Wilson Sergio Venturini

Reliability analysis of non-linear reinforced concrete frames using the response surface method

Abstract In this paper, a formulation to compute the reliability of reinforced concrete structures, in which physical and geometrical non-linearities are taken into account, is described. The adopted non-linear model allows the representation of the mechanical behaviour of concrete structures at the failure stage, which is governed by possible large displacement effects, softening behaviour of concrete and tension stiffening effects. On the other hand, the reliability model is based on adaptive failure surfaces representing the mechanical model responses. The failure surface is obtained by fitting the internal force ultimate state of the structure using a quadratic polynomial. The structural reliability index is estimated by the Rackwitz and Fiessler algorithm, which has shown to converge after a reduced amount of iterations. A parametric numerical analysis of columns and frames is presented for practical applications, where the partial safety factors proposed by international codes of practice are associated with reliability indexes.

Two-dimensional solids reinforced by thin bars using the boundary element method

In this work, a linear boundary element formulation is presented for analyzing stiffened domains. A particular sub-region technique, in which the equilibrium is preserved along interfaces without traction approximation, is adapted to model fiber immersed in a body. The integral representation is written for a whole body, requiring only the displacement along interfaces. The sub-region is then assumed to be very thin to simulate fibers only when normal forces are taken into account. The thin sub-regions then degenerate so that they can be represented by its skeleton line. The displacement field over the fiber cross-sections is assumed to be constant. In the case of 2D problems, the degrees of freedom are reduced to two components only at each fiber node. Thus, displacement integral representations for collocations defined along the fiber skeleton are needed. The quasi-singular integrals are computed by using closed expressions or employing a numerical scheme with sub-elements. An example is solved to show that the formulation is very accurate for modeling cases of domains stiffened by fibers.

On the coupling of 3D bem and fem frame model applied to elastodynamic analysis

This work presents the coupling of framed structures approached by finite elements with three-dimensional bodies represented by the boundary element method. The coupling is particularly dedicated to analyse three-dimensional half space stiffened by piles and other composite domain problems, for which static and dynamic cases have been taken into account. Some numerical examples are analysed to point out the power and the accuracy of the proposed scheme.

Boundary element for plate bending analysis

Abstract This paper is related to the applications of BEM to practical plate bending problems in engineering. Some aspects of the boundary formulations are shown, describing particular characteristics that can be considered to improved numerical solutions. Several different ways of defining the boundary element system of equations are proposed. Comparisons among them are also shown emphasizing some interesting behaviours.

Alternative formulations of the boundary element method for potential and elastic zoned problems

Abstract The use of the subregion technique is a common way to deal with potential and elastic zoned problems by the boundary element method. Although this procedure has been successfully applied in many practical engineering analyses, nonlinear solutions can be easily deteriorated in the presence of an increasing number of discretized interface lines. This paper presents an appropriate formulation to model this kind of problem, avoiding some interface approximations, and reducing, therefore, the possible errors in the final solution.

Partial safety factors for homogeneous reliability of nonlinear reinforced concrete columns

The design of reinforced concrete structures is based on the verification of the rules defined by standard specifications, where partial safety factors are introduced to ensure safety. However, the use of constant factors for different kinds of columns leads to inhomogeneous safety levels. In this paper, a new format for concrete and steel safety factors is proposed, in order to ensure uniform target reliability. These factors are given in terms of the column design parameters, such as material strengths, steel ratio, slenderness and loading eccentricity. The adopted mechanical model takes into account material and geometrical nonlinearities. The response surface technique is used to evaluate the structural reliability. Numerical applications show how the calibrated safety factors lead to a better design than those proposed by the Eurocode2.

Damage modelling of reinforced concrete beams

Analysis of reinforced concrete building floor frames can be significantly improved when appropriate material models are assumed to represent the actual global behaviour and consequently to improve the ultimate and serviceability limit state verifications. Often, the stiffness reduction and the evaluation of the ultimate strain values are carried out by means of simple models based on bending moment-curvature diagrams and therefore the shear effects are neglected. In this work, an improved model to compute either the stiffness reductions or the ultimate loads has been proposed to take into account the non-linear effects due to the shear components. A damage model is adopted to govern the stress and strain fields across the cross-section of the reinforced concrete member. Then, a simple mechanism to transfer the shear stresses not sustained by the concrete material to the shear reinforcements. The shear reinforcement force resultant is computed according to the strain component measured in the tensile principal direction. The truss model for cracked concrete beams is adopted to compute the corresponding forces applied to the shear reinforcement. The well-known model, proposed by Mazars, has been implemented to govern the concrete behaviour based on the continuum damage mechanics, while an elasto-plastic model was assumed for the bending and shear reinforcements.

Dynamic non-linear stress analysis by the mass matrix BEM

This paper deals with the elastoplastic boundary element formulation applied to transient problems. An improved procedure to take into account the non-linear influences on acceleration and velocity is proposed. Integral representations of displacements and strains are derived taking into account the following domain terms: plastic strains, mass effects, viscosity and other temperature-like effects. The required domain variable approximations are given by adopting internal cells over which shape functions are properly defined. The Kelvin static fundamental solution is used in writing the integral representations, which gives the matrix time dependent differential equation that governs the problem.

Three-dimensional transient BEM analysis

In this work the three-dimensional transient boundary element method (BEM) is studied to show its applicability when solving several problems in engineering. A general technique to combine the BEM with the finite element method (FEM) is presented, with particular emphasis on studying building structures interacting with their foundations. In order to improve the stability of the developed numerical algorithm, a modified time dependent fundamental solution was derived. The article also shows a numerical scheme to deal with time singular integrals over flat triangular and quadrangular elements.

Reliability analysis of reinforced concrete grids with nonlinear material behavior

Reinforced concrete grids are usually used to support large floor slabs. These grids are characterized by a great number of critical cross-sections, where the overall failure is usually sudden. However, nonlinear behavior of concrete leads to the redistribution of internal forces and accurate reliability assessment becomes mandatory. This paper presents a reliability study on reinforced concrete (RC) grids based on coupling Monte Carlo simulations with the response surface techniques. This approach allows us to analyze real RC grids with large number of failure components. The response surface is used to evaluate the structural safety by using first order reliability methods. The application to simple grids shows the interest of the proposed method and the role of moment redistribution in the reliability assessment.