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CONTENTS
Volume 18, Number 5, November 2004
 

Abstract
For reaching large inelastic deformations without a substantial loss in strength, the potential plastic hinge regions of the reinforced concrete structural members should be confined by adequate transverse reinforcement. Therefore, simple and realistic representation of confined concrete behaviour is needed for inelastic analysis of reinforced concrete structures. In this study, a trilinear stress-strain model is proposed for the axial behaviour of confined concrete. The model is based on experimental work that was carried out on nearly full size specimens. During the interpretation of experimental data, the buckling and strain hardening of the longitudinal reinforcement are also taken into account. The proposed model is used for predicting the stress-strain relationships of confined concrete specimens tested by other researchers. Although the proposed model is simpler than most of the available models, the comparisons between the predicted results and experimental data indicate that it can represent the stress-strain relationship of confined concrete quite realistically.

Key Words
buckling; columns (supports); confined concrete; ductility; reinforcement; strength; stress-strain curves.

Address
Alper Ilki; Istanbul Technical University, Civil Engineering Faculty, Structural and Earthquake Engineering Laboratory, 34469, Maslak, Istanbul, Turkey
Nahit Kumbasar; Istanbul Technical University, Civil Engineering Faculty, Reinforced Concrete Division, 34469, Maslak, Istanbul, Turkey
Pinar Ozdemir; Istanbul Technical University, Civil Engineering Faculty, Mechanics Department, 34469, Maslak, Istanbul, Turkey
Toshibumi Fukuta; Building Research Institute, Tsukuba, Japan

Abstract
The present paper concerns the macroscopic overall description of rheologic properties for steel wire and synthetic fibre cables under variable loading actions according to non-linear creep and/or relaxation theory. The general constitutive equations of non-linear creep and/or relaxation of tension elements - cables under one-step and the variable stress or strain inputs using the product and two types of additive approximations of the kernel functions are presented in the paper. The derived non-linear constitutive equations describe a non-linear rheologic behaviour of the cables for a variable stress or strain history using the kernel functions determined only by one-step - constant creep or relaxation tests. The developed constitutive equations enable to simulate and to predict in a general way non-linear rheologic behaviour of the cables under an arbitrary loading or straining history. The derived constitutive equations can be used for the various tension structural elements with the non-linear rheologic properties under uniaxial variable stressing or straining.

Key Words
non-linear creep of cable; non-linear relaxation of cable; variable stress or strain history; non-linear constitutive equation; kernel functions; approximations of the kernel functions.

Address
Faculty of Civil Engineering, Technical University of Kosice, Vysokoskolska 4,
042 00 Kosice, Slovak Republic

Abstract
The substance of the use of the derived non-linear creep constitutive equations under variable stress levels (see first part of the paper, Kmet 2004) is explained and the strategy of their application is outlined using the results of one-step creep tests of the steel spiral strand rope as an example. In order to investigate the creep strain increments of cables an experimental set-up was originally designed and a series of tests were carried out. Attention is turned to the individual main steps in the production and application procedure, i.e., to the one-step creep tests, definition of loading history, determination of the kernel functions, selection and definition of constitutive equation and to the comparison of the resulting values considering the product and the additive forms of the approximation of the kernel functions. To this purpose, the parametrical study is performed and the results are presented. The constitutive equations of non-linear creep of cable under variable stress history offer a strong tool for the real simulation of stochastic variable load history and prediction of realistic time-dependent response (current deflection and stress configuration) of structures with cable elements. By means of suitable stress combination and its gradual repeating various loads and times effects can be modelled.

Key Words
non-linear creep of steel rope; creep test; constitutive equation of creep under variable stress; kernel functions; parametrical study.

Address
Faculty of Civil Engineering, Technical University of Kosice, Vysokoskolska 4,
042 00 Kosice, Slovak Republic

Abstract
The Land Transport Authority of Singapore has a continuing program of highway bridge upgrading, to refurbish and strengthen bridges to allow for increasing vehicle traffic and increasing axle loads. One subject of this program has been a short span bridge taking a busy highway across a coastal inlet near a major port facility. Experiment-based structural assessments of the bridge were conducted before and after upgrading works including strengthening. Each assessment exercise comprised two separate components; a strain and acceleration monitoring exercise lasting approximately one month, and a full-scale dynamic test carried out in a single day. This paper reports the application of extreme value statistics to estimate bridge live loads using strain measurements.

