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CONTENTS
Volume 46, Number 3, May10 2013
 

Abstract
This work aims to study the effect of stiffener ties in the behavior of intermediate length open section Cold-Formed Steel (CFS) Columns under axial compression. A comparative study on the behaviour and strength of Cold Formed Steel Columns by changing the direction of projection of lips (i.e., inwards or outwards) are also done. In this work two types of sections were considered Type-I section with lip projecting outwards (hat) and Type-II section with lip projecting inwards (channel). The length of the columns is predicted by performing elastic buckling analysis using CUFSM software. The theoretical analysis is performed using DSM - S100;2007, AS/NZ: 4600-2005 and IS: 801-1975. The compression tests are carried out in a 400 kN loading frame with hinged-hinged end condition. The non-linear numerical analysis is performed using Finite Element software ANSYS 12.0 to simulate the experimental results. Extensive parametric study is carried out by varying the width and spacing of the stiffener ties. The results are compared; the effects of stiffener ties on behaviour and load carrying capacity on both types of columns are discussed.

Key Words
cold-formed steel; columns; pallet racks; axial compression member; thin walled members; distortional buckling; stiffener ties

Address
M. Anbarasu, S. Bharath Kumar and S. Sukumar: Department of Civil Engineering, Government College of Engineering, Salem - 636 011, Tamilnadu, India

Abstract
Seismic performance of critical facilities has been focused on the structural components over the past decade. However, most earthquake damages were observed to the nonstructural components during and after the earthquakes. The primary objective of this research was to develop the seismic fragility of the piping system incorporating the nonlinear Tee-joint finite element model in the full scale piping configuration installed in critical facilities. The procedure for evaluating fragility curves corresponding to the first damage state was considered the effects of the top floor acceleration sensitivities for 5, 10, 15, and 20 story linear RC and steel building systems subjected to 22 selected ground motions as a function of ground motion uncertainties. The result of this study revealed that the conditional probability of failure of the piping system on the top floor in critical facilities did not increase with increased level of story height and in fact, story level in buildings can tune the fragilities between the building and the piping system.

Key Words
fragility; seismic performance; Tee-joint; probability; uncertainty

Address
Bu Seog Ju, Yong Hee Ryu : Depatment of Civil Engineering, North Carolina State University, Raleigh, USA
Woo Young Jung : Department of Civil Engineering, GangNeung-WonJu National University, GangNeung, Korea

Abstract
This paper presents a new method to study the impact factor of an old bridge based on the model updating technique. Using the genetic algorithm (GA) by minimizing an objective function of the residuals between the measured and predicted responses, the bridge and vehicle coupled vibration models were updated. Based on the displacement relationship and the interaction force relationship at the contact patches, the vehicle-bridge coupled system can be established by combining the equations of motion of both the bridge and vehicles. The simulated results show that the present method can simulate precisely the response of the tested bridge; compared with the other bridge codes, the impact factor specified by the bridge code of AASHTO (LRFD) is the most conservative one, and the value of Chinese highway bridge design code (CHBDC) is the lowest; for the large majority of old bridges whose road surface conditions have deteriorated, calculating the impact factor with the bridge codes cannot ensure the reliable results.

Key Words
bridge; moving vehicle; model updating; impact factor; vibration

Address
Yang Liu, Xinfeng Yin, Jianren Zhang and C.S. Cai : School of Civil Engineering and Architecture, Changsha University of Science & Technology, Changsha 410004, Hunan, China
C.S. Cai : Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Abstract
Vibration isolators and anti-vibration mounts are ideal, for example, in creating floating floors for gymnasiums, or performance spaces. However, it is well-known that there are great difficulties on isolating vibration transmission in structural steel components, especially steel floors. Besides, the selection of inertia blocks, which are usually used by engineers as an effective vibration control measure, is usually based on crude methods or the experience of the engineers. Thus, no simple method or indices have been available for assessing the effect of inertia blocks on vibration isolation or stability of vibratory systems. Thus, the aims of this research are to provide further background description using a FE model and present and implement a modal approach, that was validated experimentally, the latter assisting in providing improved understanding of the vibration transmission phenomenon in steel buildings excited by a velocitysource type of excitation. A better visualization of the mean-square velocity distribution in the frequency domain is presented using the concept of modal expansion. Finally, the variation of the mean-square velocity with frequency, whilst varying mass and/or stiffness of the coupled system, is presented.

