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CONTENTS
Volume 78, Number 5, June10 2021
 


Abstract
One of the most efficient designs of solar trackers for photovoltaic panels is the single-axis tracker, which holds the panels along a torque tube that is driven by a motor at the central section. These trackers have evolved to become extremely slender structures due to mechanical optimization against static load and the need of cost reduction in a very competitive market. Owing to the corresponding decrease in mechanical resistance, some of these trackers have suffered aeroelastic instability even at moderate wind speeds, leading to catastrophic failures. In the present work, an analytical and experimental approach has been developed to study that phenomenon. The analytical study has led to identify the dimensionless parameters that govern the motion of the panel-tracker structure. Also, systematic wind tunnel experiments have been carried out on a 3D aeroelastic scale model. The tests have been successful in reproducing the aeroelastic phenomena arising in real-scale cases and have allowed the identification and a close characterization of the phenomenon. The main results have been the determination of the critical velocity for torsional galloping as a function of tilt angle and a calculation methodology for the optimal sizing of solar tracker shafts.

Key Words
single-axis tracker; wind load; aeroelastic model; torsional galloping; stability diagram; wind tunnel

Address
Eva Martínez-García, Eduardo Blanco Marigorta, Jorge Parrondo Gayo and Antonio Navarro-Manso: Department of Energy, University of Oviedo, Engineering Polytechnic School of Gijon, C/ Wifredo Ricart, s/n, Edificio Departamental Este, Campus Universitario de Viesques, 33204 Gijon, Asturias, Spain

Abstract
Inevitable source-uncertainties in geometry configuration, boundary condition, and material properties may deviate the structural dynamics from its expected responses. This paper aims to examine the influence of these uncertainties on the vibration of functionally graded beams. Finite element procedures are presented for Timoshenko beams and utilized to generate reliable datasets. A prerequisite to the uncertainty quantification of the beam vibration using Monte Carlo simulation is generating large datasets, that require executing the numerical procedure many times leading to high computational cost. Utilizing artificial neural networks to model beam vibration can be a good approach. Initially, the optimal network for each beam configuration can be determined based on numerical performance and probabilistic criteria. Instead of executing thousands of times of the finite element procedure in stochastic analysis, these optimal networks serve as good alternatives to which the convergence of the Monte Carlo simulation, and the sensitivity and probabilistic vibration characteristics of each beam exposed to randomness are investigated. The simple procedure presented here is efficient to quantify the uncertainty of different stochastic behaviors of composite structures.

Key Words
finite element analysis; artificial neural network; stochastic free vibration; functionally graded beam; Monte Carlo simulation; uncertainty quantification

Address
Minh-Chien Trinh and Hyungmin Jun: Division of Mechanical System Engineering, Jeonbuk National University, 567, Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea

Abstract
The present experimental study evaluated the seismic performance of six engineered cementitious composite (ECC) columns strengthened with carbon fiber reinforced polymer (CFRP) laminates under cyclic lateral loading. The ECC columns damaged and crushed in the first stage of cyclic tests were repaired using the ECC with a certain polyvinyl alcohol (PVA) fiber and strengthened with flexural and sheer CFRP laminates and then re-assessed under the cyclic loading. The effects of some variables were examined on lateral displacement, energy absorption and dissipation, failure modes, crack patterns, load bearing capacity and plasticity, and the obtained results were compared with those of the first stage of cyclic tests. The results showed that retrofitting the ECC columns can improve their performance, plasticity and load-bearing threshold, delayed the concrete failure, changed the failure modes and increased the energy absorbed by the strengthened columns element by over 50%.

Key Words
ECC; seismic retrofitting; square column; CFRP; experimental study

Address
Alireza Akhtari: Civil Engineering Department, Semnan Branch, Islamic Azad University, Semnan, Iran
Alireza Mortezaei, Ali Hemmati: Seismic Geotechnical and High-Performance Concrete Research Centre, Civil Engineering Department, Semnan Branch, Islamic Azad University, Semnan, Iran

Abstract
The current study considers free vibration of the spherical panel with magnetorheological (MR) fluids core and magneto-electro-elastic face sheets. The panel is subjected to electro-magnetic loads and also is located on an orthotropic visco-Pasternak elastic foundation. To describe the displacement components of the structure, the first-order shear deformation theory (FSDT) is used and the motion equations are extracted by employing Hamilton's principle. To solve the motion differential equations, Navier's method is selected as an exact analytical solution for simply supported boundary conditions. Effect of the most important parameters such as magnetic field intensity, loss factor, multi-physical loads, types of an elastic medium, geometrical properties of the panel, and also different material types for the face sheets on the results is considered and discussed in details. The outcomes of the present work may be used to design more efficient smart structures such as sensors and actuators.

