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CONTENTS
Volume 24, Number 3, September 2019
 

Abstract
Bolted joints are commonly used in civil infrastructure and mechanical assembly structures. Monitoring and identifying the connection status of bolts is the frontier problem of structural research. The existing research is mainly on the looseness of a single bolt. This article presents a study of assessing the loosening/tightening health state and identifying the loose bolt by using ultrasonic guided wave in a bolt group joint. A bolt-tightening index was proposed for evaluating the looseness of a bolt connection based on correlation coefficient. The tightening/loosening state of the bolt was simulated by changing the bolt torque. More than 180 different measurement tests for total of six bolts were conducted. The results showed that with the bolt torque increases, value of the proposed bolt-tightening index increases. The proposed bolt-tightening index trend was very well reproduced by an analytical expression using a function of the torque applied with an overall percentage error lower than 5%. The developed damage index based on the proposed bolt-tightening index can also be applied to locate the loosest bolt in a bolt group joint. To verify the effectiveness of the proposed method, a bolt group joint experiment with different positions of bolt looseness was performed. Experimental results show that the proposed approach is effective to detect and locate bolt looseness and has a good prospect of finding applications in real-time structural monitoring.

Key Words
bolt looseness; ultrasonic guided wave; correlation coefficients; structural health monitoring

Address
Yue Zhang, Dongsheng Li and Xutao Zheng: School of Civil Engineering, Dalian University of Technology, Dalian 116024, China

Abstract
Magneto-rheological fluid (MRF) rotatory dampers are normally used for controlling the constant rotation of machines and engines. In this research, such a device is proposed to act as variable stiffness device to alleviate the rotational oscillation existing in the many engineering applications, such as motor. Under such thought, the main purpose of this work is to characterize the nonlinear torque-angular displacement/angular velocity responses of an MRF based variable stiffness device in oscillatory motion. A rotational hysteresis model, consisting of a rotatory spring, a rotatory viscous damping element and an error function-based hysteresis element, is proposed, which is capable of describing the unique dynamical characteristics of this smart device. To estimate the optimal model parameters, a modified whale optimization algorithm (MWOA) is employed on the captured experimental data of torque, angular displacement and angular velocity under various excitation conditions. In MWOA, a nonlinear algorithm parameter updating mechanism is adopted to replace the traditional linear one, enhancing the global search ability initially and the local search ability at the later stage of the algorithm evolution. Additionally, the immune operation is introduced in the whale individual selection, improving the identification accuracy of solution. Finally, the dynamic testing results are used to validate the performance of the proposed model and the effectiveness of the proposed optimization algorithm.

Key Words
magneto-rheological fluid; variable stiffness device (VSD); rotational hysteresis model; parameter identification; whale optimization algorithm

Address
Yang Yu, Yancheng Li, Jianchun Li and Xiaoyu Gu: Centre for Built Infrastructure Research, School of Civil and Environmental Engineering, University of Technology Sydney, 15 Broadway, Ultimo NSW 2007, Australia


Abstract
Structural health monitoring (SHM) is of great importance to super high-rise buildings. The Shanghai Tower is currently the tallest building in China, and a complete SHM system was simultaneously constructed at the beginning of the construction of the tower. Due to the variety of sensor types and the large number of measurement points in the SHM system, an online automatic structural health assessment method with few computations and no manual intervention is needed. This paper introduces a structural health assessment method for the Shanghai Tower that uses the coefficients of an autoregressive (AR) time series model as structural state indicators. An analysis of collected data indicates that the coefficients of the AR model are affected by environmental factors, and the principal component analysis method is used to remove the influence of environmental factors. Finally, the control chart method is used to track the changes in structural state indicators, and a plan for online automatic structure health state evaluation is proposed. This method is applied to long-term acceleration and inclination data from the Shanghai Tower and successfully identifies the changes in the structural state. Overall, the structural state indicators of the Shanghai Tower are stable, and the structure is in a healthy state.

Key Words
structural health assessment; structural health monitoring; principal component analysis; Shanghai Tower; online automatic system

Address
Qilin Zhang, Xiaoxiang Tang, Jie Wuand Bin Yang: College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China

Abstract
The possibility of adopting vibration-powered wireless nodes has been largely investigated in the last years. Among the available technologies based on the piezoelectric effect, the most common ones consist of a vibrating beam covered by electroactive layers. Another energy harvesting strategy is based on the use of piezoelectric strips attached to a hosting structure subjected to dynamic loads. The hosting structure, for example, can be the system to be equipped with wireless nodes. Such strategy has received few attentions so far and no analytical studies have been presented yet. Hence, the original contribution of the present paper is concerned with the development of analytical solutions for the electrodynamic analysis and design of piezoelectric polymeric strips attached to relatively large linear elastic structural systems subjected to random vibrations at the base. Specifically, it is assumed that the dynamics of the hosting structure is dominated by the fundamental vibration mode only, and thus it is reduced to a linear elastic single-degree-of-freedom system. On the other hand, the random excitation at the base of the hosting structure is simulated by filtering a white Gaussian noise through a linear second-order filter. The electromechanical force exerted by the polymeric strip is negligible compared with other forces generated by the large hosting structure to which it is attached. By assuming a simplified electrical interface, useful new exact analytical expressions are derived to assess the generated electric power and the integrity of the harvester as well as to facilitate its optimum design.

