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CONTENTS
Volume 4, Number 5, October 2007
 

Abstract
A computer program was developed to analyze the non-linear, cyclic flexural performance of reinforced concrete structural members under various types of loading paths including non-sequential variations in axial load. This performance is significantly affected by the loading history. Different monotonic material models as well as hysteresis rules for confined and unconfined concrete and steel, some developed and calibrated against test results on material samples, were implemented in a fiber-based moment-curvature and in turn force-deflection analysis. One of the assumptions on curvature distribution along the member was based on a method developed to address the variation of the plastic hinge length as a result of loading pattern. Functionality of the program was verified by reproduction of analytical results obtained by others for several cases, and accuracy of the analytical process and the implemented models were evaluated against the experimental results from large-scale reinforced concrete columns tested under the analyzed loading cases. While the program can be used to predict the response of a member under a certain loading pattern, it can also be used to examine various analytical models and methods or refine a custom material model against test data.

Key Words
analysis; force-deflection; load pattern; material model; moment-curvature; performance; variable axial load.

Address
Asad Esmaeily and Robert J. Peterman; Civil Engineering Department, Kansas State University, Manhattan, KS 66506, USA

Abstract
Due to heterogeneities at all scales, concrete exhibits significant variability in mechanical behavior from sample to sample. An understanding of the fundamental mechanical performance of concrete must therefore be embedded in a stochastic framework. The current work attempts to address the connection between a two-dimensional concrete mesostructure and the random local material properties associated within that mesostructure. This work builds on previous work that has focused on the random configuration of concrete mesostructures. This was accomplished by developing an understanding of the effects of variations in the mortar strength and the mortar-aggregate interfacial strength in given deterministic mesostructural configurations. The results are assessed through direct tension tests that are validated by comparing experimental results of two different, pre-arranged mesostructures, with the intent of isolating the effect of local variations in strength. Agreement is shown both in mechanical property values as well as the qualitative nature of crack initiation and propagation.

Key Words
stochastic modeling; concrete; generalized method of cells; aggregate-mortar interface; fracture mechanics

Address
Nathan Tregger; Department of Civil and Environmental Engineering, McCormick School of Engineering and Applied Science, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3109, USA
David Corr; Exponent Failure Analysis Associates Inc., 185 Hansen Court, Suite 100, Wood Dale, IL, 60191, USA
Lori Graham-Brady; Department of Civil Engineering, Whiting School of Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2608, USA
Surendra Shah; Department of Civil and Environmental Engineering, McCormick School of Engineering and Applied Science, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3109, USA

Abstract
The paper presents the development of an adaptable strut-and-tie model that can be applied to the design or analysis of four-pile caps that support axial compression and biaxial flexure from a supported rectangular column. Due to an absence of relevant test data, the model is validated using nonlinear finite element analyses (NLFEA). The results indicate that the use of the proposed model would lead to safe and economical designs. The proposed model can be easily extended to any number of piles, providing a rational procedure for the design of wide range of pile caps.

Key Words
pile caps; strut-and-tie models; flexure strength; shear strength; concrete design.

Address
Rafael Alves de Souza; Departamento de Engenharia Civil, Universidade Estadual de Maringa, Av. Colombo, 5790, Bloco C67, CEP 87020-900, Maringa - PR, Brazil
Daniel Alexander Kuchma and Jung Woong Park; Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 2114 Newmark Laboratory, 205 N. Mathews Ave., 61801, Urbana - IL, USA
Tulio Nogueira Bittencourt; Departamento de Engenharia de Estruturas e Fundacoes, Escola Politecnica da Universidade de Sao Paulo, Av. Prof. Almeida Prado, trav.2, n.271, Cidade Universitaria, CEP 05508-900, Sao Paulo - SP, Brazil

Abstract
The paper presents quasi-static plane strain FE-simulations of strain localization in reinforced concrete beams without stirrups. The material was modeled with two different isotropic continuum crack models: an elasto-plastic and a damage one. In case of elasto-plasticity, linear Drucker-Prager criterion with a non-associated flow rule was defined in the compressive regime and a Rankine criterion with an associated flow rule was adopted in the tensile regime. In the case of a damage model, the degradation of the material due to micro-cracking was described with a single scalar damage parameter. To ensure the mesh-independence and to capture size effects, both criteria were enhanced in a softening regime by nonlocal terms. Thus, a characteristic length of micro-structure was included. The effect of a characteristic length, reinforcement ratio, bond-slip stiffness, fracture energy and beam size on strain localization was investigated. The numerical results with reinforced concrete beams were quantitatively compared with corresponding laboratory tests by Walraven (1978).

Key Words
bond-slip; concrete; characteristic length; damage mechanics; elasto-plasticity; nonlocal theory; reinforcement; strain localization.

Address
I. Marzec, J. Bobinski and J. Tejchman; Gdansk University of Technology Faculty of Civil and Environmental Engineering 80-952 Gdansk, Poland

Abstract
The investigation of corrosion effects on the tensile behavior of reinforced concrete (RC) members is very important in region prone to high corrosion conditions. In this article, an experimental study concerning corrosion effects on tensile behavior of RC members is presented. For this purpose, a comprehensive experimental program including 58 cylindrical reinforced concrete specimens under various levels of corrosion is conducted. Some of the specimens (44) are located in large tub containing water and salt (5% salt solution); an electrical supplier has been utilized for the accelerated corrosion program. Afterwards, the tensile behavior of the specimens was studied by means of the direct tension tests. For each specimen, the tension stiffening curve is plotted, and their behavior at various load levels is investigated. Average crack spacing, loss of cross-section area due to corrosion, the concrete contribution to the tensile response for different strain levels, and maximum bond stress developed at each corrosion level are studied, and their appropriate relationships are proposed. The main parameters considered in this investigation are: degree of corrosion (Cw), reinforcement diameter (d), reinforcement ratio (?), clear concrete cover (c), ratio of clear concrete cover to rebar diameter (c/d), and ratio of rebar diameter to reinforcement percentage (d/?).

Key Words
reinforced concrete; tension stiffening; corrosion; average crack spacing; bond slip.

Address
M. A. Shayanfar and M. Ghalehnovi; Civil Engineering Department, Iran University of Science and Technology, Narmak 16846, Tehran, Iran
A. Safiey; Civil Department, Moshanir Power Engineering Consultants, Park Prince Buildings, Vanak, Tehran, Iran


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