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CONTENTS
Volume 27, Number 6, December 2018
 

Abstract
For a long cable-stayed bridge, stay cables are its most important load-carrying components. In this paper, long-term monitoring of super-long stay cables of Sutong Bridge is introduced. A comprehensive data analysis procedure is presented, in which time domain and frequency domain based analyses are carried out. In time domain, the vibration data of several long stay cables are firstly analyzed and the standard deviation of the acceleration of stay cables, and its variation with time are obtained, as well as the relationship between in-plane vibration and out-plane vibration. Meanwhile, some vibrations such as wind and rain induced vibration are detected. Through frequency domain analysis, the basic frequencies of the stay cables are identified. Furthermore, the axial forces and their statistical parameters are acquired. To investigate the vibration deflection, an FFT-based decomposition method is used to get the modal deflection. In the end, the relationship between the vibration amplitude of stay cables and the wind speed is investigated based on correlation analysis. Through the adopted procedure, some structural parameters of the stay cables have been derived, which can be used for evaluating the component performance and corresponding management of stay cables.

Key Words
stay cable; long-term monitoring; vibration; axial force; FFT-based decomposition method

Address
Xiang Shen, Ru-jin Ma, Chun-xi Ge and Xiao-hong Hu: Department of Bridge Engineering, Tongji University, 1239 Siping Road Shanghai, China

Abstract
In this research work, nonlinear thermal buckling behavior of functionally graded (FG) plates is explored based a new higher-order shear deformation theory (HSDT). The present model has just four unknowns, by using a new supposition of the displacement field which enforces undetermined integral variables. A shear correction factor is, thus, not necessary. A power law distribution is employed to express the disparity of volume fraction of material distributions. Three kinds of thermal loading, namely, uniform, linear, and nonlinear and temperature rises over z-axis direction are examined. The non-linear governing equations are resolved for plates subjected to simply supported boundary conditions at the edges. The results are approved with those existing in the literature. Impacts of various parameters such as aspect and thickness ratios, gradient index, type of thermal load rising, on the non-dimensional thermal buckling load are all examined.

Key Words
thermal stability; functionally graded materials; refined plate theory; thermal load

Address
Ismail Bensaid, Ahmed Bekhadda and Cheikh Abdelmadjid: IS2M Laboratory, Faculty of Technology, Department of Mechanical engineering,University Abou Beckr Belkaid (UABT), Tlemcen, Algeria
Bachir Kerboua: Department of Mechanical engineering, University Abou Beckr Belkaid (UABT), Tlemcen, Algeria


Abstract
Nonlinear behavior in fluid-structure interaction (FSI) of bridge decks becomes increasingly significant for modern bridges with increasing spans, larger flexibility and new aerodynamic deck configurations. Better understanding of the nonlinear aeroelasticity of bridge decks and further development of reduced-order nonlinear models for the aeroelastic forces become necessary. In this paper, the amplitude-dependent and neutral angle dependent nonlinearities of the motion-induced loads are further highlighted by series of computational fluid dynamics (CFD) simulations. An effort has been made to investigate a semi-analytical time-domain model of the nonlinear motion induced loads on the deck, which enables nonlinear time domain simulations of the aeroelastic responses of the bridge deck. First, the computational schemes used here are validated through theoretically well-known cases. Then, static aerodynamic coefficients of the Great Belt East Bridge (GBEB) cross section are evaluated at various angles of attack, leading to the so-called nonlinear backbone curves. Flutter derivatives of the bridge are identified by CFD simulations using forced harmonic motion of the cross-section with various frequencies. By varying the amplitude of the forced motion, it is observed that the identified flutter derivatives are amplitude-dependent, especially for A_2* and H_2* parameters. Another nonlinear feature is observed from the change of hysteresis loop (between angle of attack and lift/moment) when the neutral angles of the cross-section are changed. Based on the CFD results, a semi-analytical time-domain model for describing the nonlinear motion-induced loads is proposed and calibrated. This model is based on accounting for the delay effect with respect to the nonlinear backbone curve and is established in the state-space form. Reasonable agreement between the results from the semi-analytical model and CFD demonstrates the potential application of the proposed model for nonlinear aeroelastic analysis of bridge decks.

Key Words
computational fluid dynamics; flutter derivatives; nonlinear aeroelasticity; nonlinear semi-analytical model

Address
Christian Grinderslev: Department of Wind Energy, Technical University of Denmark (Riso Campus), 4000 Roskilde, Denmark
2COWI, 8000 Aarhus, Denmark
Mikkel Lubek: COWI, 8000 Aarhus, Denmark
Zili Zhang: Department of Engineering, Aarhus University, 8000 Aarhus, Denmark



Abstract
In order to reveal the independent relationship between track irregularity and wind loads, the stochastic characteristics of train-bridge coupling systems subjected to wind loads were investigated by the multi-sample calculation. The vehicle was selected as 23 degrees of freedom dynamical model, and the bridge was described by three-dimensional finite element model. It was assumed that the wind loads were random processes with strong spatial correlation, while the track irregularities were stationary random ones. As a case study, a high-speed train running on a cable-stayed bridge subjected to wind loads was studied. The effect of rail irregularities was deemed to be independent of the effect of wind excitations on the coupling system in the same wind circumstance for the same project, leading to the conclusion that the effect of wind loads and moving vehicle could be calculated separately. The variance results of the stochastic responses of vehicle-bridge coupling system under the action of wind loads and rail irregularities together were equivalent to the sum of the variance of the responses induced by each excitation. Therefore, when one of the input excitations is different, only the effect of changed loads needs to be assessed. Moreover, the new calculated results were combined with the effect of unchanged loads to present the stochastic response of coupling system subjected to the different excitations, reducing the cost of computations. The stochastic characteristics, the CFD (cumulative distribution function) of the coupling system with different wind velocities, vehicle speed, and vehicle marshalling were studied likewise.

