace in compressive side is simulated and the sufficient ductility of steel brace in tensive side can be expected.
    This Chapter presents a literature review of seismic fragility functions for reinforced concrete road and railway bridges. It first covers the main issues in fragility analysis, such as the systems for classification of bridges, methods for deriving fragility functions, intensity measures, damage states and damage measures. A section is dedicated to the way the uncertainties regarding the seismic action, geometry, material properties and modelling are treated in existing studies. The Chapter deals also with the recent developments that examine special issues which were not addressed in the first generation of fragility curves. They refer to damaged and retrofitted bridges, the effects of corrosion, skew, spatial variability of the seismic action and liquefaction. Finally, a method for fast fragility analysis of regular bridges is presented. The method applies to bridges with continuous deck monolithically connected to the piers or supported on elastomeric bearings and with free or constrained transverse translation at the abutments.
    A literature review on the seismic strengthening of reinforced concrete buildings, using steel bracings, infills and shear walls, is presented. Extensive experimental testing and numerical analyses of elements and structures have demonstrated the feasibility and effectiveness of all three measures for the increase of global strength and stiffness. In certain cases, they provide additional energy dissipation and help reducing irregularities. The selection of the most appropriate technique is based on desired performance levels and on economic and, possibly, other non-technical criteria. The results of previous studies clearly show that infilling an existing bay with reinforced concrete provides the highest increase in strength and stiffness. These studies also indicate that precast panels, steel bracings and masonry infills strengthened with fibre-reinforced polymers or textile-reinforced mortars are able to offer the same degree of improvement. The results available in literature, complemented by parametric numerical analyses, may provide the basis for the development of design guidelines with emphasis on strength and stiffness characteristics and on detailing of the connection between new and existing elements. Indeed, the development of models and their implementation in analysis software is a necessary step towards the wider application of these strengthening techniques.
    The seismic vulnerability of stone masonry buildings is studied on the basis of their fragility curves. In order to account for out-of-plane failure modes, normally disregarded in past studies, linear static Finite Element analysis in 3D of prototype regular buildings is performed using a nonlinear biaxial failure criterion for masonry. More than 1100 analyses are carried out, so as to cover the practical range of the most important parameters, namely the number of storeys, percentage of side length in exterior walls taken up by openings, wall thickness, plan dimensions and number of interior walls, type of floor and pier height-to-length ratio. Results are presented in the form of damage and fragility curves. The fragility curves correspond well to the damage observed in masonry buildings after strong earthquakes and are in good agreement with other fragility curves in the literature. They confirm what is already known, namely that buildings with stiff floors or higher percentage of load-bearing walls are less vulnerable, and that large openings, taller storeys, larger number of storeys, higher wall slenderness and higher ratio of clear height to horizontal length of walls increase the vulnerability, but show also by how much.
    Fragility curves are constructed for prototype regular RC frame and wall-frame buildings designed and detailed per EC 2 and EC 8. The aim is to evaluate how the Eurocodes achieve their seismic performance goals for RC buildings designed to them. These goals seem to be met in a consistent and uniform way across all types of buildings considered and their geometric or design parameters, except for concrete walls of Ductility Class Medium, which may fail early in shear despite their design against it per EC 8. In fact they do not perform much better than those in braced systems per EC 2 alone.
    Nonlinear response-history analyses (NLRHA) are performed for 16 five- or eight-storey prototype regular reinforced concrete wall-frame (“dual”) buildings designed to Eurocode 8 for ductility class (DC) M (Medium) or H (High), in order to evaluate the higher-mode inelastic magnification of shear forces in the walls. The percentage of the total base shear taken by the walls in the 16 buildings covers evenly a range from 37 to 90 %. The NLRHA results show that there is indeed post-elastic amplification of wall shear forces due to higher modes, but that it is overestimated by the current approach for DC H in Eurocode 8. A recently proposed rational modification of that approach, which suits better modal response spectrum analysis (MRSA), is also evaluated; it is found to be in better overall agreement with NLRHA in an analysis context, but, in general, to underestimate inelastic shears above the ground storey in a design context. The design envelope of wall shears in “dual” buildings prescribed in Eurocode 8 is found to cover such shortfalls; it should be extended to wall systems as well, even in case the current approach in Eurocode 8 is modified per the recent, more rational proposals. The current design envelope of wall moments in Eurocode 8 does not safeguard against plastic hinging at upper levels; even if the linear design envelope is anchored to the moment resistance of the base section, instead of the moment from elastic analysis with the design (i.e., reduced) spectrum, it does not preclude flexural plastic hinging in upper storeys. Although for the design of walls safe-sided envelopes and approximations may be appropriate, for the purposes of evaluation of seismic performance and vulnerability of walls, NLRHA seems to be the only means to estimate with certain confidence the post-elastic higher mode effects on wall shears.
