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CONTENTS
Volume 23, Number 3, March 2019
 

Abstract
In the present paper, the nonlocal strain gradient refined model is used to study the thermal stability of sandwich nanoplates integrated with piezoelectric layers for the first time. The influence of Kerr elastic foundation is also studied. The present model incorporates two small-scale coefficients to examine the size-dependent thermal stability response. Elastic properties of nanoplate made of functionally graded materials (FGMs) are supposed to vary through the thickness direction and are estimated employing a modified power-law rule in which the porosity with even type of distribution is approximated. The governing differential equations of embedded sandwich piezoelectric porous nanoplates under hygrothermal loading are derived through Hamilton\'s principle where the Galerkin method is applied to solve the stability problem of the nanoplates with simply-supported edges. It is indicated that the thermal stability characteristics of the porous nanoplates are obviously influenced by the porosity volume fraction and material variation, nonlocal parameter, strain gradient parameter, geometry of the nanoplate, external voltage, temperature and humidity variations, and elastic foundation parameters.

Key Words
porous materials; thermal stability; refined plate theory; nonlocal strain gradient theory; hygrothermal environment; elastic substrate

Address
Behrouz Karami and Davood Shahsavari: Department of Mechanical Engineering, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran

Abstract
Seismic isolation systems employ structural control that protect both buildings and vibration-sensitive contents from destructive effects of earthquakes. Structural control is divided into three main groups: passive, active, and semi-active. Among them, semi-active isolation systems, which can reduce floor displacements and accelerations concurrently, has gained importance in recent years since they don\'t require large power or pose stability problems like active ones. However, their seismic performance may vary depending on the variations that may be observed in the mechanical properties of semi-active devices and/or seismic isolators. Uncertainties relating to isolators can arise from variations in geometry, boundary conditions, material behavior, or temperature, or aging whereas those relating to semi-active control devices can be due to thermal changes, inefficiencies in calibrations, manufacturing errors, etc. For a more realistic evaluation of the seismic behavior of semi-active isolated buildings, such uncertainties must be taken into account. Here, the probabilistic behavior of semi-active isolated buildings under historical pulse-like near-fault earthquakes is evaluated in terms of their performance in preserving structural integrity and protecting vibration-sensitive contents considering aforementioned uncertainties via Monte-Carlo simulations of 3-story and 9-story semi-active isolated benchmark buildings. The results are presented in the form of fragility curves and probability of failure profiles.

Key Words
semi-active isolation; probabilistic behavior; historical pulse-like earthquakes; uncertainty; fragility curve

Address
Seda Öncü-Davas and Cenk Alhan: Department of Civil Engineering, Istanbul University-Cerrahpaşa, Turkey

Abstract
This paper introduces an appropriate technique to estimate the weighting matrices used in the linear quadratic regulator (LQR) method for active structural control. For this purpose, a parameter is defined to regulate the relationship between the structural energy and control force. The optimum value of the regulating parameter, is determined for single degree of freedom (SDOF) systems under seismic excitations. In addition, the suggested technique is generalized for multiple degrees of freedom (MDOF) active control systems. Numerical examples demonstrate the robustness of the proposed method for controlled buildings under a wide range of seismic excitations.

Key Words
active control; LQR method; weighting matrices; seismic excitation; modal space

Address
Behrang Moghaddasie: Department of Civil Engineering, Ferdowsi University of Mashhad, P.O. Box 91775-1111, Mashhad, Iran
Ali Jalaeefar: Department of Civil Engineering, North Tehran Branch, Islamic Azad University, Tehran, Iran

Abstract
Moving train load parameters, including train speed, axle spacing, gross train weight and axle weights, are identified based on strain-monitoring data. In this paper, according to influence line theory, the classic moving force identification method is enhanced to handle time-varying velocity of the train. First, the moments that the axles move through a set of fixed points are identified from a series of pulses extracted from the second derivative of the structural strain response. Subsequently, the train speed and axle spacing are identified. In addition, based on the fact that the integral area of the structural strain response is a constant under a unit force at a unit speed, the gross train weight can be obtained from the integral area of the measured strain response. Meanwhile, the corrected second derivative peak values, in which the effect of time-varying velocity is eliminated, are selected to distribute the gross train weight. Hence the axle weights could be identified. Afterwards, numerical simulations are employed to verify the proposed method and investigate the effect of the sampling frequency on the identification accuracy. Eventually, the method is verified using the real-time strain data of a continuous steel truss railway bridge. Results show that train speed, axle spacing and gross train weight can be accurately identified in the time domain. However, only the approximate values of the axle weights could be obtained with the updated method. The identified results can provide reliable reference for determining fatigue deterioration and predicting the remaining service life of railway bridges.

