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Volume 33, Number 2, October25 2019

A semi analytical method is employed to analyze free vibration characteristics of uniform and stepped functionally graded circular cylindrical shells under complex boundary conditions. The analytical model is established based on multi-segment partitioning strategy and first-order shear deformation theory. The displacement functions are handled by unified Jacobi polynomials and Fourier series. In order to obtain continuous conditions and satisfy complex boundary conditions, the penalty method about spring technique is adopted. The solutions about free vibration behavior of functionally graded circular cylindrical shells were obtained by approach of Rayleigh-Ritz. To confirm the dependability and validity of present approach, numerical verifications and convergence studies are conducted on functionally graded cylindrical shells under various influencing factors such as boundaries, spring parameters et al. The present method apparently has rapid convergence ability and excellent stability, and the results of the paper are closely agreed with those obtained by FEM and published literatures.

Key Words
uniform and stepped functionally graded circular cylindrical; free vibration; multi-segment partitioning strategy; penalty method; complex boundary conditions

College of Shipbuilding Engineering, Harbin Engineering University, Harbin, 150001, P.R. China.

This paper presents an efficient approach to generate a new empirical formula to predict the axial compression capacity (ACC) of circular concrete-filled tube (CCFT) columns using the artificial neural network (ANN). A total of 258 test results extracted from the literature were used to develop the ANN models. The ANN model having the highest correlation coefficient (R) and the lowest mean square error (MSE) was determined as the best model. Stability analysis, sensitivity analysis, and a parametric study were carried out to estimate the stability of the ANN model and to investigate the main contributing factors on the ACC of CCFT columns. Stability analysis revealed that the ANN model was more stable than several existing formulae. Whereas, the sensitivity analysis and parametric study showed that the outer diameter of the steel tube was the most sensitive parameter. Additionally, using the validated ANN model, a new empirical formula was derived for predicting the ACC of CCFT columns. Obviously, a higher accuracy of the proposed empirical formula was achieved compared to the existing formulae.

Key Words
artificial neural network; axial compression capacity; circular concrete-filled tube; empirical formula

(1) Viet-Linh Tran, Duc-Kien Thai, Seung-Eock Kim:
Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-Dong, Gwangjin-Gu, Seoul 05006, South Korea;
(2) Viet-Linh Tran, Duc-Kien Thai:
Department of Civil Engineering, Vinh University, Vinh 461010, Vietnam.

Based on a four-variable shear deformation shell theory, the free vibration analysis of functionally graded graphene platelet-reinforced composite (FGGPRC) doubly-curved shallow shells with different boundary conditions is investigated in this work. The doubly-curved shells are composed of multi nanocomposite layers that are reinforced with graphene platelets. The graphene platelets are uniformly distributed in each individual layer. While, the volume faction of the graphene is graded from layer to other in accordance with a novel distribution law. Based on the suggested distribution law, four types of FGGPRC doubly-curved shells are studied. The present shells are assumed to be rested on elastic foundations. The material properties of each layer are calculated using a micromechanical model. Four equations of motion are deduced utilizing Hamilton's principle and then converted to an eigenvalue problem employing an analytical method. The obtained results are checked by introducing some comparison examples. A detailed parametric investigation is performed to illustrate the influences of the distribution type of volume fraction, shell curvatures, elastic foundation stiffness and boundary conditions on the vibration of FGGPRC doubly-curved shells.

Key Words
doubly-curved nanocomposite shells; functionally graded; graphene platelets; vibration; elastic foundations; four-variable shell theory

(1) Mohammed Sobhy:
Department of Mathematics and Statistics, Faculty of Science, King Faisal University, P.O. Box 400, Hofuf 31982, Saudi Arabia;
(2) Ashraf M. Zenkour:
Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia;
(3) Mohammed Sobhy, Ashraf M. Zenkour:
Department of Mathematics, Faculty of Science, Kafrelsheikh University, Kafrelsheikh 33516, Egypt.

This paper describes a study of the mapping relationship between the vertical deformation of bridge structures and rail deformation of high-speed railway, taking the interlayer interactions of the bridge subgrade CRTS II ballastless slab track system (HSRBST) into account. The differential equations and natural boundary conditions of the mapping relationship between the vertical deformation of bridge structures and rail deformation were deduced according to the principle of stationary potential energy. Then an analytical model for such relationship was proposed. Both the analytical method proposed in this paper and the finite element numerical method were used to calculate the rail deformations under three typical deformations of bridge structures and the evolution of rail geometry under these circumstances was analyzed. It was shown that numerical and analytical calculation results are well agreed with each other, demonstrating the effectiveness of the analytical model proposed in this paper. The mapping coefficient between bridge structure deformation and rail deformation showed a nonlinear increase with increasing amplitude of the bridge structure deformation. The rail deformation showed an obvious "following feature"; with the increase of bridge span and fastener stiffness, the curve of rail deformation became gentler, the track irregularity wavelength became longer, and the performance of the rail at following the bridge structure deformation was stronger.

