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CONTENTS
Volume 63, Number 2, July25 2017
 

Abstract
The size-dependent behavior of single walled carbon nanotubes (SWCNT) embedded in the elastic medium and subjected to the initial axial force is investigated using the mixed finite element method. The SWCNT is assumed to be Euler- Bernoulli beam incorporating nonlocal theory developed by Eringen. The mixed finite element model shows its great advantage of dealing with nonlocal behavior of SWCNT subjected to a concentrated load owing to the existence of two coefficients alpha1 and alpha2. This is the first numerical approach to deal with a puzzling fact of nonlocal theory with concentrated load. Numerical examples are performed to show the accuracy and efficiency of the present method. In addition, parametric study is carefully carried out to point out the influences of nonlocal effect, the elastic medium, and the initial axial force on the behavior of the carbon nanotubes.

Key Words
nonlocal continuum theory; mixed finite element method; elastic medium; carbon nanotubes

Address
Tuan Ngoc Nguyen, Nam-Il Kim and Jaehong Lee : Department of Architectural Engineering, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul 05006, South Korea

Abstract
In this study, the optimum parameters of Tuned Mass Dampers (TMDs) are proposed using Gravity Search Algorithm (GSA) and Particle Swarm Optimization (PSO) to reduce the responses of the structures. A MATLAB program is developed to apply the new approach to the benchmark 10 and 40-story structures. The obtained results are compared to those of other optimization methods used in the literature to verify the developed code. To show the efficiency and accuracy of the proposed methods, nine far-field and near-field worldwide earthquakes are applied to the structures. The results reveal that in the 40-story structure, GSA algorithm can reduce the Relative Displacement (RD) and Absolute Acceleration (AA) up to 43% and 21%, respectively while the PSO decreases them by 50% and 25%, respectively. In contrast, both GSA and PSO algorithms reduce the RD and AA about 29% and 21% for the 10-story structure. Furthermore, using the proposed approach the required TMD parameters reduce by 47% and 63% in the 40 and 10-story buildings in comparison with the referenced ones. Result evaluation and related comparison indicate that these methods are more effective even by using smaller TMD parameters resulting in the reduction of acting force from TMD, having smaller stiffness and damping factors while being more cost effective due to its decreased parameters. In other words, the TMD with optimum parameters can play a positive role in both tall and typical structures.

Key Words
GSA algorithm; PSO algorithm; TMD; optimization; vibration reduction

Address
Nadia M. Mirzai : School of Civil Engineering, College of Engineering, the University of Tehran, Tehran, Iran
Seyed Mehdi Zahrai : Center of Excellence for Engineering and Management of Civil Infrastructures, School of Civil Engineering, College of Engineering, the University of Tehran, Tehran, Iran
Fatemeh Bozorgi : School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran

Abstract
In this paper, using consistent couple stress theory and Hamilton\'s principle, the free vibration analysis of Euler- Bernoulli nano-beams made of bi-directional functionally graded materials (BDFGMs) with small scale effects are investigated. To the best of the researchers\' knowledge, in the literature, there is no study carried out into consistent couple-stress theory for free vibration analysis of BDFGM nanostructures with arbitrary functions. In addition, in order to obtain small scale effects, the consistent couple-stress theory is also applied. These models can degenerate into the classical models if the material length scale parameter is taken to be zero. In this theory, the couple-tensor is skew-symmetric by adopting the skew-symmetric part of the rotation gradients as the curvature tensor. The material properties except Poisson\'s ratio are assumed to be graded in both axial and thickness directions, which it can vary according to an arbitrary function. The governing equations are obtained using the concept of Hamilton principle. Generalized differential quadrature method (GDQM) is used to solve the governing equations for various boundary conditions to obtain the natural frequencies of BDFG nano-beam. At the end, some numerical results are presented to study the effects of material length scale parameter, and inhomogeneity constant on natural frequency.

