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CONTENTS
Volume 15, Number 5, November 2013
 

Abstract
In this paper, a new first-order shear deformation beam theory based on neutral surface position is developed for bending and free vibration analysis of functionally graded beams. The proposed theory is based on assumption that the in-plane and transverse displacements consist of bending and shear components, in which the bending components do not contribute toward shear forces and, likewise, the shear components do not contribute toward bending moments. The neutral surface position for a functionally graded beam which its material properties vary in the thickness direction is determined. Based on the present new first-order shear deformation beam theory and the neutral surface concept together with Hamilton\'s principle, the motion equations are derived. To examine accuracy of the present formulation, several comparison studies are investigated. Furthermore, the effects of different parameters of the beam on the bending and free vibration responses of functionally graded beam are discussed.

Key Words
functionally graded beam; first shear deformation theory; neutral surface position

Address
(1) Mohammed Bouremana, Mohammed Sid Ahmed Houari, Abdelouahed Tounsi, Abdelhakim Kaci and El Abbas Adda Bedia: Laboratoire des Materiaux et Hydrologie, Universite de Sidi Bel Abbes, BP 89 Cite Ben M\'hidi, 22000 Sidi Bel Abbes, Algerie;
(2) Mohammed Sid Ahmed Houari: Departement de genie civil, Universite de Mascaraa, Algerie;
(3) Abdelouahed Tounsi: Universite de Sidi Bel Abbes, Faculte de Technologie, Departement de Genie Civil, BP 89 Cite Ben M\'hidi, 22000 Sidi Bel Abbes, Algerie.

Abstract
This paper focuses on thermal post-buckling analysis of functionally graded beams with temperature dependent physical properties by using the total Lagrangian Timoshenko beam element approximation. Material properties of the beam change in the thickness direction according to a power-law function. The beam is clamped at both ends. In the case of beams with immovable ends, temperature rise causes compressible forces and therefore buckling and post-buckling phenomena occurs. It is known that post-buckling problems are geometrically nonlinear problems. Also, the material properties (Young\'s modulus, coefficient of thermal expansion, yield stress) are temperature dependent: That is the coefficients of the governing equations are not constant in this study. This situation suggests the physical nonlinearity of the problem. Hence, the considered problem is both geometrically and physically nonlinear. The considered highly non-linear problem is solved considering full geometric non-linearity by using incremental displacement-based finite element method in conjunction with Newton-Raphson iteration method. In this study, the differences between temperature dependent and independent physical properties are investigated for functionally graded beams in detail in post-buckling case. With the effects of material gradient property and thermal load, the relationships between deflections, critical buckling temperature and maximum stresses of the beams are illustrated in detail in post-buckling case.

Key Words
functionally graded material; temperature dependent physical properties; thermal postbuckling analysis; total lagrangian finite element model

Address
Turgut KOCATURK and Şeref Doğuşan AKBAŞ: Y

Abstract
The seismic performance of six types of weak-axis steel moment connections was investigated through cyclic testing of six full-scale specimens. These weak-axis moment connections were the column-tree type, WUF-B type, FF-W type, WFP type, BFP-B type and DST type weak-axis connections. The testing results showed that each of these weak-axis connection types achieved excellent seismic performance, except the WFP and the WUF-B types. The WFP and WUF-B connections displayed poor seismic performance because a fracture appeared prematurely at the weld joint due to stress concentrations. The column-tree type connection showed the best seismic behavior such that the story drift ratio could reach 5%.

Key Words
moment resisting frame; weak-axis steel moment connection; cyclic testing; seismic performance; ductility

Address
(1) Kangmin Lee, Rui Li: Department of Architectural Engineering Chungnam National University, Daejeon, Korea;
(2) Heetaek Jung: 3D Engineering Co., Ltd., Seoul, Korea;
(3) Kyunghwan Oh: Engineering & Construction Group, Samsung C&T Corporation, Seoul, Korea.

