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Volume 26, Number 6, March25 2018

This paper contains a consistent couple-stress theory to capture size effects in Euler-Bernoulli nano-beams made of three-directional functionally graded materials (TDFGMs). These models can degenerate into the classical models if the material length scale parameter is taken to be zero. In this theory, the couple-stress tensor is skew-symmetric and energy conjugate to 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 all three axial, thickness and width directions, which it can vary according to an arbitrary function. The governing equations are obtained using the concept of minimum potential energy. Generalized differential quadrature method (GDQM) is used to solve the governing equations for various boundary conditions to obtain the natural frequencies of TDFG nano-beam. At the end, some numerical results are performed to investigate some effective parameter on buckling load. In this theory the couple-stress tensor is skew-symmetric and energy conjugate to the skew-symmetric part of the rotation gradients as the curvature tensor.

Key Words
Euler-Bernoulli nano-beams; buckling analysis; consistent couple-stress theory; Three-directional functionally graded materials (TDFGMs); size effect; generalized differential quadrature method (GDQM)

(1) Amin Hadi, Abbas Rastgoo:
School of Mechanical Engineering, University of Tehran, Tehran, Iran;
(2) Mohammad Zamani Nejad:
Department of Mechanical Engineering, Yasouj University, P.O. Box: 75914-353, Yasouj, Iran;
(3) Mohammad Hosseini:
Department of Mechanical Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran.

The main goal of this research is to examine the in-plane and out-of-plane forced vibration of a curved nanocomposite microbeam. The in-plane and out-of-plane displacements of the structure are considered based on the first order shear deformation theory (FSDT). The curved microbeam is reinforced by functionally graded carbon nanotubes (FG-CNTs) and thus the extended rule of mixture is employed to estimate the effective material properties of the structure. Also, the small scale effect is captured using the strain gradient theory. The structure is rested on a nonlinear orthotropic viscoelastic foundation and is subjected to concentrated transverse harmonic external force, thermal and magnetic loads. The derivation of the governing equations is performed using energy method and Hamilton's principle. Differential quadrature (DQ) method along with integral quadrature (IQ) and Newmark methods are employed to solve the problem. The effect of various parameters such as volume fraction and distribution type of CNTs, boundary conditions, elastic foundation, temperature changes, material length scale parameters, magnetic field, central angle and width to thickness ratio are studied on the frequency and force responses of the structure. The results indicate that the highest frequency and lowest vibration amplitude belongs to FGX distribution type while the inverse condition is observed for FGO distribution type. In addition, the hardening-type response of the structure with FGX distribution type is more intense with respect to the other distribution types.

Key Words
forced vibration; curved nanocomposite microbeam; strain gradient theory; viscoelastic foundation; DQ-IQ-Newmark method

(1) Farshid Allahkarami, Mansour Nikkhah-bahrami:
Department of Mechanical and Aerospace Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran;
(2) Maryam Ghassabzadeh Saryazdi:
Vehicle Technology Research Institute, Amirkabir University of Technology, Tehran, Iran.

To better understand the influence of hollow ratio on the hollow concrete-filled circular steel tubular (H-CFT) stub columns under axial compression and to propose the design formula of ultimate bearing capacity for H-CFT stub columns, 3D finite element analysis and laboratory experiments were completed to obtain the load-deformation curves and the failure modes of H-CFT stub columns. The changes of the confinement effect between core concrete and steel tube with different hollow ratios were discussed based on the finite element results. The result shows that the axial stress of concrete and hoop stress of steel tube in H-CFT stub columns are decreased with the increase of hollow ratio. After the yield of steel, the reduction rate of longitudinal stress and the increase rate of circumferential stress for the steel tube slowed down. The confinement effect from steel tube on concrete also weakened slowly with the increase of hollow ratio. Based on the limit equilibrium method, a simplified formula of ultimate bearing capacity for the axially loaded H-CFT stub columns was proposed. The predicted results showed satisfactory agreement with the experimental and numerical results.