Key Words
vibration analysis; sandwich structures; spherical panels; magnetorheological fluids; magneto-electro-elastic materials; elastic foundation

Address
Javad Kargar, Ali Ghorbanpour Arani, Ehsan Arshid and Mohsen Irani Rahaghi: Department of Solid Mechanics, Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran

Abstract
This paper presents a theoretical approach of the structures reinforced with bonded FRP composites, taking into account loading model, shear lag effect and the thermal effect. These composites are used, in particular, for rehabilitation of structures by stopping the propagation of the cracks. They improve rigidity and resistance, and prolong their lifespan. In this paper, an original model is presented to predict and to determine the stresses concentration at the FRP end, with the new theory analysis approach. The model is based on equilibrium and deformations compatibility requirements in and all parts of the strengthened beam, i.e., the concrete beam, the FRP plate and the adhesive layer. The theoretical predictions are compared with other existing solutions. The numerical resolution was finalized by taking into account the physical and geometric properties of materials that may play an important role in reducing the stress values. This solution is general in nature and may be applicable to all kinds of materials.

Key Words
interfacial stresses; RC beam; strengthening; shear lag effect; thermal effects; composite materials

Address
Hassaine Daouadji Tahar, Rabahi Abderezak, Benferhat Rabia: Civil Engineering Department, University of Tiaret, Algeria;
Laboratory of Geomatics and Sustainable Development, University of Tiaret, Algeria
Abdelouahed Tounsi: YFL (Yonsei Frontier Lab), Yonsei University, Seoul, Korea; LMH Laboratory, Civil Engineering Department, University of Sidi Bel Abbes, Algeria; Department of Civil and Environmental Engineering, King Fahd University Saudi Arabia

Abstract
This paper presents a comparative study of analytical method, finite element method (FEM) and Multilayer Perceptron (MLP) for analysis of a contact problem. The problem consists of a functionally graded (FG) layer resting on a half plane and pressed with distributed load from the top. Firstly, analytical solution of the problem is obtained by using theory of elasticity and integral transform techniques. The problem is reduced a system of integral equation in which the contact pressure are unknown functions. The numerical solution of the integral equation was carried out with Gauss-Jacobi integration formulation. Secondly, finite element model of the problem is constituted using ANSYS software and the two-dimensional analysis of the problem is carried out. The results show that contact areas and the contact stresses obtained from FEM provide boundary conditions of the problem as well as analytical results. Thirdly, the contact problem has been extended based on the MLP. The MLP with three-layer was used to calculate the contact distances. Material properties and loading states were created by giving examples of different values were used at the training and test stages of MLP. Program code was rewritten in C++. As a result, average deviation values such as 0.375 and 1.465 was obtained for FEM and MLP respectively. The contact areas and contact stresses obtained from FEM and MLP are very close to results obtained from analytical method. Finally, this study provides evidence that there is a good agreement between three methods and the stiffness parameters has an important effect on the contact stresses and contact areas.

Key Words
contact problem; functionally graded layer; theory of elasticity; finite element method; multilayer perceptron; quarter plane

Address
Murat Yaylaci, Mojgen Yayli: Department of Civil Engineering, Recep Tayyip Erdogan University, 53100, Rize, Turkey
Ecren Uzun Yaylaci: Surmene Faculty of Marine Science, Karadeniz Technical University, 61530, Trabzon, Turkey
Hasan Olmez: Department of Marine Engineering Operations, Karadeniz Technical University, 61530, Trabzon, Turkey
Ahmet Birinci: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey

Abstract
Early detection of small concrete crack or reinforcement corrosion is necessary for Structural Health Monitoring (SHM). Global vibration based methods are advantageous over local methods because of simple equipment installation and cost efficiency. Among vibration based techniques, FRF based methods are preferred over modal based methods. In this study, a new coupled method using frequency response function (FRF) and proper orthogonal modes (POM) is proposed by using the dynamic characteristic of a damaged beam. For the numerical simulation, wave finite element (WFE), coupled with traditional finite element (FE) method is used for effectively incorporating the damage related information and faster computation. As reported in literature, hybrid combination of wave function based wave finite element method and shape function based finite element method can addresses the mid frequency modelling difficulty as it utilises the advantages of both the methods. It also reduces the dynamic matrix dimension. The algorithms are implemented on a three-dimensional reinforced concrete beam. Damage is modelled and studied for two scenarios, i.e., crack in concrete and rebar corrosion. Single and multiple damage locations with different damage length are also considered. The proposed methodology is found to be very sensitive to both single- and multiple- damage while being computationally efficient at the same time. It is observed that the detection of damage due to corrosion is more challenging than that of concrete crack. The similarity index obtained from the damage parameters shows that it can be a very effective indicator for appropriately indicating initiation of damage in concrete structure in the form of spread corrosion or invisible crack.

