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CONTENTS
Volume 20, Number 6, December 2017
 

Abstract
The effects of vertical component of earthquakes on torsional amplification due to mass eccentricity in seismic responses of base-isolated structures subjected to near-field ground motions are studied in this paper. 3-, 6- and 9-story superstructures and aspect ratios of 1, 2 and 3 have been modeled as steel special moment frames mounted on Triple Concave Friction Pendulum (TCFP) bearings considering different period and damping ratios. Three-dimensional linear superstructures resting on nonlinear isolators are subjected to both 2 and 3 component near-field ground motions. Effects of mass eccentricity and vertical component of 25 near-field earthquakes on the seismic responses including maximum isolator displacement and base shear as well as peak superstructure acceleration are studied. The results indicate that the effect of vertical component on the responses of asymmetric structures, especially on the base shear is significant. Therefore, it can be claimed that in the absence of the vertical component, mass eccentricity has a little effect on the base shear increase. Additionally, the impact of this component on acceleration is remarkable so the roof acceleration of a nine-story structure has been increased 1.67 times, compared to the case that the structure is subjected to only horizontal components of earthquakes.

Key Words
TCFP isolators;near-field earthquakes; eccentricity;vertical component; steel frames

Address
Nasim Partovi Mehr and Faramarz Khoshnoudian: Faculty of Civil Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
Hamed Tajammolian: Faculty of Civil Engineering, Yazd University, Yazd, Iran

Abstract
In this research, an active vibration suppression of a smart beam having piezoelectric sensor and actuators is investigated by designing separate controllers comprising a linear quadratic regulator and a neural network. Firstly, design of a smart beam which consists of a cantilever aluminum beam with surface bonded piezoelectric patches and a designed mechanism having a micro servomotor with a mass attached arm for obtaining variations in the frequency response function are presented. Secondly, the frequency response functions of the smart beam are investigated experimentally by using different piezoelectric patch combinations and the analytical models of the smart beam around its first resonance frequency region for various servomotor arm angle configurations are obtained. Then, a linear quadratic regulator controller is designed and used to simulate the suppression of free and forced vibrations which are performed both in time and frequency domain. In parallel to simulations, experiments are conducted to observe the closed loop behavior of the smart beam and the results are compared as well. Finally, active vibration suppression of the smart beam is investigated by using a linear controller with a neural network based adaptive element which is designed for the purpose of overcoming the undesired consequences due to variations in the real system.

Key Words
active vibration suppression; system identification; piezoelectricity; linear quadratic regulator; artificial neural network; performance comparison

Address
Onur Akin: Aerospace Engineer, Department of Aerospace Engineering,Middle East Technical University, Ankara, Turkey
Melin Şahin: Department of Aerospace Engineering, Middle East Technical University, Ankara, Turkey

Abstract
The normalised version of bispectrum, the so-called bicoherence, has often proved a reliable method of damage detection on engineering applications. Indeed, higher-order spectral analysis (HOSA) has the advantage of being able to detect non-linearity in the structural dynamic response while being insensitive to ambient vibrations. Skewness in the response may be easily spotted and related to damage conditions, as the majority of common faults and cracks shows bilinear effects. The present study tries to extend the application of HOSA to damage localisation, resorting to a neural network based classification algorithm. In order to validate the approach, a non-linear finite element model of a 4-meters-long cantilever beam has been built. This model could be seen as a first generic concept of more complex structural systems, such as aircraft wings, wind turbine blades, etc. The main aim of the study is to train a Neural Network (NN) able to classify different damage locations, when fed with bispectra. These are computed using the dynamic response of the FE nonlinear model to random noise excitation.

Key Words
structural health monitoring; damage detection; higher-order spectral analysis; bispectrum; neural network; non-linear vibrations; breathing crack

Address
M. Civera and C. Surace: Department of Structural, Building and Geotechnical engineering, Politecnico di Torino, Corso Duca degli Abruzzi, 24, Turin, 10129, Italy
Zanotti Fragonara: School of Aerospace, Transportation and Manufacturing, Cranfield University, College Road, Cranfield, MK43 0AL, United Kingdom



Abstract
In this study, the reliability of nonlinear ultrasonic modulation based fatigue crack detection is improved using a feature-level data fusion approach. When two ultrasonic inputs at two distinct frequencies are applied to a specimen with a fatigue crack, modulation components at the summation and difference of these two input frequencies appear. First, the spectral amplitudes of the modulation components and their spectral correlations are defined as individual features. Then, a 2D feature space is constructed by combining these two features, and the presence of a fatigue crack is identified in the feature space. The effectiveness of the proposed fatigue crack detection technique is experimentally validated through cyclic loading tests of aluminum plates, full-scale steel girders and a rotating shaft component. Subsequently, the improved reliability of the proposed technique is quantitatively investigated using receiver operating characteristic analysis. The uniqueness of this study lies in (1) improvement of nonlinear ultrasonic modulation based fatigue crack detection reliability using feature-level data fusion, (2) reference-free fatigue crack diagnosis without using the baseline data obtained from the intact condition of the structure, (3) application to full-scale steel girders and shaft component, and (4) quantitative investigation of the improved reliability using receiver operating characteristic analysis.

