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CONTENTS
Volume 54, Number 5, June10 2015
 

Abstract
This is an experimental study to investigate the behaviour of piled raft system in different types of sandy soil. A small scale \"prototype\" model was tested in a sand box with load applied to the foundation through a compression jack and measured by means of load cell. The settlement was measured at the raft by means of dial gauges, three strain gauges were attached on piles to measure the strains and calculate the load carried by each pile in the group. Nine configurations of group (1X2, 1X3, 1X4, 2X2, 2X3, 2X4, 3X3, 3X4 and 4X4) were tested in the laboratory as a free standing pile group (the raft not in contact with the soil) and as a piled raft (the raft in contact with the soil), in addition to tests for raft (unpiled) with different sizes. It is found that when the number of piles within the group is small (less than 4), there is no evident contribution of the raft to the load carrying capacity. The failure load for a piled raft consisting of 9 piles is approximately 100% greater than free standing pile group containing the same number of piles. This difference increases to about 4 times for 16 pile group. The piles work as settlement reducers effectively when the number of piles is greater than 6 than when the number of piles is less than 6. The settlement can be increased by about 8 times in (1X2) free standing pile group compared to the piled raft of the same size. The effect of piled raft in reducing the settlement vanishes when the number of piles exceeds 6.

Key Words
piled raft; bearing capacity; group; geometry

Address
Mohammed Y. Fattah: Department of Building and Construction Engineering University of Technology, Baghdad, Iraq
Mustafa A. Yousif and Sarmad M.K. Al-Tameemi: Civil Engineering Department, Al-Mustansiriya University, Baghdad, Iraq

Abstract
Many novel materials exhibit a property of different elastic moduli in tension and compression. One such material is graphene, a wonder material, which has the highest strength yet measured. Investigations on buckling problems for structures with different moduli are scarce. To address this new problem, firstly, the nondimensional expression of the relation between offset of neutral axis and deflection curve is derived based on the phased integration method, and then using the energy method, load-deflection relation of the rod is determined; Secondly, based on the improved constitutive model for different moduli, large deformation finite element formulations are developed and combined with the arc-length method, finite element iterative program for rods with different moduli is established to obtain buckling critical loads; Thirdly, material mechanical properties tests of graphite, which is the raw material of graphene, are performed to measure the tensile and compressive elastic moduli, moreover, buckling tests are also conducted to investigate the buckling behavior of this kind of graphite rod. By comparing the calculation results of the energy method and finite element method with those of laboratory tests, the analytical model and finite element numerical model are demonstrated to be accurate and reliable. The results show that it may lead to unsafe results if the classic theory was still adopted to determine the buckling loads of those rods composed of a material having different moduli. The proposed models could provide a novel approach for further investigation of non-linear mechanical behavior for other structures with different moduli.

Key Words
different moduli; buckling compression rod; analytical method; numerical method; laboratory tests

Address
Wenjuan Yao, Jianwei Ma, Jinling Gao: Department of Civil Engineering, Shanghai University, Shanghai 200072, China
Yuanzhong Qiu: SEGOC, West Rd., Houston TX 77041, USA

Abstract
This paper aims to fill the technical gap on the elastic buckling behavior of functionally graded material (FGM) grid systems under inplane loads on which few research has been done. Material properties of an FG beam are assumed to vary smoothly in the thickness direction according to power and exponential laws. Based on a hybrid-stress finite element formulation, buckling solutions for FGM grid systems consisting of various aspect ratios and material gradation are provided. The numerical results demonstrate that the aspect ratio and material gradation play an important role in the buckling behavior of FGM grid systems. We believe that the new results obtained from this study, will be very useful to designers and researchers in this field.