Key Words
Euler-Bernoulli nano-beams; free vibration; consistent couple-stress theory; bi-directional functionally graded materials (BDFGMs); size effect; generalized differential quadrature method (GDQM)

Address
Mohammad Zamani Nejad : Mechanical Engineering Department, Yasouj University, P. O. Box: 75918-74831, Yasouj, Iran
Amin Hadi and Ali Farajpour : School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran

Abstract
Static and dynamic instability of a viscoelastic carbon nanotube (CNT) embedded on a thermo-elastic foundation are investigated, in this research. The CNT is modeled based on Euler-Bernoulli beam (EBB) and nonlocal small scale elasticity theory is utilized to analyze the structure. Governing equations of the system are derived using Hamilton‟s principle and differential quadrature (DQ) method is applied to solve the partial differential equations. The effects of variable axial load and diverse boundary conditions on static/vibration instability are studied. To verify the result of the DQ method, the Galerkin weighted residual approach is used for the instability analysis. It is observed appropriate agreement for results of two different solution methods and satisfactory accuracy with those obtained in prior studies. The results of this work could be useful for engineers and designers in order to produce and design nano/micro structures in thermo-elastic medium.

Key Words
static and vibration analysis; viscoelastic; armchair and zigzag CNT; variable axial load; thermo-elastic foundation

Address
Saeed Amir, Mehdi Khani and Pedram Dashti : Department of Solid Mechanics, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran
Ali Reza Shajari : Mechanical Properties Research Lab (MPRL), Faculty of Mechanical Engineering, K.N. Toosi University of Technology, No. 17, Pardis St., Mollasadra Ave., Vanak Square, Tehran, Iran

Abstract
An identification method for determination of the parameters of the rheological models of dampers made of viscoelastic material is presented. The models have two, three or four parameters and the model equations of motion contain derivatives of the fractional order. The results of dynamical experiments are approximated using the trigonometric function in the first part of the procedure while the model parameters are determined as the solution to an appropriately defined optimization problem. The particle swarm optimization method is used to solve the optimization problem. The validity and effectiveness of the suggested identification method have been tested using artificial data and a set of real experimental data describing the dynamic behavior of damper and a fluid frequently used in dampers. The influence of a range of excitation frequencies used in experiments on results of identification is also discussed.

Key Words
viscoelastic dampers and fluids, fractional rheological models, identification of parameter

Address
Roman Lewandowski and Maciej Przychodzki : Institute of Structural Engineering, Poznan University of Technology, ul. Piotrowo 5, 60-965 Poznan, Poland
Mieczyslaw Slowik : Institute of Civil Engineering, Poznan University of Technology, ul. Piotrowo 5, 60-965 Poznan, Poland

Abstract
This paper presents the parametric numerical analysis on the ultimate bearing capacity of the purlin-sheet roofs connected by standing seam clips. The effects of several factors on failure modes and ultimate bearing capacity of the purlins are studied, including setup of anti-sag bar, purlin type, sheet thickness and connection type et al. A simplified design formula is proposed for predicting the ultimate bearing capacity of purlins. Results show that setting the anti-sag bars can improve the ultimate bearing capacity and change the failure modes of C purlins significantly. The failure modes and ultimate bearing capacity of C purlins are significantly different from those of Z purlins, in the purlin-sheet roof connected by standing seam clips. Setting the anti-sag bars near the lower flange is more favorable for increasing the ultimate bearing capacity of purlins. The ultimate bearing capacity of C purlins increases slightly with sheet thickness increasing from 0.6 mm to 0.8 mm. The ultimate bearing capacity of the purlin-sheet roofs connected by standing seam clips is always higher than those by self-drilling screws. The predictions of the proposed design formulas are relatively in good agreement with those of EN 1993-1-3: 2006, compared with GB 50018-2002.