Key Words
hollow ratio; ultimate bearing capacity; finite element method; composite action

(1) Fa-xing Ding, Qiang Fu, Tao Zhang, Liping Wang, Chang-jing Fang:
School of Civil Engineering, Central South University, Changsha, Hunan Province, 410075, P.R. China;
(2) Chang-jing Fang:
National Engineering Laboratory for High Speed Railway Construction, Changsha, 410075, P.R. China;
(3) Chang-jing Fang:
Engineering Technology Research Center for Prefabricated Construction Industrialization of Hunan Province, P.R. China.

This paper presents a numerical investigation on the mechanical performance of concrete-filled dual steel tubular columns of circular section subjected to concentric axial load. A three-dimensional numerical model is developed and validated against a series of experimental tests. A good agreement is obtained between the experimental and numerical results, both in the peak load value and in the ascending and descending branches of the load-displacement curves. By means of the numerical model, a parametric study is carried out to investigate the influence of the main parameters that determine the axial capacity of double-tube columns, such as the member slenderness, inner and outer steel tube thicknesses and the concrete grade – of both the outer concrete ring and inner core –, including ultra-high strength concrete. A total number of 163 numerical simulations are carried out, by combining the different parameters. Specific indexes are defined (Strength Index, Concrete-Steel Contribution Ratio, Inner Concrete Contribution Ratio) to help rating the relative mechanical performance of dual steel tubular columns as compared to conventional concrete-filled steel tubular columns, and practical design recommendations are subsequently given.

Key Words
finite element model; concrete-filled dual steel tubular columns; ultra-high strength concrete; slender columns

Instituto de Ciencia y Tecnologĺa del Hormigon (ICITECH), Universitat Politècnica de València, Valencia, Spain.

Concrete filled steel tubular (CFST) laced columns have been widely used in high rise buildings in China. Compared to solid-web columns, this type of columns has a larger cross-section with less weight. In this paper, four concrete filled steel tubular laced columns consisting of 4 main steel-concrete tubes were tested under cyclic loading. Hysteresis and failure mechanisms were studied based on the results from the lateral cyclic loading tests. The influence of each design parameter on restoring forces was investigated, including axial compression ratio, slenderness ratio, and the size of lacing tubes. The test results show that all specimens fail in compression-bending-shear and/or compression-bending mode. Overall, the hysteresis curves appear in a full bow shape, indicating that the laced columns have a good seismic performance. The bearing capacity of the columns decreases with the increasing slenderness ratio, while increases with an increasing axial compression ratio. For the columns with a smaller axial compression ratio (< 0.3), their ductility is increased. Furthermore, with the increasing slenderness ratio, the yield displacement increases, the bending failure characteristic is more obvious, and the hysteretic loops become stouter. The results obtained from the numerical analyses were compared with the experimental results. It was found that the numerical analysis results agree well with the experimental results.

Key Words
concrete filled steel tubular laced columns; seismic performance; low cyclic loading; restoring force model; hysteresis curve

(1) Zhi Huang, Li-Zhong Jiang, Yao Luo, Wang-Bao Zhou:
School of Civil Engineering, Central South University, Changsha 410075, China;
(2) Zhi Huang:
School of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;
(3) Y. Frank Chen:
School of Engineering and Technology, Southwest University, Chongqing 400716, China;
(4) Y. Frank Chen:
Department of Civil Engineering, The Pennsylvania State University, Middletown 17057, PA, USA.