Key Words
train-bridge systems; stochastic characteristic; independent relationship; wind loads; track irregularity

Address
Siyu Zhu: 1College of Environment and Civil Engineering, Chengdu University of Technology, 610059, Chengdu, Sichuan, P.R. China
Yongle Li, Koffi Togbenou and Chuanjin Yu: Department of Bridge Engineering, Southwest Jiaotong University, 610031, Chengdu, Sichuan, P.R. China
Tianyu Xiang: Department of Civil and Structural Engineering, The Xihua University, 610031, Chengdu, Sichuan, P.R. China

Abstract
The concentration fields in the proximities of a local gas emission source are experimentally analyzed in several combinations of wind incidences and source emissions. These conditions are determined by the plume buoyancy, emission velocity and incident flow wind speed. Concentration measurements are performed by an aspirating probe in a boundary layer wind tunnel. The analysis included the mean concentration values and the intensity of concentration fluctuations in a neutral atmospheric boundary layer flow. Different configurations are tested: an isolated stack in a homogeneous terrain and a stack with a bluff body in close proximity, located windward and leeward from the emission source. The experimental mean concentration values are contrasted with Gaussian profiles and the dilution factor is analyzed with respect to the empirical curves of the minimum dilution. Finally, a study on the plume intermittency is performed in a cross-sectional plane near the emission source. It is possible to highlight the following observations: a) plume vertical asymmetry in the case of an isolated emission source, b) significant differences in the dispersion process related to the relative location of the emission source and bluff body effects, and c) different probabilistic behavior of the concentration fluctuation data in a cross-sectional measurement plane inside the plume.

Key Words
plume dispersion; wind tunnel modeling; tracer gas; probabilistic analysis

Address
Adrián R. Wittwer: Laboratorio de Aerodinámica, Facultad de Ingeniería, Universidad Nacional del Nordeste (UNNE),
Postal Code 3500, Resistencia, Chaco, Argentina
Acir M. Loredo-Souza: Laboratório de Aerodinâmica das Construções, Universidade Federal do Rio Grande do Sul,
PO Box 15035, PC 91501-970, Porto Alegre, Rio Grande do Sul, Brazil
Edith B. Camano Schettini: Instituto de Pesquisas Hidráulicas, Universidade Federal do Rio Grande do Sul, PC 91501-970, Porto Alegre, Rio Grande do Sul, Brazil
Hugo G. Castro: Instituto de Modelado e Innovación Tecnológica, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET),
Universidad Nacional del Nordeste, PC 3500, Resistencia, Chaco, Argentina




Abstract
This paper presents a free vibration analysis of size-dependent functionally graded (FG) nanobeams with all surface effects considerations on the basis of modified couple stress theory. The material properties of FG nanobeam are assumed to vary according to power law distribution. Based on the Euler–Bernoulli beam theory, the modeled nanobeam and its equations of motion are derived using Hamilton\'s principle. An analytical method is used to discretize the model and the equation of motion. The model is validated by comparing the benchmark results with the obtained results. Results show that the vibration behavior of a nanobeam is significantly influenced by surface density, surface tension and surface elasticity. Also, it is shown that by increasing the beam size, influence of surface effect reduces to zero, and the natural frequency tends to its classical value.

Key Words
surface effects; vibration analysis; modified couple stress theory; functionally graded material; nanobeam

Address
Farzad Ebrahimi and Hamed Safarpour: Mechanical Engineering department, faculty of engineering, Imam Khomeini International University, Qazvin, Iran

Abstract
Wind loading of a tall building built amidst a group of buildings in urban environment is always greatly affected by shielding effects. Wind tunnel tests were carried out to assess the shielding provided by a row of low-rise or medium-rise buildings upstream a square-section tall building of height-to-breadth ratio 6. Mean and dynamic wind loads on the tall building were measured at different wind incidence angles and presented as interference factors (IFs). It is found that presence of a row of upstream buildings provides significant shielding to the tall building. At normal wind incidence, the mean along-wind loads and all components of fluctuating wind loads on the tall building are always reduced by shielding. Vortex shedding seems to still occur on the upper exposed part of the tall building but the vortex excitation levels are largely reduced. The degree of shielding is found to depend on a number of arrangement parameters of the row of upstream buildings. Empirical equations are proposed to quantify the shielding effect based on the wind tunnel data.

Key Words
tall buildings; shielding effect; interference; wind loads

Address
G.B. Zu: Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong
2Grandland Group, No.2098, Shennan East Road, Shenzhen, Guangdong, China
K.M. Lam: Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong




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