Key Words
standing seam clip; purlin-sheet roof; failure mode; ultimate bearing capacity; diaphragm effect

Address
Yingying Zhang : State Key Laboratory for Geomechanics and Deep Underground Engineering, Jiangsu Key Laboratory of Environmental Impact
and Structural Safety in Engineering, China University of Mining and Technology, Xuzhou Jiangsu, 221116, China
Xiaoguang Song : Shandong Academy of building research, Jinan Shandong, 250031, China
Qilin Zhang : College of Civil Engineering, Tongji University, Shanghai, 20092, China

Abstract
The paper presents the studies carried out on low velocity impact of Ultra high performance concrete (UHPC) panels of size 350X350X10 mm3 and 350X350X15 mm3. The panels are cast with 2 and 2.5% micro steel fibre and compared with UHPC without fiber. The panels are subjected to low velocity impact, by a drop-weight hemispherical impactor, at three different energy levels of 10, 15 and 20 J. The impact force obtained from the experiments are compared with numerically obtained results using finite element method, theoretically by energy balance approach and empirically by nonlinear multigenetic programming. The predictions by these models are found to be in good coherence with the experimental results.

Key Words
ultra high performance concrete; low velocity impact; finite element; energy balance model; empirical relation

Address
Prabhat R. Prem, Mohit Verma, A. Ramachandra Murthy, J. Rajasankar and B.H. Bharatkumar : CSIR-Structural Engineering Research Centre, Chennai, 600 113, India

Abstract
Glass fiber reinforced gypsum panels (GFRG) are hollow panels made from modified gypsum plaster and reinforced with chopped glass fibers. The hollow cores of panels can be filled with in-situ concrete/reinforced concrete or insulation material to increase the structural strength or the thermal insulation, respectively. GFRG panels are unfilled when used as partition walls. As load bearing walls, the panels are filled with M 20 grade concrete (reinforced concrete filling) in order to resist the gravity and lateral loads. The study was conducted in two stages: First stage involves formulation of the alternate light weight mix by conducting experimental investigations to obtain the optimum combination of phosphogypsum and shredded thermocol. In the second stage the alternate mixes are filled in GFRG panels and experimental investigations are conducted to compare the performance against panels filled with conventional M 20 mix.

Key Words
hollow panels; phosphogypsum; shredded thermocol; light weight; low cost

Address
Nithya Nandan and R. Renjith : Department of Civil Engineering, Sree Buddha College of Engineering, Alappuzha, Kerala, India

Abstract
In this paper a vibration study on orthotropic elliptic paraboloid shells with openings is carried out by using a hybrid stress finite element. The formulation of the element is based on Hellinger-Reissner variational principle. The element is developed by combining a hybrid plane stress element and a hybrid plate element. Natural frequencies of orthotropic elliptic paraboloid shells with and without openings are presented. The influence of aspect ratio, height ratio, opening ratio and material angle on the frequencies and mode shapes are investigated.

Key Words
elliptic paraboloid shell;, shell; opening; assumed stress hybrid element; finite element; free vibration

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

Abstract
The Self-Organization Feature Map as an unsupervised network is very widely used these days in engineering science. The applied network in this paper is the Self Organization Feature Map with constant weights which includes Kohonen Network. In this research, Reinforced Concrete Shear Wall buildings with different stories and heights are analyzed and a database consisting of measured fundamental periods and characteristics of 78 RC SW buildings is created. The input parameters of these buildings include number of stories, height, length, width, whereas the output parameter is the fundamental period. In addition, using Genetic Algorithm, the structure of the Self-Organization Feature Map algorithm is optimized with respect to the numbers of layers, numbers of nodes in hidden layers, type of transfer function and learning. Evaluation of the SOFM model was performed by comparing the obtained values to the measured values and values calculated by expressions given in building codes. Results show that the Self-Organization Feature Map, which is optimized by using Genetic Algorithm, has a higher capacity, flexibility and accuracy in predicting the fundamental period.