Key Words
high-speed railway; bridge structure; analytical model; mapping relationship

(1) Yulin Feng, Lizhong Jiang, Wangbao Zhou, Zhipeng Lai, Xilin Chai:
School of Civil Engineering, Central South University, Changsha, 410075, China;
(2) Lizhong Jiang, Wangbao Zhou:
National Engineering Laboratory for High Speed Railway Construction, Central South University, Changsha, 410075, China;
(3) Zhipeng Lai:
Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA.

Cold-formed steel (CFS) sections when used as primary load carrying members often require additional strengthening for retrofitting purposes. In some cases, it is also necessary to reduce deflections in order to satisfy serviceability requirements. The introduction of angle sections, screwed to the webs so as to act as external stiffeners, has the potential to both increase flexural strength as well as reduce deflections. This paper presents the results of ten four-point bending tests, on built-up CFS sections, both open and closed, with different stiffening arrangements. In the laboratory tests, the stiffening arrangements increased the moment capacity and stiffness of the CFS beams by up to 85% and 100% respectively. The increase in moment capacity was more evident for the open sections, while that reduction in deflection was largest for the closed sections.

Key Words
cold-formed steel; experiment; stiffening arrangements; buckling; flexural strength

(1) M. Adil Dar:
Department of Civil Engineering, Indian Institute of Technology Delhi, New Delhi, India;
(2) N. Subramanian:
Consulting Engineer, Maryland, USA;
(3) Amer I. Rather, A.R. Dar:
Department of Civil Engineering, National Institute of Technology Srinagar, J&K, India;
(4) James B.P. Lim, Krishanu Roy:
Department of Civil & Environmental Engineering, University of Auckland, New Zealand;
(5) M. Anbarasu:
Department of Civil Engineering, Government College of Engineering Salem, Tamil Nadu, India.

The present study aims to investigate the ultimate strength and geometric nonlinear behavior of composite plates containing initial imperfection subjected to combined end-shortening strain and lateral pressure loading by using a semi-analytical method. In this study, the first order shear deformation plate theory is considered with the assumption of large deflections. Regarding in-plane boundary conditions, two adjacent edges of the laminates are completely held while the two others can move straightly. The formulations are based on the concept of the principle of minimum potential energy and Newton-Raphson technique is employed to solve the nonlinear set of algebraic equations. In addition, Hashin failure criteria are selected to predict the failures. Further, two distinct models are assumed to reduce the mechanical properties of the failure location, complete ply degradation model, and ply region degradation model. Degrading the material properties is assumed to be instantaneous. Finally, laminates having a wide range of thicknesses and initial geometric imperfections with different intensities of pressure load are analyzed and discuss how the ultimate strength of the plates changes.

Key Words
ultimate strength; geometric nonlinearity; lateral pressure; initial imperfection; Ritz method; Hashin criteria

New Technologies and Engineering Department, Shahid Beheshti University, G.C, Tehran, Iran.

The main objective of this paper is to study the axial buckling and post-buckling of geometrically imperfect single-layer graphene sheets (GSs) under in-plane loading in the theoretical framework of the nonlocal strain gradient theory. To begin with, a graphene sheet is modeled by a two-dimensional plate subjected to simply supported ends, and supposed to have a small initial curvature. Then according to the Hamilton*$39;s principle, the nonlinear governing equations are derived with the aid of the classical plate theory and the von-karman nonlinearity theory. Subsequently, for providing a more accurate physical assessment with respect to the influence of respective parameters on the mechanical performances, the approximate analytical solutions are acquired via using a two-step perturbation method. Finally, the authors perform a detailed parametric study based on the solutions, including geometric imperfection, nonlocal parameters, strain gradient parameters and wave mode numbers, and then reaching a significant conclusion that both the size-dependent effect and a geometrical imperfection can't be ignored in analyzing GSs.

