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CONTENTS
Volume 68, Number 3, November10 2018
 

Abstract
This paper presents the experimental study of nine pieces of reinforced concrete (RC) short columns. RC short columns were tested with cyclic loading with displacement control under the influence of constant axial load with load index of 0.2. Three columns within the tested nine columns are reference columns which have the details of the reinforcement given in the modern regulations and six of them are 150 mm and 100 mm externally collared columns. In addition to the parameter of the collar spacing, aspect ratio (as=2-1.5-1) is also considered as a parameter. The data obtained from experimental results have shown that externally collar contributes significantly to increasing the shear resistance of RC short columns and limiting the shear dominant behavior. It has been observed that the effectiveness of the externally collar increases with the decrease of the aspect ratio.

Key Words
RC short column; aspect ratio; collar; cyclic loading; stiffness; ductility; energy dissipation

Address
Tamer Dirikgil and Oguz Dugenci: Department of Civil Engineering, Erciyes University, 38280 Talas, Kayseri, Turkey

Abstract
The real behavior of the RC structures constructed based on the assumed specifications of the used materials is matched with the designed ones when the assumed and the applied specifications in construction are the same. Despite in the construction phase of the reinforced concrete (RC) structures always it is tried to implement the same specifications of materials as given in the executive drawings, but considering the unpredicted/uncontrolled parameters that affect the specification of materials, always there is a deviation between the constructed and the designed materials\' specifications. The objective of this paper is to submit a guideline for the evaluation of the strength and damage to the existing RC structures encountered deviation in materials\' strengths. To achieve this goal, the lateral strength (plastic behaviors) and damage to twenty-five RC moment-resisting frames (MRFs) are studied by applying the inelastic analysis. In this study, a couple of concrete and reinforcement strengths\' deviations are investigated. The obtained results indicate that in general, there is a semi-linear relationship between the deviation in the strength of reinforcement and the changes in the lateral strength values of the MRFs. The relative effect of the deviation in the strength of reinforcements is more than the relative effect of the deviation in the concrete strength on the damage rate. The obtained results could be a guideline for the engineers in the survey of the existing buildings encountered deviation in materials\' strengths during their construction phase.

Key Words
deviation; strength; material; inelastic; vulnerability; damage

Address
Ali Massumi: Department of Civil Engineering, Faculty of Engineering, Kharazmi University, Tehran, 15719-14911, Iran
Kabir Sadeghi: Civil Engineering and Environmental Faculty, Near East University, North Cyprus, Via Mersin 10, Turkey
Ehsan Moshtagh: School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran

Abstract
In this paper, an extended finite element method is proposed to analyze both geometric and material non-linear behavior of general Functionally Graded Material (FGM) plate-shell type structures. A user defined subroutine (UMAT) is developed and implemented in Abaqus/Standard to study the elastoplastic behavior of the ceramic particle-reinforced metal-matrix FGM plates-shells. The standard quadrilateral 4-nodes shell element with three rotational and three translational degrees of freedom per node, S4, is extended in the present study, to deal with elasto-plastic analysis of geometrically non-linear FGM plate-shell structures. The elastoplastic material properties are assumed to vary smoothly through the thickness of the plate-shell type structures. The nonlinear approach is based on Mori-Tanaka model to underline micromechanics and locally determine the effective FGM properties and self-consistent method of Suquet for the homogenization of the stress-field. The elasto-plastic behavior of the ceramic/metal FGM is assumed to follow Ludwik hardening law. An incremental formulation of the elasto-plastic constitutive relation is developed to predict the tangent operator. In order to to highlight the effectiveness and the accuracy of the present finite element procedure, numerical examples of geometrically non-linear elastoplastic functionally graded plates and shells are presented. The effects of the geometrical parameters and the volume fraction index on nonlinear responses are performed.

Key Words
composites; finite element method (FEM); functionally graded; non-linear analysis; numerical methods; plate/shell structures

Address
Hanen Jrad, Jamel Mars and Fakhreddine Dammak: Engineering Production Mechanics and Materials Unit (UGPMM), National Engineering School of Sfax, University of Sfax, B.P. 1173-3038, Sfax, Tunisia
Mondher Wali:
1) Department of Mechanical Engineering, College of Engineering, King Khalid University, Abha, Saudi Arabia
2) Engineering Production Mechanics and Materials Unit (UGPMM), National Engineering School of Sfax,
University of Sfax, B.P. 1173-3038, Sfax, Tunisia

Abstract
A numerical study is performed to investigate the impacts of thermal loading on the vibration and buckling of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) conical shells. Thermo-mechanical properties of constituents are considered to be temperature-dependent. Considering the shear deformation theory, the energy functional is derived, and applying the variational differential quadrature (VDQ) method, the mass and stiffness matrices are obtained. The shear correction factors are accurately calculated by matching the shear strain energy obtained from an exact three-dimensional distribution of the transverse shear stresses and shear strain energy related to the first-order shear deformation theory. Numerical results reveal that considering temperature-dependent material properties plays an important role in predicting the thermally induced vibration of FG-CNTRC conical shells, and neglecting this effect leads to considerable overestimation of the stiffness of the structure.