Key Words
fundamental period; Reinforced Concrete Shear Wall (RC SW) buildings; Genetic Algorithm (GA); nonlinear regression analysis; Self-Organization Feature Map (SOFM)

Address
Mehdi Nikoo : Young Researchers and Elite Club, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
Marijana Hadzima-Nyarko : Faculty of Civil Engineering, University of J.J. Strossmayer, 31000 Osijek, Vladimira Preloga 3, Croatia
Faezehossadat Khademi : Civil and Environmental Engineering Department, Illinois Institute of Technology, Chicago, USA
Sassan Mohasseb : Technical Director Smteam Gmbh, 8706 Meilen, Switzerland

Abstract
A local damage identification method using measured structural static displacement is proposed in this study. Based on the residual force vector deduced from the static equilibrium equation, residual strain energy (RSE) is introduced, which can localize the damage in the element level. In the case of all the nodal displacements are used, the RSE can localize the true location of damage, while incomplete displacement measurements are used, some suspicious damaged elements can be found. A model updating method based on static displacement response sensitivity analysis is further utilized for accurate identification of damage location and extent. The proposed method is verified by two numerical examples. The results indicate that the proposed method is efficient for damage identification. The advantage of the proposed method is that only limited static displacement measurements are needed in the identification, thus it is easy for engineering application.

Key Words
damage identification; residual force vector; strain energy; incomplete static displacement; model updating

Address
Y.J. Ou : School of Mechanics and Construction Engineering, Jinan University, Guangzhou, Guangdong 510632, P.R. China
Z.R. Lu : Department of Applied Mechanics, Sun Yat-sen University, Guangzhou, Guangdong Province, 510006, P.R. China
J.J. Zhu : Department of Applied Mechanics, Sun Yat-sen University, Guangzhou, Guangdong Province, 510006, P.R. China
/Key Laboratory of Earthquake Monitoring and Disaster Mitigation Technology,
Earthquake Administration of Guangdong Province, CEA, Guangzhou 510070, P.R. China

Abstract
In this paper, the incremental nonlinear dynamic analysis is used to evaluate the seismic performance of steel moment frame structures. To this purpose, three special moment frame structure with 5, 10 and 15 stories are designed according to the Iran\'s national building code for steel structures and the provisions for design of earthquake resistant buildings (2800 code). Incremental Nonlinear Analysis (IDA) is performed for 15 different ground motions, and responses of the structures are evaluated. For the immediate occupancy and the collapse prevention performance levels, the probability that seismic demand exceeds the seismic capacity of the structures is computed based on FEMA350. Also, fragility curves are plotted for three high-code damage levels using HASUS provisions. Based on the obtained results, it is evident that increase in the height of the frame structures reduces the reliability level. In addition, it is concluded that for the design earthquake the probability of exceeding average collapse prevention level is considerably larger than high and full collapse prevention levels.9.

Key Words
incremental nonlinear dynamic analysis; reliability of structures; fragility curves; seismic demand and capacity; performance level; confidence level

Address
Majid Khorami : Facultad de Arquitectura y Urbanismo, Universidad Tecnológica Equinoccial, Calle Rumipamba s/n y Bourgeois, Quito, Ecuador
/Facultad de Ingeniería Civil y Ambiental, Escuela Politécnica Nacional, Ladrón de Guevara E11-253L, Quito, Ecuador
Masoud Khorami, Hedayatollah Motahar : Civil Engineering Department, Islamic Azad University, Tehran, Iran
Mohammadfarid Alvansazyazdi : Facultad Ingeniería Ciencias Físicas y Matemática, Carrera Ingeniería Civil, Universidad Central del Ecuador, Quito, Ecuador
/Facultad de Arquitectura, Universidad Laica Eloy Alfaro de Manabí, Ecuador
Mahdi Shariati : Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran
/UTM Construction Research Centre, Faculty of Civil Engineering, Universiti, Teknologi Malaysia (UTM),
81310 Johor Bahru, Johor, Malaysia
Abdolrahim Jalali : Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran
M.M. Tahir : UTM Construction Research Centre, Faculty of Civil Engineering, Universiti, Teknologi Malaysia (UTM),
81310 Johor Bahru, Johor, Malaysia



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