Key Words
steel floor; FE simulations; dynamic response; velocity excitation

Address
Max D.C. Magalhaes : Department of Structural Engineering, Federal University of Minas Gerais, Belo Horizonte 31270-901, Brazil

Abstract
A dynamic load protocol has been used to experimentally simulate fatigue behavior in coldformed metal panels with screwed connections under wind loading. The specific protocol adopted is an adaptation of SIDGERS, originally developed for non-metallic membranes, which is composed of levels each under increasing load values. A total of 19 tests were performed on 3.35 m long by 0.91 m wide panels, identified as Type B-wide rib and Type E, both with screw connections at the edge and at the center, thus conforming two-span specimens. In some configurations the panels were fixed at the valleys, whereas crestfixed connections were also investigated. Reinforcing the connections by means of washers was also investigated to evaluate their efficiency in improving fatigue capacity. The experimental results show maximum load capacities in improved connections with washers of approximately twice of those with classical connections.

Key Words
old-formed plates; connections; dynamic load protocol; fatigue failure; roof systems; SIDGERS; steel; testing

Address
Antonio J. Garcia-Palencia : Department of Civil Engineering, University of New Hampshire, Durham, NH 03824, USA
Luis A. Godoy : Structures Department, FCEFyN, National University of Cordoba, and CONICET, P.O. Box 916, Cordoba 5000, Argentina

Abstract
This paper describes new optimization strategy that offers significant improvements in performance over existing methods for geometry design of frame structures. In this study, an imperialist competitive algorithm (ICA) and ant colony optimization (ACO) are combined to reach to an efficient algorithm, called Imperialist Competitive Ant Colony Optimization (ICACO). The ICACO applies the ICA for global optimization and the ACO for local search. The results of optimal geometry for three benchmark examples of frame structures, demonstrate the effectiveness and robustness of the new method presented in this work. The results indicate that the new technique has a powerful search strategies due to the modifications made in search module of ICACO. Higher rate of convergence is the superiority of the presented algorithm in comparison with the conventional mathematical methods and non hybrid heuristic methods such as ICA and particle swarm optimization (PSO).

Key Words
geometry optimization; hybrid optimization method; frame structure; imperialist competitive algorithm; ant colony optimization

Address
Mojtaba Sheikhi and Ali Ghoddosian : Faculty of Mechanical Engineering, Semnan University, Semnan 35195-363, Iran

Abstract
This paper presents responses of the free end of a cantilever micro beam under the effect of an impact force based on the modified couple stress theory. The beam is excited by a transverse triangular force impulse modulated by a harmonic motion. The Kelvin–Voigt model for the material of the beam is used. The considered problem is investigated within the Bernoulli-Euler beam theory by using energy based finite element method. The system of equations of motion is derived by using Lagrange\'s equations. The obtained system of linear differential equations is reduced to a linear algebraic equation system and solved in the time domain by using Newmark average acceleration method. In the study, the difference of the modified couple stress theory and the classical beam theory is investigated for the wave propagation. A few of the obtained results are compared with the previously published results. The influences of the material length scale parameter on the wave propagation are investigated in detail. It is clearly seen from the results that the classical beam theory based on the modified couple stress theory must be used instead of the classical theory for small values of beam height.

Key Words
wave propagation; modified couple stress theory; microbeam

Address
Turgut Kocaturk and Şeref Doğuşcan Akbaş : Department of Civil Engineering, Yildiz Technical University, Davutpasa Campus, 34210 Esenler-Istanbul, Turkey

Abstract
Vibration-based structural identification has become an important tool for structural health monitoring and safety evaluation. However, various kinds of uncertainties (e.g., observation noise) involved in the field test data obstruct automation system identification for accurate and fast structural safety evaluation. A practical way including a data preprocessing procedure and a vector backward auto-regressive (VBAR) method has been investigated for practical bridge identification. The data preprocessing procedure serves to improve the data quality, which consists of multi-level uncertainty mitigation techniques. The VBAR method provides a determinative way to automatically distinguish structural modes from extraneous modes arising from uncertainty. Ambient test data of a cantilever beam is investigated to demonstrate how the proposed method automatically interprets vibration data for structural modal estimation. Especially, structural identification of a truss bridge using field test data is also performed to study the effectiveness of the proposed method for real bridge identification.

Key Words
structural identification; ambient vibration; automate; uncertainty; signal processing

Address
J. Zhang : Key Laboratory of C&RC Structures of the Ministry of Education, Southeast University, Nanjing 210096, China
F.L. Moon : Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA
T. Sato : International Institute for Urban Systems Engineering, Southeast University, Nanjing 210096, China


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