Key Words
electrospun piezoelectric nanofibers; energy harvesting; random vibrations; smart structures; structural monitoring

Address
Francesco Trentadue: Department of Civil Engineering and Architecture, Technical University of Bari, via Orabona 4, 70125 Bari, Italy
Giuseppe Quaranta: Department of Structural and Geotechnical Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy
Claudio Maruccio: Department of Innovation Engineering, University of Salento, via Monteroni, 73100 Lecce, Italy
Giuseppe C. Marano: Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, corso Duca degli Abruzzi 24, 10129 Torino, Italy

Abstract
The shear keys are set in a seismic isolation system to resist the long-term service loadings, and are cut off to isolate the earthquakes. This paper investigated the influence of shear keys on the seismic performance of a vertical spring-viscous damper-concave Coulomb friction isolation system by an incremental dynamic analysis (IDA) and a performance-based assessment. Results show that the cutting off process of shear keys should be simulated in a numerical analysis to accurately predict the seismic responses of isolation system. Ignoring the cutting off process of shear keys usually leads to untrue seismic responses in a numerical analysis, and many of them are unsafe for the design of isolated structure. And those errors will be increased by increasing the cutting off force of shear keys and decreasing the spring constant of shear keys, especially under a feeble earthquake. The viscous damping action postpones the cutting off time of shear keys during earthquakes, and reduces the seismic isolation efficiency. However, this point can be improved by increasing the spring constant of shear keys.

Key Words
seismic isolation; shear key; friction; viscous damper; spring

Address
Biao Wei, Chaobin Li, Xiaolong Jia, Xuhui He and Menggang Yang: School of Civil Engineering, Central South University, 22 Shaoshan South Road, Changsha, China;
National Engineering Laboratory for High Speed Railway Construction,
22 Shaoshan South Road, Changsha, China


Abstract
The stiffness of shape memory alloy (SMA) spring while in actuation is represented by an empirical model that is derived from the logistic differential equation. This model correlates the stiffness to the alloy temperature and the functionality of SMA spring as active variable stiffness actuator (VSA) is analyzed based on factors that are the input conditions (activation current, duty cycle and excitation frequency) and operating conditions (pre-stress and mechanical connection). The model parameters are estimated by adopting the nonlinear least square method, henceforth, the model is validated experimentally. The average correlation factor of 0.95 between the model response and experimental results validates the proposed model. In furtherance, the justification is augmented from the comparison with existing stiffness models (logistic curve model and polynomial model). The important distinction from several observations regarding the comparison of the model prediction with the experimental states that it is more superior, flexible and adaptable than the existing. The nature of stiffness variation in the SMA spring is assessed also from the Dynamic Mechanical Thermal Analysis (DMTA), which as well proves the proposal. This model advances the ability to use SMA integrated mechanism for enhanced variable stiffness actuation. The investigation proves that the stiffness of SMA spring may be altered under controlled conditions.

Key Words
shape memory alloy spring; helical tension spring; joule heating; variable stiffness actuation; nonlinear differential equation; experimental analysis; DMTA

Address
Bhagoji B. Sul and K. Dhanalakshmi: Department of Instrumentation and Control Engineering, National Institute of Technology, Tiruchirappalli, India

Abstract
In this research, we propose an energy harvesting structure with a flexible blade element vibrating at its first mode to maximize the power output of the piezoelectric material. For this purpose, a piezoelectric patch was attached on the blade element used in a small-scale wind turbine, and air load was applied with a suitable angle of attack in the stall zone. The aerodynamic load created by air excitation vibrates the blade element in its first natural frequency and maximizes the voltage output of the piezoelectric patch. The variation of power outputs with respect to electrical resistance, air speed, and extra mass is experimentally investigated for various cases. An analytical model is constituted using a single-mode blade element with piezoelectric patch dynamics, and the power outputs of the obtained model are compared with experimental results.

