Gianluca Ranzi

Time-Dependent Behaviour of Concrete Structures

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

A complete dynamic approach to the Generalized Beam Theory cross-section analysis including extension and shear modes

A simple and efficient ‘complete dynamic approach’ is proposed, and named GBT-D, to evaluate a suitable basis of modes for the elastic analysis of thin-walled members in the framework of Generalized Beam Theory (GBT). The basis includes conventional and non-conventional modes, the latter accounting for transverse extension and membrane shear strain of the plate elements forming the cross-section, which identically vanish in the former set. The method relies on the solution of two distinct eigenvalue problems, governing the in-plane and the out-of-plane free oscillations of a segment of a thin-walled beam. Both the eigenvalue problems, differential in origin, and defined on a one-dimensional spatial domain, are transformed into an algebraic problem by means of a discretization carried out at the cross-section middle line. Numerical examples are then presented to outline the ease of use of the proposed method considering a single plate, an open cross-section and a partially closed one. Member analyses are also performed for the simplest boundary conditions, to validate the accuracy of the proposed GBT-D approach against finite element method results and analytical solutions, highlighting the importance in including the non-conventional modes.

Time analysis of composite beams with partial interaction using available modelling techniques: A comparative study

Abstract This paper presents a comparison of available numerical structural formulations for the short- and long-term analysis of composite beams with partial shear interaction. The finite difference method, the finite element method, the direct stiffness method and the exact analytical model have been considered, and both the instantaneous analysis and the time analysis based on the age-adjusted effective modulus method (AEMM) have been carried out. For modelling based on the first two of these formulations, a spatial discretisation and a discretisation in the time domain are required, while only the time discretisation needs to be specified for the direct stiffness method. The results obtained using these formulations are compared qualitatively and their accuracy is estimated, adopting the exact analytical model as a benchmark reference with the objective of establishing the minimum spatial discretisations required to keep the error within an acceptable tolerance. These comparisons are carried out for simply supported beams, propped cantilevers and fixed ended beams, from which the qualitative behaviour of these formulations in the modelling of continuous beams can also be deduced.

Macroscopic electrical field distribution and field-induced surface stresses of needle-shaped field emitters.

One major concern since the development of the field ion microscope is the mechanical strength of the specimens. The macroscopic shape of the imaging tip greatly influences field-induced stresses and there is merit in further study of this phenomenon from a classical perspective. Understanding the geometrical, as opposed to localized electronic, factors that affect the stress might improve the quality and success rate of atom probe experiments. This study uses macroscopic electrostatic principles and finite element modelling to investigate field-induced stresses in relation to the shape of the tip. Three two-dimensional idealized models are considered, namely hyperbolic, parabolic and sphere-on-orthogonal-cone; the shapes of which are compared to experimental tips prepared by electro-polishing. Three dimensional morphologies of both a nano-porous and single-crystal aluminium tip are measured using electron tomography to quantitatively test the assumption of cylindrical symmetry for electro-polished tips. The porous tip was prepared and studied to demonstrate a fragile specimen for which such finite element studies could determine potential mechanical failure, prior to any exhaustive atom probe investigation.

Short- and Long-term Analytical Solutions for Composite Beams with Partial Interaction and Shear-lag Effects

This paper presents an analytical model for the short- and long-term analysis of composite steel-concrete beams with partial shear interaction and accounting for shear-lag effects. The material properties of the concrete have been assumed to be time-dependent and have been modelled by means of the algebraic methods while the remaining materials forming the cross-section have been supposed to behave in a linear-elastic manner. The global balance condition of the problem has been obtained by means of the principle of virtual work and, integrating this by parts, the governing system of differential equations and corresponding boundary conditions have been determined. Analytical expressions for both short- and long-term solutions have been derived and, to outline their ease of use, a number of case studies relevant for bridge applications have been proposed.

Time-Dependent Analysis of Long-Span, Concrete-Filled Steel Tubular Arch Bridges

Concrete-filled steel tubular (CFST) arch bridges have gained popularity over the last decades for use in long-span applications. At service conditions, these bridges are influenced significantly by the time-dependent behavior of the concrete. This paper presents a finite-element model that was developed using commercial finite-element software and is capable of describing the time-dependent behavior. The proposed approach can account for the construction process, time effects, and geometric nonlinearity. The time-dependent behavior of the core concrete in the arch ribs was modeled using European guidelines and the integral-type creep law, implemented with the finite-element model with a user-defined subroutine. The accuracy of the proposed method was validated against real site measurements recorded for a representative arch bridge. As part of this work, the necessity of considering the variation of the time of first loading and the geometric nonlinearity has been discussed. Finally, a simplified method was developed based on the results of the refined finite-element model and is recommended for possible use in day-to-day routine design.