Key Words
railway bridges; strain-monitoring data; moving train loads; identification; influence line

Address
Hao Wang, Qingxin Zhu and Jianxiao Mao: Key Laboratory of C&PC Structures of Ministry of Education, Southeast University, Nanjing, China
Jian Li: Department of Civil, Environmental and Architectural Engineering, The University of Kansas, Lawrence, Kansas, USA
Suoting Hu and Xinxin Zhao: Railway Engineering Research Institute, China Academy of Railway Sciences, Beijing, China

Abstract
To ensure high quality data being used for data mining or feature extraction in the bridge structural health monitoring (SHM) system, a practical sensor fault diagnosis methodology has been developed based on the similarity of symmetric structure responses. First, the similarity of symmetric response is discussed using field monitoring data from different sensor types. All the sensors are initially paired and sensor faults are then detected pair by pair to achieve the multi-fault diagnosis of sensor systems. To resolve the coupling response issue between structural damage and sensor fault, the similarity for the target zone (where the studied sensor pair is located) is assessed to determine whether the localized structural damage or sensor fault results in the dissimilarity of the studied sensor pair. If the suspected sensor pair is detected with at least one sensor being faulty, field test could be implemented to support the regression analysis based on the monitoring and field test data for sensor fault isolation and reconstruction. Finally, a case study is adopted to demonstrate the effectiveness of the proposed methodology. As a result, Dasarathy\'s information fusion model is adopted for multi-sensor information fusion. Euclidean distance is selected as the index to assess the similarity. In conclusion, the proposed method is practical for actual engineering which ensures the reliability of further analysis based on monitoring data.

Key Words
structural health monitoring; sensor fault diagnosis; similarity; symmetric structure responses; multi-sensor information fusion; evidential reasoning

Address
Xiang Xu, Qiao Huang, Yuan Ren and Dan-Yang Zhao: School of Transportation, Southeast University, Sipailou, Xuanwu District, Nanjing 210-096, People

Abstract
Shanghai Tower is a 632-meter super high-rise building located in an area with wind and active earthquake. A sophisticated structural health monitoring (SHM) system consisting of more than 400 sensors has been built to carry out a long-term monitoring for its operational safety. In this paper, a reduced-order model including 31 elements was generated from a full model of this super tall building. An iterative regularized matrix method was proposed to tune the system parameters, making the dynamic characteristic of the reduced-order model be consistent with those in the full model. The updating reduced-order model can be regarded as a benchmark model for further analysis. A long-term monitoring for structural dynamic characteristics of Shanghai Tower under different construction stages was also investigated. The identified results, including natural frequency and damping ratio, were discussed. Based on the data collected from the SHM system, the dynamic characteristics of the whole structure was investigated. Compared with the result of the finite element model, a good agreement can be observed. The result provides a valuable reference for examining the evolution of future dynamic characteristics of this super tall building.

Key Words
super high-rise building; reduced-order model; regularization matrix method; dynamic characteristics; modal identification

Address
Hai-Bei Xiong and Ji-Xing Cao: Department of Disaster Mitigation for Structures, Tongji University, 1239 Siping Road, Shanghai, China
Feng-Liang Zhang: School of Civil and Environmental Engineering, Harbin Institute of Technology (HIT), Shenzhen, China;
Department of Disaster Mitigation for Structures, Tongji University, 1239 Siping Road, Shanghai, China
Xiang Ou: East China Region of Tahoe (Group) Co. Ltd., Shanghai, China
Chen-Jie Chen: Shanghai Architectural Design & Research (Co., Ltd.), Shanghai, China


Abstract
The frequently excessive vibrations presented in civil structures during seismic events or service conditions may result in users\' discomfort, or worst, in structures failure, producing economic and even human casualties. This work contributes in proposing the synthesis of a nonlinear optimal control strategy for semiactive structural control, with the main characteristic that the synthesis considers both the structure model and the semiactive actuator nonlinear dynamics, which produces a nonlinear system that requires a nonlinear controller design. The aim is to reduce the unwanted vibrations in the response of civil structures, by means of intelligent fluid semiactive actuator such as the Magnetorheological Damper (MRD), which is a device with a low level of power consumption. The civil structures for which the proposed control methodology can be applied are those admitting a state-dependent coefficient factorized representation model, such as buildings, bridges, among others. A scaled model of a three storey building is analyzed as a case study, whose dynamical response involves displacement, velocity and acceleration of each one of the storeys, subjected to the North-South component of the September 19th., 2017, Puebla-Morelos (7.1M), Mexico earthquake. The investigation rests on comparing the structural response over time for two different conditions: with no control device installed and with one MRD installed between the first floor and the ground, where a nonlinear optimal signal for the MRD input voltage is determined. Simulation results are presented to show the effectiveness of the proposed controller for reducing the building\'s dynamical response.

Key Words
structural control; nonlinear optimal control; MR damper; seismic protection; semiactive control

Address
Joaquin Contreras-Lopez, Fernando Ornelas-Tellez and Elisa Espinosa-Juarez: Faculty of Electrical Engineering, Universidad Michoacana de San Nicolas de Hidalgo,
Ciudad Universitaria, 58030, Morelia, Mich., Mexico



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