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Roumen Petrov - One of the best experts on this subject based on the ideXlab platform.
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the effect of nb on the strain rate and temperature Dependent Behaviour of quenching partitioning steels
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2021Co-Authors: Florian Vercruysse, Carola Celadacasero, Bernd M Linke, Patricia Verleysen, Roumen PetrovAbstract:Abstract In present study, the potential of Niobium (Nb) to improve the mechanical properties of Quenched and Partitioning (Q&P) steels is investigated. To this purpose, the effect of the addition of Nb on the microstructure and mechanical properties of a Q&P steel was thoroughly analysed with special attention on the thermal stability of the retained austenite. The mechanical properties of the Nb Q&P steel were determined using static and dynamic tensile tests and compared to the properties of a Nb free Q&P steel. Additionally, the effect of test temperature was quantified by static and dynamic tests in the temperature range from −40 °C to +80 °C. It was found that the Nb induced grain refinement together with the stabilising effect of Nb on the retained austenite, resulted in an excellent low temperature Behaviour of the Nb Q&P steel. Additionally, the high temperature, dynamic Behaviour of the Nb micro-alloyed steel was improved with respect to the Nb free steel due to the presence of a large fraction of retained austenite with low thermal stability.
Gianluca Ranzi - One of the best experts on this subject based on the ideXlab platform.
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state of the art on the time Dependent Behaviour of composite steel concrete structures
Journal of Constructional Steel Research, 2013Co-Authors: Gianluca Ranzi, Graziano Leoni, Riccardo ZandoniniAbstract:Abstract Composite steel–concrete structures represent an efficient and economical form of construction for building and bridge applications. This paper presents the current state of the art on the time-Dependent Behaviour of composite steel–concrete members, i.e. columns, slabs and beams, and how this influences both service and ultimate conditions. In the case of beams, only H-shaped or box steel sections with solid and composite slabs have been considered. In the initial part of the paper, a brief outline of the main aspects related to the time-Dependent Behaviour of the concrete is provided. This is followed by the description of the work carried out to date on the long-term response of composite columns, slabs and beams considered separately. In the case of composite columns, particular attention has been devoted to the influence of time effects on the ultimate response, role of confinement at service conditions and possible occurrence of creep buckling. Very limited work has been carried out to date on the long-term response of composite slabs. Because of this, only brief considerations are provided on this solution while still presenting recent research dealing with the development of shrinkage gradients through the slab thickness when cast on steel decks. The work outlined on composite beams has been categorised according to different design issues, which include shear-lag effects, the shear deformability of the steel beam, influence of time effects on the ultimate response, prestressing, time-Dependent buckling, and sequential casting of the slab. Recommendations for possible future research work are provided in the concluding remarks.
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time Dependent Behaviour of concrete filled steel tubular columns analytical and comparative study
Magazine of Concrete Research, 2012Co-Authors: Yue Geng, Gianluca Ranzi, Yuyin Wang, Sumei ZhangAbstract:Concrete-filled steel tubes (CFSTs) have become popular in both building and bridge applications over recent decades, and an accurate prediction of their time-Dependent response is necessary. Different models are currently available to describe the time-Dependent Behaviour of concrete and these have been shown to lead to very different predictions. In the particular case of CFST members, there is no recommended method for service calculations for routine design. The particularity of the long-term Behaviour of CFSTs relies on the fact that no moisture exchange occurs between the core concrete and the surrounding environment due to the presence of the steel section. In this context, this paper recommends a suitable concrete model using 81 tests for benchmarking purposes. An extensive parametric study is carried out to evaluate the influence of time effects for geometric cross-sections commonly used in real bridge applications. Finally, different algebraic methods suitable for design calculations are applied...
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time Dependent Behaviour of expansive concrete filled steel tubular columns
Journal of Constructional Steel Research, 2011Co-Authors: Yuyin Wang, Gianluca Ranzi, Yue Geng, Sumei ZhangAbstract:Abstract Expansive concrete-filled steel tubes (ECFST) are commonly used in modern building and bridge applications. Despite their popularity, limited attention has been devoted to investigate the time-Dependent Behaviour of such elements. This paper intends to provide new experimental data for the benchmarking of numerical models. Particular attention is devoted to ECFST elements first loaded at quite early concrete ages, e.g. 5 days after concrete casting, to reflect the construction site practice. Eleven ECFST short columns were subjected to different levels of sustained axial loads applied at different concrete ages. Seven columns were then tested to failure to evaluate the long-term effects on their ultimate capacity. The accuracy of four currently available concrete models, EC2, MC90, AFREM and B3, in predicting the long-term response of ECFST elements was investigated based on the related experimental results. Investigation shows that the assumption of linear creep can apply to ECFST elements with initial concrete compressive stresses up to approximately 80% of the concrete strength, rather than the normally accepted upper limit of 40%–50%. During the service life, confinement does not affect the performance of ECFST elements. Model EC2 is adequate to predict the time-Dependent response of ECFST elements.