Abstract
The design optimization of a cold-formed steel portal frame building is considered in this paper. The proposed genetic algorithm (GA) optimizer considers both topology (i.e., frame spacing and pitch) and cross-sectional sizes of the main structural members as the decision variables. Previous GAs in the literature were characterized by poor convergence, including slow progress, that usually results in excessive computation times and/or frequent failure to achieve an optimal or near-optimal solution. This is the main issue addressed in this paper. In an effort to improve the performance of the conventional GA, a niching strategy is presented that is shown to be an effective means of enhancing the dissimilarity of the solutions in each generation of the GA. Thus, population diversity is maintained and premature convergence is reduced significantly. Through benchmark examples, it is shown that the efficient GA proposed generates optimal solutions more consistently. A parametric study was carried out, and the results included. They show significant variation in the optimal topology in terms of pitch and frame spacing for a range of typical column heights. They also show that the optimized design achieved large savings based on the cost of the main structural elements; the inclusion of knee braces at the eaves yield further savings in cost, that are significant.

Key Words
optimization; cold-formed steel; portal frames; niching; real-coded genetic algorithm

Address
(1) D.T. Phan: Department of Civil Engineering, Universiti Tunku Abdul Rahman, Kuala Lumpur, 53300, Malaysia;
(2) J.B.P. Lim and W. Sha: School of Planning, Architecture and Civil Engineering, Queen\'s University Belfast, David Keir Building, Belfast, BT9 5AG, UK;
(3) T.T. Tanyimboh: Department of Civil and Environmental Engineering, University of Strathclyde, John Anderson Building, Glasgow, G4 0NG, UK.

Abstract
This paper summarises the results of a numerical study on the non linear response of steel concentric braced frames under monotonic and cyclic loads, using force-based finite elements with section fibre discretisation. The first part of the study is addressed to analyse the single brace response. A parametric analysis was carried out and discussed to evaluate the accuracy of the model, examining the influence of the initial camber, the material modelling, the type of force-based element, the number of integration points and the number of fibers. The second part of the paper is concerned with the modelling issues of whole braced structures. The effectiveness of the modelling approach is verified against the nonlinear static and dynamic behaviour of different type of bracing configurations. The model sensitivity to brace-to-brace interaction and the capability of the model to mimic the response of complex bracing systems is analyzed. The influence of different approaches for modelling the inertia, the equivalent viscous damping and the brace hysteretic response on the overall structural response are also investigated. Finally, on the basis of the performed numerical study general modelling recommendations are proposed.

Key Words
steel concentric bracing; buckling; cyclic response; numerical modelling; seismic analysis

Address
M. D\'Aniello, G. La Manna Ambrosino, F. Portioli and R. Landolfo: Department of Structures for Engineering and Architecture, University of Naples \"Federico II\", Via Forno Vecchio 36, 80134 Naples, Italy.

Abstract
Experimental studies and finite element analyses have been carried out to establish the effect of tendon damage on the structural behavior of concrete filled tubular tied arch girder (CFTA girder). The damage of tendon is considered in different stages by varying the number of damaged cables in the tendon. Static and dynamic structural parameters are observed at each stage. The results obtained from the experiments and numerical studies have been compared to validate the studies. The tendons whose damage can significantly affect the stiffness of the CFTA girder are identified by performing the sensitivity analysis. The locations in the girder which are sensitive to the tendon damage are also identified.

Key Words
bridge design; composite structure; concrete filled tubular tied arch; CFTA girder; dynamic behavior; static behaviour

Address
(1) Thuy Dung Vu and Dookie Kim: Department of Civil and Environmental Engineering, Kunsan National University, Jeonbuk, Republic of Korea;
(2) Sang Yoon Lee: Structural Engineering & Bridge Research Division, Korea Institute of Construction Technology, Gyeonggi-Do, Republic of Korea;
(3) Sandeep Chaudhary: Department of Civil Engineering, Malaviya National Institute of Technology Jaipur, India.


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