This paper presents post-buckling responses of a simply supported laminated composite beam subjected to a non-follower axially compression loads. In the nonlinear kinematic model of the laminated beam, total Lagrangian approach is used in conjunction with the Timoshenko beam theory. In the solution of the nonlinear problem, incremental displacement-based finite element method is used with Newton-Raphson iteration method. There is no restriction on the magnitudes of deflections and rotations in contradistinction to von-Karman strain displacement relations of the beam. The distinctive feature of this study is post-buckling analysis of Timoshenko Laminated beams full geometric non-linearity and by using finite element method. The effects of the fibber orientation angles and the stacking sequence of laminates on the post-buckling deflections, configurations and stresses of the composite laminated beam are illustrated and discussed in the numerical results. Numerical results show that the above-mentioned effects play a very important role on the post-buckling responses of the laminated composite beams.

Key Words
composite laminated beams; post-buckling analysis; Timoshenko Beam Theory; total lagragian; Finite Element Method

Department of Civil Engineering, Bursa Technical University, Yıldırım Campus, Yıldırım, Bursa 16330, Turkey.

The inclusion of a ductile steel bracing as means of repairing an earthquake-damaged bridge bent is evaluated and experimentally assessed for the purposes of restoring the damaged bent's strength and stiffness and further improving the energy dissipation capacity. The study is focused on substandard reinforced concrete multi-column bridge bents constructed in the 1950 to mid-1970 in the United States. These types of bents have numerous deficiencies making them susceptible to seismic damage. Large-scale experiments were used on a two-column reinforced concrete bent to impose considerable damage of the bent through increasing amplitude cyclic deformations. The damaged bent was then repaired by installing a ductile fuse steel brace in the form of a buckling-restrained brace in a diagonal configuration between the columns and using post-tensioned rods to strengthen the cap beam. The brace was secured to the bent using steel gusset plate brackets and post-installed adhesive anchors. The repaired bent was then subjected to increasing amplitude cyclic deformations to reassess the bent performance. A subassemblage test of a nominally identical steel brace was also conducted in an effort to quantify and isolate the ductile fuse behavior. The experimental data from these large-scale experiments were analyzed in terms of the hysteretic response, observed damage, internal member loads, as well as the overall stiffness and energy dissipation characteristics. The results of this study demonstrated the effectiveness of utilizing ductile steel bracing for restoring the bent and preventing further damage to the columns and cap beams while also improving the stiffness and energy dissipation characteristics.

Key Words
bridge bent; buckling-restrained brace; repair; reinforced concrete; subassemblage; testing

(1) Ramiro Bazaez:
Departamento de Obras Civiles, Universidad Técnica Federico Santa María, Valparaíso, Chile;
(2) Peter Dusicka:
Department of Civil and Environmental Engineering, Portland State University, Portland, OR, USA.

This paper studies shear and tensile behaviors of headed stud connectors in double skin composite (DSC) structure. Firstly, 11 push-out tests and 11 tensile tests were performed to investigate the ultimate shear and tensile behaviors of headed stud in DSC shear wall, respectively. The main parameters investigated in this test program were height and layout of headed stud connectors. The test results reported the representative failure modes of headed studs in DSC structures subjected to shear and tension. The shear-slip and tension-elongation behaviors of headed studs in DSC structures were also reported. Influences of different parameters on these shear-slip and tension-elongation behaviors of headed studs were discussed and analyzed. Analytical models were also developed to predict the ultimate shear and tensile resistances of headed stud connectors in DSC shear walls. The developed analytical model incorporated the influence of the dense layout of headed studs in DSC shear walls. The validations of analytical predictions against 22 test results confirmed the accuracy of developed analytical models.

Key Words
headed stud; shear resistance; tensile resistance; sandwich shear wall; double skin composite shear wall; steel-concrete composite shear wall

(1) Jia-Bao Yan, Zhe Wang:
School of Civil Engineering / Key Laboratory of Coast Civil Structure Safety of Ministry of Education, Tianjin University, Tianjin 300350, China;
(2) Tao Wang, Xiao-Ting Wang:
Key Laboratory of Earthquake Engineering and Engineering Vibration, Institute of Engineering Mechanics, CEA, Harbin 150080, China;
(3) Xiao-Ting Wang:
Department of Civil Engineering, Tsinghua University, Beijing 100084, China.