Key Words
bridge assessment; extreme value statistics; structural health monitoring; bridge live loading.

Address
Pilate Moyo; Department of Civil Engineering, University of Cape Town, Rondebosch 7701, Cape Town, Republic of South Africa
James Mark William Brownjohn; School of Engineering, University of Plymouth, Drake Circus, Plymouth PL4 8AA, United Kingdom
Piotr Omenzetter; Department of Civil and Environmental Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand

Abstract
In this study, the endochronic theory was used to investigate the collapse of thick-walled tubes subjected to external pressure and axial tension. The experimental and theoretical findings of Madhavan et al. (1993) for thick-walled tubes of 304 stainless steel subjected to external pressure and axial tension were compared with the endochronic simulation. Collapse envelopes for various diameter-to-thickness tubes under two different pressure-tension loadings were involved. It has been shown that the experimental results were aptly described by the endochronic approach demonstrated from comparison with the theoretical prediction employed by Madhavan et al. (1993). Furthermore, by using the rate-sensitivity function of the intrinsic time measure proposed by Pan and Chern (1997) in the endochronic theory, our theoretical analysis was extended to investigate the viscoplastic collapse of thick-walled tubes subjected to external pressure and axial tension. It was found that the pressure-tension collapse envelopes are strongly influenced by the strain-rate during axial tension. Due to the hardening of the metal tube of 304 stainless steel under a faster strain-rate during uniaxial tension, the size of the tension-collapse envelope increases.

Key Words
endochronic theory; viscoplastic collapse; thick-walled tubes, external pressure and axial tension.

Address
Kuo-Long Lee; Department of Mechanical Engineering, Far East College, Tainan, Taiwan, R.O.C.
Kao-Hua Chang; Dept. of Engineering Science, National Cheng Kung University, Tainan, Taiwan, R.O.C.

Abstract
In seismic analysis of moment-resisting frames, beam-column connections are often modeled with rigid joint zones. However, it has been demonstrated that, in ductile reinforced concrete (RC) moment-resisting frames designed based on current codes (to say nothing of older non-ductile frames), the joint zones are in fact not rigid, but rather undergo significant shear deformations that contribute greatly to global drift. Therefore, the

Key Words
reinforced concrete; beam-column connection; joint shear deformation; bond slip; numerical modeling; frame analysis.

Address
Morehead State University, Department of Industrial and Engineering Technology, 309 Lloyd Cassity, Morehead, Kentucky, 40351, U.S.A.

Abstract
An assumed strain finite strip method(FSM) using the non-periodic B-spline for a shell is presented. In the present method, the shape function based on the non-periodic B-splines satisfies the Kronecker delta properties at the boundaries and allows to introduce interior supports in much the same way as in a conventional finite element formulation. In the formulation for a shell, the geometry of the shell is defined by non-periodic B3-splines without any tangential vectors at the ends and the penalty function method is used to incorporate the drilling degrees of freedom. In this study, new assumed strain fields using the non-periodic B-spline function are proposed to overcome the locking problems. The strip formulated in this way does not posses any spurious zero energy modes. The versatility and accuracy of the new approach are demonstrated through a series of numerical examples.

Key Words
finite strip; non-periodic B-spline function; assumed strain method.

Address
Hyun-Seok Hong and Kyeong-Ho Kim; Chungsuk Engineering, Structural Div.,
57 Karak-dong, Songpa-gu, Seoul, Korea
Chang-Koon Choi; Department of Civil and Environmental Engineering, KAIST, Daejeon 305-701, Korea

Abstract
This paper presents the micro-mechanical modeling for predicting concrete behavior under compressive loading. The model is able to represent the heterogeneities in the microstructure up to three phases, i.e., aggregate particles, matrix and interfaces. The smeared crack concept based on non-linear fracture mechanics is implemented in order to formulate the constitutive relation for each component. The splitting tensile strength is considered as a fracture criterion for cracking in micro-level. The finite element method is employed to simulate the model based on plane stress condition by using quadratic triangular elements. The validation of the model is verified by comparing with the experimental results.
The influence of tensile strength from both aggregate and matrix phases on the concrete compressive strength is demonstrated. In addition, a guideline on selecting appropriate tensile strength for each phase to obtain specified concrete compressive strength is also presented.

Key Words
micro-mechanical modeling; finite element method; non-linear fracture mechanics; concrete material; compressive behavior

Address
Dept. of Civil Engineering, Chulalongkorn University, Bangkok 10330, Thailand

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