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time Dependent Behaviour of concrete structures
2010Co-Authors: R I Gilbert, Gianluca RanziAbstract:Time-Dependent Deformation Background Creep of Concrete Shrinkage of Concrete Time-Analysis - The Basic Problem Material Properties Concrete Steep Reinforcement References Design for Serviceability - Deflection and Crack Control Introduction Design Objectives and Criteria Design Actions Design Criteria for Servicability Maximum Span-to-Depth Ration Minimum Thickness Deflection Control by Simplified Calculation Crack Control References Uncracked Sections - Axial Loading Preamble The Effective Modulus Method The Principle of Superposition - Step-by-Step Method The Age-Adjusted Effect Modulus Method (AEMM) The Rate of Creep Method (RCM) Comparison of Methods of Analysis Uncracked Sections - Axial Force and Uniaxial Bending Uncracked Sections - Axial Force and Biaxial Bending Introductory Remarks Overview of Cross-Sectional Analysis Short-Term Analysis of Reinforced or Prestressed Concrete Cross Sections Long-Term Analysis of Reinforced or Prestressed Concrete Cross Sections Using the Age Adjusted Effective Modulus Long-Term Analysis of Reinforced Prestressed Concrete Cross Section Using the Step-by-Step Procedure Composite Steel-Concrete Cross Sections References Cracked Sections Introductory Remarks Short-Term Analysis Time-Dependent Analysis (AEMM) Short- and Long-Term Analysis Using the Step-by-Step Method References Members and Structures Introductory Remarks Deflection of Statically Determinate Beams Statically Indeterminate Beams and Slabs Two-Way Slab Systems Slender Reinforced Concrete Columns Temperature Effects Concluding Remarks References Stiffness Method and Finite Element Modelling Introduction Overview of the Stiffness Method Member Loads Time Analysis Using AEMM Time Analysis Using SSM Time Analysis Using the Finite Element Method Analysis of Cracked Members References Appendix: Analytical Formulations - Euler-Bernoulli Beam Model
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a steel concrete composite beam model with partial interaction including the shear deformability of the steel component
Engineering Structures, 2007Co-Authors: Gianluca Ranzi, Alessandro ZonaAbstract:Abstract This paper presents an analytical model for the analysis of steel–concrete composite beams with partial shear interaction including the shear deformability of the steel component. This model is obtained by coupling an Euler–Bernoulli beam for the reinforced concrete slab to a Timoshenko beam for the steel beam. The composite action is provided by a continuous shear connection which enables relative longitudinal displacements to occur between the two components. The balance conditions are derived using the principle of virtual work and the weak form of the problem is presented. The steel of the beam and the steel of the slab reinforcement are modelled by using linear elastic laws, while the time-Dependent Behaviour of the slab concrete is included by using a general linear viscous–elastic integral-type constitutive law. The numerical solution is obtained by means of the finite element method implementing a time-stepping procedure. The derived displacement-based finite elements are tested and their performance is discussed. Extensive numerical simulations are carried out on approximately 200 realistic simply supported and three-span composite beams to evaluate the effects of the shear deformability of the steel member on the overall structural response. The numerical results obtained with the proposed model are compared to those of the composite beam model with partial shear interaction that does not include the shear deformability of the steel beam to determine under which conditions shear deformations of the steel component need to be considered in the analysis of composite systems and to evaluate how these are affected by the shear connection stiffness and by the redistributions due to the time-Dependent Behaviour of the concrete slab.
D F Malan - One of the best experts on this subject based on the ideXlab platform.
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manuel rocha medal recipient simulating the time Dependent Behaviour of excavations in hard rock
Rock Mechanics and Rock Engineering, 2002Co-Authors: D F MalanAbstract:Although hard rock is not usually associated with large creep deformation, data collected from the tunnels and stopes of the deep South African gold mines illustrates significant time-Dependent Behaviour. Apart from application in mining, a better understanding of the time-Dependent Behaviour of crystalline rock is required to analyse the long term stability of nuclear waste repositories and to design better support for deep civil engineering tunnels in these rock types. To illustrate the subtle problems associated with using viscoelastic theory to simulate the time-Dependent Behaviour of hard rock, a viscoelastic convergence solution for the incremental enlargement of a tabular excavation is discussed. Data on the time-Dependent deformation of a tunnel developed in hard rock further illustrates the limitations of the theory, as it is unable to simulate the fracture zone around these excavations. To simulate the rheology of the fracture zone, a continuum viscoplastic approach was developed and implemented in a finite difference code. This proved more successful in modelling the time-Dependent closure of stopes and squeezing conditions in hard rock tunnels. A continuum approach, however, has limitations in areas where the squeezing Behaviour is dominated by the time-Dependent Behaviour of prominent discontinuities such as bedding planes. To overcome this problem, a viscoplastic displacement discontinuity technique was developed. This, combined with a tessellation approach, leads to more realistic modelling of the time-Dependent Behaviour of the fracture zone around excavations.
