1,721,180 research outputs found
Nonlinear creep, Poisson's ratio, and creep-damage interaction of concrete in compression
No full textThe results of a set of short-term uniaxial creep tests at low, medium, high, and very high stress levels are presented. Creep Poisson's ratio evolution with time, nonlinear creep strain amplification, damage growth, and creep-damage interaction of concrete in compression are investigated. The electronic speckle pattern interferometry (ESP1) technique has been used to measure transverse strain also when, at high stress levels, cracks grow and propagate in concrete specimens. Under high stresses, creep Poisson 's ratio shows variation in lime, associated with damage evolution. Moreover, tests showed that for medium to high stress levels, nonlinear creep strain amplification occurs, even if not accompanied by significant concrete damage
Modelling single-lap shear tests on masonry elements strengthened by FRCM
No full textFiber Reinforced Cementitious Matrices (FRCM) are composite materials adopted nowadays for the structural strengthening of masonry structures to improve the seismic behavior. To guarantee the efficiency of the strengthening system, a proper bond between the FRCM and the masonry substrate should be ensured. For the mechanical characterization of the composite materials, indeed, bond tests are performed together with tensile tests. In the present work, numerical simulations of single-lap shear tests on carbon FRCMs applied on a brick masonry substrate are performed with the objective of calibrating a proper bond-slip law. In particular, the carbon fibers are singularly modelled, adopting a micro-modeling approach for the FRCM system. Nonlinearities are lumped into interface elements at the matrix-to-fiber interface, to reproduce the failure modes typically encountered in experimental tests, where a relative slip between the fibers and the matrix can often occur. Comparisons between numerical results and results obtained from experimental tests performed on the same FRCM systems, allowed for the calibration of bond-slip laws, which could be also implemented in the numerical modeling of experimental tests on full-scale strengthened masonry walls
Intermediate debonding of FRP strengthened RC beams
No full textIn the present paper, first results of an experimental campaign concerning failure due to intermediate crack-induced (IC) debonding of r.c. beams strengthened with FRP composites (both CFRP plates and sheets) are presented. The simply-supported beam scheme with distributed load has been adopted in order to have a loading condition closest as possible to real cases. During tests, mid-span deflection, compressive strain, mean tensile elongation, crack opening and FRP strains along the beams have been measured. Effect of FRP reinforcement on flexural strength and ductility has been studied. Moreover, IC debonding type of failure has been identified from FRP strain distribution profiles. It is shown that IC debonding occurs where FRP strain is high but a gradient of it (associated with shear stresses along the interface according to classical beam theory) is also present. A characteristic behavior of FRP strain profile close to debonding has been also identified. Finally, comparison between experimental and theoretical prediction of FRP strains along the beam, close to failure, showed another way to localize intermediate crack debonding from FRP strain profile
Nonlinear creep damage model for concrete under uniaxial compression
No full textAn isotropic model for creep damage of concrete under uniaxial compression is proposed, where the combined effect of nonlinear viscous strain evolution and crack nucleation and propagation at high stress levels is considered. Strain splitting assumption is used for creep and damage contributions. Creep is modeled by a modified version of solidification theory. As usual in the modeling of damage of concrete, a damage index based on positive strains is introduced. As particular cases, the proposed model reduces to linear viscoelasticity for long time low stress levels whereas, for very high stresses, tertiary creep causing failure at a finite time can be described. The effect of strength variation with time is also included. The model is numerically implemented to perform time integration of nonlinear equations by means of a modified version of exponential algorithm. The model is validated through comparison with experimental results. Some numerical examples are also presented, where the roles of concrete ageing and strength variation with time are investigated
FRCM/SRG strengthened masonry in diagonal compression: experimental results and analytical approach proposal
Full text linkThe in-plane shear behavior of masonry panels strengthened with Fiber Reinforced Cementitious Matrices (FRCM) or Steel Reinforced Grouts (SRG) is investigated in this research in order to propose an improved analytical approach able to predict the maximum capacity. With this purpose, a dedicated experimental campaign is developed through the execution of diagonal compression tests. Experimental outcomes highlight the crucial role of the mortar matrix in enhancing the shear capacity of the masonry panels. The careful analysis of the different contributions during the tests allows the development of predictive formulas taking into account the role of the matrix and of the cracked masonry
A design oriented fibre-based model for simulating the long-term behaviour of RC beams: Application to beams cast in different stages
Full text linkThe present paper describes a numerical model that has been developed in order to analyse the long-term behaviour of beams built in different stages under serviceability loads, considering the effect of delayed deformations due to creep and shrinkage. The rheological model for concrete creep is based on a modified version of the solidification theory while the cross section behaviour is described by means of a fibre model. The paper discusses the performances of the model in simulating the behaviour of two fullscale beams tested by the authors
