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Valutazione numerica e sperimentale della performance sismica di edifici in c.a. a bassa duttilita. Edifici a nucleo e a soletta piena
Full text linkThis work reports seismic evaluation of two particular types of R/C (Reinforced concrete) buildings with low ductility capacity: U-core buildings, or torsionally deformable buildings, and flat-slab buildings. In both cases, although for different reasons, these structures are extremely vulnerable to seismic action. Special studies with suitable approaches are therefore demanded from seismic standard codes when treating them.
Examples of U-core buildings are the typical multi-storey apartment blocks built in Italy between 1960s and 1980s with a rigid central staircase, i.e. the R/C U-core, in most cases designed only to withstand static actions, in accordance with the design code of those year. The main structural problem of these buildings is due to their in-plan irregularity, which involves torsional behavior when subjected to seismic events. This produces large displacements and deformations of the flexible edge and strong forces on the R/C U-core which represents the stiff edge.
Flat-slab buildings do not generally have problems of low torsional stiffness since, if well designed, they are equipped with perimeter frames or walls. One of the major problems lies in the connections between the slabs and their supporting columns. Large bending moments and shear forces are concentrated at the connections, which are thus susceptible to non-ductile, sudden, brittle punching failure. Connections failing in this mode have very little post-failure rotational capacity or ductility.
A common issue for these two types of buildings is therefore the low energy dissipation capacity due to limited ductility, which is translated by standard seismic codes in quite low values of the behavior factor 'q'. A good estimation of this factor, by non-linear numerical analysis or experimental evidences, is mandatory for a proper seismic evaluation of the cited buildings.
Based on the foregoing, the present work is therefore divided into four parts. All of them are first preceded by a short Introduction where the major features of these buildings and an overview of the current developments in research and seismic codes are presented.
Part 1 studies non-linear static methods (pushover methods) for seismic assessment of both regular and irregular buildings. The applicability and validity of such methods, in particular of the advanced pushover methods specifically developed to be applied to irregular structures, to the case study of U-core buildings is evaluated. As most innovative result, Part 1 presents and discusses the validation of the N1 method, a non-linear static procedure recently proposed by Bosco et al. and its improvement, in order to make it completely conform to the widely accepted N2 method. In the present work, the N1 corrected method is presented and validated with reference to a set of 22 steel and R/C frames, with regular and irregular mass distribution in height. This analytical correction was facilitated by introducing coefficient , calculated in closed form from the results of modal analysis. N1 thus becomes equivalent to N2 and can be used for practical applications in the same conditions, with the advantage that it can easily be adapted to the modern Displacement-Based Seismic Design of structures.
Part 2, which is essential for the numerical study conducted on U-core and flat-slab buildings, reports the validation of the adopted non-linear numerical model through the reproduction of three experimental tests on complex RC walls found in literature.
Part 3 represents the main part of this thesis. It covers seismic analysis of U-core buildings by means of non-linear dynamic analyses aimed at numerical evaluation of behavior factor 'q', to be used in less expensive but still reliable spectral response analyses. Numerical evaluation was conducted on suitably designed case studies representing typical U-core buildings designed only for gravity loads with various core eccentricities and torsional stiffness.
Part 4 reports study of flat-slab buildings for which provisions in seismic design codes are still largely deficient. Results from experimental tests on four specimens of full-scale flat-plate slab-edge column connections made with normal concrete and steel fiber-reinforced lightweight concrete are reported. Quasi-static reversed cyclic loading was applied to the specimens to investigate their ductile behavior. Analytical interpretation of the experimental results in order to obtain correspondent values of ductility capacity and behavior factor is also reported. Numerical simulation and extension of the results have been also conduced. This part of the research is still far from being exhaustive and is suitable for further future investigatio
Studio del comportamento sismico di edifici a nucleo in c.a. e valutazione del fattore di struttura
No full textValidation and improvement of N1 method for pushover analysis
No full textMany of the non-linear static methods for seismic assessment of buildings according to modern structural codes are based on the well-known N2 procedure. A more intuitive pushover procedure, N1, has recently been proposed. Its main advantage is that the explicit evaluation of an equivalent SDOF system is not required. The N1 method has been proved to provide the same accuracy as N2, but only when a lateral load distribution proportional to the first mode shape is involved. After a brief description of the main differences between the two methods, an improved version, the N1 corrected method, is presented here. It is more consistent with N2, also when constant acceleration lateral load patterns are applied. The N1 corrected method is validated according to an extensive parametric investigation of a set of case studies on steel and R/C frames, with regular and irregular mass distribution in height. © 2013 Elsevier Ltd
Validazione del metodo N1 per le analisi di pushover e proposta di un fattore correttivo
No full textI “metodi statici non lineari” per la valutazione della risposta sismica delle strutture hanno conquistato negli anni un consenso sempre più ampio nella comunità scientifica, tanto che il loro uso è oggi consentito da diverse normative antisismiche. Nella maggior parte dei casi l’approccio di tali codici si basa sul metodo N2 proposto da Fajfar [1], il quale richiede, come passaggio intermedio, la definizione di un sistema SDOF equivalente ad un grado di libertà. Questo rende l’applicazione di tali metodi piuttosto macchinosa e concettualmente complessa.
Per superare tale limite sono stati proposti alcuni metodi che escludono il passaggio per il sistema SDOF e determinano la risposta sismica della struttura attraverso una più semplice analisi modale della struttura opportunamente corretta. Tra questi risulta di particolare interesse il metodo N1 proposto da Ghersi et al. [2,3]. Tale metodo fornisce gli stessi risultati del metodo N2 per quanto concerne l’analisi statica non lineare con distribuzione di forze proporzionale al primo modo di vibrare, mentre per quanto concerne l’analisi statica non lineare con distribuzione di forze proporzionale alle masse di piano presenta dei risultati differenti sovrastimando la risposta sismica per periodi principali della struttura alti e sottostimandola nel caso opposto.
Nel presente articolo viene effettuata una validazione del metodo N1 attraverso un’ampia analisi parametrica su telai di acciaio e c.a. caratterizzati da diversi periodi propri e gradi di regolarità.
Viene inoltre proposta una possibile correzione al metodo che, nelle analisi di pushover con distribuzione di forze costante permette di ottenere dei risultati vicini a quelli che si otterrebbero utilizzando il metodo N2 di oramai comprovata validità
Prove sperimentali di rottura ciclica di nodi soletta-pilastro in calcestruzzo armato e simulazione numerica
No full textComparative cyclic tests of exterior flat slab–column connections in normal concrete and fiber-reinforced lightweight aggregate concrete
No full textPost-peak load ductility of connections in reinforced concrete structures is essential to ensure structural integrity against seismic action and also to prevent local failure, which may lead to progressive collapse. The purpose of this study is to evaluate how the addition of steel fibers to the concrete mix can improve the hysteretic behavior of exterior flat-plate slab-column connections. Experimental tests were carried out on four full-scale exterior slab-column assemblies, all with the same geometries, reinforcement layout and concrete compressive strength. Two reference specimens were cast in normal concrete, and two in lightweight aggregate steel fiber-reinforced concrete. Quasi-static reversed cyclic loading was applied to the specimens. Ductile behavior was observed for all connections, with a marked increase in drift capacity, ductility and dissipative capacity in those made of fiber-reinforced lightweight concrete. Moreover first steps towards the numerical simulation of flat slabs to columns structures are presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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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