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CONTENTS
Volume 39, Number 6, September25 2011
 

Abstract
The present paper deals with the identification of a concentrated damage in an elastic parabolic arch through the minimization of an objective function which measures the differences between numerical and experimental values of static displacements. The damage consists in a notch that reduces the height of the cross section at a given abscissa and therefore causes a variation in the flexural stiffness of the structure. The analytical values of static displacements due to applied loads are calculated by means of the principle of virtual work for both the undamaged and damaged arch. First, pseudo-experimental data are used to study the inverse problem and investigate whether a unique solution can occur or not. Various damage intensities are considered to assess the reliability of the identification procedure. Then, the identification procedure is applied to an experimental case, where displacements are measured on a prototype arch. The identified values of damage parameters, i.e., location and intensity, are compared to those obtained by means of a dynamic identification technique performed on the same structure.

Key Words
damage identification; inverse problems; arches; experimental static displacements

Address
Annalisa Greco: Department of Civil and Environmental Engineering, University of Catania, v.le A.Doria 6, 95125 Catania, Italy
Annamaria Pau: Department of Structural and Geotechnical Engineering, La Sapienza University of Rome, via A. Gramsci 53, 00197 Roma, Italy

Abstract
An efficient edge-based smoothed finite element method (ES-FEM) has been recently developed for solving solid mechanics problems. The ES-FEM uses triangular elements that can be generated easily for complicated domains. In this paper, the complexity study of the ES-FEM based on triangular elements is conducted in detail, which confirms the ES-FEM produces higher computational efficiency compared to the FEM. Therefore, the ES-FEM offers an excellent platform for adaptive analysis, and this paper presents an efficient adaptive procedure based on the ES-FEM. A smoothing domain based energy (SDE) error estimate is first devised making use of the features of the ES-FEM. The present error estimate differs from the conventional approaches and evaluates error based on smoothing domains used in the ES-FEM. A local refinement technique based on the Delaunay algorithm is then implemented to achieve high efficiency in the mesh refinement. In this refinement technique, each node is assigned a scaling factor to control the local nodal density, and refinement of the neighborhood of a node is accomplished simply by adjusting its scaling factor. Intensive numerical studies, including an actual engineering problem of an automobile part, show that the proposed adaptive procedure is effective and efficient in producing solutions of desired accuracy.

Key Words
smoothed finite element method (SFEM); edge-based smoothed finite element method (ESFEM); complexity study; adaptive analysis; error estimate; local refinement

Address
L. Chen: Centre for Advanced Computations in Engineering Science (ACES), National University of Singapore, 9 Engineering Drive 1, Singapore 117576; Institute of High Performance Computing, Singapore 138632
J. Zhang: Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576
K.Y. Zeng: Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576
P.G. Jiao: School of Mechanical Engineering, Shandong University, Jinan 250061, P.R. China

Abstract
Estimation of damage probability of buildings under a future earthquake is an essential issue to ensure the seismic reliability. Fragility curves are useful tools for showing the probability of structural damage due to earthquakes as a function of ground motion indices. The purpose of this study is to compare the damage probability of R/C buildings with low and high level of strength and ductility through fragility analysis. Two different types of sample buildings have been considered which represent the building types mentioned above. The first one was designed according to TEC-2007 and the latter was designed according to TEC-1975. The pushover curves of sample buildings were obtained via pushover analyses. Using 60 ground motion records, nonlinear time-history analyses of equivalent single degree of freedom systems were performed using bilinear hysteretic model and peak-oriented hysteretic model with stiffness – strength deterioration for each scaled elastic spectral displacement. The damage measure is maximum inter-story drift ratio and each performance level considered in this study has an assumed limit value of damage measure. Discrete damage probabilities were calculated using statistical methods for each considered performance level and elastic spectral displacement. Consequently, continuous fragility curves have been constructed based on the lognormal distribution assumption. Furthermore, the effect of hysteresis model parameters on the damage probability is investigated.

Key Words
earthquake damage; fragility curves; hysteretic models; degradation

Address
Muzaffer Borekci and Murat S. Kircil: Department of Civil Engineering, Yildiz Technical University, Istanbul, Turkey

Abstract
This paper presents an efficient version of Hilbert-Huang transform for nonlinear nonstationary systems analyses. An ensemble empirical mode decomposition (EEMD) is introduced to alleviate the problem of mode mixing between intrinsic mode functions (IMFs) decomposed by EMD. Yet the problem has not been fully resolved when a signal of a similar scale resides in different IMF components. Instead of using a trial and error method to select the \"best\" outcome generated by EEMD, a hybrid algorithm based on EEMD and EMD is proposed for multi-mode signal processing. The developed approach comprises the steps from a bandpass filter design for regrouping modes of the IMFs obtained from EEMD, to the mode extraction using EMD, and to the assessment of each mode in the marginal spectrum. A simulated two-mode signal is tested to demonstrate the efficiency and robustness of the approach, showing average relative errors all equal to 1.46% for various noise levels added to the signal. The developed approach is also applied to a real bridge structure, showing more reliable results than the pure EMD. Discussions on the mode determination are offered to explain the connection between modegrouping form on the one hand, and mode-grouping performance on the other.

