Frédéric Dufour

A Simple and Efficient Intensity Measure to Account for Nonlinear Structural Behavior

A fundamental issue that arises in the framework of probabilistic seismic risk analysis is the choice of ground motion intensity measures (IMs). A new structure-specific IM, namely, relative average spectral acceleration (ASAR), is being proposed herein, and a comparison with current IMs is performed based on (1) a large data set of recorded earthquake signals; (2) numerical analyses conducted with state-of-the-art finite element (FE) models, representing actual load-bearing walls and frame structures, and validated against experimental test; and (3) systematic statistical analyses of the results. According to a comparative study of the case of nonlinear structural behavior, the ASAR proves to be the most efficient IM with respect to demand parameters, such as maximum interstory drift, frequency drop and maximum ductility demand. Beyond the sufficiency and simplicity of its formulation, which allow for the use of existing ground motion prediction models, the ASAR offers a promising IM for performance-based seismic design and/or assessment.

Cracking analysis of reinforced concrete structures

The study of a reinforced concrete beam tested under four-point bending is proposed in this article. An analysis of the behaviour of this beam from the global response down to local information such as cracking is performed. In order to describe the progressive degradation of the beam, a damage model is used, associated to a stress-based non-local regularisation method. A post-treatment of the finite element analysis is then performed to characterise the cracking pattern (crack spacing and crack opening). In this article, two different methods of post-treatment are compared: the topological search and continuous/discontinuous crack opening and the global/local analysis. Results show the capability of both methods to give a good estimation of cracking. Finally, the main advantages and drawbacks of both methods are underlined.

Modelling rainfall-induced mudflows using FEMLIP and a unified hydro-elasto-plastic model with solid-fluid transition

The paper describes a proposed unified hydro-elasto-plastic model with a solid–fluid transition. The model associates a hydro-elasto-plastic model for partially saturated geomaterials, a Bingham’s viscous law, and the transition criterion between solid and fluid states. The model describes both solid and fluid behaviours of partially saturated geomaterials in a unified framework. In addition, this paper describes a novel Finite Element Method with Lagrangian Integration Points (FEMLIP) formulation for solving hydro-mechanical problems. Based on the equilibrium equation of momentum and the continuity equation of water flow, the formulation was developed and implemented in a FEMLIP tool. Bishop’s effective stress expression and proper water retention diagrams were taken into account. A heuristic column and real rainfall-induced mudflows were also simulated and analysed in this study. The permeability effects of partially saturated soil and the effective cohesion were considered, which included consideration of the entire mudflow process. The results were proven satisfactorily in a qualitative fashion.

Force fluctuations on a wall in interaction with a granular lid-driven cavity flow

The force fluctuations experienced by a boundary wall subjected to a lid-driven cavity flow are investigated by means of numerical simulations based on the discrete-element method. The time-averaged dynamics inside the cavity volume and the resulting steady force on the wall are governed by the boundary macroscopic inertial number, the latter being derived from the shearing velocity and the confinement pressure imposed at the top. The force fluctuations are quantified through measuring both the autocorrelation of force time series and the distributions of grain-wall forces, at distinct spatial scales from particle scale to wall scale. A key result is that the grain-wall force distributions are entirely driven by the boundary macroscopic inertial number, whatever the spatial scale considered. In particular, when the wall scale is considered, the distributions are found to evolve from nearly exponential to nearly Gaussian distributions by decreasing the macroscopic inertial number. The transition from quasistatic to dense inertial flow is well identified through remarkable changes in the shapes of the distributions of grain-wall forces, accompanied by a loss of system memory in terms of the mesoscale force transmitted toward the wall.

A multifiber beam model coupling torsional warping and damage for reinforced concrete structures

This paper is dedicated to the numerical modelling of structures using multifibre beam elements. A formulation is developed to account for torsional warping in the deformations of an arbitrary-shaped composite cross section. The resulting warping profiles are validated by comparison with the axial displacements obtained by three-dimensional modelling of beams in torsion. The warping kinematics is then implemented in a Timoshenko multifibre beam element. The material is modelled by a 3D damage law, and warping is updated throughout the computations to account for damage evolution. Non-linear parameters of the constitutive model are identified using a genetic algorithm. A comparison of torque–twist curves predicted with enhanced and classical beam elements to experimental curves highlights the importance of including warping in the model. The analysis of damage patterns further ascertains the effect of warping.

