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CONTENTS
Volume 37, Number 5, March10 2011
 

Abstract
The study of the early age concrete properties is becoming more important, as the thermal effects and the shrinkage, even in the first hours, could generate cracks, increasing the permeability of the structure and being able to induce problems of durability and functionality in the same ones. The detailed study of the stresses development during the construction process can be decisive to keep low the cracking levels. In this work a computational model, based on the finite element method, was implemented to simulate the early age concrete behavior and, specially, the evaluation of the cracking risk. The finite element analysis encloses the computational modeling of the following phenomena: chemical, thermal, moisture diffusion and mechanical which occur at the first days after the concrete cast. The developed software results were compared with experimental values found in the literature, demonstrating an excellent approach for all the implemented analysis.

Key Words
concrete; early age; thermal analysis; moisture transport; mechanical analysis.

Address
Mauren Aurich: Department of Civil Engineering, Pontifical Catholic University of Rio Grande do Sul, Brazil
Americo Campos Filho: Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil
Tulio Nogueira Bittencourt: Polytechnical School, University of Sao Paulo, Brazil
Surendra P. Shah: Department of Civil Engineering, Northwestern University, Evanston, IL, USA

Abstract
In a flexible multi-body dynamic system the typical topological optimization method for structures cannot be directly applied, as the stiffness varies with position. In this paper, the topological optimization of the flexible multi-body dynamic system is converted into structural optimization using the equivalent static load method. First, the actual boundary conditions of the control system and the approximate stiffness curve of the mechanism are obtained from a flexible multi-body dynamical simulation. Second, the finite element models are built using the absolute nodal coordination for different positions according to the stiffness curve. For efficiency, the static reanalysis method is utilized to solve these finite element equilibrium equations. Specifically, the finite element equilibrium equations of key points in the stiffness curve are fully solved as the initial solution, and the following equilibrium equations are solved using a reanalysis method with an error controlled epsilon algorithm. In order to identify the efficiency of the elements, a non-dimensional measurement is introduced. Finally, an improved evolutional structural optimization (ESO) method is used to solve the optimization problem. The presented method is applied to the optimal design of a die bonder. The numerical results show that the presented method is practical and efficient when optimizing the design of the mechanism.

Key Words
topological optimization; the flexible multi-body dynamic system; die bonder; epsilon algorithm.

Address
Zhi-Jun Yang: Faculty of ElectroMechanical Engineering, Guangdong University of Technology, 510006, China; Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA
Xin Chen: Faculty of ElectroMechanical Engineering, Guangdong University of Technology, 510006, China
Robert Kelly: Faculty of ElectroMechanical Engineering, Guangdong University of Technology, 510006, China

Abstract
The dynamic response of a Timoshenko beam on a tensionless Pasternak foundation is investigated by assuming that the beam is subjected to a concentrated harmonic load at its middle. This action results in the creation of lift-off regions between the beam and the foundation that effect the character of the response. Although small displacements for the beam and the foundation are assumed, the problem becomes nonlinear since the contact/lift-off regions are not known at the outset. The governing equations of the beam, which are coupled in deflection and rotation, are obtained in both the contact and lift-off regions. After removing the coupling, the essentials of the problem (the contact regions) are determined by using an analytical-numerical method. The results are presented in figures to demonstrate the effects of some parameters on the extent of the contact lengths and displacements. The results are also compared with those of Bernoulli-Euler, shear, and Rayleigh beams. It is observed that the solution is not unique; for a fixed value of the frequency parameter, more than one solution (contact length) exists. The contact length of the beam increases with the increase of the frequency and rotary-inertia parameters, whereas it decreases with increasing shear foundation parameter.

Key Words
Timoshenko beam; Pasternak foundation; lift-off.

