An adaptive version of the capacity spectrum method is proposed to estimate deformation demands of steel moment-resisting frames under seismic loads. Its computational attractiveness and capability of providing satisfactory predictions of seismic demands in comparison with those obtained by other advanced nonlinear static procedures in literature are examined. Both effectiveness and accuracy of these approximated methods based on pushover analysis are verified through an extensive comparative study involving both regular and irregular steel moment-resisting frames. The results obtained by nonlinear static procedures and nonlinear dynamic time-history analysis under spectrum-compatible accelerograms are eventually compared. The proposed procedure generally gives a more accurate solution than that obtained from the other nonlinear static procedures.
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Seismic Application of Pentamode Lattices
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.
CRITICAL REVIEW OF SEISMIC DESIGN CRITERIA FOR CHEVRON CONCENTRICALLY BRACED FRAMES: THE ROLE OF THE BRACE-INTERCEPTED BEAM
Steel chevron concentrically braced frames are expected to dissipate seismic energy by yielding of the brace under tension, while both beam and columns behave elastically. Besides the strength, also the stiffness of the brace-intercepted beam plays a key role to avoid unfavourable mechanisms. However, no codified requirements are provided to assure adequate beam rigidity. In order to examine this aspect, in the first part of this paper the main results of a numerical parametric study devoted to investigate the mutual interaction between the beam vertical deflection and the brace ductility demand are described. The second part of this article investigates the efficiency of both EC8 and AISC341-10 seismic provisions on the global performance of chevron bracings, particularly focusing on the design of the beam of the braced bays. The results of incremental dynamic analyses performed on several structures confirm the primary importance of the flexural stiffness of the beam.
INFLUENCE OF THE CYCLIC BEHAVIOUR OF BEAM-TO-COLUMN CONNECTION ON THE SEISMIC RESPONSE OF REGULAR STEEL FRAMES
The work presented is aimed at the investigation of the influence of beam-to-column connection typologies on the seismic response of MR-Frames designed according to the Theory of Plastic Mechanism Control (TPMC). The investigated typologies are four partial strength connections designed in order to obtain the same flexural resistance. The first three joints are partial-strength semi-rigid connections while the fourth one is a beam-to-column connection equipped with friction pads properly designed to assure the earthquake input energy dissipation. Beam-to-column joints are modelled by means of rotational inelastic spring elements located at the ends of the beams whose moment-rotation curve is characterized by a cyclic behaviour accounting for stiffness and strength degradation and pinching phenomenon. The parameters characterizing the joints cyclic hysteretic behaviour have been calibrated on the base of experimental results aiming to the best fitting. The prediction of the structural response has been carried out by means of IDA analyses.
PREFACE
Structural health monitoring (SHM) received in the last twenty years, a growing interest by both research- ers and professional, as pointed out by the number of monitoring systems installed today in various countries of the world. The main reasons for this development are on one side the limitations related to the use of traditional methods based on visual inspections an on the other side the great potential offered by automatic systems of damage detection allowing a preventive as- sessment of the seismic vulnerability and a sensible reduction of maintenance costs.
The aim of the Special Issue is to report recent advances in this field and successful applications. It includes eight papers from several research groups that have devoted considerable effort to the identification of techniques and tools for the application of SHM to structures in seismic areas.
For seismic-prone structures, health monitoring can sometimes take advantage of real responses recorded during strong earthquakes. These occurrences have a strategic importance both for the advancement of knowledge on the behavior of structures under strong seismic actions and for the calibration of realistic and reliable numerical models, able to reproduce the structural behavior and to formulate a diagnosis about possible sources of damage. Furthermore, the possibility to assess the seismic vulnerability based on data recorded on the monitored structure, opens new avenues in maintenance policies, shifting from a traditional ‘scheduled maintenance’, to a ‘condition-based maintenance’, carried out ‘on demand’, basing on the current structural condition.
The papers by Saisi et al. and Ranieri et al. tackle this subject for the specific case of historical masonry towers which are important landmarks in the Italian heritage, severely hit by the Emilia earthquake of 2012. The two papers present techniques for assessing the seismic vulnerability by means of preventive non destructive testing based on ambient vibrations taking into account environmental changes as well. The preventive assessment of the seismic vulnerability is also the subject of the paper by Ditommaso et al. focusing on irregular buildings, for which heavy structural damage is often due to torsional effects. In the paper is proposed a simplified experimental approach to identify the existence of torsional modes, based on a limited number of sensors thus limiting the cost of the monitoring system.
In recent years, thanks to the innovations in sensors and techniques for data transmission, a challenging goal for permanent seismic monitoring networks is developing: the possibility to quickly detect damage in a structure affected by a seismic event. This can be of crucial importance both for the management and coordination of immediate safety interventions or evacuation operation, and also for planning of the first operations of repair of damaged structures. In the aftermath of a strong seismic event this assessment is carried out using traditional methods based primarily on visual inspection that require quite a long time to cover the entire area affected by the earthquake, and also lead to estimations which are typically conservative, certainly on the safety side, but with a correspondent increase of the costs related to the downtime of the structure. Furthermore they may fail if damage is not visibly evident. Networks of sensors permanently installed on the structure supported by efficient damage identification algorithms constitute a promising alternative, able to provide real time information and a quick assessment of the damage state of the building after a seismic event. The application of algorithms for damage detection and localization to structures in seismic prone areas is the subject of the papers by Guéguen et al., Ponzo et al, Limongelli et al. and Benzoni et. al. that present numerical and experimental applications of different damage localization algorithms based on modal or non modal features. Sharing the same challenging goal the paper by Hernandez-Garcia et al. implements a robust data-driven methodology for detecting, locating and quantifying changes not only in linear structural parameters but also in nonlinear characteristics of chain-like systems.
