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CONTENTS
Volume 37, Number 6, March25 2011
 

Abstract
For seismic resistant design of critical structures, a dynamic analysis, either response spectrum or time history is frequently required. Owing to the lack of recorded data and the randomness of earthquake ground motion that may be experienced by structure in the future, usually it is difficult to obtain recorded data which fit the requirements (site type, epicenteral distance, etc.) well. Therefore, the artificial seismic records are widely used in seismic designs, verification of seismic capacity and seismic assessment of structures. The purpose of this paper is to develop a numerical method using Artificial Neural Network (ANN) and wavelet packet transform in best basis method which is presented for the decomposition of artificial earthquake records consistent with any arbitrarily specified target response spectra requirements. The ground motion has been modeled as a non-stationary process using wavelet packet. This study shows that the procedure using ANN-based models and wavelet packets in best-basis method are applicable to generate artificial earthquakes compatible with any response spectra. Several numerical examples are given to verify the developed model.

Key Words
artificial ground motion; wavelet packet transform; best basis algorithm; generalized regression neural network; target spectrum.

Address
A. Asadi: Department of Civil Engineering, Yazd University, Yazd, Iran
M. Fadavi: Department of Computer Engineering, Shomal University, Amol, Iran
A. Bagheri: Center of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering, Iran University of Science & Technology, PO Box 16765-163, Narmak, Tehran 16846, Iran
G. Ghodrati Amiri: Center of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering, Iran University of Science & Technology, PO Box 16765-163, Narmak, Tehran 16846, Iran

Abstract
This study determines the water length effects on the modal behavior of a prototype arch dam using Operational and Analytical Modal Analyses. Achievement of this purpose involves construction of a prototype arch dam-reservoir-foundation model under laboratory conditions. In the model, reservoir length was taken to be as much as three times the dam height. To determine the experimental dynamic characteristics of the arch dam using Operational Modal Analysis, ambient vibration tests were implemented for empty reservoir and three different reservoir water lengths. In the ambient vibration tests, the dam was vibrated by natural excitations provided from small impact effects and the response signals were measured using sensitive accelerometers. Operational Modal Analysis software process signals collected from the ambient vibration tests, and Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification techniques estimated modal parameters of the dams. To validate the experimental results, 3D finite element model of the prototype arch dam was modeled by ANSYS software for empty reservoir and three different reservoir water lengths, and dynamic characteristics of each model were determined analytically. At the end of the study, experimentally and analytically identified dynamic characteristics compared to each other. Also, changes on the natural frequencies along to water length are plotted as graphs. Results suggest that reservoir water complicates the modal behavior of the arch dam significantly.

Key Words
ambient vibration test; dynamic characteristic; prototype arch dam-reservoir-foundation model; water length effect.

Address
Baris Sevim: Department of Civil Engineering, Gumu hane University, 29100, Gumu hane, Turkey
Alemdar Bayraktar: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey
Ahmet Can Altunisik: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey

Abstract
A concept of crash energy absorbing (CEA) lattice structure for an inflatable morphing vehicle body (Lee et al. 2008) has been investigated as a method of providing rigidity and energy absorption capability during a vehicular collision (Lee et al. 2007). A modified analytical model for the CEA lattice structure design is described in this paper. The modification of the analytic model was made with a stiffness approach for the elastic region and updated plastic limit analysis with a pure plastic bending deformation concept and amended elongation factors for the plastic region. The proposed CEA structure is composed of a morphing lattice structure with movable thin-walled members for morphing purposes, members that will be locked in designated positions either before or during the crash. What will be described here is how to model the CEA structure analytically based on the energy absorbed by the CEA structure.

Key Words
lattice structure; crashworthiness; crash energy absorption; inflatable morphing body.