Key Words
piezo-aeroelastic structure; energy harvesting; flexible structure; aerodynamic excitation

Address
Fevzi C. Bolat: Department of Mechanical Engineering, Bolu Abant İzzet Baysal University, Bolu, Turkey
lim Sivrioglu: Department of Mechanical Engineering, Gebze Technical University, Gebze, Turkey

Abstract
Control of vibrations against extraordinary excitations such as wind and earthquake is very important to the protection of life and financial concerns. One of the methods of structural control is to use Tuned Liquid Damper (TLD), however due to the nature of TLD only one sloshing frequency can be created when the water is sloshing. Among various ideas proposed to compensate this problem, by changing the angle of some rotatable baffles embedded inside a TLD, a frequency range is created such that these baffles are tuned manually at different frequencies. In this study, the effect of cross sectional shape of container with rotating baffles on seismic behavior of TLD is experimentally studied. For this purpose, rectangular and cylindrical containers are designed and used to suppress the vibrations of a Single Degree-Of-Freedom (SDOF) structure under harmonic and earthquake excitations considering three baffle angles. The results show that the rectangular-shaped damper reduces the structural response in all load cases more than the damper with a cylindrical shape, such that maximum differences of two dampers to reduce the structural displacement and structural acceleration are 5.5% and 3% respectively, when compared to the cases where no baffles are employed.

Key Words
rectangular TLD; cylindrical TLD; SDOF structure; rotatable baffles; shaking table test

Address
Seyed Mehdi Zahrai: Center of Excellence for Engineering and Management of Infrastructures, School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
Sirous Kakouei: School of Civil Engineering, University of Tehran, Tehran, Iran


Abstract
Offshore drilling has become a key process for obtaining oil. Offshore platforms have many applications, including oil exploration and production, navigation, ship loading and unloading, and bridge and causeway support. However, vibration problems caused by severe environmental loads, such as ice, wave, wind, and seismic loads, threaten the functionality of platform facilities and the comfort of workers. These concerns may result in piping failures, unsatisfactory equipment reliability, and safety concerns. Therefore, the vibration control of offshore platforms is essential for assuring structural safety, equipment functionality, and human comfort. In this study, an optimal multiple tuned mass damper (MTMD) system was proposed to mitigate the excessive vibration of a three-dimensional offshore platform under ice and earthquake loadings. The MTMD system was designed to control the first few dominant coupled modes. The optimal placement and system parameters of the MTMD are determined based on controlled modal properties. Numerical simulation results show that the proposed MTMD system can effectively reduce the displacement and acceleration responses of the offshore platform, thus improving safety and serviceability. Moreover, this study proposes an optimal design procedure for the MTMD system to determine the optimal location, moving direction, and system parameters of each unit of the tuned mass damper.

Key Words
offshore platform; multiple tuned mass dampers; vibration control; ice load; earthquake engineering

Address
Chih-Shiuan Lin: National Rail Transit Electrification and Automation Engineering Technology Research Center,
The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong;
Department of Civil Engineering and Engineering Mechanics, Columbia University, New York, NY, USA
Feifei Liu and Jigang Zhang:School of Civil Engineering, Qingdao University of Technology, Qingdao, Shandong, China
Jer-Fu Wang: Department of Civil and Disaster Prevention Engineering, National United University, Miaoli, Taiwan
Chi-Chang Lin: Department of Construction Engineering, Chaoyang University of Technology, Taichung, Taiwan;
Department of Civil Engineering, National Chung Hsing University, Taichung, Taiwan



Abstract
This study was carried out to evaluate the natural zeolite in producing green concrete as an effort to prevent global warming and environmental impact problems of cement industries. To achieve this target, two types of natural zeolites applied to study physical, chemical and compressive strength of concrete containing different percentages of zeolites. The results in comparison with control samples indicate that compressive strength of zeolites mixes increases with the 15% replacement of zeolite instead of cement in all types of samples. In the water-cement ratio of 0.6, results showed an increase in the compressive strength of all percentages of replacement. This results indicate that using natural zeolites could be produced a green concrete by a huge reduction and saving in the consumption of cement.

Key Words
cement; natural zeolite; green concrete; compressive strength

Address
Nguyen Thoi Trung: Division of Computational Mathematics and Engineering, Institute for Computational Science,
Ton Duc Thang University, Ho Chi Minh City, Viet Nam;
Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
Nima Alemi: Faculty of Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran
James H. Haido: Department of Civil Engineering, College of Engineering, University of Duhok, Kurdistan Region, Iraq
Mahdi Shariati: Institute of Research and Development, Duy Tan University, Da Nang 550000, Viet Nam
Seyedata Baradaran: Department of Civil Engineering, Islamic Azad University, Shabestar Branch, Shabestar, Iran
Salim T. Yousif: Department of civil engineering, Al-Qalam University College, Kirkuk, Iraq



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