Key Words
Grid system; functionally graded materials; buckling; hybrid finite element

Address
K. Darilmaz, M. Gunhan Aksoylu and Yavuz Durgun: Civil Engineering Department, İstanbul Technical University, Maslak, Sariyer, 34469, Istanbul, Turkey

Abstract
For a systematic study on wind-induced vibration characteristics of large hyperbolic cooling towers with different feature sizes, the pressure measurement tests are finished on the rigid body models of three representative cooling towers with the height of 155 m, 177 m and 215 m respectively. Combining the refined frequency-domain algorithm of wind-induced responses, the wind-induced average response, resonant response, background response, coupling response and wind vibration coefficients of large cooling towers with different feature sizes are obtained. Based on the calculating results, the parametric analysis on wind-induced vibration of cooling towers is carried out, e.g. the feature sizes, damping ratio and the interference effect of surrounding buildings. The discussion shows that the increase of feature sizes makes wind-induced average response and fluctuating response larger correspondingly, and the proportion of resonant response also gradually increased, but it has little effect on the wind vibration coefficient. The increase of damping ratio makes resonant response and the wind vibration coefficient decreases obviously, which brings about no effect on average response and background response. The interference effect of surrounding buildings makes the fluctuating response and wind vibration coefficient increased significantly, furthermore, the increase ranges of resonant response is greater than background response.

Key Words
large hyperbolic cooling towers; wind tunnel test; wind-induced vibration characteristics; wind vibration coefficient; parametric analysis

Address
Shitang Ke: Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Road, Nanjing 210016, China
Yaojun Ge, Lin Zhao: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
Yukio Tamura: Center of Wind Engineering Research, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan

Abstract
The dynamic response and the mooring line tension of a 1/75 scale model of spar-type platform for 2.5 MW floating offshore wind turbine subject to one-dimensional regular harmonic wave are investigated numerically and verified by experiment. The upper part of wind turbine which is composed of three rotor blades, hub and nacelle is modeled as a lumped mass the scale model and three mooring lines are pre-tensioned by means of linear springs. The coupled fluid rigid body interaction is numerically simulated by a coupled FEM-cable dynamics code, while the experiment is performed in a wave tank with the specially-designed vision and data acquisition system. The time responses of surge, heave and pitch motions of the scale platform and the mooring line tensions are obtained numerically and the frequency domainconverted RAOs are compared with the experiment.

Key Words
spar-type floating platform; scale model; response amplitude operator (RAO); mooring line tension; fluid-rigid body interaction simulation; wave tank experiment

Address
E.Y. Choi, J.R. Cho, Y.U. Cho, W.B. Jeong, S.B. Lee: School of Mechanical Engineering, Pusan National University, Busan 609-735, Korea
S.P. Hong, H.H. Chun: Global Core Research Center for Ships and Offshore Plants, Pusan National University, Busan 609-735, Korea

Abstract
In this paper, a simple n-order refined theory based on neutral surface position is developed for bending and frees vibration analyses of functionally graded beams. The present theory is variationally consistent, uses the n-order polynomial term to represent the displacement field, does not require shear correction factor, and gives rise to transverse shear stress variation such that the transverse shear stresses vary parabolically across the thickness satisfying shear stress free surface conditions. The governing equations are derived by employing the Hamilton\'s principle and the physical neutral surface concept. The accuracy of the present solutions is verified by comparing the obtained results with available published ones.

Key Words
mechanical properties; vibration; functionally graded; deformation; modeling

Address
Lazreg Hadji, T. Hassaine Daouadji, A. Tounsi: Universite Ibn Khaldoun, BP 78 Zaaroura, 14000 Tiaret, Algerie
Lazreg Hadji, T. Hassaine Daouadji, A. Tounsi and E.A. Bedia: Laboratoire des Materiaux & Hydrologie, Universite de Sidi Bel Abbes, 22000 Sidi Bel Abbes, Algerie

Abstract
This paper investigates the load model for single footfall trace of human walking. A large amount of single person walking load tests were conducted using the three-dimensional gait analysis system. Based on the experimental data, Fourier series functions were adopted to model single footfall trace in three directions, i.e. along walking direction, direction perpendicular to the walking path and vertical direction. Function parameters such as trace duration time, number of Fourier series orders, dynamic load factors (DLFs) and phase angles were determined from the experimental records. Stochastic models were then suggested by treating walking rates, duration time and DLFs as independent random variables, whose probability density functions were obtained from experimental data. Simulation procedures using the stochastic models are presented with examples. The simulated single footfall traces are similar to the experimental records.

