Artistas sobre outras Obras

Propõe-se neste número uma estética da materialidade, em cada um dos quinze artigos aqui reunidos. Há nas propostas artísticas a expansão estética que se alarga com tranquilidade a partir da materialidade das obras de arte. As matérias suportam a arte, o seu pensamento, a sua estética. A relação com o público está em permanente adiamento e, ao mesmo tempo, renovação. As matérias, enunciadas pelos artistas, suportam o seu próprio resgate, na possibilidade de uma relação. O espectador, o novo curador, segue em construção no processo.info:eu-repo/semantics/publishedVersio

Vibrazioni nelle passerelle ciclopedonali: un caso di studio

Le eccessive vibrazioni del Millennium Bridge a Londra nel giorno dell’inaugurazione hanno portato all’attenzione dell’opinione pubblica e della comunità scientifica il problema delle vibrazioni laterali delle passerelle ciclopedonali. Tali strutture, per questioni di rappor-to tra i carichi agenti e le luci che le caratterizzano, tendono ad essere molto deformabili, e dunque potenzialmente sensibili a tale fenomeno. In questo lavoro si illustrano i risultati di un’analisi numerica condotta per studiare il com-portamento dinamico di una passerella pedonale recentemente costruita a Milano. L’obiettivo dello studio è il confronto delle caratteristiche dinamiche dell’opera prima e do-po un intervento di consolidamento, resosi necessario a seguito dell’elevato livello di vibra-zioni lamentato dai fruitori della struttura nel periodo immediatamente successivo all’inaugurazione. Il comportamento dinamico è stato analizzato mediante un modello nu-merico, validato attraverso il confronto con rilievi strumentali eseguiti dopo il consolida-mento. Il modello ha permesso di dimostrare l’efficacia dell’intervento di consolidamento, attraverso l’applicazione di due differenti criteri di verifica del comportamento dinamico. Tali metodi risultano particolarmente adatti al caso di affollamenti significativi. I risultati confermano la bontà delle scelte adottate in fase di consolidamento (peraltro già testimonia-ta dall’ottimo comportamento della struttura) e mostra come, a differenza di casi analoghi recenti, il rimedio possa essere semplice e anche decisamente economico

Simplified Approach for the Evaluation of the Bearing Capacity of Deep R/C Tunnels Exposed to Fire

The structural performance of R/C tunnels when exposed to fire can represent a critical aspect in the design phase, due to some specific issues concerning this kind of infrastructures such as (a) the geometry of the fire compartment (favouring the development of severe fire scenarios) and (b) the inherent structural redundancy (due to the almost axisymmetry and to the presence of the surrounding soil). Both aspects foster the development of sizable indirect actions which can definitely modify the initial state of stress in the lining. It follows the need for a reliable evaluation of the fire performance of tunnels exposed to high temperature, considering the decay of the material properties due to the rise of the temperature, while taking into account the evolution of the internal actions due to the restrained thermal dilation. This task often entails the implementation of complex thermo-mechanical non-linear analyses, which can be properly implemented via advanced finite elements codes capable of multi-physics simulations (as for example Abaqus or Safir). In this regard, in the present study, a simplified approach is described for the non-linear analysis of deep R/C tunnels exposed to fire, based on the main assumption of (I) plane section and (II) axysimmetrically loaded lining. These assumptions allow to reduce the problem into a sectional analysis which can be even implemented in common worksheets. In the paper, the overall method is described, showing sensitivity analyses regarding some parameters involved in the assessment of the structural performance of tunnels exposed to fire

Modeling R/C Columns in Fire according to Different Constitutive Models for Heated Concrete

Several analytical formulations aimed to describe concrete behavior at high temperature have been proposed in the past two decades, all of them taking into ac-count transient and creep strains, either explicitly or implicitly. The main objective of this paper is to compare some of the different models available in the literature, with reference to the structural behavior of long R/C columns exposed to fire, since these structural members are rather sensitive to thermally-induced deformations. To this end, a number of significant examples based on real-scale fire tests, characte-rized by well-defined geometry, loads and boundary conditions, were simulated numerically, by adopting different formulations for concrete behavior in compres-sion. In terms of displacements and time to failure, the comparisons clearly show that by using accurate constitutive laws (including – implicitly or explicitly – all strain components) a correct evaluation of the time to failure is possible. At the same time, no significant differences are observed among the various models, which means that the development of further models is not fully justified

Effetto irrigidente dell'aderenza armatura-calcestruzzo in presenza di alta temperatura

The two aspects of bond in R/C structures – anchoring ability and tension stiffening - have attracted scanty attention in the recent past with reference to fire and high temperature. However, since many R/C structures survive a fire and require their residual safety level to be assessed, knowing bond properties and having information on tension stiffening in residual conditions is a necessary step to reliably analyze the structural behavior past a fire. In this preliminary research project, tension stiffening is investigated by testing a number of R/C tension members exposed to a severe fire (temperature-time curve comprised between the hydrocarbon and the standard fires in the first 20 minutes, and exposure to 750°C for further 100 minutes). After two hours, the average temperature of the specimens was close to 650°C. An electric furnace was used for heating the specimens, once at a time, and natural cooling followed inside the furnace. After the removal from the furnace, each specimen was loaded by applying opposite forces to the rebar at the end sections, to check the effectiveness of tension stiffening in residual conditions and specifically to evaluate the residual bond stiffness. Ten specimens made of three self-compacting mixes (fc = 50, 80 and 90 MPa) were tested. Tension stiffening turned out to be rather weak, as it occurred only in 5 specimens, where bond stiffness exhibited a marked decrease compared to the virgin situation, as confirmed by modelling bar-concrete interaction via a simple axisymmetric slip-based model

