International Concrete Abstracts Portal

SP-205 Nonlinear finite element analysis (NLFEA) of reinforced concrete is close to being a practical tool for everyday use by design engineers. The first in this collection of 18 papers takes a critical look at the accuracy of this analysis procedure, then identifies and discusses reasons for caution in applying nonlinear analysis methods. Subsequent papers cover topics that include: * Seismic behavior predictions of structures; * Three-dimensional cyclic analysis of compressive diagonal shear failure; * Finite element analysis of shear columns; and * Simulation strategies to predict seismic response of reinforced concrete structures. Designers and researchers who use NLFEA models and procedures for reinforced concrete must be experienced and cautious. The papers in this volume will enable the users to better understand modeling, analysis, and interpretation of results.

Finite element analyses were conducted of as-built and seismically retrofitted RC bridge columns tested at UCSD. The as-built columns were provided with the same rectangular cross section and shear reinforcement, resulting in approximately the same shear capacity, but were designed to fail at different levels of ductility in either a brittle or flexural shear failure. This was accomplished by adjusting the shear force demand by varying the column height (or aspect ratio) and the grade of longitudinal reinforcement. In the analysis the challenge was to capture the overall force-deformation hysteretic behavior and failure mechanism, as well as the individual deformation components of flexure and shear. The analysis focuses on the shear behavior of concrete under large tensile strains and calibrates the shear stress capacity to the concrete component of the UCSD shear model, which reduces as a function of curvature ductility at the critical section. Also, the shear modulus is reduced in proportion to the ratio of cracked to gross flexural stiffness. The results show that a relatively simple design oriented shear capacity model can be used to calibrate the required shear parameters of the 3-D plasticity concrete model. In the paper, detailed finite element analyses are conducted to assess the shear force capacity and post-peak deformation response of shear dominated RC bridge columns.

A computationally efftcient procedure is presented for analyzing the performance of reinforced concrete structures under cyclic loading. A rigid-body-spring network is used as a basis of a material representation. Concrete is modeled as an assemblage of discrete particles interconnected along their boundaries through flexible interfaces. Random geometry is introduced using Voronoi diagrams in order to reduce mesh bias on crack propagation. Rather than averaging the effects of reinforcing over a regional material volume, rein-forcing bars are explicitly modeled using line elements with nonlinear linkage springs. The spring network has the advantage to model material discontinuities and provides realistic predictions of concrete cracking. The network performance is demonstrated through analyses of reinforced concrete columns under cyclic loading. Numerical results reasonably agree with experimental observations in terms of load carrying capacity and crack propagation. Deterioration of load carrying capacity due to shear failure after or before yielding of main reinforcing steel is discussed through the numerical predictions.

The lattice model provides equivalent continuum formulations for a variety of constitutive equations. In this study, the Compression Field Theory (1) developed by Vecchio and Collins is re-formulated in the form of an equivalent lattice model and developed further for cyclic loading, beyond the scope of the original model. The RC column experiments of UCSD (2) are then analyzed by the method, for which the equivalent lattice model shows acceptable agreement with the experiments. It is noted that shear failure during cyclic loading after yielding of the flexural reinforcement is captured by the method, which is a characteristic feature of this numerical simulation.

Seismic analyses of reinforced concrete structures are performed using the finite element method. A shake table test of a lightly reinforced concrete three story frame building and a shake table test of a seismically designed shear wall are simulated. The effects of modeling boundary conditions and of considering the initial micro-cracking of concrete on natural frequency change are investigated. These parameters are used to calibrate finite element models to experimental models. The simulations predict the overall seismic behavior of reinforced concrete structures. However, the analyses of both structures showed that accuracy of material degradation is lacking and the computational efficiency of such models needs improvement for large-scale seismic analyses.