Key Words
fatigue crack; nonlinear ultrasonic modulation; structural health monitoring; data fusion; receiver operating characteristic analysis

Address
Hyung Jin Lim, Yongtak Kim, Hoon Sohn, Ikgeun Jeon and Peipei Liu: Department of Civil and Environmental Engineering, Korea Advanced Institute for Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea


Abstract
Maintenance plays a critical role in the bridge industry, but actual practices show many limitations because of traditional, 2D-based information systems. It is necessary to develop a new generation of maintenance information management systems for more reliable decision making in bridge maintenance. Enhancing current work processes requires a BIM-based 3D digital model that can use information from the whole lifecycle of a project (design, construction, operation, and maintenance) through continuous exchanges and updates from each stakeholder. This study describes the development of a data scheme for maintenance of cable-stayed bridges. We implemented the proposed system for a cable-stayed bridge and discussed its effectiveness.

Key Words
bridge maintenance; BIM; 3D digital model; data schema; cable-stayed bridge

Address
Chang-su Shim, Hwirang Kang and Ngoc Son Dang: Department of Civil Engineering, Chung-Ang University, 310 - 434, 84 Heukseok-ro, Dongjak-gu, Seoul, Republic of Korea
Deokkeun Lee: Long Span Bridge Management Center, Korea Infrastructure Safety & Technology Corporation
131, Sadeul-ro, Jinju-si, Gyeongsangnam-do, Republic of Korea


Abstract
This disquisition proposes a nonlocal strain gradient beam theory for thermo-mechanical dynamic characteristics of embedded smart shear deformable curved piezoelectric nanobeams made of porous electro-elastic functionally graded materials by using an analytical method. Electro-elastic properties of embedded curved porous FG nanobeam are assumed to be temperature-dependent and vary through the thickness direction of beam according to the power-law which is modified to approximate material properties for even distributions of porosities. It is perceived that during manufacturing of functionally graded materials (FGMs) porosities and micro-voids can be occurred inside the material. Since variation of pores along the thickness direction influences the mechanical and physical properties, so in this study thermo-mechanical vibration analysis of curve FG piezoelectric nanobeam by considering the effect of these imperfections is performed. Nonlocal strain gradient elasticity theory is utilized to consider the size effects in which the stress for not only the nonlocal stress field but also the strain gradients stress field. The governing equations and related boundary condition of embedded smart curved porous FG nanobeam subjected to thermal and electric field are derived via the energy method based on Timoshenko beam theory. An analytical Navier solution procedure is utilized to achieve the natural frequencies of porous FG curved piezoelectric nanobeam resting on Winkler and Pasternak foundation. The results for simpler states are confirmed with known data in the literature. The effects of various parameters such as nonlocality parameter, electric voltage, coefficient of porosity, elastic foundation parameters, thermal effect, gradient index, strain gradient, elastic opening angle and slenderness ratio on the natural frequency of embedded curved FG porous piezoelectric nanobeam are successfully discussed. It is concluded that these parameters play important roles on the dynamic behavior of porous FG curved nanobeam. Presented numerical results can serve as benchmarks for future analyses of curve FG nanobeam with porosity phases.

Key Words
curved piezoelectric FG nanobeam; porous materials; thermo-mechanical vibration; nonlocal strain gradient theory; smart materials; winkler-pasternak foundations

Address
Farzad Ebrahimi, Mohsen Daman and Ali Jafari: Department of Mechanical Engineering, Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran P.O.B. 16818-34149


Abstract
Piezoelectric composite laminates are a powerful material system that offers vast options to improve structural behavior. Successful design of piezoelectric adaptive structures and testing of control laws call for highly accurate, reliable and numerically efficient numerical tools. This paper puts focus onto linear and geometrically nonlinear static and dynamic analysis of smart structures made of such a material system. For this purpose, highly efficient linear 3-node and 4-node finite shell elements are proposed. Both elements employ the Mindlin-Reissner kinematics. The shear locking effect is treated by the discrete shear gap (DSG) technique with the 3-node element and by the assumed natural strain (ANS) approach with the 4-node element. Geometrically nonlinear effects are considered using the co-rotational approach. Static and dynamic examples involving actuator and sensor function of piezoelectric layers are considered.

