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CONTENTS
Volume 28, Number 5, March30 2008
 

Abstract
In this paper, experimental and theoretical investigations of the effect of the mean moment on the response and collapse of circular thin-walled tubes subjected to cyclic bending are discussed. To highlight the influence of the mean moment effect, three different moment ratios r (minimum moment/ maximum moment) of -1, -0.5 and 0, respectively, were experimentally investigated. It has been found that the moment-curvature loop gradually shrinks with the number of cycles, and becomes stable after a few cycles for symmetric cyclic bending (r = -1). However, the moment-curvature loop exhibits ratcheting and increases with the number of cycles for unsymmetric cyclic bending (r = -0.5 or 0). In addition, although the three groups of tested specimens had three different moment ratios, when plotted in a log-log scale, three parallel straight lines describe the relationship between the controlled moment range and the number of cycles necessary to produce buckling. Finally, the endochronic theory combined with the principle of virtual work was used to simulate the relationship among the moment, curvature and ovalization of thin-walled tubes under cyclic bending. An empirical formulation was proposed for simulating the relationship between the moment range and the number of cycles necessary to produce buckling for thin-walled tubes subjected to cyclic bending with different moment ratios. The results of the experimental investigation and the simulation are in good agreement with each other.

Key Words
moment-controlled; mean moment; moment ratio; thin-walled tube; cyclic bending; ovalization; collapse; endochronic theory.

Address
Kao-Hua Chang and Wen-Fung Pan: Dept. of Engineering Science, National Cheng Kung University, Tainan, Taiwan 701, R.O.C.
Kuo-Long Lee: Dept. of Computer Application Engineering, Far East College, Tainan County, Taiwan 744, R.O.C.

Abstract
In this paper the effect of prestressing force on the first flexural natural frequency of beams is studied. Finite element technique is used to model the beam-tendon system, and the prestressing force
is applied in the form of initial tension in the tendon. It is shown that the effect of prestressing force on the first natural frequency depends on bonded and unbonded nature of the tendon, and also on the eccentricity of tendon. For the beams with bonded tendon, the prestressing force does not have any appreciable effect on the first flexural natural frequency. However, for the beams with unbonded tendon, the first natural frequency significantly changes with the prestressing force and eccentricity of the tendon. If the eccentricity of tendon is small, then the first natural frequency decreases with the prestressing force and if the eccentricity is large, then the first flexural natural frequency increases with the prestressing force. Results of the present study clearly indicate that the first natural frequency can not be used as an easy indicator for detecting the loss of prestressing force, as has been attempted in some of the past studies.

Key Words
prestressed beam; flexural natural frequency; bonded and unbonded tendon; prestressing force.

Address
O. R. Jaiswal: Dept. of Applied Mechanics, Visvesvaraya National Institute of Technology, Nagpur 440 011, India

Abstract
This article deals the theory for solving an inverse problem of plate structures using the frequency-domain information instead of classical time-domain delays or free vibration eigenmodes or eigenvalues. A reduced set of output parameters characterizing the defect is used as a regularization technique to drastically overcome noise problems that appear in imaging techniques. A deconvolution scheme from an undamaged specimen overrides uncertainties about the input signal and other coherent noises. This approach provides the advantage that it is not necessary to visually identify the portion of the signal that contains the information about the defect. The theoretical model for Quantitative nondestructive evaluation, the relationship between the real and ideal models, the finite element method (FEM) for the
forward problem, and inverse procedure for detecting the defects are developed. The theoretical formulation is experimentally verified using dynamic responses of a steel plate under impact loading at
several points. The signal synthesized by FEM, the residual, and its components are analyzed for different choices of time window. The noise effects are taken into account in the inversion strategy by designing a filter for the cost functional to be minimized. The technique is focused toward a exible and rapid inspection of large areas, by recovering the position of the defect by means of a single accelerometer, overriding experimental calibration, and using a reduced number of impact events.

Key Words
inverse problem; quantitative non-destructive evaluation (QNDE); real and ideal model; finite element method (FEM); impact testing; noise effect.

Address
Sang-Youl Lee: Dept. of Civil and Engineering, Hanyang University, 17 Haedang-Dong, Sungdong-Gu,
Seoul 133-791, Korea
Guillermo Rus: Dept. of Structural Mechanics, University of Granada, Politecnico de Fuentenueva,
18071 Granada, Spain
Taehyo Park: Dept. of Civil and Engineering, Hanyang University, 17 Haedang-Dong, Sungdong-Gu,
Seoul 133-791, Korea

Abstract
Numerical integration is an efficient approach for nonlinear dynamic analysis. In this paper, general category of the implicit integration errors will be discussed. In order to decrease the errors, Dynamic Relaxation method with modified time step (MFT) will be used. This procedure leads to an alternative algorithm which is very general and can be utilized with any implicit integration scheme. For numerical verification of the proposed technique, some single and multi degrees of freedom nonlinear dynamic systems will be analyzed. Moreover, results are compared with both exact and other available solutions. Suitable accuracy, high efficiency, simplicity, vector operations and automatic procedures are the main merits of the new algorithm in solving nonlinear dynamic problems.