    Closed-form solutions are developed for the elastic response of symmetric bridges, continuous over two, three or four spans, for unidirectional earthquake perpendicular to the deck, i.e., in the transverse or the vertical direction. The ends of the deck are restrained along the earthquake component, but free to rotate within the plane of bending. The mass and flexural rigidity of the deck are uniform along its length and modelled as continuous. The piers may support the deck through flexible bearings or are rigidly connected to it, in which case a lumped mass is considered at the connection. For each bridge geometry a nonlinear transcendental equation is derived for the modal circular frequency and solved numerically. Closed formulas are given for participation factors, participating masses, the curvature and end slope of the deck and the end reactions. The frequency and cumulative participating mass of the important modes are plotted as a function of the dimensionless total stiffness of the piers. Parametric analyses are presented for the mass of the piers, the location of the intermediate piers and their relative stiffness. They suggest that the eigenvalues are close to those of a simply supported beam with uniformized transverse restraint and mass, except for relatively high pier stiffness and for a single pier of medium stiffness. Even if this approximation is made, modal shapes and elastic forces or deformations of the deck may differ significantly from those of a simply supported beam with uniformized parameters and should be calculated with the closed-form expressions presented. The analytical first-mode frequency and the one from the single-mode method of the AASHTO code in general agree very well, but the first mode participating masses from the two methods may differ.
    It is an essential step in urban earthquake risk assessment to compile inventory databases of elements at risk and to make a classification on the basis of pre-defined typology/taxonomy definitions. Typology definitions and the classification system should reflect the vulnerability characteristics of the systems at risk, e.g. buildings, lifeline networks, transportation infrastructures, etc., as well as of their sub-components in order to ensure a uniform interpretation of data and risk analyses results. In this report, a summary of literature review of existing classification systems and taxonomies of the European physical assets at risk is provided in Chapter 2. The identified main typologies and the classification of the systems and their sub-components, i.e. SYNER-G taxonomies, for Buildings, Utility Networks, Transportation Infrastructures and Critical Facilities are presented in Chapters 3, 4, 5 and 6, respectively.
    Fragility curves are constructed for a portfolio of prototype regular RC frame and wall-frame buildings designed and detailed to EC2 and EC8. The aim is to use EC8’s own seismic performance assessment methods and criteria for existing buildings to evaluate how EC8 achieves its performance goals for new RC buildings. The overall conclusion is that these goals are met in a very consistent and uniform way across all types of buildings considered and their geometric or design parameters, except for RC walls of Ductility Class Medium, which may fail early in shear despite their design against it according to EC8. In fact, these walls do not perform much better than those of braced systems designed to EC2 alone. Therefore, wall shear seems to be an aspect in EC8 worth looking at again. Another finding is that the slenderness limits and the lateral bracing requirements of EC2 for 2nd-order effects under factored gravity loads place severe restrictions on the size of columns and walls, which, although ignored in ordinary seismic design practice, materially impact the outcome of the design and, to a smaller extent, the seismic fragilities of the building’s members. Mixing in the same building columns with significantly different stiffness most often penalizes the more flexible ones and not the stiff, despite their smaller deformation capacities. Another finding is that the reduction in fragility from higher design peak ground accelerations is disproportionately low. Even buildings designed for gravity loads only, but in full accordance to EC2, possess substantial seismic resistance.
    : Fragility curves are developed for stone masonry buildings with rigid or flexible floors and varying number of storeys, plan dimensions, number of interior cross-walls, storey height, percentage of side length of exterior walls taken up by openings and wall height-to-thickness ratio. Linear Finite Element analysis in 3D is performed and a nonlinear biaxial failure criterion for masonry is used. Buildings with rigid floors and higher percentage of load-bearing walls are less vulnerable. Larger openings, taller storeys, larger number of storeys, higher wall slenderness and higher ratio of clear height to the horizontal dimension of the walls increase the vulnerability.
    The seismic design of multi-storey precast structures is at present not covered by specific provisions of the European seismic codes. To fill such gap, capacity design criteria for multi-storey precast concrete frames with hinged beams are presented. Based on the same approach prescribed by the codes for monolithic cast-in-place frames, a distribution of floor forces and a value of behaviour factor are assumed to perform a static linear analysis of the structure. A parametric study aimed to validate the design method is carried out by varying, within the range of practical interest, the main design parameters such as the number of storeys, the mass-over-stiffness ratios and the stiffness characteristics of the columns. The results of dynamic modal analyses and non linear static analyses show that the proposed method can safely be applied to ordinary multi-storey concrete precast frames characterized by structural regularity and limited flexibility.