Key Words
ensemble empirical mode decomposition; filter design; intrinsic mode function; multi modes; signal processing

Address
Jeng-Wen Lin: Department of Civil Engineering, Feng Chia University, Taichung, Taiwan 407, R.O.C.

Abstract
We have developed a lightweight aggregate (LWA) concrete made by expanding fine sediments dredged from the Shihmen Reservoir (Taiwan) with high heat. In this study, the performance of the concrete and of prestressed concrete beams made of the sedimentary LWA were tested and compared with those made of normal-weight concrete (NC). The test results show that the lightweight concrete (LWAC) exhibited comparable time-dependent properties (i.e., compressive strength, elastic modulus, drying shrinkage, and creep) as compared with the NC samples. In addition, the LWAC beams exhibited a smaller percentage of prestress loss compared with the NC beams. Moreover, on average, the LWAC beams could resist loading up to 96% of that of the NC beams, and the experimental strengths were greater than the nominal strengths calculated by the ACI Code method. This investigation thus established that sedimentary LWA can be recommended for structural concrete applications.

Key Words
lightweight aggregate; lightweight aggregate concrete; prestressed concrete beam; shrinkage; creep; flexural strength

Address
How-Ji Chen, Wen-Po Tsai and Te-Hung Liu: Department of Civil Engineering, National Chung-Hsing University, No. 250, Kuo Kuang Road, Taichung, Taiwan, R.O.C.
Chao-Wei Tang: Department of Civil Engineering & Engineering Informatics, Cheng-Shiu University, No. 840,
Chengcing Road, Niaosong District, Kaohsiung City, Taiwan, R.O.C.

Abstract
The buckling capacity of the cylindrical shells depends on two geometric ratios of L/R and R/t. However the effect of thickness variation on the behavior of the shells is more complicated and the buckling strength of them is sensitive to the magnitude and shape of geometric imperfections. In this paper the effects of thickness variation and geometric imperfections on the buckling and postbuckling behavior of cylindrical shells are experimentally investigated. The obtained results are presented under the effect of uniform lateral pressure. It is found in this investigation that the buckling mode can be generated in the whole length of the shell, if the thickness variation is low.

Key Words
cylindrical shells; experiments; buckling; post buckling; varying thickness; uniform external pressure

Address
Sirous Aghajari and Karim Abedi: Faculty of Civil Engineering, Sahand University of Technology, Tabriz, Iran
Hossein Showkati: Department of Civil Engineering, Urmia University, Urmia, Iran

Abstract
The aim of the present article is to study the micropolar thermoelastic interactions in an infinite Kelvin-Voigt type viscoelastic thermally conducting plate. The coupled dynamic thermoelasticity and generalized theories of thermoelasticity, namely, Lord and Shulman\'s and Green and Lindsay\'s are employed by assuming the mechanical behaviour as dynamic to study the problem. The model has been simplified by using Helmholtz decomposition technique and the resulting equations have been solved by using variable separable method to obtain the secular equations in isolated mathematical conditions for homogeneous isotropic micropolar thermo-viscoelastic plate for symmetric and skew-symmetric wave modes. The dispersion curves, attenuation coefficients, amplitudes of stresses and temperature distribution for symmetric and skew-symmetric modes are computed numerically and presented graphically for a magnesium crystal.

Key Words
micropolar thermoviscoelastic plate; secular equations; dispersion curves; attenuation coefficients

Address
Rajneesh Kumar: Department of Mathematics, Kurukshetra University, Kurukshetra, Haryana, 136 119, India
Geeta Partap: Department of Mathematics, Dr. B.R. Ambedkar National Institute of Technology, Jalandhar, Punjab, 144011, India

Abstract
Response spectra of earthquake ground motions are important in the earthquake-resistant design and reliability analysis of structures. The formulation of the response spectrum in the frequency domain efficiently computes and evaluates the stochastic response spectrum. The frequency information of the excitation can be described using different functional forms. The shapes of the calculated response spectra of the excitation show strong magnitude and site dependency, but weak distance dependency. In this paper, to compare the effect of the earthquake ground motion variables, the contribution of these sources of variability to the response spectrum\'s uncertainty is calculated by using a stochastic analysis. The analytical results show that earthquake source factors and soil condition variables are the main sources of uncertainty in the response spectra, while path variables, such as distance, anelastic attenuation and upper crust attenuation, have relatively little effect. The presented formulation of dynamic structural response in frequency domain based only on the frequency information of the excitation can provide an important basis for the structural analysis in some location that lacks strong motion records.

Key Words
probabilistic; Fourier amplitude; response spectra; design earthquake

Address
Azad Yazdani: Department of Civil Engineering, Faculty of Engineering, University of Kurdistan, Sanandaj, Iran
Tsuyoshi Takada: Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan


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