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On the benefits of using in the joints of R/C frames subjected to seismic loading
No full textThe extensive research activity carried out in the last few decades on fibre-reinforced concrete is showing - beyond any doubt - that FRC has very interesting properties for structural applications. The dispersion of short fibres - made of steel, polymers, carbon, etc. - in the concrete mass brings in a 'crack bridging' effect, which prevents or delays cracking, and – at the same time – provides concrete with a ductile behaviour both in tension and in compression, given that relatively large amounts of stiff fibres are used (as for example in high-performance fibre- reinforced concretes - HPFRC). As a result, the use of such concretes significantly improves significantly the structural performance of R/C members, not only under static and fatigue loading, but also under dynamic and seismic loading. However, considering their higher costs and more complex technology, high-performance fibre-reinforced concretes are only suitable for critical areas of R/C beams and columns, as well as in the beam-column joints of R/C frames, where the actual Italian code (2008) may require an excessive amount of reinforcement, even in the design of low-ductility members.
Within this context, this paper investigates the benefits of using high-performance fibre-reinforced concretes in the nodal and inelastic regions of R/C seismic-resistant plane frames are investigated in this paper, focusing on typical frames which are commonly found in residential buildings, with two-four bays and two-eight storeys. Ordinary mixes (C25/30 and C40/50) are adopted for frame members, while higher-performance fibre-reinforced mixes (FRC40/50 and FRC80/85) are used in critical areas and in the joints. The joint regions are modelled with or without rigid end-sections and in-plane stiffness of the floors is introduced as well. The frames are designed according to the actual Italian code (NTC08) and to EC8. In the nonlinear static analyses, considering either triangular or uniform load profiles, a diffused-plasticity model is adopted for frame members. In terms of global capacity and ductility, the use of HPFRC - instead of plain concrete – offers considerable benefits
Telai in C.A. con F.R.C. nelle zone di nodo - valutazione del fattore di struttura
No full textAccording to the capacity design approach, reinforced concrete frame structures must have high ductility, which is achieved by adopting a high amount of transverse reinforcement in dissipative regions. The use of Fibre Reinforced Concrete (FRC) can solve the problem due to the development of significant residual tensile strength. To date, it is widely known that the use of FRC results in an improvement of the structural performance for elements subjected to gravitational and cyclic loads. Benefits can be achieved in terms of shear strength, ductility, cracking behaviour, energy dissipation, tolerance to damage and fatigue. However, the available studies on FRC and new HPFRC (High Performance Fibre Reinforced Concrete) focus on individual members only.
This paper aims to investigate the overall effects of using FRC materials in dissipative regions of RC framed regular structures. To this scope, a numerical investigation is run to simulate the seismic behaviour of plane RC frames with or without FRC in inelastic zones and beam-column joints. The frames are analysed by means of non-linear static analysis with distributed plasticity and fibre sections. The behaviour of simple and mixed frames is compared in terms of capacity curves and, therefore, behaviour factor q. The variables taken into examination are investigated by means of statistical analysis (ANOVA and Tuckey test). Mainly the adoption of fibre reinforced concrete in dissipative zones of mixed frames proves an increase in the behaviour factor q compared to concrete frames ordinar
Valutazione della rigidezza di telai in c.a. soggetti ad azioni sismiche
No full textNella progettazione antisismica, la stima della rigidezza effettiva degli elementi in calcestruzzo armato (c.a.) gioca un ruolo determinante quando si adottano metodi di analisi lineare. Il presente studio vuole evidenziare come le NTC08 e l'EC8 non tengano conto di tutti i parametri che influenzano la determinazione della rigidezza effettiva ed in taluni casi possono risultare non conservativi, specialmente nei telai in cui l’effetto P-Δ può divenire significativo. Il presente studio mira ad investigare l'influenza dei fattori essenziali per il calcolo della rigidezza effettiva di telai in c.a. soggetti ad azioni sismiche. A tal fine sono stati esaminati cinque differenti approcci tratti dalla letteratura scientifica e da norme internazionali di progettazione. Si è fatto riferimento ad un edificio reale in c.a. per il quale è stata studiata e discussa l’importanza delle seguenti varabili: quantità di armatura longitudinale, entità del carico assiale applicato, resistenza a compressione del calcestruzzo e tipologia di trave (travi a spessore di solaio e travi alte). Sono stati poi valutati gli eventuali effetti del secondo ordine in termini di spostamenti di piano risultanti da analisi eseguite allo Stato Limite di Danno per azioni sismiche. I risultati delle analisi portano a concludere che la rigidezza effettiva è particolarmente sensibile al rapporto geometrico di armatura. Inoltre, il valore limite inferiore secondo NTC08 e EC8, pari al 50% della rigidezza integrale, risulta essere piuttosto un valore superiore per i casi di basso rapporto di carico assiale. Infine, si mostrerà che in alcuni casi le due normative tendono a sottostimare gli effetti del secondo ordine nell’analisi sismica allo Stato Limite di Danno
Effective stiffness in regular R/C frames subjected to seismic loads
No full textCurrent design codes and technical recommendations often provide rough indications on how to assess effective stiffness of Reinforced Concrete (R/C) frames subjected to seismic loads, which is a key factor when a linear analysis is performed. The Italian design code (NTC-2008), Eurocode 8 and ACI 318 do not take into account all the structural parameters affecting the effective stiffness and this may not be on the safe side when second-order P-Δ effects may occur.
