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CONTENTS
Volume 8, Number 1, February 2011
 

Abstract
A numerical model that can simulate the nonlinear behavior of ultra high strength fiberreinforced concrete (UHSFRC) structures subject to monotonic loadings is introduced. Since engineering material properties of UHSFRC are remarkably different from those of normal strength concrete and engineered cementitious composite, classification of the mechanical characteristics related to the biaxial behavior of UHSFRC, from the designation of the basic material properties such as the uniaxial stressstrain relationship of UHSFRC to consideration of the bond stress-slip between the reinforcement and surrounding concrete with fiber, is conducted in this paper in order to make possible accurate simulation of the cracking behavior in UHSFRC structures. Based on the concept of the equivalent uniaxial strain, constitutive relationships of UHSFRC are presented in the axes of orthotropy which coincide with the principal axes of the total strain and rotate according to the loading history. This paper introduces a criterion to simulate the tension-stiffening effect on the basis of the force equilibriums, compatibility conditions, and bond stress-slip relationship in an idealized axial member and its efficiency is validated by comparison with available experimental data. Finally, the applicability of the proposed numerical model is established through correlation studies between analytical and experimental results for idealized UHSFRC beams.

Key Words
ultra high performance concrete (UHPC); steel fiber-reinforced concrete (SFRC); tensionstiffening model; tensile properties; finite element analysis.

Address
Chaekuk Na and Hyo-Gyoung Kwak: Dept. of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahak-ro, Yuseong-gu, Daejeon, Korea, 305-701

Abstract
The behaviour of concrete confined by transverse reinforcement is a classical topic. Numerous studies have been conducted to establish the stress-strain relationships for concrete under various confining reinforcement arrangements. Many empirical and semi-empirical formulas exist. Simplified analytical models have also been proposed to evaluate the increase in the strength and ductility of confined concrete. However, relatively few studies have been conducted to utilise advanced computational models for a realistic simulation of the behaviour of concrete confined by transverse reinforcement. As a matter of fact, high fidelity simulations using the latest numerical solvers in conjunction with advanced material constitutive models can be a powerful means to investigating the mechanisms underlying the confining effects of different reinforcement schemes. This paper presents a study on the use of high fidelity finite element models for the investigation of the behaviour of concrete confined by stirrups, as well as the interpretation of the numerical results. The development of the models is described in detail, and the essential modelling considerations are discussed. The models are then validated by simulating representative experimental studies on short columns with different confining reinforcement schemes. The development and distribution of the confining stress and the subsequent increase in the axial strength are examined. The models are shown to be capable of reproducing the behaviour of the confined concrete realistically, paving a way for systematic parametric studies and investigation into complicated confinement, load combination, and dynamic loading situations.

Key Words
concrete; confinement; transverse reinforcement; numerical simulation; finite element model; confining mechanisms.

Address
Zhenhuan Song and Yong Lu: Institute for Infrastructure and Environment, Joint Institute of Civil and Environmental Engineering, School of Engineering, The University of Edinburgh, The Kings Buildings, Edinburgh, EH9 3JL, UK

Abstract
Force-deformation modelling of cracked reinforced concrete is essential for a displacementbased seismic assessment of structures and can be achieved by fibre-element analysis of the cross-section of the major lateral resisting elements. The non-linear moment curvature relationship obtained from fibreelement analysis takes into account the significant effects of axial pre-compression and contributions by the longitudinal reinforcement. Whilst some specialised analysis packages possess the capability of incorporating fibre-elements into the modelling (e.g., RESPONSE 2000), implementation of the analysis on EXCEL is illustrated in this paper. The outcome of the analysis is the moment-curvature relationship of the wall cross-section, curvature at yield and at damage control limit states specified by the user. Few software platforms can compete with EXCEL in terms of its transparencies, versatility and familiarity to the computer users. The program has the capability of handling arbitrary cross-sections that are without an axis of symmetry. Application of the program is illustrated with examples of typical cross-sections of structural walls. The calculated limiting curvature for the considered cross-sections were used to construct displacement profiles up the height of the wall for comparison with the seismically induced displacement demand.

Key Words
deformation modelling; cracked reinforced concrete; deflection; EXCEL; spreadsheets; fibre element analysis; RESPONSE 2000.

