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CONTENTS
Volume 19, Number 2, January30 2005
 

Abstract
An automatic modal identification program is developed for continuous extraction of modal parameters of three cable-supported bridges in Hong Kong which are instrumented with a long-term monitoring system. The program employs the Complex Modal Indication Function (CMIF) algorithm for identifying modal properties from continuous ambient vibration measurements in an on-line manner. By using the LabVIEW graphical programming language, the software realizes the algorithm in Virtual Instrument (VI) style. The applicability and implementation issues of the developed software are demonstrated by using one-year measurement data acquired from 67 channels of accelerometers permanently installed on the cable-stayed Ting Kau Bridge. With the continuously identified results, variability in modal vectors due to varying environmental conditions and measurement errors is observed. Such an observation is very helpful for selection of appropriate measured modal vectors for structural health monitoring use.

Key Words
cable-stayed bridge; monitoring system; automatic modal identification; complex modal indication function; modal vector; modal variability.

Address
Y. Q. Ni
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

K. Q. Fan
School of Information, Wuyi University, Jiangmen, Guangdong 529020, P. R. China

G. Zheng
Chongqing Communications Research and Design Institute, Chongqing 400067, P. R. China

J. M. Ko
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Abstract
There are several ways of decreasing the vibration energy of structures. One of which is special damping layers made of various viscoelastic materials are widely applied in structures subjected to dynamic loading. In this study, a cantilever beam, partially covered by damping a constraining layers, is investigated by using Finite Element method (FEM). The frequency and system loss factor are evaluated. The effects of different physical and geometrical parameters on the natural frequency and system loss factors are discussed.

Key Words
damping; sandwich beam; finite element.

Address
Department of Mechanical Engineering, Ataturk University, Erzurum, Turkey

Abstract
A new load/displacement parameter method is proposed for the simultaneous control of applied loads and structural displacements at one or more points. The procedure is based on a generalized Riks?method, which utilizes load/displacement parameters as scaling factors to analyze post-buckling phenomena including snap-through or snap-back. The convergence characteristics are improved by employing new relaxation factors through an incremental displacement parameter, particularly in a region that exhibits severe numerical instability. The improved performance is illustrated by means of a numerical example.

Key Words
post-buckling phenomena; snap-back phenomena; incremental load parameter; incremental displacement parameter; relaxation factor; relaxation method.

Address
Young-Doo Kwon
School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Korea

Hyun-Wook Kwon
Graduate School, Mechanical Engineering Department, Kyungpook National University, Korea

Beom-Soo Lim
Agency for Defense Development, Korea

Abstract
For reducing construction cost, two-plate-girder bridges are getting popular in Japan. This type of bridge employs a PC slab, which is often cast-in-place. In such a case, concrete is not usually cast over the whole slab at one time: some portions are constructed earlier than the rest. Therefore, a construction joint is inevitably created. Due to the drying shrinkage of concrete, tension stress may occur in concrete slab. High tensile stress can be expected near the construction joint where concretes with different ages meet. Moreover, prestressing is not applied over the whole length of slab at one time. This may also serve as a source of tensile stress in the slab. Thus there is a chance that cast-in-place PC slab, especially near the construction joint, may be subjected to tensile cracking. In the present study, stress states near the construction joint in the cast-in-place PC slab of a two-plate-girder bridge are investigated numerically. The finite element method is employed and the three-dimensional analysis is conducted to see the influence of dry shrinkage and prestressing. The stress states in the PC slab thus obtained are discussed. The simplified model of a plate girder for this class of analysis is also proposed.

Key Words
two-plate-girder bridge; cast-in-place PC slab; stress state; construction joint; finite element analysis.

Address
Eiki Yamaguchi
Department of Civil Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu 804-8550, Japan

Fumio Fukushi
Japan Steel Tower Co. Ltd., Kitahama, Wakamatsu, Kitakyushu 808-0023, Japan

Naoki Hirayama
Department of Civil Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu 804-8550, Japan

Takemi Kubo
Japan Steel Tower Co. Ltd., Kitahama, Wakamatsu, Kitakyushu 808-0023, Japan

Yoshinobu Kubo
Department of Civil Engineering, Kyushu Institute of Technology, Tobata, Kitakyushu 804-8550, Japan

Abstract
In the design of reinforced concrete beams, it is a standard practice to use the yield stress of the steel reinforcement for the evaluation of the flexural strength. However, because of strain hardening, the tensile strength of the steel reinforcement is often substantially higher than the yield stress. Thus, it is a common belief that the actual flexural strength should be higher than the theoretical flexural strength evaluated with strain hardening ignored. The possible increase in flexural strength due to strain hardening is a two-edge sword. In some cases, it may be treated as strength reserve contributing to extra safety. In other cases, it could lead to greater shear demand causing brittle shear failure of the beam or unexpected greater capacity of the beam causing violation of the strong column-weak beam design philosophy. Strain hardening may also have certain effect on the flexural ductility. In this paper, the effects of strain hardening on the post-peak flexural behaviour, particularly the flexural strength and ductility, of reinforced normal- and high-strength concrete beams are studied. The results reveal that the effects of strain hardening could be quite significant when the tension steel ratio is relatively small.

Key Words
flexural ductility; high-strength concrete; reinforced concrete beams.

Address
J. C. M. Ho, F. T. K. Au and A. K. H. Kwan
Department of Civil Engineering, The University of Hong Kong, Hong Kong

Abstract
An assumed stress quadrilateral thin/moderately thick plate element HQP4 based on the Mindlin/Reissner plate theory is proposed. The formulation is based on Hellinger-Reissner variational principle. Static and free vibration analyses of plates are carried out. Numerical examples are presented to show that the validity and efficiency of the present element for static and free vibration analysis of plates. Satisfactory accuracy for thin and moderately thick plates is obtained and it is free from shear locking for thin plate analysis.

Key Words
Reissner-Mindlin plate; hybrid finite element; static analysis; free vibration.

Address
Department of Civil Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

Abstract
A steel silo traditionally consists of a cylindrical and a conical shell. In order to facilitate emptying operations, the cylinder is placed on local supports. This may lead to dangerous stress concentrations and eventually to local instability of the cylindrical wall. In this contribution, the locally supported cylinder is strengthened by means of ring stiffeners and longitudinal stiffeners and the effect of their dimensions on the buckling stress is investigated. This study leads to a number of diagrams, each of them representing the effect of one of the dimensions on the buckling stress. In each diagram, the failure pattern corresponding to the buckling stress is indicated.

Key Words
buckling; cylinders; local supports; stiffeners.

Address
Wesley Vanlaere, Rudy Van Impe, Guy Lagae and Thomas Maes
Laboratory for Research on Structural Models, Ghent University, Technologiepark-Zwijnaarde 904, B-9052 Zwijnaarde, Belgium

Abstract
This study considers the buckling of orthotropic cylindrical thin shells with material non-homogeneity in the thickness direction, under torsion, which is a power function of time. The dynamic stability and compatibility equations are obtained first. Applying Galerkin\'s method then applying Ritz type variational method to these equations and taking the large values of loading parameters into consideration, analytic solutions are obtained for critical parameter values. Using those results, the effects of the periodic and power variations of Young\'s moduli and density, ratio of Young\'s moduli variations, loading parameters variations and the power of time in the torsional load expression variations are studied via pertinent computations. It is concluded that all these factors contribute to appreciable effects on the critical parameters of the problem in question.

Key Words
torsion; buckling; non-homogeneous orthotropic material; cylindrical shell; critical torsional loads; dynamic factor.

Address
Department of Civil Engineering, Suleyman Demirel University, Isparta, Turkey


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