SEISMIC FINITE ELEMENT ANALYSIS OF AN EXISTING OLD CONCRETE STRUCTURE BY USING MULTIFIBER BEAMS: INTRODUCTION OF AN ADAPTIVE PUSHOVER METHOD

The study of the seismic vulnerability of existing structures is an important issue. Many researches have been developed in order to investigate the structural behavior of these structures, and extract the basic informations needed to establish retrofitting guidelines in order to reduce the seismic risk to acceptable levels. The most accurate analysis procedure for the structures subjected to strong ground motions is the time-history analysis. This method is time-consuming though for application in all practical purposes. The necessity for faster methods that would ensure a reliable structural assessment or design of structures subjected to seismic loading led to the pushover analysis. Pushover analysis is a non-linear static analysis based on the assumption that structures oscillate predominantly in the first mode or in the lower modes of vibration during a seismic event. The present work deals with seismic vulnerability assessment of an old existing reinforced concrete structure – Perret tower – located in Grenoble, France. After a brief description of the structure in exam, a preliminary computation of the mass of the building and the definition of every existing section are performed. A simplified 3D numerical model is carried out using a finite element code based on multifiber beams approach. Firstly, a non-linear temporal dynamic analysis is performed, then a conventional and adaptive pushover analysis is carried out. The results obtained of the studied cases are then compared: it is observed that the conventional pushover analysis should be adjusted in order to take into account the change of dynamic characteristics due to the formation of plastic mechanisms. Finally, the tower critical levels in term of damage are highlighted. 3491 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 3491-3505

Induced Anisotropic Gas Permeability of Concrete due to Coupled Effect of Drying and Temperature

An experimental campaign is carried out to study the effect of drying shrinkage and temperature on multi-directional gas permeability of dry concrete. Thermal loadings up to 250°C are applied on concrete samples in cylinder (11×22) and dog-bone forms (total length of 61 cm). Samples are sliced for permeability measurements. Permeabilities in longitudinal and radial directions are addressed. The cylinder samples are first sliced then dried or heated whilst the dog-bone samples are first dried or heated then sliced. The average of initial intrinsic permeability for the slices (5 cm height, 11 cm diameter) obtained from the (11×22) samples is found isotropic and equal to 2.93×10-17 m2. In this case, drying shrinkage is isotropic. Furthermore, it is shown that for the dog-bone samples, drying shrinkage may induce micro-cracks preferentially in a certain direction which induces permeability anisotropy. Finally, the evolution of the normalized intrinsic permeability with respect to initial permeability versus temperature is found isotropic. An exponential fitting of intrinsic permeability versus temperature is found based on experimental measurements.

Modelling strategies of prestressing tendons and reinforcement bars in concrete structures

This contribution presents an original approach to improve the modeling of steel rebars and prestressing tendons in concrete structures at a reduced cost. Classical 1D meshes and models typically used for civil engineering applications tend to provoke strain localization due to the geometrical singularity and are thus unable to reproduce local mechanical effects. Complete 3D models can be applied in some cases, however their accuracy at the local scale comes at the cost of engineering work on the meshes, especially for complex structures. The 1D-3D model presented in this contribution generates an equivalent volume for the steel bars, based on existing 1D models. Its 3D stiffness and stress state are computed, and then condensed on its interface with the concrete. The condensed degree of freedom are then linked to the surrounding concrete elements by kinematic relations. The presented approach is validated on different representative cases, and is able to predict the 3D effects of the bars and tendons at the local scale. In particular it provides the representativeness and mesh stability of a full 3D model, without the need for a complex mesh.

Adaptive Zooming Method for the Simulation of Quasi-Brittle Materials

A method to simulate concrete structures (quasi-brittle material) with localized nonlinearities is presented. Based on Guyan’s condensation, it consists in replacing the elastic zones of the structure by their equivalent rigidities (super-elements). The nonlinear computation is then performed only on the zones of interest (ie, damaged). As new damaged zones may appear, the proposed method monitors the evolution of the system and re-integrates previously condensed areas if necessary. This method, applied on different tests cases, allows a substantial computation economy.

Massive Structure Monitoring: Relevance of Surface Strain Measurement

Most of large civil engineering concrete structures have been instrumented for decades with embedded sensors. To prevent the eventual loss of data, complementary instrumentation of external surface has recently been deployed. This new instrumentation can take different forms but in all cases, to avoid damaging the structure, it will be only superficially anchored. Near the outer surfaces, thermo-hydromechanical concrete behaviour is more sensitive to varying environmental conditions than in the centre of the structures. Therefore, the strain measured near the outer surfaces is not identical to the strain measured by embedded sensors. Consequently the methods of classical physical-statistical analysis must be reviewed. Using a thermo-hydro-mechanical finite element modeling calibrated on a representative concrete and applied on a current part of a thick structure, this work confirms a dependence of strain on the depth. First results show that the depth impact affects both kinetic and amplitude strain.