Address
Nelson Lam: Dept. of Civil & Environmental Engineering, c/o School of Engineering, University of Melbourne, Parkville, 3010, Australia
John Wilson: Swinburne University of Technology, Hawthorne, Victoria, Australia
Elisa Lumantarna: Dept. of Civil & Environmental Engineering, c/o School of Engineering, University of Melbourne, Parkville, 3010, Australia

Abstract
A theoretical model known as the modified rotating-angle softened-truss model (MRA-STM), which is a modification of Rotating-Angle Softened-Truss Model and Modified Compression Field Theory, is presented for the analysis of reinforced concrete membranes in shear. As an application, shear strength and behaviour of reinforced concrete exterior beam-column joints are analysed using the MRA-STM combining with the deep beam analogy. The joints are considered as RC panels and subjected to vertical and horizontal shear stresses from adjacent columns and beams. The strut and truss actions in a beamcolumn joint are represented by the effective transverse compression stresses and a softened concrete truss in the proposed model. The theoretical predictions of shear strength of reinforced concrete exterior beamcolumn joints from the proposed model show good agreement with the experimental results.

Key Words
beam-column joints; shear strength; softened-truss model; reinforced concrete.

Address
Simon H.F. Wong: Hyder Consulting (Hong Kong) Ltd, 47/F Hopewell Centre, 183 Queen\'s Road East, Hong Kong
J.S. Kuang: Dept. of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Abstract
Existing reinforced concrete (RC) beams can be strengthened with externally bolted steel plates to the sides of beams. The effectiveness of this type of bolted side-plate (BSP) beam can however be affected by partial interaction between the steel plates and RC beams due to the mechanical slip of bolts. To avoid over-estimation of the flexural strength and ensure accurate prediction of the loaddeformation response of the beams, the effect of partial interaction has to be properly considered. In this paper, a special non-linear macro-finite-element model that takes into account the effects of partial interaction is proposed. The RC beam and the steel plates are modelled as two different elements, interacting through discrete groups of bolts. A layered method is adopted for the formulation of the RC beam and steel plate elements, while a special non-linear model based on a kinematic hardening assumption for the bolts is used to simulate the bolt group effect. The computer program SiBAN was developed based on the proposed approach. Comparison with the available experimental results shows that SiBAN can accurately predict the partial interaction behaviour of the BSP beams. Further numerical simulations show that the interaction between the RC beam and the steel plates is greatly reduced by the formation of plastic hinges and should be considered in analyses of the strengthened beams.

Key Words
partial interaction; strengthening; reinforced concrete; plate; finite element; slip.

Address
W.H. Siu and R.K.L. Su: Dept. of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, PRC

Abstract
In this paper, FEM procedure is applied to the static and dynamic analyses of R/C structures. Simple R/C shell structure is solved by using FEM procedures and the experimental evaluations are performed to represent the applicability of FEM procedure to R/C structures. Also, R/C columns are analyzed numerically and experimentally. On the basis of these results, FEM procedures are applied to the R/C cooling tower structures assembled by huge R/C shell structure and a lot of discrete R/C columns. In this analysis, the parallel computing procedures are introduced into these analyses to reduce the computational effort. The dynamic performances of R/C cooling tower are also solved by the application of parallel computations as well. From the numerical analyses, the conventional FEM procedures combined with computational technologies enables us to design the huge R/C structures statically and dynamically.

Key Words
FEM analysis; R/C shell; parallel computing; computational mechanics.

Address
Takashi Hara: Dept. of Civil Engineering and Architecture, Tokuyama College of Technology, Gakuendai, Shunan 745-8585, Japan

Abstract
Severe element distortion problem is observed in finite element mesh while performing numerical simulations of high velocity steel projectiles penetration/perforation of concrete targets using finite element method (FEM). This problem of element distortion in Lagrangian formulation of FEM can be resolved by using element erosion methodology. Element erosion approach is applied in the finite element program by defining failure parameters as a condition for element elimination. In this study strain parameters for both compression and tension at failure are used as failure criteria. Since no direct method exists to determine these values, a calibration approach is used to establish suitable failure strain values while performing numerical simulations of ogive-nose steel projectile penetration/perforation into concrete target. A range of erosion parameters is suggested and adopted in concrete penetration/perforation tests to validate the suggested values. Good agreement between the numerical and field data is observed.

Key Words
high velocity impact; concrete; ogive-nose projectile; perforation; penetration; element erosion.

Address
Md. Jahidul Islam: Dept. of Civil Engineering, National University of Singapore, # 1 Engineering Drive 2, Singapore 117576
Zishun Liu: Institute of High Performance Computing, Fusionopolis Way, #16-16 Connexis, Singapore 138632
Somsak Swaddiwudhipong: Dept. of Civil Engineering, National University of Singapore, # 1 Engineering Drive 2, Singapore 117576

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