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time Dependent Behaviour of deep level tabular excavations in hard rock
Rock Mechanics and Rock Engineering, 1999Co-Authors: D F MalanAbstract:Although hard rock is not usually associated with large creep deformation, significant time-Dependent Behaviour is observed in the tabular excavations of the South African gold mines. Time-Dependent closure data was collected in stopes of the Ventersdorp Contact Reef and Vaal Reef. This data typically consists of a primary closure phase after blasting, followed by a steady-state closure phase. This closure Behaviour is the result of the rheology of the fracture zone around these excavations and the time-Dependent extension of this zone following a mining increment. An elasto-viscoplastic approach was developed to simulate the time-Dependent nature of the fracture zone. This model proved successful in simulating the experimental closure profiles. It appears that the closure data may provide useful diagnostic information of the stress conditions in the fracture zone ahead of the face. This may possibly be used to identify hazardous conditions such as areas prone to face bursting. The effect of preconditioning on the time-Dependent closure Behaviour is also illustrated.
M A Bradford - One of the best experts on this subject based on the ideXlab platform.
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time Dependent in plane Behaviour and buckling of concrete filled steel tubular arches
Engineering Structures, 2011Co-Authors: M A BradfordAbstract:Abstract This paper presents a theoretical analysis for the time-Dependent Behaviour and buckling of concrete-filled steel tubular (CFST) circular arches due to shrinkage and creep of the concrete core under a sustained uniform radial load. The algebraically tractable age-adjusted effective modulus method is used to model the time-Dependent Behaviour of the concrete core, based on which the differential equations of equilibrium for the time-Dependent analysis of CFST arches are derived and analytical solutions for the long-term displacements, stresses and internal forces of CFST arches under the sustained load are obtained. It is shown that the visco-elastic effects of creep and shrinkage of the concrete core have significant long-term effects on the in-plane structural Behaviour of CFST arches. The long-term radial and axial displacements, as well as the bending moment, increase substantially with time. For a CFST arch with a low area ratio of the steel tube to the concrete core, the long-term deformations may be excessive and affect the serviceability of the CFST arch. The increases of the long-term stresses in the steel tube with time are significant, while the long-term stresses in the concrete core decrease with time and may change from compressive to tensile if the time is sufficiently long. It is demonstrated that the time-Dependent change of the equilibrium configuration of the CFST arch can lead to a buckling configuration being attained in the time domain under a sustained load, which is lower than the buckling loads of the CFST arch under short-term loading. The solution for the possible prebuckling structural life for time-Dependent creep buckling of deep CFST arches is derived and can be used to determine the effects of various parameters on the creep buckling of a CFST arch.
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analytical solutions for the time Dependent Behaviour of composite beams with partial interaction
International Journal of Solids and Structures, 2006Co-Authors: Gianluca Ranzi, M A BradfordAbstract:Abstract This paper presents a generic modelling for the time-Dependent analysis of composite steel–concrete beams with partial shear interaction that occurs due to the deformation of the shear connection. The time effects considered in this modelling are those that arise from shrinkage and creep deformations of the concrete slab, and these effects are modelled using algebraic representations such as those of the age-adjusted effective modulus method (AEMM) and the mean stress method (MS), which are viscoelastic models for concrete deformation that can be stated algebraically. The generic model lends itself to closed form solutions for the analysis of composite beams subjected to a generic applied loading under a variety of end conditions. In this paper, the generic model is applied for the time-Dependent analysis of composite beams that are simply supported and encastre, and to a propped cantilever, that are subjected to uniformly distributed loading and shrinkage deformations. Various representations of the structural Behaviour of these beams are given in closed form which can also be used to benchmark available modelling techniques, i.e. finite element and finite difference formulations, which require a spatial discretisation to be specified as well as the time discretisation to perform a time analysis.
Annachristina Eilers - One of the best experts on this subject based on the ideXlab platform.
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time Dependent Behaviour of quasar proximity zones at z 6
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Frederick B Davies, Joseph F Hennawi, Annachristina EilersAbstract:Since the discovery of $z\sim 6$ quasars two decades ago, studies of their Ly$\alpha$-transparent proximity zones have largely focused on their utility as a probe of cosmic reionization. But even when in a highly ionized intergalactic medium, these zones provide a rich laboratory for determining the timescales that govern quasar activity and the concomitant growth of their supermassive black holes. In this work, we use a suite of 1D radiative transfer simulations of quasar proximity zones to explore their time-Dependent Behaviour for activity timescales from $\sim10^3$ to $10^8$ years. The sizes of the simulated proximity zones, as quantified by the distance at which the smoothed Ly$\alpha$ transmission drops below 10% (denoted $R_p$), are in excellent agreement with observations, with the exception of a handful of particularly small zones that have been attributed to extremely short $\lesssim 10^4$ year lifetimes. We develop a physically motivated semi-analytic model of proximity zones which captures the bulk of their equilibrium and non-equilibrium Behaviour, and use this model to investigate how quasar variability on $\lesssim10^5$ year timescales is imprinted on the distribution of observed proximity zone sizes. We show that large variations in the ionizing luminosity of quasars on timescales of $\lesssim10^4$ years are disfavored based on the good agreement between the observed distribution of $R_p$ and our model prediction based on "lightbulb" (i.e. steady constant emission) light curves.