A probabilistic three-dimensional finite element study on simply-supported composite floor beams

Composite steel-concrete beams are commonly used as flooring in buildings. The composite action between slab and steel joist is typically provided by shear connectors welded to the top of the steel joist and embedded in the concrete. This paper investigates the effects of material uncertainties on the numerically simulated structural response of simply-supported beam tests reported in the literature by means of Monte Carlo simulation (MCS). The numerical analyses are performed using a three-dimensional finite element model developed using the commercial software ABAQUS and capable of predicting the response of composite steel-concrete members as well as the influence of the shear connectors without having to rely on shear connection load-slip curves obtained from push-out tests. All materials are assumed to behave in a non-linear fashion. Contact regions between the concrete and steel elements are simulated using surface-to-surface and embedment techniques. The statistical information on the structural response obtained from MCS using different realisation sizes is compared and discussed. For the particular case studies considered in this paper it can be concluded that even a reduced number of realisations can already provide meaningful statistical representations of the structural response of the considered composite floor beams.

An atomistic study of dislocation-solute interaction in Mg-Al alloys

In this study, atomistic simulations are performed to study the dislocation-Al solute effect on the strength of Mg alloys. At temperature T = 0 K, molecular mechanics simulations are carried out to investigate the effect of Al solute concentration on the Peierls stress of basal plane edge and screw dislocations. It is found that the Peierls stress will increase by at least one order of magnitude when 0.25 at% Al atoms are randomly introduced in the Mg alloys with an edge dislocation. Generally, the Peierls stress will increase with the increase of the Al concentration up to 8 at%. Furthermore, the interaction between the basal plane edge dislocation and Al solute at T = 300 K is studied using molecular dynamics. It appears from the simulations that the critical shear stress increases with the Al solute concentration. Comparing with the solute effect at T = 0 K, however, the critical shear stress at T = 300 K is lower since the kinetic energy of the atoms can help the dislocation conquer the energy barriers created by the Al atoms.

A Numerical Study of Turbulent Mixed Convection in a Smooth Horizontal Pipe

This paper presents a numerical study aimed at identifying a suitable turbulence model to describe the fully developed turbulent mixed convention of air in smooth horizontal pipes. The flow characteristics considered here are relevant to those typically observed in ventilated hollow core slab (VHCS) applications and, because of this, the adopted geometry and boundary conditions are represented by the Reynolds number and Richardson number of about 23,000 and 1.04, respectively. Empirical expressions available in the literature are used as reference to evaluate the accuracy of different turbulence models in predicting the dimensionless velocity (u+) and temperature (T+) profiles as well as the Nusselt number (Nu). Among the turbulence models considered, the standard and realizable k-e models provide the best overall predictions of u+, T+, and Nu in the fully developed flow, and the former is recommended for the modeling of VHCS systems as it gives slightly better estimates of the Nu values.

A Personalized Electronic Movie Recommendation System Based on Support Vector Machine and Improved Particle Swarm Optimization

With the rapid development of ICT and Web technologies, a large an amount of information is becoming available and this is producing, in some instances, a condition of information overload. Under these conditions, it is difficult for a person to locate and access useful information for making decisions. To address this problem, there are information filtering systems, such as the personalized recommendation system (PRS) considered in this paper, that assist a person in identifying possible products or services of interest based on his/her preferences. Among available approaches, collaborative Filtering (CF) is one of the most widely used recommendation techniques. However, CF has some limitations, e.g., the relatively simple similarity calculation, cold start problem, etc. In this context, this paper presents a new regression model based on the support vector machine (SVM) classification and an improved PSO (IPSO) for the development of an electronic movie PRS. In its implementation, a SVM classification model is first established to obtain a preliminary movie recommendation list based on which a SVM regression model is applied to predict movies’ ratings. The proposed PRS not only considers the movie’s content information but also integrates the users’ demographic and behavioral information to better capture the users’ interests and preferences. The efficiency of the proposed method is verified by a series of experiments based on the MovieLens benchmark data set.