Key Words
human walking load; footfall load model; three dimensional gait analysis technique; stochastic model

Address
Yixin Peng, Jun Chen and Guo Ding: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China

Yixin Peng: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China

Abstract
In this paper, the influence of the polled direction of piezoelectric materials on the stress distribution is studied under time-harmonic dynamical load (time-harmonic Lamb\'s problem). The system considered in this study consists of piezoelectric covering layer and piezoelectric half-plane, and the harmonic dynamical load acts on the free face of the covering layer. The investigations are carried out by utilizing the exact equations of motion and relations of the linear theory of electro-elasticity. The plane-strain state is considered. It is assumed that the perfect contact conditions between the covering layer and halfplane are satisfied. The boundary value problems under consideration are solved by employing Fourier exponential transformation techniques with respect to coordinates directed along the interface line. Numerical results on the influence of the polled direction of the piezoelectric materials such as PZT-5A, PZT-5H, PZT-4 and PZT-7A on the normal stresses, shear stresses and electric potential acting on the interface plane are presented and discussed. As a result of the analyses, it is established that the polled directions of the piezoelectric materials play an important role on the values of the studied stresses and electric potential.

Key Words
piezoelectric layered material; polled direction; time-harmonic dynamical loading; stress distribution; resonance behavior

Address
Nihat ilhan and Nagihan Koc: Department of Civil Engineering, Yildiz Technical University, 34220 Esenler, Istanbul, Turkey

Abstract
Ethylene tetrafluoroethylene (ETFE) foil is a novel structural material which has being used in shell and spatial structures. This paper studies biaxial creep property of ETFE foil by creep tests and numerical simulation. Biaxial creep tests of cruciform specimens were performed using three stress ratios, 1:1, 2:1 and 1:2, which showed that creep coefficients in biaxial tension were much smaller than those in uniaxial one. Then, a reduction factor was introduced to take account of this biaxial effect, and relation between the reduction factor and stress ratio was established. Circular bubble creep test and triangle cushion creep test of ETFE foil were performed to verify the relation. Interpolation was adopted to consider creep stress and reduction factor was involved to take account of biaxial effect in numerical simulation. Simulation results of the bubble creep test embraced a good agreement with those measuring ones. In triangle cushion creep test, creep displacements from numerical simulation showed a good agreement with those from creep test at the center and lower foil measuring points.

Key Words
ETFE foil; cushion; biaxial creep; creep test; creep simulation

Address
Yintang Li and Minger Wu: Department of Structural Engineering, Tongji University, Shanghai, China

Abstract
In this paper, free vibrations of a clamped-clamped double-walled carbon nanotube (DWNT) under axial force is studied. By utilizing Euler–Bernoulli beam theory, each layer of DWNT is modeled as a beam. In this analysis, nonlinear form of interlayer van der Waals (vdW) forces and nonlinearities aroused from mid-plane stretching are also considered in the equations of motion. Further, direct application of multiple scales perturbation method is utilized to solve the obtained equations and to analyze free vibrations of the DWNT. Therefore, analytical expressions are found for vibrations of each layer. Linear and nonlinear natural frequencies of the system and vibration amplitude ratios of inner to outer layers are also obtained. Finally, the results are compared with the results obtained by Galerkin method.