Address
Irfan Coskun: Faculty of Civil Engineering, Yildiz Technical University, Davutpasa Campus, 34210 Esenler,
Istanbul, Turkey
Hasan Engin: Faculty of Civil Engineering,I stanbul Technical University, 34469 Maslak, Istanbul, Turkey
Ayfer Tekin: Faculty of Civil Engineering, Yildiz Technical University, Davutpasa Campus, 34210 Esenler, Istanbul, Turkey

Abstract
The paper investigates the dynamic buckling behaviour of a laminated composite stiffened cylindrical shell using the commercial finite element code ABAQUS. The numerical model of the composite shell is validated by static tests. In particular, the experimental collapse test is numerically simulated by a quasi static analysis carried out by both ABAQUS/Standard and ABAQUS/Explicit. The behaviour in the post-buckling field and the collapse load obtained by the analyses are close to the experimental data. The validated model is then used to study the dynamic buckling behaviour with ABAQUS/Explicit. The effects of the loading magnitude and of the loading duration are investigated, implementing in the analysis also first-ply failure criteria. It is observed that the dynamic buckling load is highly affected by the loading duration.

Key Words
dynamic buckling; composite stiffened shell; pulse loading; failure; finite element analysis.

Address
S.N. Patel: Department of Aerospace Engineering, Politecnico di Milano, Milano, Italy
C. Bisagni: Department of Aerospace Engineering, Politecnico di Milano, Milano, Italy
P.K. Datta: Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur, India

Abstract
Three dimensional solutions for free vibrations analysis of functionally graded fiber reinforced cylindrical panel are presented, using differential quadrature method (DQM). The orthotropic panel is simply supported at the edges and is assumed to have an arbitrary variation of reinforcement volume fraction in the radial direction. Suitable displacement functions that identically satisfy the simply supported boundary condition are used to reduce the equilibrium equations to a set of coupled ordinary differential equations with variable coefficients, which can be solved by differential quadrature method to obtain natural frequencies. The main contribution of this work is presenting useful results for continuous grading of fiber reinforcement in the thickness direction of a cylindrical panel and comparison with similar discrete laminate composite ones. Results indicate that significant improvement is found in natural frequency of a functionally graded fiber reinforced composite panel due to the reduction in spatial mismatch of material properties.

Key Words
DQM; functionally graded fiber reinforced; free vibrations; orthotropic; panel.

Address
M.H. Yas, B. Sobhani Aragh and M. Heshmati: Department of Mechanical Engineering, Razi University, Kermanshah, Iran

Abstract
In structural engineering there are randomness inherently exist on determination of the loads, strength, geometry, and so on, and the manufacturing of the structural members, workmanship etc. Thus, objective and constraint functions of the optimization problem are functions that depend on those randomly natured components. The constraints being the function of the random variables are evaluated by using reliability index or performance measure approaches in the optimization process. In this study, the minimum weight of a space truss is obtained under the uncertainties on the load, material and crosssection areas with harmony search using reliability index and performance measure approaches. Consequently, optimization algorithm produces the same result when both the approaches converge. Performance measure approach, however, is more efficient compare to reliability index approach in terms of the convergence rate and iterations needed.

Key Words
reliability based design optimization; reliability index approach; performance measure approach; harmony search.

Address
Vedat Togan: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey
Ayse T. Daloglu: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey
Halil Karadeniz: Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2628 CN, Delft, The Netherlands

Abstract
For efficient calculation of natural modes of structures, a numerical scheme which accelerates convergence of the subspace iteration method by employing accelerated starting Lanczos vectors was proposed in 2005. This paper is an extension of the study. The previous study simply showed feasibility of the proposed method by analyzing structures with smaller degrees of freedom. While, the present study verifies efficiency of the proposed method more rigorously by comparing closeness of conventional and accelerated starting vectors to genuine eigenvectors. This study also analyzes an example structure with larger degrees of freedom and more complex constraints in order to investigate applicability of the proposed method.

Key Words
natural modes of structures; eigenvector; subspace iteration method; accelerated starting vector.

Address
B.W. Kim: Marine Structure & Plant Research Department, KORDI/MOERI, Daejeon, Korea
H.J. Jung: Department of Civil and Environmental Engineering, KAIST, Daejeon, Korea
S.Y. Hong: Marine Structure & Plant Research Department, KORDI/MOERI, Daejeon, Korea


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