Key Words
active structure; linear shell element; piezoelectricity; co-rotational FEM; geometric nonlinearity

Address
Gil Rama and Manfred W. Zehn: Department of Structural Analysis, Berlin Institute of Technology, Germany
Dragan Z. Marinkovic: Department of Structural Analysis, Berlin Institute of Technology, Germany;
Faculty of Mechanical Engineering, University of Nis, Serbia


Abstract
The main goal of this study is to investigate the hysteretic behavior of polyurethane rubber springs in compression with and without precompression. The precompression is introduced to provide rigid force in the behavior, and thereby a precompressed rubber spring can be used for a restoring element. For the goal, this study prepares nine rubber springs for three suites which are all cylindrical in shape with a hole at the center. The rubber springs in each suite have different dimensions of diameter and length but have similar shape factors; thus, they are designed to have a similar compressive stiffness. Three rubber springs from the nine are tested with increasing compressive strain up to 30% strain to investigate the behavior of the rubber springs without precompression as well as the effect of the loading strain. The nine springs are compressed up to 30% strain with increasing precompressive strain from 0 to 20% at increments of 5%. The study analyzes the effective stiffness and damping ratio of the rubber springs with and without precompression, and the rigid force of the precompressed rubber springs is discussed. Finally, this study suggests a regression method to determine the minimum required precompression to eliminate residual strain after unloading.

Key Words
rubber spring; hysteretic behavior; precompression; self-centering; smart damper

Address
Eunsoo Choi: Department of Civil Engineering, Hongik University, Seoul 04066, Korea
Jong-Su Jeon: Department of Civil Engineering, Andong National University, Andong, Gyeongsanbuk-do 36729, Korea
Junwon Seo: Department of Civil and Environmental Engineering, South Dakota State University, Brookings, SD 57007, USA

Abstract
Wear and aging associated damage is a severe problem for safety and maintenance of engineering structures. To acquire structural operational state and provide warning about different types of damage, research on damage identification has gained increasing popularity in recent years. Among various damage identification methods, the Lamb wave-based methods have shown promising suitability and potential for damage identification of plate-type structures. In this paper, a comprehensive study was presented to elaborate four remarkable aspects regarding the Lamb wave-based damage identification method for plate-type structures, including wave velocity, signal denoising, image reconstruction, and sensor layout. Conclusions and path forward were summarized and classified serving as a starting point for research and application in this area.

Key Words
Lamb wave; damage identification; structural health monitoring; wavelet denoising; finite element modeling; sensor array; beamforming

Address
Zijian Wang: Department of Dam Safety Management, Nanjing Hydraulic Research Institute, Nanjing, 210029, China
Pizhong Qiao: Department of Civil and Environmental Engineering and Composite Materials and Engineering Center,
Washington State University, Pullman, WA, 99164-2910, USA
Binkai Shi: School of Mechanics and Materials, Hohai University, Nanjing, 210098, PR China


Abstract
Cable force monitoring is an essential and critical part of the safety evaluation of cable-supported bridges. A reasonable cable force monitoring scheme, particularly, sensor placement related to accurate safety assessment and budget cost-saving becomes a major concern of bridge administrative authorities. This paper presents optimal sensor placement for cable force monitoring by selecting representative sensor positions, which consider the spatial correlativeness existing in the cable group. The limited sensors would be utilized for maximizing useful information from the monitored bridges. The maximum information coefficient (MIC), mutual information (MI) based kernel density estimation, as well as Pearson coefficients, were all employed to detect potential spatial correlation in the cable group. Compared with the Pearson coefficient and MIC, the mutual information is more suitable for identifying the association existing in cable group and thus, is selected to describe the spatial relevance in this study. Then, the bond energy algorithm, which collects clusters based on the relationship of surrounding elements, is used for the optimal placement of cable sensors. Several optimal placement strategies are discussed with different correlation thresholds for the cable group of Nanjing No.3 Yangtze River Bridge, verifying the effectiveness of the proposed method.

Key Words
structural health monitoring (SHM); spatial correlation; bridges; optimum design; sensor/sensor placement

Address
Shunlong Li and Hui Li: Department of Bridge and Tunnel engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin 150090, China
Jialin Dong: Zhejiang Scientific Research Institute of Transport, 188 Gangyang, Hangzhou 311305, China
Wei Lu: Department of Civil and Environment Engineering, Harbin Institute of Technology (Shenzhen),
HIT Campus of Xili University Town, Shenzhen 518055, China
Wencheng Xu and Yao Jin: CCCC Highway Consultants CO., Ltd. (HPDI), 85 Deshengmenwai Street, Beijing 100000, China


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