Key Words
SHM; wave finite element; coupled numerical technique; corrosion detection; crack detection; FRF signature

Address
Rahul Dev Kundu: Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India and Intern, CSIR-Structural Engineering Research Centre (CSIR-SERC), Chennai 600113, India
Saptarshi Sasmal: Special and Multifunctional Structures Laboratory, CSIR-Structural Engineering Research Centre (CSIR-SERC), CSIR-Structural Engineering Research Centre (CSIR-SERC), Chennai, 600113, India

Abstract
Composite material-due to low density-causes weight savings, which results in lower fuel consumption of transport vehicles. The aim of the research was to change the existing base-plate of the aluminum airplane container with the composite sandwich plate in order to reduce the weight of the containers of cargo aircrafts. The newly constructed sandwich plate consists of aluminum honeycomb core and composite face-sheets. The face-sheets consist of glass or carbon or hybrid fiber layers. The orientations of the fibers in the face-sheets were 0o, 90o and +-45o. Multi-objective optimization method was elaborated for the newly constructed sandwich plates. Based on the design aim, the importance of the objective functions (weight and cost of sandwich plates) was the same (50%). During the optimization nine design constraints were considered: stiffness, deflection, facing stress, core shear stress, skin stress, plate buckling, shear crimping, skin wrinkling, intracell buckling. The design variables were core thickness and number of layers of the face-sheets. During the optimization both the Weighted Normalized Method of the Excel Solver and the Genetic Algorithm Solver of Matlab software were applied. The mechanical properties of composite face-sheets were calculated by Laminator software according to the Classical Lamination Plate Theory and Tsai-Hill failure criteria. The main added-value of the study is that the multi-objective optimization method was elaborated for the newly constructed sandwich structures. It was confirmed that the optimal new composite sandwich construction-due to weight savings and lower fuel consumption of cargo aircrafts - is more advantageous than conventional all-aluminum container.

Key Words
composite sandwich plates; weight and cost optimization; airplane container; weighted normalized method; genetic algorithm

Address
Alaa Al-Fatlawi: Faculty of Mechanical Engineering and Informatics, University of Miskolc, H-3515 Miskolc, Egyetemváros, Hungary; Faculty of Mechanical Engineering, University of Kufa, 54001 Al-Najaf, Iraq
Karoly Jarmai, Gyorgy Kovacs: Faculty of Mechanical Engineering and Informatics, University of Miskolc, H-3515 Miskolc, Egyetemváros, Hungary

Abstract
Previous studies have suggested the maximum experimental story shear force of beam-column joint frame does not reach its theoretical value due to beam-column joint failure when the column-to-beam moment capacity ratio was close to 1.0. It was also pointed out that under a certain amount of axial force, an axial collapse and a sudden decrease of lateral load-carrying capacity may occur at the joint. Although increasing joint transverse reinforcement could improve the lateral load-carrying capacity and axial load-carrying capacity of beam-column joint frame, the conditions considering varying axial force were still not well investigated. For this purpose, 7 full-scale specimens with no-axial force and 14 half-scale specimens with varying axial force are designed and subjected to static loading tests. Comparing the experimental results of the two types of specimens, it has indicated that introducing the varying axial force leads to a reduction of the required joint transverse reinforcement ratio which can avoid the beam-column joint failure. For specimens with varying axial force, to prevent beam-column joint failure and axial collapse, the lower limit of joint transverse reinforcement ratio is acquired when given a column-to-beam moment capacity ratio.

Key Words
exterior beam-column joint; column-to-beam moment capacity ratio; axial force; joint transverse reinforcement ratio; joint failure; axial collapse

Address
Yanbing Hu, Masaki Maeda: Graduate School of Engineering, Tohoku University, Sendai, Japan
Yusuke Suzuki: Graduate School of Engineering, Osaka City University, Osaka, Japan
Kiwoong Jin: Graduate School of Science and Technology, Meiji University, Kawasaki, Japan

Abstract
however, the differences between seismic responses of these two types of underground structures, especially the vault radian influencing the seismic responses of arch structures are not clarified. This paper presents a detailed numerical investigation on the seismic responses of arch underground structures with different vault radians, and aims to illustrate the rule that vault radian affects the seismic responses of underground structures. Five arch underground structures are built for nonlinear soil-structure interaction analysis. The internal forces of the structural components of the underground structures only under gravity are discussed detailedly, and an optimum vault radian for perfect load-carrying functionality of arch underground structures is suggested. Then the structures are analyzed under seven scaled ground motions, amounting to a total of 35 dynamic calculations. The numerical results show that the vault radian can have beneficial effects on the seismic response of the arch structure, compared to the rectangular underground structures, causing the central columns to suffer smaller axial force and horizontal deformation. The conclusions provide some directive suggestions for the seismic design of the arch underground structures.

Key Words
arch underground structures; vault radian; static analysis; seismic performance; optimum radian

Address
Chao Ma: School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, No. 15 Yongyuan Road Daxing District, Beijing, China
Dechun Lu: Institute of Geotechnical and Underground Engineering, Beijing University of Technology, No. 100 Pingleyuan, Chaoyang District, Beijing, China
Chengzhi Qi: School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, No. 15 Yongyuan Road Daxing District, Beijing, China
Xiuli Du: Institute of Geotechnical and Underground Engineering, Beijing University of Technology, No. 100 Pingleyuan, Chaoyang District, Beijing, China


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