Key Words
double-walled; carbon nanotube; nonlinear vibrations; Galerkin method; Perturbation method

Address
Ali Hajnayeb: Mechanical Engineering Department, Engineering Faculty, Shahid Chamran University of Ahvaz, Ahvaz, Iran
S.E. Khadem: Mechanical & Aerospace Engineering Department, Tarbiat Modares University, P.O. Box 14115-177, Tehran, Iran

Abstract
In this paper, we introduce a new method for assembly of shipbuilding blocks at sea and present its feasibility focusing on structural safety. The core concept of this method is to assemble ship building blocks by use of bolting, gluing and welding techniques at sea without dock facilities. Due to its independence of dock facilities, shipyard construction capability could be increased considerably by the proposed method. To show the structural safety of this method, a bulk carrier and an oil tanker were employed, and we investigated the structural behavior of those ships to which the new block assembly method was applied. The ship hull models attached with connective parts are analyzed in detail through finite element analyses, and the cargo capacity of the bulk carrier is briefly discussed as well. The results of these studies show the potential for applying this new block assembly method to practical shipbuilding.

Key Words
ship blocks assembly method; shipbuilding, structural design; stress assessment; finite element method

Address
Bilin Zhang: Hyundai Heavy Industries (Shanghai) R & D Co., Ltd, 498 Guoshoujing Road, Shanghai 201-203, Republic of China
Seung-Hwan Boo: Division of Ocean Systems Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu 305-701, Daejeon, Republic of Korea
Jin-Gyun Kim: Department of System Reliability, Korea Institute of Machinery and Materials, 156, Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea

Abstract
This study aimed to investigate the random vibration characteristic of train-slab track-bridge interaction system subjected to both track irregularities and earthquakes by use of pseudo-excitation method (PEM). Each vehicle subsystem was modeled by multibody dynamics. A three-dimensional rail-slabgirder-pier finite element model was created to simulate slab track and bridge subsystem. The equations of motion for the entire system were established based on the constraint condition of no jump between wheel and rail. The random load vectors of equations of motion were formulated by transforming track irregularities and seismic accelerations into a series of deterministic pseudo-excitations according to their respective power spectral density (PSD) functions by means of PEM. The time-dependent PSDs of random vibration responses of the system were obtained by step-by-step integration method, and the corresponding extreme values were estimated based on the first-passage failure criterion. As a case study, an ICE3 high-speed train passing a fifteen-span simply supported girder bridge simultaneously excited by track irregularities and earthquakes is presented. The evaluated extreme values and the PSD characteristic of the random vibration responses of bridge and train are analyzed, and the influences of train speed and track irregularities (without earthquakes) on the random vibration characteristic of bridge and train are discussed.

Key Words
train-slab track-bridge interaction; random vibration; pseudo-excitation method; earthquake; track irregularity

Address
Zhi-ping Zeng: School of Civil Engineering, Railway Campus, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China; Department of Architecture, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-Ku, Yokohama-shi, Kanagawa, 221-8686, Japan; National Engineering Laboratory for High-Speed Railway Construction, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China
Xian-feng He: School of Civil Engineering, Railway Campus, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China
Yan-gang Zhao: School of Civil Engineering, Railway Campus, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China; Department of Architecture, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-Ku, Yokohama-shi, Kanagawa, 221-8686, Japan
Zhi-wu Yu: School of Civil Engineering, Railway Campus, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China; National Engineering Laboratory for High-Speed Railway Construction, Central South University,
22 Shao-shan-nan Road, Changsha, Hunan 410075, China
Ling-kun Chen: National Engineering Laboratory for High-Speed Railway Construction, Central South University,
22 Shao-shan-nan Road, Changsha, Hunan 410075, China; College of civil science and technology, Yangzhou University, 88 Yang-zhou-da-xue-nan Road, Yangzhou, Jiangsu 225127, China
Wen-tao Xu: National Engineering Laboratory for High-Speed Railway Construction, Central South University,
22 Shao-shan-nan Road, Changsha, Hunan 410075, China; School of Mechanics and Engineering Science, Zhengzhou University, 100 Ke-xue Road, Zhengzhou,
Henan 450001, China
Ping Lou: School of Civil Engineering, Railway Campus, Central South University, 22 Shao-shan-nan Road, Changsha, Hunan 410075, China; National Engineering Laboratory for High-Speed Railway Construction, Central South University,
22 Shao-shan-nan Road, Changsha, Hunan 410075, China


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