Difettosità superficiali e loro implicazioni tecnico-legali negli elementi prefabbricati in calcestruzzo

Investitori, committenti, imprese appaltatrici e produttrici, progettisti e direttori dei lavori si trovano spesso a dover fronteggiare varie problematiche che emergono dopo la messa in opera degli elementi prefabbricati (colonne, travi, lastre alveolari, pannelli di facciata, …). Le superfici di tali elementi possono presentare – anche a pochi mesi dal completamento dell’opera - difetti le cui cause sono spesso di non facile identificazione, siano esse ricon-ducibili al processo di produzione e di immagazzinaggio, ovvero alla movimentazione ed al montaggio. Tali difetti, pur non compromettendo funzionalità e sicurezza, riducono il valo-re della costruzione o per lo meno ne compromettono l’immagine, che nell’odierna società tende spesso a far tutt’uno con la sostanza, e possono – in qualche caso – ridurre la durabi-lità strutturale e non, tema oggi scottante. In questo lavoro vengono presentati sia una ras-segna dei principali difetti spesso riscontrabili negli elementi prefabbricati, nell’ottica del calcestruzzo a vista, sia alcune considerazioni sugli aspetti legali conseguenti alla difettosi-tà. Dilavamenti e rigonfiamenti, efflorescenze e striature, macchie e segni indotti dall’armatura, sedimentazione/essudazione e segregazione, scheggiature e bolle d’aria, di-somogeneità di colore e fuoripiano o disallineamenti possono essere in gran parte evitati - o mitigati - introducendo nei documenti contrattuali e progettuali precise condizioni sulla qualità superficiale dei manufatti, trasformando così possibili contestazioni legali a poste-riori in chiare disposizioni contrattuali e progettuali accettate a priori dalle parti

Preliminary results on tension stiffening in heat-exposed R/C tension members

Tension stiffening and - more generally – bond has attracted scanty attention in the recent past with reference to fire and high temperature, probably because of scholars’ and designers’ opinion that sectional collapse due to the strength loss in the rebars be more critical than the overall collapse due to bond loss. (As a matter of fact, bond losses involve the length of the reinforcement and not a single section, as bending). However, since many R/C structures survive a fire and require their residual safety level to be assessed, knowing bond properties and having information on tension stiffening in residual conditions is a necessary step to reliably analyze the structural behavior past a fire. In this research project, tension stiffening is investigated by testing a number of reinforced tension members initially exposed to a rather severe fire (750°C) to investigate concrete spalling. Later, the specimens were kept at 750°C for a time length sufficient to have a uniform thermal field in excess of 600°C, in and around the bar. An electric furnace pre-heated to 750°C was used. After the removal from the furnace, each specimen was tested by applying opposite forces at the end sections, to check the effectiveness of tension stiffening in residual conditions. The total number of specimens in this preliminary experimental campaign was 11. Three self-compacting mixes were used (fc = 50, 80 and 90 MPa). Only 5 specimens showed a non-negligible tension stiffening, and the justification of such weak effect was found in the marked bond sensitivity to high temperature, something that was also checked by means of a rather simple slip-based model

Study on the effects of cooling phase and construction technology on the fire performance of R/C tunnels

Fire performance of tunnels can represent a critical issue in the design phase, even more than for other structures and infrastructures, due to some inherent features such as (a) the fire compartment geometry often leading to very high temperature (also making difficult the intervention of fire brigades), (b) the structural redundancy caused by soil restraint fostering the development of relevant indirect actions, and (c) the high compression state in the lining (all the more during the fire exposure) that increases the spalling propensity and severity. Within this context, the role played by key parameters such as lining thickness and stiffness are investigated by comparing the fire performance of two different technological solutions for the lining: (1) traditional cast-in-situ lining and (2) pre-cast segmental tunnel lining. The fire scenario also considers the cooling phase, in order to discuss the main critical points to be solved when facing the final stage of the fire. 3D finite element analyses have been performed, proving that (I) higher thickness and stiffness does not necessarily correspond to a higher safety factor due to the indirect actions, and (II) fire cooling phase (if any) can be even more critical than the heating phase

On the extension of a plastic-damage model to high temperature and fire

Concrete modelling at high temperature is not an easy task due to its highly nonlinear behaviour. When simple members are at issue, numerical modelling can take advantage of uniaxial constitutive laws. On the contrary, when walls and tunnel linings have to be studied, finite elements must be used, thus requiring the multiaxial constitutive behaviour to be properly defined. The aim of this paper is to show the calibration of the main parameters of a plastic-damage model via inverse analysis, on the basis of experimental data available in the literature, regarding tests under multiaxial state of stress in both hot and residual conditions. The results indicate that there is a strong dependence on temperature of the ratio between biaxial and uniaxial compressive strength in hot conditions, while it is definitely less evident in residual conditions. As regards the variation of the shape of the failure surface with temperature, it appears to be of secondary importance