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CONTENTS
Volume 34, Number 5, March30 2010
 

Abstract
In this paper, free vibration analyses of a parallel placed twin pipe system simulated by simply supported-simply supported and fixed-fixed Euler-Bernoulli beams resting on Winkler elastic soil are presented. The motion of the system is described by a homogenous set of two partial differential equations, which is solved by a simulation method called the Differential Transform Method (DTM). Free vibrations of an elastically connected twin pipe system are realized by synchronous and asynchronous deflections. The results of the presented theoretical analyses for simply supported Euler-Bernoulli beams are compared with existing ones in open literature and very good agreement is demonstrated.

Key Words
Differential Transform Method; DTM; elastic soil; vibration; twin beam; twin pipeline.

Address
Muge Balkaya: Department of Civil Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
Ahmet Sa lamer: Department of Civil Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
Metin O. Kaya: Faculty of Aeronautics and Astronautics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

Abstract
Composite beams using bolts to attach steel plates to the side faces of existing reinforced concrete (RC) coupling beams can enhance both their strength and deformability. The behavior of those composite beams differs substantially from the behavior of typical composite beams made up of steel beams and concrete slabs. The former are subjected to longitudinal, vertical and rotational slips, while the latter only involve longitudinal slip. In this study, a mixed analysis method was adopted to develop the fundamental equations for accurate prediction of the load-carrying capacity of steel plate strengthened RC coupling beams. Then, a rigid plastic analysis technique was used to cope with the full composite effect of the bolt group connections. Two theoretical models for the determination of the strength of mediumlength plate strengthened coupling beams based on mixed analysis and rigid plastic methods are presented. The strength of the strengthened coupling beams is derived. The vertical and longitudinal slips of the steel plates and the shear strength of the anchor-bolt connection group is considered. The theoretical models are validated by the available experimental results presented in a companion paper. The strength of the specimens predicted from the mixed analysis model is found to be in good agreement with that from the experimental results.

Key Words
coupling beam; strengthening; steel plate; bolt connection; slip strain; partial interaction; rigid plastic; mixed analysis.

Address
Y. Zhu: School of Civil Engineering, The University of Guang Zhou, China
R.K.L. Su: Department of Civil Engineering, The University of Hong Kong, Hong Kong, China

Abstract
This paper presents topology optimization of geometrically and materially nonlinear structures using a bi-directional evolutionary optimization (BESO) method. To maximum the stiffness of nonlinear structures under prescribed design load, the complementary work is selected as the objective function of the optimization. An optimal design can be obtained by gradually removing inefficient material and adding efficient ones. The proposed method can be applied to a series of geometrically and/or materially nonlinear structures. The results show considerable differences in topologies and stiffness of the optimal designs for linear and nonlinear structures. It is found that the optimal designs for nonlinear structures are much stiffer than those for linear structures when large design loads (which result in significantly nonlinear deformations) are applied.

Key Words
bi-directional evolutionary structural optimization (BESO); topology optimization; complementary work.

Address
X. Huang: School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476V, Melbourne 3001, Australia
Y.M. Xie: School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476V, Melbourne 3001, Australia

Abstract
A problem formulation and solution methodology for design optimization of laminated thinwalled composite beams of generic section is presented. Objective functions and constraint equations are given in the form of beam stiffness. For two different problems one for open section and the other for closed section, the objective function considered is bending stiffness about x-axis. Depending upon the case, one can consider bending, torsional and axial stiffnesses. The different search and optimization algorithm, known as Evolution Strategies (ES) has been applied to find the optimal fibre orientation of composite laminates. A multi-level optimization approach is also implemented by narrowing down the size of search space for individual design variables in each successive level of optimization process. The numerical results presented demonstrate the computational advantage of the proposed method \"Evolution strategies\" which become pronounced to solve optimization of thin-walled composite beams of generic section.

Key Words
evolution strategies; laminate sequence; optimization; composite; penalty approach.

Address
S. Rajasekaran: Department of Infrastructure Engineering, PSG College of Technology, Coimbatore, 641004 Tamilnadu, India

Abstract
Excessive stay cable vibrations can cause severe problems for cable-stayed bridges. In this paper a semiactive Magnetorheological (MR) damper is investigated to reduce cable vibrations. The control-oriented cable-damper model is first established; a computer simulation for the cable-damper system is carried out; and finally a MR damper is experimentally used to reduce the cable vibration in a laboratory environment using a semiactive control algorithm. Both the simulation and experimental results show that the semiactive MR damper achieves better control results than the corresponding passive damper.

Key Words
Magnetorheological (MR) damper; cable vibration control; semiactive control.

Address
W.J. Wu: Mustang Engineering, Houston, TX 77084, USA
C.S. Cai: Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, USA

Abstract
The objective of this paper is to apply a new proposed accuracy indicator to quantify the true and false modes for Eigensystem Realization Algorithm using output-based responses. First, a discrete mass-spring system and a simply supported continuous beam were modelled using finite element method. Then responses are simulated under random excitation. Natural Excitation Technique using only response measurements is applied to compute the impulse responses. Eigensystem Realization Algorithm is employed to identify the modal parameters on the simulated responses. A new accuracy indicator, Normalized Occurrence Number-NON, is developed to quantitatively partition the realized modes into true and false modes so that the false portions can be disregarded. Numerical simulation demonstrates that the new accuracy indicator can determine the true system modes accurately.

Key Words
modal identification; eigensystem realization algorithm; accuracy indicator; normalized occurrence number.

Address
Shuqing Wang: College of Engineering, Ocean University of China, Qingdao 266100, China
Fushun Liu: College of Engineering, Ocean University of China, Qingdao 266100, China

Abstract
In this study, the effect of superstructure-abutment continuity on the distribution of live load effects among the girders of integral abutment bridges (IABs) is investigated. For this purpose, two and three dimensional finite element models of several single-span, symmetrical integral abutment and simply supported (jointed) bridges (SSBs) are built and analyzed. In the analyses, the effect of various superstructure properties such as span length, number of design lanes, girder size and spacing as well as slab thickness are considered. The results from the analyses of two and three dimensional finite element models are then used to calculate the live load distribution factors (LLDFs) for the girders of IABs and SSBs as a function of the above mentioned parameters. LLDFs for the girders are also calculated using the AASHTO formulae developed for SSBs. Comparison of the analyses results revealed that the superstructure-abutment continuity in IABs produces a better distribution of live load effects among the girders compared to SSBs. The continuity effects become more predominant for short span IABs. Furthermore, AASHTO live load distribution formulae developed for SSBs lead to conservative estimates of live load girder moments and shears for short-span IABs.

Key Words
bridge; integral abutment; continuity; girder; live load distribution.

Address
Murat Dicleli: Department of Engineering Sciences, Middle East Technical University, 06531 Ankara, Turkey
Semih Erhan: Department of Engineering Sciences, Middle East Technical University, 06531 Ankara, Turkey

Abstract
The spectral content of ground displacement of the 10 largest last California earthquakes is studied. Specifically, the power spectral density function of the critical horizontal-component record of the closest-to-epicenter station is analyzed in each case. The results permit to state that horizontal ground displacement is a narrow-band process. This fact was previously noticed while trying to solve the largebase-displacement problem in isolated structures and it was fundamental in the solution of this issue; however, these preliminary results were limited in number to enable a statement like the foregoing one. Thus, the broader results presented herein were necessary.

Key Words
ground motion; displacement; spectral characterization; California earthquakes; base isolation.

Address
C.A. Morales: Departamento de Mecanica, Universidad Simon Bolivar, Apdo. 89000, Caracas 1080A, Venezuela


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