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CONTENTS
Volume 9, Number 1, January 2012
 

Abstract
In bridge engineering, maintenance strategies and thus budgetary demands are highly influenced by construction type and quality of design. Nowadays bridge owners and planners tend to include life-cycle cost analyses in their decision processes regarding the overall design trying to optimize structural reliability and durability within financial constraints. Smart permanent and short term monitoring can reduce the associated risk of new design concepts by observing the performance of structural components during prescribed time periods. The objectives of this paper are the discussion and analysis of influence line or influence field approaches in terms of (a) an efficient incorporation of monitoring information in the structural performance assessment, (b) an efficient characterization of performance indicators for the assessment of structures, (c) the ability of optimizing the positions of sensors of a monitoring system, and (d) the ability of checking the robustness of the monitoring systems applied to a structure. The proposed influence line- model correction approach has been applied to an integrative monitoring system that has been installed for the performance assessment of an existing three-span jointless bridge.

Key Words
influence lines model correction approach; model updatimg; fiber optical monitoring systems; LVDT monitoring systems; proof loading.

Address
Alfred Strauss,Roman Wendner and Konrad Bergmeister : Department of Civil Engineering and Natural Hazards, University of Natural Resources and Life Sciences, Vienna, A-1190, Austria
Dan M. Frangopol : Department of Civil and Environmental Engineering, ATLSS Engineering Research Center, Lehigh Univ., 117 ATLSS Dr., Bethlehem, PA 18015-4729, USA

Abstract
In this study, a Lamb-wave based damage detection approach is proposed for damage localization in plate. A sensor network consisting of three PZT wafer type actuators/sensors is used to generate and detect Lamb waves. To minimize the complication resulted from the multimode and dispersive characteristics of Lamb waves, the fundamental symmetric Lamb mode, S0 is selectively generated through designing the excitation frequency of the narrowband input signal. A damage localization algorithm based upon the configuration of the PZT sensor network is developed. Time-frequency analysis method is applied to purify the raw signal and extract damage features. Experimental result obtained from aluminum plate verified the proposed damage localization approach.

Key Words
damage localizations; structural health monitoring; Lamb waves; piezoelectric sensors; timefrequency analysis.

Address
Xinglong Liu : National Engineering Laboratory for System Integration of High Speed Train (South), CSR Qingdao Sifang, Co., Ltd, Qingdao, China
Chengxu Zhou and Zhongwei Jiang : Department of Mechanical Engineering, Engineering Faculty, Yamaguchi University, Ube, Japan

Abstract
Mechanical dampers have been proved to be one of the most effective countermeasures for vibration mitigation of stay cables in various cable-stayed bridges over the world. However, for long stay cables, as the installation height of the damper is restricted due to the aesthetic concern, using passive dampers alone may not satisfy the control requirement of the stay cables. In this connection, semi-active MR dampers have been proposed for the vibration mitigation of long stay cables. Although various studies have been carried out on the implementation of MR dampers on stay cables, the optimal damping performance of the cable-MR damper system has yet to be evaluated. Therefore, this paper aims to investigate the effectiveness of MR damper as a semi-active control device for the vibration mitigation of stay cable. The mathematical model of the MR damper will first be established through a performance test. Then, an efficient semi-active control strategy will be derived, where the damping of MR damper will be tuned according to the dynamic characteristics of stay cable, in order to achieve optimal damping of cable-damper system. Simulation study will be carried out to verify the proposed semi-active control algorithm for suppressing the cable vibrations induced by different loading patterns using optimally tuned MR damper. Finally, the effectiveness of MR damper in mitigating multi modes of cable vibration will be examined theoretically

Key Words
long stay cable; vibration mitigation; MR damper; semi-active control; performance evaluation.

Address
Hongwei Huang : State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China
Limin Sun : Department of Bridge Engineering, Tongji University, Shanghai, China
Xiaolu Jiang : Shanghai Urban Construction Design & Research Institute, Shanghai, China

Abstract
It is not easy to experimentally obtain the FRF (Frequency Response Function) matrix corresponding to a full set of DOFs (degrees of freedom) for a dynamic system. Utilizing FRF data measured at specific positions, with DOFs less than that of the system, as constraints to describe a damaged system, this study identifies parameter matrices such as mass, stiffness and damping matrices of the system, and provides a damage identification method from their variations. The proposed parameter identification method is compared to Lee and Kim

Key Words
frequency response function; dynamic stiffness matrix; receptance; damping matrix; minimization.

Address
Salam Rahmatalla : Faculty of Department of Civil and Environmental Engineering, University of Iowa, Iowa City, USA
Hee-Chang Eun : Faculty of Department of Architectural Engineering, Kangwon National University, Chuncheon, Korea
Eun-Taik Lee : Faculty of Department of Architectural Engineering, Chung-Ang University, Seoul, Korea

Abstract
In this research, a typical tension-leg type of floating platform incorporated with an innovative concept of underwater tuned liquid column damper system (UWTLCD) is studied. The purpose of this study is to improve the structural safety by means of mitigating the wave induced vibrations and stresses on the offshore floating Tension Leg Platform (TLP) system. Based on some encouraging results from a previous study, where a Tuned Liquid Column Damper (TLCD) system was employed in a floating platform system to reduce the vibration of the main structure, in this study, the traditional TLCD system was modified and tested. Firstly, the orifice-tube was replaced with a smaller horizontal tube and secondly, the TLCD system was combined into the pontoon system under the platform. The modification creates a multipurpose pontoon system associated with vibration mitigation function. On the other hand, the UWTLCD that is installed underwater instead would not occupy any additional space on the platform and yet provide buoyancy to the system. Experimental tests were performed for the mitigation effect and parameters besides the wave conditions, such as pontoon draught and liquid-length in the TLCD were taken into account in the test. It is found that the accurately tuned UWTLCD system could effectively reduce the dynamic response of the offshore platform system in terms of both the vibration amplitude and tensile forces measured in the mooring tethers.

Key Words
vibration mitigation; Under Water TLCD (UWTLCD); TLP system; experimental water tank testing; stochastic analysis.

Address
Hsien Hua Lee and H.H. Juang : Department of Marine Environment and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan


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