    The research presented in this article deals with the seismic retrofit of bridge piers with rectangular hollow cross-section using fiber-reinforced polymer (FRP) jackets. A two-level numerical approach that combines finite element method (FEM) analyses and fiber modeling is proposed. The FEM is used to study the effect of FRP jackets on the properties of concrete. The analyses show that the existing empirical laws for FRP-confined concrete are not suitable for piers with hollow cross-section, as the effect of confinement is not uniform within the cross-section and the stress–strain curves show softening after peak strength. Fiber modeling is used to study the global behavior of reinforced concrete piers with rectangular hollow cross-section wrapped with FRP jackets. To account for confinement, the properties of the concrete fibers are modified according to the results of the FEM analyses. The proposed method is validated against experimental results and used for an extensive parametric study. It is found that the effectiveness of jacketing is conditioned by the axial load, longitudinal reinforcement, and jacket dimensions. An empirical design equation is formulated on the basis of the numerical analyses.
    Pseudo-dynamic tests on a large-scale model of an existing six-pier bridge were performed at the ELSA laboratory using the substructuring technique. Two physical pier models were constructed and tested in the laboratory, while the deck, the abutments and the remaining four piers were numerically modeled on-line. These tests on a large-scale model of an existing bridge are the first to have been performed considering non-linear behavior for the modeled substructure. Asynchronous input motion, generated for the specific bridge site, was used for the abutments and the pier bases. Three earthquake tests with increasing intensities were carried out, aimed at the assessment of the seismic vulnerability of a typical European motorway bridge designed prior to the modern generation of seismic codes. The experimental results confirm the poor seismic behavior of the bridge, evidenced by irregular distribution of damage, limited deformation capacity, tension shift effects and undesirable failure locations. Copyright © 2004 John Wiley & Sons, Ltd.
    Cyclic tests on two large-scale models of existing bridge piers with rectangular hollow cross-section were performed in the ELSA laboratory. The prototype structure is an existing reinforced concrete highway bridge constructed in Austria in 1975. The piers presented several seismic deficiencies and consequently they showed poor hysteretic behaviour and limited deformation capacity as well as undesirable failure modes that do not comply with the requirements of modern codes for seismic-resistant structures. Experimental data are compared to numerical and empirical predictions. Copyright © 2003 John Wiley & Sons, Ltd.
    On 31 October and 1 November 2002, two earthquakes took place in the Italian region of Molise. 29 deaths were reported, while many buildings collapsed or suffered major damage. The tectonics of the earthquakes and historic seismicity of the area are briefly described. The distribution of damage and macroseismic intensity are confronted with the current seismic zonation of the region. In particular, the paper deals with the damage suffered by different types of structures, namely masonry and RC buildings and historical churches. The observed damage is mainly attributed to the poor quality of the materials and execution of construction, lack of maintenance and protective devices (e.g., steel ties), as well as to structural interventions. Reference is made to the management of the post-earthquake emergency.
    Within the VAB research program, pseudodynamic tests on a large-scale model of an existing bridge were carried out in the reaction wall of the ELSA laboratory of the Joint Research Centre, applying the substructuring method. Asynchronous input motion, generated for the bridge site, was considered at the base of the piers and abutments. Non-linear substructuring was successfully implemented for the first time at world level by considering appropriate hysteretic models for the substructured piers. Pre-test numerical calculations were canied out in order to define the simplest, yet accurate analytical models for the substructured piers. The results of cyclic tests on large-scale specimens and refined analyses were used to calibrate the models for the numerical piers. The pseudodynamic test was simulated numerically and the results were compared to the experimental ones in order to evaluate the effectiveness of the implemented substructuring technique and adopted models. Three pseudodynamic tests with increasing input intensity were carried out. For every earthquake amplitude the performance of the piers, in terms of drift and ductility demands, dissipated energy and degree of damage, was studied, along with the distribution of damage among the piers. On the basis of the experimental results and the observed damage level, the perfotmance of the bridge was assessed for the earthquakes with different intensity.
    This paper presents a simplified method for the capacity design of multi-storey precast concrete frames with hinged beams. A parametric study shows that in the field of ordinary and less flexible structures, the proposed simplified method can be applied with full reliability, in the same way as prescribed by the seismic codes for monolithic cast-in-situ structures. Moreover, the displacement ductility capacity of a range of multi-storey precast concrete structures designed according to the proposed method is investigated.
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