Key Words
FG-CNTRC conical shells; vibration and buckling; variational formulation; thermal loading

Address
Jalal Torabi and Reza Ansari: Department of Mechanical Engineering, University of Guilan, P.O. Box 3756, Rasht, Iran

Abstract
The Nonlinear dynamic response of a sandwich plate subjected to the low velocity impact is theoretically and experimentally investigated. The Hertz law between the impactor and the plate is taken into account. Using the Extended High Order Sandwich Panel Theory (EHSAPT) and the Ritz energy method, the governing equations are derived. The skins follow the Third order shear deformation theory (TSDT) that has hitherto not reported in conventional EHSAPT. Besides, the three dimensional elasticity is used for the core. The nonlinear Von Karman relations for strains of skins and the core are adopted. Time domain solution of such equations is extracted by means of the well-known fourth-order Runge-Kutta method. The effects of core-to-skin thickness ratio, initial velocity of the impactor, the impactor mass and position of the impactor are studied in detail. It is found that these parameters play significant role in the impact force and dynamic response of the sandwich plate. Finally, some low velocity impact tests have been carried out by Drop Hammer Testing Machine. The results are compared with experimental data acquired by impact testing on sandwich plates as well as the results of finite element simulation.

Key Words
sandwich plate; low velocity impact; extended high order sandwich panel theory; geometrical nonlinearity; experiment

Address
Sattar Jedari Salami: Department of Mechanical Engineering, Damavand Branch, Islamic Azad University, Damavand, Iran
Soheil Dariushi: Department of Composite, Iran Polymer and Petrochemical Institute, Tehran-Karaj Highway, Pajuhesh Boulevard, Tehran, Iran

Abstract
The execution of an experiment is a complex affair. It includes the preparation of test specimens, the measurement process itself and also the evaluation of the experiment as such. Financial requirements can differ significantly. In contrast, the cost of numerical simulations can be negligible, but what is the credibility of a simulated experiment? Discussions frequently arise concerning the methodology used in simulations, and particularly over the geometric model used. Simplification, rounding or the complete omission of details are frequent reasons for differences that occur between simulation results and the results of executed experiments. However, the creation of a very complex geometry, perhaps all the way down to the resolution of the very structure of the material, can be complicated. The subject of the article is therefore a means of creating the material structure of concrete contained in a test specimen. Because a complex approach is taken right from the very start of the numerical simulation, maximum agreement with experimental results can be achieved. With regard to the automation of the process described, countless material structures can be generated and randomly produced samples simulated in this way. Subsequently, a certain degree of randomness can be observed in the results obtained, e.g., the shape of the failure – just as is the case with experiments. The first part of the article presents a description of a complex approach to the creation of a geometry representing real concrete test specimens. The second part presents a practical application in which the numerical simulation of the compressive testing of concrete is executed using the generated geometry.

Key Words
heterogeneity; material structure; noise; pressure test; smoothed particle hydrodynamics

Address
Martin Husek and Jiri Kala: Institute of Structural Mechanics, Faculty of Civil Engineering, Brno University of Technology, Veveri 331/95, 602 00 Brno, Czech Republic

Abstract
Heterogeneous structure and, particularly, low resistance to tension stresses leads to different mechanical properties of the concrete in different loading situations. To solve this problem, the tension zone of concrete elements is reinforced. Development of the cracks, however, becomes even more complicated in the presence of bar reinforcement. Direct tension test is the common layout for analyzing mechanical properties of reinforced concrete. This study investigates scatter of the test results related with arrangement of bar reinforcement. It employs results of six elements with square 60x60 mm cross-section reinforced with one or four 5 mm bars. Differently to the common research practice (limited to the average deformation response), this study presents recordings of numerous strain gauges, which allows to monitor/assess evolution of the deformations during the test. A simple procedure for variation assessment of elasticity modulus of the concrete is proposed. The variation analysis reveals different deformation behavior of the concrete in the prisms with different distribution of the reinforcement bars. Application of finite element approach to carefully collected experimental data has revealed the effects, which were neglected during the test results interpretation stage.

