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.

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.

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.

When designing concrete structures, fatigue related problems are not among the first that come to mind. However, structures subjected to strong cyclic loads such as those associated with destructive earthquakes experience strength and stiffness degradation that are most aptly described as a low-cycle fatigue phenomenon and are related to the damage accumulated under such loading. This paper briefly discusses the various elements of a rational, i.e. mechanics-based design methodology. Results of an experimental test program are summarized, in which 4-inch cubes with or without fiber reinforcement are subjected to uni- and biaxial cyclic compression until failure. The review concludes with a brief review of the various aspects of material behavior that need to be modeled, if the response of reinforced concrete members is to be simulated numerically.

Confinement is the key to the performance of reinforced concrete structures when ductility demands are of primary interest. Hence dilatancy and restraining effects are critical for the behavior of reinforced concrete under seismic environments. In fact, restrained dilatancy is the determinant factor ensuring strength and ductility of reinforced concrete members in compression. In this paper, the issue of the dilatancy of concrete at different levels of active confinement is revisited. Experimental observations on 150x300 mm concrete cylinders, which were recently tested in a large capacity triaxial chamber, are presented. For the analysis of the dilatancy data, the elastoplastic concrete model known as the Extended Leon Model is applied. The study is focused on the volumetric behavior of concrete, which in plasticity terminlogy refers to inelastic dilatancy and the concomitant issue of normality. In particular, the test data is examined within the framework of the non-associated flow theory of plasticity. In this context, the origin of discontinuous failure mechanisms in the high confinement regime is questioned, where inelastic dilatancy together with the loss of axisymmetry are the primary reasons for localized failure in the form of discontinuous faulting.