Address
Dong Wook Lee: R&D Center, Samsung Techwin, Seongnam-Si, Kyunggi-Do 463-400, Korea
Zheng-Dong Ma: Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
Noboru Kikuchi: Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA

Abstract
This paper investigates the aspect-ratio locking of the isoparametric 8-node quadrilateral (QUAD8) element. An important finding is that, if finite element solution is carried out with in exact arithmetic (i.e., with no truncation and round off errors), the locking tendency of the element is completely avoided even for aspect-ratios as high as 100000. The current finite element codes mostly use floating point arithmetic. Thus, they can only avoid this locking for aspect-ratios up to 100 or 1000. A novel method is proposed in the paper to avoid aspectratio locking in floating point computations. In this method, the offending terms of the strain-displacement matrix (i.e., B-matrix) are multiplied by suitable scaling factors to avoid ill-conditioning of stiffness matrix. Numerical examples are presented to demonstrate the efficacy of the method. The examples reveal that aspect-ratio locking is avoided even for aspect-ratios as high as 100000.

Key Words
finite element method; quadrilateral element; aspect-ratio; locking; exact arithmetic; illconditioning.

Address
S. Rajendran: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

Abstract
In this paper an analytical model is presented that addresses the compressive response of short-fiber reinforced concrete members (FRC) with hooked steel fibers. This model is applicable to a wide range of concrete strengths and accounts for the interaction between the cover spalling and the concrete core confinement induced by transverse steel stirrups and also for buckling of longitudinal reinforcing bars. The load-shortening curves generated here analytically fit existing experimental data well.

Key Words
compression; fibers; concrete; confinement; cover spalling; buckling; load-axial strain curves; bearing capacity.

Address
G. Campione: Dipartimento di Ingegneria Strutturale e Geotecnica, Universita di Palermo, Viale delle Scienze, Palermo 90128, Italy

Abstract
Deciding on an optimal sensor placement (OSP) is a common problem encountered in many engineering applications and is also a critical issue in the construction and implementation of an effective structural health monitoring (SHM) system. The present study focuses with techniques for selecting optimal sensor locations in a sensor network designed to monitor the health condition of Dalian World Trade Building which is the tallest in the northeast of China. Since the number of degree-of-freedom (DOF) of the building structure is too large, multi-modes should be selected to describe the dynamic behavior of a structural system with sufficient accuracy to allow its health state to be determined effectively. However, it\'s difficult to accurately distinguish the translational and rotational modes for the flexible structures with closely spaced modes by the modal participation mass ratios. In this paper, a new method of the OSP that computing the mode shape matrix in the weak axis of structure by the simplified multi-DOF system was presented based on the equivalent rigidity parameter identification method. The initial sensor assignment was obtained by the QR-factorization of the structural mode shape matrix. Taking the maximum off-diagonal element of the modal assurance criterion (MAC) matrix as a target function, one more sensor was added each time until the maximum off-diagonal element of the MAC reaches the threshold. Considering the economic factors, the final plan of sensor placement was determined. The numerical example demonstrated the feasibility and effectiveness of the proposed scheme.

Key Words
optimal sensor placement (OSP); finite element method (FEM); modal assurance criterion (MAC); series of multidegree-of freedom; high-rise building.

Address
Ting-Hua Yi: Faculty of Infrastructure Engineering, State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Ganjingzi District, Linggong Road 2, Dalian, 116024, P.R. China
Hong-Nan Li: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Siping Road 1239, Shanghai, 200092, P.R. China
Ming Gu: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Siping Road 1239, Shanghai, 200092, P.R. China

Abstract
Developing accurate prediction models for deterioration behavior represents a challenging but essential task in comprehensive Infrastructure Management Systems. The challenge may be a result of the lack of historical data, impact of unforeseen parameters, and/or the past repair/maintenance practices. These realities contribute heavily to the noticeable variability in deterioration behavior even among similar components. This paper introduces a novel approach to predict the deterioration of any infrastructure component. The approach is general as it fits any component, however the prediction is custom for a specific item to consider the inherent impacts of expected and unexpected parameters that affect its unique deterioration behavior.

Key Words
IMS; deterioration modeling; Markov chains; optimization; repair and maintenance.

Address
O.A. Hosny: The American University in Cairo, Cairo, Egyp
A.A. Elhakeem: AUC/KAUST, The American University in Cairo, Cairo, Egypt
T. Hegazy: University of Waterloo, Waterloo, Canada


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