The Guest Editor wishes to thank Dr. Gianmario Benzoni, Editor of the Journal, for his support, supervision and assistance in producing this special issue and also all the Authors and the Reviewers for their work and contribution.
SEISMIC MICROZONING IN ITALY: A BRIEF HISTORY AND RECENT EXPERIENCES
In an attempt to develop a diffuse culture of prevention against earthquakes, the Italian government has issued an Act (June 24 2009 n. 77) and subsequent ordinances which regulate the distribution of resources to those regions that promote studies of seismic microzoning (SM). As a result, during the last years the number of Ital ian SM studies has been increasing and posing many new problems and challenges to administrators and scientists. The aim of this paper is to illustrate, through some salient examples of recent SM cases in Italy, the emerging philosophy of a new current of SM studies that can be called ordinary, namely projects devoted to seismic risk reduction through the usual preventive activities that can be carried out by local governments with the contribution of local officials and professionals, to the maximum extent possible. While the SM studies are generally elaborate, slow and costly, the logic of ordinary SM projects is that they have to be performed smoothly, rather quickly and cheaply. The aim is not to obtain the best SM product at any price but the best SM project consistent with efficient administrators who really want to reduce seismic risk in their regions.
In order to reach this objective and to guarantee the quality of the products, local governments (regions, provinces and municipalities) now have new duties; that is, they must undertake, in conjunction with scientists and researchers, positive actions to improve the state of knowledge and of practice in seismic risk reduction in the region, to increase the number and technical capacities of local professionals and public officials, to reduce time and investigation costs through drawing up basic geological maps also in digital format at an appropriate scale for SM, issuing specific valuable SM guidelines, implementing digital models of the territory, creating regional seismological, geological and geotechnical data banks, controlling the accuracy of the obtained SM maps and so on.
The paper focuses on unprecedented problems posed by the new legislation also to scientists and researchers, who are now called by administrators and public officials to play different scientific roles than in the past in order to contribute to advancing the frontiers of knowledge as well as the state of practice through experience transfer and applications.
EFFECT OF SUPPORT FLEXIBILITY AND SOIL – STRUCTURE INTERACTION ON SEISMIC RISK ANALYSIS OF HARP TYPE CABLE STAYED BRIDGES
Seismic risk analysis including soil – structure interaction (SSI) of harp type cable stayed bridge with support flexibility is presented which can be used for preliminary estimate of its probability of failure. The risk analysis procedure uses the format of probabilistic risk analysis (PRA) and considers the band limited white noise at the bedrock as the seismic input. The bridge deck is modeled as a beam supported on springs at different points. The coupled stiffness matrix of the springs is determined by a separate 2D static analysis of cable-tower-deck system in which flexibility of the tower base due to soil-structure interaction is included. Damping due to soil is incorporated by the equivalent modal energy method. The response of the bridge deck is obtained by the response spectrum method of analysis for multi-degree of freedom system. The PRA includes uncertainties of responses due to the variation in ground motion, material property, modeling and method of analysis, and uncertainties of the capacity due to the variation of ductility factor and damage concentration effect. Failure mode of the bridge is assumed to be bending failure of the bridge deck at the point of maximum bending moment. Probability of failure of the bridge deck is determined by First Order Second Moment (FOSM) theory of reliability analysis. A three span double plane cable stayed bridge is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations. The parameters include, base flexibility, degree of correlation of ground motion, angle of incidence of earthquake, ratio of the components of ground motion. Study shows that flexible base condition provides significantly less value of probability of failure as compared to the fixed base. Further, angles of incidence, degree of correlation and ratio of components of ground motion have considerable effects on the probability of failure.
INNOVATIVE CONNECTIONS FOR TIMBER PANEL BUILDINGS
Cross-laminated timber panel buildings are gaining a growing interest of the scientific community due to significant technical advantages, such as the material sustainability, the high fire resistance and quick constructability. Nevertheless, it is well known that timber panels themselves are not able to dissipate a significant amount of energy during an earthquake. In fact, the design of a Cross-Lam building is carried out in order to dissipate the energy in the steel connections (hold-downs or angle brackets) which govern the seismic performance. The paper here presented proposes to substitute the classical hold-downs, which usually exhibit a limited dissipation capacity, with an innovative type of dissipative angle. The new connection, called XL-stubs, apply the concept usually adopted for designing the hysteretic metallic dampers ADAS (Added Damping and Stiffness). In particular, the hourglass shape allows a better spread of plasticization resulting in a high dissipation capacity. In order to characterize the force-displacement response under cyclic loads of XL-stubs an experimental campaign is carried out comparing the hysteretic behavior of the classical hold-down with that of the proposed dissipative angle.