This paper presents a study on the factors influencing the effective stiffness of R/C beams, columns and walls under seismic forces. Five different approaches are adopted and analyzed in order to evaluate the effective stiffness of R/C members, in accordance with the scientific literature and the international design codes. Furthermore, the paper discusses the outcomes of a parametric analysis performed on an actual R/C building and analyses the main variables, namely reinforcement ratio, axial load ratio, concrete compressive strength, and type of shallow beams. The second-order effects are investigated and the resulting displacements related to the Damage Limit State (DLS) under seismic loads are discussed. Although the effective stiffness increases with steel ratio, the analytical results show that the limit of 50% of the initial stiffness turns out to be the upper bound for small values of axial-load ratio, rather than a lower bound as indicated by both Italian NTC-2008 and EC8. As a result, in some cases the current Italian and European provisions tend to underestimate second-order P-Δ effects, when the DLS is investigated under seismic loading
Rinforzo attivo mediante CFRP di solai nuovi a travetti in c.a.p. soggetti ad insufficienza statica
No full textGli errori di natura progettuale ed esecutiva, che possono causare gravi insufficienze stati-che anche su strutture di recente costruzione, richiedono spesso interventi poco invasivi, risolutivi e durevoli al tempo stesso, visto l’elevato costo di dismissione e ricollocazione delle sovrastrutture. Quando le insufficienze statiche riguardano non solo requisiti di resi-stenza, bensì anche un recupero di rigidezza degli elementi inflessi, il problema richiede una particolare valutazione al fine di recuperare e salvaguardare i requisiti di stato limite di esercizio della costruzione. Nel caso in questione sarà descritto l’intervento di recupero e rinforzo strutturale attuato su un campo di impalcato di ca. 180 m2 posto al piano terra di un edificio a telaio in C.A. adibito a civili abitazioni, realizzato nel 2006, che sin dai primi anni di vita aveva manifestato dissesti statici nel solaio e fessurazioni nelle sovrastrutture, a causa di insufficienza nelle armature metalliche poste in opera. Il lavoro presentato riporta le fasi dello studio effettuato in sede progettuale, per il dimensionamento dei rinforzi, e in sede applicativa per l’attuazione degli stessi che è avvenuta dopo controllati sollevamenti dell'impalcato, al fine di conferire maggiore rigidezza al nuovo sistema strutturale. Sono riportati i risultati sperimentali relativi alle procedure di rinforzo e di controllo in fase di nuovo collaudo della struttura. L’esito raggiunto alla luce della problematica affrontata mostra come le tecniche innovative, ove opportunamente utilizzate, possano essere di notevole aiuto per il progettista in tutte quelle situazioni di rinforzo e riparazione locale così come riconosciute ai sensi di legge nel § 8.4.3 del D.M. 14/01/2008
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Cracking analysis of FRC beams under sustained long-term loading
No full textCrack formation within concrete members undergoing flexural loading is a complex mechanism, which governs the serviceability and durability of concrete structures. As for reinforced concrete (RC) members, a number of works based on empirical or theoretical approaches are published in the scientific literature. All the models propose a formulation for the estimation of crack spacing and crack width taking into account several parameters. Mechanical properties of concrete matrix, reinforcement ratio, concrete cover, bar diameter and size effect are the most influencing parameters on the cracking pattern of RC members, while tension stiffening can be influential as well. In Fiber Reinforced Concrete (FRC) elements the presence of short fibers modifies the crack pattern within the members due to the development of a residual tensile stress and greater toughness. Normally the number of cracks within the length of FRC members is higher while the mean crack spacing and the crack width are lower. In fact the crack bridging effect of fibers consists in post-cracking stresses between the crack faces. Such mechanism is mainly governed by the interface bond between fiber and concrete matrix. Therefore, the volume fraction and the geometrical properties of fibers strongly influence the overall contribution in the cracking phenomena. A limited number of design codes have taken into account the modified behaviour of FRC members (especially in the case of steel fibers) by providing specific equations for crack width. This work presents the results of an experimental campaign on RC beams subjected to sustained service loads and environmental exposure for 72 months. In some beams, short steel or polyester fibers were added to the concrete matrix. The results presented in the paper show that the addition of fibres in concrete reduces both flexural displacements and crack widths, by modifying also the long-term behaviour of FRC members
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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