Masonry columns strengthened with FRCM system: Numerical and experimental evaluation
No full textThe present paper deals with the structural behavior of masonry columns strengthened by a system made up of composite fiber grids embedded in cementitious matrix; in particular, the problem has been investigated both numerically and by performing experimental tests. Starting from the Spoelstra-Monti's iterative approach for FRP-confined concrete columns, an analytical approach for FRCM-reinforced masonry columns has been proposed. In particular, critical equations have been improved or modified in order to properly taken into account masonry behaviour and its interaction with the composite reinforcement. A 3D numerical description of the problem has been also considered, by using the commercial code MIDAS FEA, based on a macro-model approach; masonry and the reinforcement system have been considered as homogeneous materials with non-linear constitutive behaviour. The two models have been compared in order to understand the capabilities and limitations of the 1D model. To calibrate the models, some tests on plain masonry columns and composite specimens have been carried out at the University of Bologna and described. Other tests on strengthened columns, coming from the same experimental campaign, have been used to validate the capability of the models. Different types of reinforcement have been considered. Further validation has been obtained by considering also experimental results coming from literature data. The comparison of the numerical models with the experimental outcomes shows a good matching both in terms of mode of failure and quantitative behavior
Connessioni dissipative per strutture prefabbricate esistenti: il sistema Sismocell
No full textNell’ambito delle tecnologie di riduzione della vulnerabilità sismica di strutture prefabbricate esistenti,
l’obiettivo di individuare un sistema che non si limitasse alla realizzazione di un semplice fissaggio tra
elementi strutturali ha portato allo sviluppo del sistema Sismocell, realizzato grazie ad una collaborazione
tra l’Università di Bologna, Dipartimento di ingegneria civile, chimica, ambientale e dei materiali (Dicam), il
laboratorio Ciri - Edilizia e Costruzioni, e il gruppo Reglass di Minerbio (Bo).
La soluzione proposta consente di creare connessioni dissipative, in particolare in corrispondenza del nodo
trave-pilastro, attraverso il montaggio di dispositivi in acciaio e fibra di carbonio in grado di concentrare il
danneggiamento in punti prestabiliti e ridurre gli effetti dell’azione sismica sugli elementi strutturali
esistenti. L’applicazione è semplice e permette di ridurre l’onerosità, in termini di costi e di invasività, di
interventi spesso difficili da realizzare in edifici industriali per le interferenze con le attività produttive.
Il sistema prevede, nel caso di travi di copertura semplicemente appoggiate sui pilastri, la realizzazione di
una connessione con un comportamento di vincolo a fusibile in grado di dissipare energia per effetto della
deformazione plastica dei dispositivi.
Il sistema permette dunque di coniugare l’esigenza di collegare tra loro gli elementi strutturali e quella di
mantenere valori di sollecitazione contenuti sul pilastro, grazie all’effetto di fusibile del dispositivo in grado
di tagliare l’effetto dei principali picchi dell’accelerazione sismica, riducendo gli effetti del sisma sulla
struttur
BRICK MASONRY COMPRESSIVE STRENGTH EVALUATION: COMPARISON BETWEEN PREDICTIVE MODELS
No full textThe evaluation of the safety of existing masonry constructions is a crucial and current issue, especially for countries characterized by the presence of a large number of old and ancient buildings. Within the vulnerability assessment procedures, one of the most important phases is the evaluation of the material mechanical properties. Several investigation techniques can be adopted, classified as non-destructive, minor-destructive and destructive according to their invasiveness on the construction. To determine the compressive strength of existing masonries, tests on the masonry components, bricks and mortar, can be performed. Predictive models or empiric formulations can then be used to evaluate the masonry compressive strength, starting from the mechanical properties of the constituents. The objective of the present research is to determine the capability of predictive models to effectively capture the masonry compressive strength. For this purpose, a database of experimental tests conducted on existing masonry buildings was collected, together with data from the literature. In particular, the masonry compressive strength and the strength of bricks and mortar were experimentally evaluated or derived from previous researches. The predictive models chosen were applied, using the brick and mortar properties, and the results were correlated with the masonry compressive strength, considered as a reference. The investigated masonries were classified according to their strength and different ranges were considered to better evaluate the reliability of the formulations adopted. Critical analyses about the suitability of each predictive model will be presented, with a particular focus about the determination of the compressive strength for poorquality masonries, for which a modified analytical formulation was calibrated
Analytical and experimental seismic analysis of base-isolated R.C. frame structures in the nonlinear range
No full textSeismic analysis of isolated plane R. C. frame structures is performed in the nonlinear range. R. C. structural elements (beam/columns) is divided in subelements, and an hysteretic model is developed. Isolators are modeled as shear-deformable elements with curvilinear branches for loading/unloading. Dissipated energy is evaluated in order to estimate structural damage. The numerical results are compared with those obtained from an extensive experimental dynamic campaign performed on shaking-table on a base-isolated 1/3-scale three-story R. C. building
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