Key Words
geometric imperfection; perturbation method; graphene sheets; nonlocal strain gradient theory

(1) Yang Gao, Wan-shen Xiao:
State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, 410082, China;
(2) Haiping Zhu:
School of Computing, Engineering and Mathematics, Western Sydney University, Locked, Bag 1797, Penrith, NSW2751, Australia.

Concrete filled steel tubular (CFST) column joints with composite beams have been widely used as lateral loading resisting elements in civil infrastructure. To better utilize these innovative joints for the application of structural seismic design and analysis, it is of great importance to investigate the dynamic behavior of the joint under cyclic loading. With this aim in mind, a novel phenomenal model has been put forward in this paper, in which a Bouc-Wen hysteresis component is employed to portray the strength and stiffness deterioration phenomenon caused by increment of loading cycle. Then, a modified chicken swarm optimization algorithm was used to estimate the optimal model parameters via solving a global minimum optimization problem. Finally, the experimental data tested from five specimens subjected to cyclic loadings were used to validate the performance of the proposed model. The results effectively demonstrate that the proposed model is an easy and more realistic tool that can be used for the pre-design of CFST column joints with reduced beam section (RBS) composite beams.

Key Words
composite joint; cyclic loading; hysteresis; model identification; chicken swarm optimization

(1) Yang Yu, Bijan Samali, Mohsen Askari:
Centre for Infrastructure Engineering, Western Sydney University, Penrith, NSW2751, Australia;
(2) Chunwei Zhang:
School of Civil Engineering, Qingdao University of Technology, Qingdao, Shandong 266033, China;
(3) Yang Yu:
School of Civil and Environmental Engineering, University of Technology Sydney, Ultimo, NSW2007, Australia.

The natural fibre composites are termed as bio-composites. They have shown a promising replacement to the current carbon/glass fibre reinforced composites as environmental friendly materials in specific applications. Natural fibre reinforced composites are potential materials for various engineering applications in automobile, railways, building and Aerospace industry. The natural fibre selected to fabricate the composite material is plant-based fibre e.g., sisal fibre. Sisal fibre is a suitable reinforcement for use in composites on account of its low density, high specific strength, and high hardness. Epoxy is a thermosetting polymer which is used as a resin in natural fibre reinforced composites. Hand lay-up technique was used to fabricate the composites by reinforcing sisal fibres into the epoxy matrix. Composites were prepared with the unidirectional alignment of sisal fibres. Test specimens with different fibre orientations were prepared. The fabricated composites were tested for mechanical properties. Impact test, tensile test, flexural test, hardness test, compression test, and thermal test of composites had been conducted to assess its suitability in industrial applications. Scanning electron microscopy (SEM) test revealed the microstructural information of the fractured surface of composites.

Key Words
sisal fibre; bio-composites; epoxy; flexural; tensile; matrix; SEM

School of Mechanical Engineering, SASTRA Deemed University, Thanjavur, Tamilnadu 613402, India.

This is the first attempt to consider the nonlinear bending analysis of porous functionally graded (FG) thick annular and circular nanoplates resting on Kerr foundation. The size effects are captured based on modified couple stress theory (MCST). The material properties of the porous FG nanostructure are assumed to vary smoothly through the thickness according to a power law distribution of the volume fraction of the constituent materials. The elastic medium is modeled by Kerr elastic foundation which consists of two spring layers and one shear layer. The governing equations are extracted based on Hamilton's principle and two variables refined plate theory. Utilizing generalized differential quadrature method (GDQM), the nonlinear static behavior of the nanostructure is obtained under different boundary conditions. The effects of various parameters such as material length scale parameter, boundary conditions, and geometrical parameters of the nanoplate, elastic medium constants, porosity and FG index are shown on the nonlinear deflection of the annular and circular nanoplates. The results indicate that with increasing the material length scale parameter, the nonlinear deflection is decreased. In addition, the dimensionless nonlinear deflection of the porous annular nanoplate is diminished with the increase of porosity parameter. It is hoped that the present work may provide a benchmark in the study of nonlinear static behavior of porous nanoplates.

Key Words
nonlinear bending; FG porous; annular and circular nanoplates; modified couple stress theory; Kerr medium

(1) Amirmahmoud Sadoughifar, Fatemeh Farhatnia, Mohsen Izadinia, Sayed Behzad Tal:
Department of Civil Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, Iran, 8514143131, Iran;
(2) Fatemeh Farhatnia:
Department of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Isfahan, Iran, 8418148499, Iran;
(3) Sayed Behzad Talaeitaba:
Department of Civil Engineering, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Isfahan, Iran.

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