Key Words
reinforced concrete; heterogeneous material; tension tests; numerical modeling; stochastic effects; arrangement of reinforcement; deformations

Address
Ronaldas Jakubovskis and Viktor Gribniak: Laboratory of Innovative Building Structures, Vilnius Gediminas Technical University (VGTU), Sauletekio av. 11, Vilnius LT-10223, Lithuania
Rimantas Kupliauskas: Department of Storm-Water Network, Grinda Ltd., Eiguliu str. 32, Vilnius LT-03150, Lithuania
Arvydas Rimkus:
1) Laboratory of Innovative Building Structures, Vilnius Gediminas Technical University (VGTU), Sauletekio av. 11, Vilnius LT-10223, Lithuania
2) Laboratory of Composite Materials, VGTU, Vilnius, Lithuania

Abstract
This paper deals with the dynamic stability of nanocomposite pipes conveying pulsating ferrofluid. The pipe is reinforced by carbon nanotubes (CNTs) where the agglomeration of CNTs are considered based on Mori-Tanaka model. Due to the existence of CNTs and ferrofluid flow, the structure and fluid are subjected to axial magnetic field. Based on Navier-Stokes equation and considering the body forced induced by magnetic field, the external force of fluid to the pipe is derived. For mathematical modeling of the pipe, the first order shear deformation theory (FSDT) is used where the energy method and Hamilton\'s principle are used for obtaining the motion equations. Using harmonic differential quadrature method (HDQM) and Bolotin\'s method, the motion equations are solved for calculating the excitation frequency and dynamic instability region (DIR) of the structure. The influences of different parameters such as volume fraction and agglomeration of CNTs, magnetic field, structural damping, viscoelastic medium, fluid velocity and boundary conditions are shown on the DIR of the structure. Results show that with considering agglomeration of CNTs, the DIR shifts to the lower excitation frequencies. In addition, the DIR of the structure will be happened at higher excitation frequencies with increasing the magnetic field.

Key Words
dynamic stability; pulsating ferrofluid flow; CNTs; agglomeration; magnetic field

Address
Hemat Ali Esmaeili, Mehran Khaki and Morteza Abbasi: Department of Mechanical Engineering, Sari Branch, Islamic Azad University, Sari, Iran

Abstract
This paper presents the shear buckling analysis of symmetrically laminated cross-ply plates resting on Pasternak foundation under pure in-plane uniform shear load. The classical laminated plate theory is used for the shear buckling analysis of laminated plates. The Rayleigh-Ritz method with novel plate shape functions is proposed to solve the differential equations and a computer programming is developed to obtain the shear buckling loads. Finally, the effects of the plate aspect ratios, boundary conditions, rotational restraint stiffness, translational restraint stiffness, thickness ratios, modulus ratios and foundation parameters on the shear buckling of the laminated plates are investigated.

Key Words
laminated composite plates; shear buckling; Rayleigh Ritz method; Pasternak foundation

Address
Umut Topal: Department of Civil Engineering, Faculty of Technology, Karadeniz Technical University, Trabzon, Turkey
Ebrahim Nazarimofrad: Department of Civil Engineering, Bu Ali Sina University, Hamedan, Iran
Seyed Ebrahim Sadat Kholerdi: Department of Civil Engineering, Malayer University, Malayer, Iran

Abstract
Bi-stable structure can be stable in both its extended and coiled forms. For the un-stressed thin cylindrical shell, the strain energy expressions are deduced by using a theoretical model in terms of only two parameters. Based on the principle of minimum potential energy, the bi-stable behaviors of the cylindrical shells are investigated. The results indicate that the isotropic cylindrical shell does not have the second stable configuration and laminated cylindrical shells with symmetric or antisymmetric layup of fibers have the second stable state under some confined conditions. In the case of antisymmetric laminated cylindrical shell, the analytical expressions of the stability are derived based on the extremal principle, and the shell can achieve a compact coiled configuration without twist deformation in its second stable state. In the case of symmetric laminated cylindrical shell, the explicit solutions for the stability conditions cannot be deduced. Numerical results show that stable configuration of symmetric shell is difficult to achieve and symmetric shell has twist deformation in its second stable form. In addition, the roll-up radii of the antisymmetric laminated cylindrical shells are calculated using the finite element package ABAQUS. The results show that the value of the roll-up radii is larger from FE simulation than from theoretical analysis. By and large, the predicted roll-up radii of the cylindrical shells using ABAQUS agree well with the theoretical results.

Key Words
cylindrical shell; bi-stable; two-parameter model; strain energy

Address
Yaopeng Wu, Erle Lu and Shuai Zhang: School of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, P R China


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