Key Words
Modified Dynamic Relaxation; implicit time integration; nonlinear dynamic analysis.

Address
M. Rezaiee-Pajand and J. Alamatian: Dept. of Civil Engineering, Ferdowsi University, P. O. Box 91775-1111, Mashhad, Iran

Abstract
The purpose of this study is to investigate the linear earthquake behavior of the frame structures including subsoil with different stiffening members and to compare the results of each frame considered. These comparisons are made separately for displacement, bending moments and axial forces for frames with different storey and bay numbers for the time history and the modal analyses. The results
of both methods are also compared. The results of the frames with subsoil are also compared with the results of the frames without subsoil. It is concluded that all stiffening members considered in this study decrease the lateral displacement of the frame and the bending moment of the columns and increase the axial force in the columns and that configuration of the bracing members come out to be an important parameter in braced frames since the frames with the same type of bracing give different results depending on configuration. It is also concluded that, in general, the absolute maximum displacements of the frames modeled with subsoil are larger than those of the frames modeled without subsoil.

Key Words
earthquake behavior; frame structure; moment-resisting frames; braced frames; frames with shear walls; stiffening members; comparative study.

Address
Y. I. Ozdemir and Y. Ayvaz: Dept. of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey

Abstract
In this study, a numerical procedure based on the finite element method for materially and geometrically nonlinear analysis of reinforced and prestressed concrete slender columns with arbitrary
section subjected to combined biaxial bending and axial load is developed. In order to overcome the low computer efficiency of the conventional section integration method in which the reinforced concrete
section is divided into a large number of small areas, an efficient section integration method is used to determine the section tangent stiffness. In this method, the arbitrary shaped cross section is divided into several concrete trapezoids according to boundary vertices, and the contribution of each trapezoid to section stiffness is determined by integrating directly the trapezoid. The space frame flexural theory is utilized to derive the element tangent stiffness matrix. The nonlinear full-range member response is traced by an updated normal plane arc-length solution method. The analytical results agree well with the experimental ones.

Key Words
slender concrete column; section integration method; finite element; arc-length algorithm; nonlinear analysis.

Address
T.J. Lou and Y.Q. Xiang: Dept. of Civil Engineering, Zhejiang University, Hangzhou 310027, China

Abstract
A four-node plate finite element for the analysis of laminated composites which is developed using strain gradient notation is presented. The element is based on a first-order shear deformation theory and on the equivalent lamina assumption. Strains and stresses can be calculated at different points through the thickness of the plate. They are averaged values due to the equivalent lamina assumption. A shear correction factor is used as the transverse shear strain is taken to be constant over the plate thickness while its actual variation is parabolic. Strain gradient notation, which is physically interpretable, allows for the detailed a-priori analysis of the finite element model. The polynomial expansions are inspected and spurious terms responsible for modeling errors are identified in the shear strains polynomial expansions. The element is corrected by simply removing the spurious terms from the shear strains expansions. The element is implemented into a FORTRAN finite element code in two versions; namely, with and without spurious terms. Results are compared to show the effects of the spurious terms on the solutions. It is also shown that a refined mesh composed of corrected elements provides solutions which approximate very well the analytical solutions, validating the procedure.

Key Words
laminated composites; plates; finite elements; locking; strain gradient notation; parasitic shear.

Address
Joao Elias Abdalla Filho: Programa de Pos-Graduacao em Engenharia Mecanica, Pontificia Univ Catolica do Parana (PUCPR) Rua Imaculada Conceicao, 1155- Prado Velho - 80215-901, Curitiba - PR, Brasil
Curso de Engenharia de Producao Civil, Universidade Tecnologica Federal do Parana (UTFPR) Rua Sete de Setembro, 3165 . Centro . 80000-000, Curitiba . PR, Brasil
Ivan Moura Belo and Michele Schunemann Pereira:
Programa de Pos-Graduacao em Engenharia Mecanica, Pontificia Univ Catolica do Parana (PUCPR) Rua Imaculada Conceicao, 1155- Prado Velho - 80215-901, Curitiba - PR, Brasil


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