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CONTENTS
Volume 29, Number 3, June20 2008
 

Abstract
This paper presents a new identification approach to prestress force. Firstly, a bridge deck is modeled as a prestressed Timoshenko beam. The time domain responses of the beam under sinusoidal excitation are studied based on modal superposition. The prestress force is then identified in the time domain by a system identification approach incorporating with the regularization of the solution. The orthogonal polynomial function is used to improve the noise effect and obtain the derivatives of modal responses of the bridge. Good identification results are obtained from only the first few measured modal
data under both sinusoidal and impulsive excitations. It is shown that the proposed method is insensitive to the magnitude of force to be identified and can be successfully applied to indirectly identify the
prestress force as well as other physical parameters, such as the flexural rigidity and shearing rigidity of a beam even under noisy environment.

Key Words
identification; prestress force; time domain; vibration.

Address
Z. R. Lu and J. K. Liu: School of Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China
S. S. Law: Civil and Structural Engineering Department, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, P. R. China

Abstract
This paper presents a nonlinear finite element procedure accounting for the effects of geometric as well as material nonlinearities for reinforced concrete bridge piers supported by laminated
rubber bearings. Reinforced concrete bridge piers supported by laminated rubber bearings and carrying a cyclic load were analyzed by using a special purpose, nonlinear finite element program, RCAHEST. For
reinforced concrete, the proposed robust nonlinear material model captures the salient response characteristics of the bridge piers under cyclic loading conditions and addresses with the influence of
geometric nonlinearity on post-peak response of the bridge piers by transformations between local and global systems. Seismic isolator element to predict the behaviors of laminated rubber bearings is also
developed. The seismic performance of reinforced concrete bridge piers supported by laminated rubber bearings is assessed analytically. The results show good correlation between the experimental findings and
numerical predictions, and demonstrate the reliability and robustness of the proposed analytical model. Additionally, the studies and discussions presented in this investigation provide an insight into the key behavioral aspects of reinforced concrete bridge piers supported by laminated rubber bearings.

Key Words
reinforced concrete bridge piers; laminated rubber bearings; nonlinear material model; geometric nonlinearity; seismic isolator element; seismic performance.

Address
T. H. Kim and Y. J. Kim: Civil Engineering Research Team, Daewoo Institute of Construction Technology, 60 Songjuk-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-210, Korea
H. M. Shin: Dept. of Civil and Environmental Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-746, Korea

Abstract
In the present study, the discrete singular convolution (DSC) method is developed for buckling analysis of columns and thin plates having different geometries. Regularized Shannon?s delta (RSD) kernel is selected as singular convolution to illustrate the present algorithm. In the proposed approach, the derivatives in both the governing equations and the boundary conditions are discretized by the method of DSC. The results obtained by DSC method were compared with those obtained by the other numerical and analytical methods.

Key Words
buckling; plates; discrete singular convolution; columns; circular plate; skew plate.

Address
Omer Civalek: Akdeniz University, Faculty of Engineering, Civil Engineering Department, Division of Mechanics, Antalya, Turkiye
Altug Yavas: Bal kesir University, Faculty of Engineering, Civil Engineering Department, Division of Structures, Bal kesir, Turkiye

Abstract
The use of metric trial functions to represent the real stress field in what is called the unsymmetric finite element formulation is an effective way to improve predictions from distorted finite elements. This approach works surprisingly well because the use of parametric functions for the test functions satisfies the continuity conditions while the use of metric (Cartesian) shape functions for the trial functions attempts to ensure that the stress representation during finite element computation can retrieve in a best-fit manner, the actual variation of stress in the metric space. However, the issue of how to handle situations where there is locking along with mesh distortion has never been addressed. In this paper, we show that the use of a consistent definition of the constrained strain field in the metric space can ensure a lock-free solution even when there is mesh distortion. The three-noded Timoshenko beam element is used to illustrate the principles. Some significant conclusions are drawn regarding the optimal strategy for finite element modelling where distortion effects and field-consistency requirements have to be reconciled simultaneously.

Key Words
mesh distortion; locking; unsymmetric parametric-metric formulation; metric trial function; Timoshenko theory; three-node beam element.

Address
Surendra Kumar: CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore 560 037, India
G. Prathap: CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore 560 037, India

Abstract
Applications of fracture mechanics in the strength analysis of ceramic materials have been lately studied by many researchers. Various test specimens have been proposed in order to investigate the
fracture resistance of cracked bodies under mixed mode conditions. Double Cleavage Drilled Compression (DCDC) specimen, with a hole offset from the centerline is a configuration that is frequently used in subcritical crack growth studies of ceramics and glasses. This specimen exhibits a strong crack path stability that is due to the strongly negative T-stress term. In this paper the maximum tensile stress (MTS) criterion is employed for investigating theoretically the initiation of brittle fracture in the DCDC specimen under mixed mode conditions. It is shown that the T-stress has a significant influence on the predicted fracture load and the crack initiation angle. The theoretical results suggest that brittle fracture in the DCDC specimen is controlled by a combination of the singular stresses (characterized by KI and KII) and the non-singular stress term, T-stress.

Key Words
brittle fracture; mixed mode loading; ceramic materials; fracture analysis.

Address
M.R. Ayatollahi and S. Bagherifard: Fatigue and Fracture Research Laboratory, Department of Mechanical Engineering, Iran University of Science and Technology, Narmak, 16846, Tehran, Iran

Abstract
Noting that damage occurrence of offshore jacket platforms is concentrated in two structural regions that are in the vicinity of still water surface and close to the seabed, a damage detection method
by using only partial measurement of vibration in a suspect region was presented in this paper, which can not only locate damaged members but also evaluate damage severities. Then employing an experiment
platform model under white-noise ground excitation by shaking table and using modal parameters of the first three modes identified by a scalar-type ARMA method on undamaged and damaged structures, the feasibility of the damage detection method was discussed. Modal parameters from eigenvalue analysis on the structural FEM model were also used to help the discussions. It is demonstrated that the damage detection algorithm is feasible on damage location and severity evaluation for broken slanted braces and it is robust against the errors of baseline FEM model to real structure when the principal errors is formed
by difference of modal frequencies. It is also found that Z-value changes of modal shapes also play a role in the precise detection of damage.

Key Words
damage detection; offshore jacket platform; model experiment; partial measurement; shaking table.

Address
Shi Xiang, Li Hua-jun, Yang Yong-chun and Gong Chen:
Engineering College, Ocean University of China, Qingdao, 266100, China

Abstract
Some unreasonable results from the use of a popular thick plate element are discovered from the analysis of a raft foundation and a pile cap in Hong Kong. To overcome the problems, the authors have developed a new shear deformable beam which can be extended to a general quadrilateral shear deformable plate. The behaviour of this new element under several interesting cases is investigated, and it is demonstrated that the new element possesses very high accuracy under different depth/span ratios, and the results from this new element are good even for a coarse mesh.

Key Words
thick plate; shear deformation; finite element.

Address
Cheng Y.M.: Dept. of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Kong
Law C.W.: Housing Department, Hong Kong SAR Government, Hong Kong

Abstract
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Key Words
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Address
Hakan Gokda: Dept. of Mechanical Engineering, College of Engineering and Architecture, Uluda University, Gorukle, Bursa 16059, Turkey

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