This paper deals with the low velocity impact response and dynamic stresses of composite sandwich truncated conical shells (STCS) with compressible or incompressible core. Impacts are assumed to occur normally over the top face-sheet and the interaction between the impactor and the structure is simulated using a new equivalent three-degree-of-freedom (TDOF) spring-mass-damper (SMD) model. The displacement fields of core and face sheets are considered by higher order and first order shear deformation theory (FSDT), respectively. Considering continuity boundary conditions between the layers, the motion equations are derived based on Hamilton's principal incorporating the curvature, in-plane stress of the core and the structural damping effects based on Kelvin-Voigt model. In order to obtain the contact force, the displacement histories and the dynamic stresses, the differential quadrature method (DQM) is used. The effects of different parameters such as number of the layers of the face sheets, boundary conditions, semi vertex angle of the cone, impact velocity of impactor, trapezoidal shape and in-plane stresses of the core are examined on the low velocity impact response of STCS. Comparison of the present results with those reported by other researchers, confirms the accuracy of the present method. Numerical results show that increasing the impact velocity of the impactor yields to increases in the maximum contact force and deflection, while the contact duration is decreased. In addition, the normal stresses induced in top layer are higher than bottom layer since the top layer is subjected to impact load. Furthermore, with considering structural damping, the contact force and dynamic deflection decrees.

Key Words
low velocity impact; STCS; structural damping; dynamic stresses; higher order theory

(1) A. Azizi:
Department of Mechanical and Aerospace Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran;
(2) S.M.R. Khalili:
Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran;
(3) S.M.R. Khalili:
Department of Applied Mechanics, Indian Institute of Technology Delhi, New Delhi, 110016, India;
(4) S.M.R. Khalili:
Center of Excellence for Research in Advanced Materials and Structures, Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Pardis Street, Molassadra Avenue, Vanak Square, Tehran, Iran;
(5) K. Malekzadeh Fard:
Malek Ashtar University of Technology, Department of Mechanical Engineering, 4th Kilameter, Makhsous RD, Tehran, Iran.

In this study, the seismic performance of the connections between L-shaped columns composed of concrete-filled steel tubes (L-CFST columns) and H-beams used in high-rise steel frame structures was investigated. Seven full-scale specimens were tested under quasi-static cyclic loading. The variables studied in the tests included the joint type, the axial compression ratio, the presence of concrete, the width-to-thickness ratio and the internal extension length of the side plates. The hysteretic response, strength degradation, stiffness degradation, ductility, plastic rotation capacity, energy dissipation capacity and the strain distribution were evaluated at different load cycles. The test results indicated that both the corner and exterior joint specimens failed due to local buckling and crack within the beam flange adjacent to the end of the side plates. However, the failure modes of the interior joint specimens primarily included local buckling and crack at the end plates and curved corners of the beam flange. A design method was proposed for the flexural capacity of the end plate connection in the interior joint. Good agreement was observed between the theoretical and test results of both the yield and ultimate flexural capacity of the end plate connection.

Key Words
L-shaped column composed of concrete-filled steel tubes; side plate connection; end plate connection; experimental study; seismic performance; high-rise steel structure residential building; flexural capacity

(1) Zhihua Chen:
State Key Laboratory of Hydraulic Engineering Simulation and Safety, China;
(2) Wang Zhang, Zhihua Chen, Qingqing Xiong:
School of Civil Engineering, Tianjin University, Tianjin, China;
(3) Zhihua Chen:
School of Civil Engineering, Tianjin University, Tianjin, China;
(4) Ting Zhou:
School of Architecture, Tianjin University, Tianjin, China;
(5) Xian Rong, Yansheng Du:
School of Civil and Transportation Engineering, Hebei University of Technology, Tianjin, 300401, China;
(6) Qingqing Xiong:
School of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang, 050043, China.

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