The two-dimensional design and behavior of typical reinforced concrete (RC) structures has been extensively studied in the past several decades. Such design requires knowledge of the constitutive behavior of reinforced concrete elements subjected to a biaxial state of stress. These constitutive models were accurately derived from experimental test data on representative reinforced concrete panel elements. The true behavior of many large complex structures, however, requires knowledge of the constitutive laws of RC elements subjected to a triaxial state of stress. The goal of the proposed work is to develop new constitutive relations for RC elements subjected to a triaxial state of stress. To accomplish this task, largescale tests on representative concrete panels need to be conducted. The University of Houston is equipped with a unique universal panel testing machine that was used for this purpose. This universal panel tester is the only one of its kind in the United States, and the only one in the world that allows for both displacement and forcecontrolled load application through its newly upgraded servo-control system. The panel tester enhanced the understanding of the in-plane shear behavior of reinforced concrete elements. Recently, 20 additional hydraulic cylinders were mounted in the out-of-plane direction of the universal panel tester to facilitate testing of concrete elements subjected to tridirectional shear stresses. The addition of these cylinders makes the panel tester the only one of its kind in the world that is capable of applying such combinations of stresses on full-scale reinforced concrete elements. This paper presents the details of the mounting and installation of the additional hydraulic cylinders on the universal panel tester, and preliminary results of large-scale tests of a series of RC panels subjected to three-dimensional shear loads.

This paper presents a brief study of sequentially linear analysis compared with traditional nonlinear analysis to simulate the response of small-scale, slender beams made of mortar and of engineered cementitious composites, a class of highperformance fiber-reinforced cement-based composites, subjected to four-point bending. Nonlinear finite element analyses are conducted using a smeared crack model and sequentially linear analyses are conducted using in one case, a saw-tooth model to preserve fracture energy dissipation. Load-displacement response, tensile strain history, cracking behavior, and convergence of the analyses are discussed. It was found that both the nonlinear and sequential linear analyses were able to predict similar load-displacement responses as well as similar cracking patterns and failure modes as were observed in the experiments. The sequential linear analyses were easily able to simulate a snapback behavior. Mesh sensitivity was also observed in the analyses, as fracture energy-dependent models were not adopted for these comparisons.

Two-way flat slabs are extensively used in many structures, such as buildings, shopping centers, and parking garages, because their static efficiency allows to attain large span-depth ratios and to have more spaced columns. The reduction of the thickness, however, is limited by serviceability requirements (resulting from deflection criteria) and by the ultimate limit state of punching shear. This collapse mode has been widely studied in the past, with reference to ordinary conditions, but very limited attention has been devoted to the punching resistance of flat slabs in fire conditions, an issue which is of primary concern, especially in the case of parking garages. This paper deals with two key aspects of slab punching in fire conditions. The first is the sizable reduction of the punching resistance in a typical slab-column assembly, because of the thermally induced damage caused by the exposure to the fire, that is modeled by means of the temperature-time curve ISO-834 (that fits very well the points given by ASTM E119-08a. The second is the sizable load increase due to the redistribution of the internal forces ensuing from the fire that is modeled through a realistic fire scenario based on the available information coming from real car fires. Even if these two phenomena do not necessarily occur simultaneously in a real fire, they both testify to what extent punching shear can be critical for reinforced concrete slabs in fire.

The causes of the nonlinear behavior of concrete until failure are numerous and complex, particularly for nonmonotonic and rapid loadings. A model is presented coupling damage and plasticity including several effects: development and closure of cracks, damping, compaction, and strain rate effects. The idea is to describe, with the same tools, a wide variety of problems, the model is of explicit form, and what makes possible its implementation into explicit numerical scheme well adapted to the treatment of fast dynamic problems. In this context, the finite element "Abaqus explicit" code is used, and the model has been successfully applied during the past few years to model a large range of complex reinforced concrete structures subjected to severe loadings. In this paper, the main model concepts are presented, and some examples of numerical simulations are given and compared with experimental data. The applications proposed are related to quasi-static loading as well as to rapid loading (impact); in particular, one of them is within the framework of an experiment linked to the design of a reinforced concrete rock-shed gallery located in the French Alps. The results show the relevance of the modelling used, which makes some real numerical experiments very useful for complex structures and/or extreme loadings.

Eight full-scale panels were tested to study the constitutive relationships of elements (panels) made of reinforced concrete with steel fibers (RCSF). The RCSF panels were subjected to biaxial tensile-compressive loadings. The principal variables of the testing program were: (a) percent of steel fibers by volume, Vf, (b) length-to-diameter ratio of steel fibers, Lf/Df, (c) reinforcing bar ratio in the tensile direction, ?, and (d) the tensile strain, eT, in the direction of the principal applied tensile stress. Based on the test results, analytical expressions are proposed for the smeared stress-strain curves of cracked steel fiber concrete (SFC) in tension and compression, as well as the smeared tensile stress-strain curves of mild steel embedded in SFC. The proposed analytical expressions take into account the effect of the steel fiber index, Wf (Wf = Vf Lf /Df).