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CONTENTS
Volume 2, Number 1, January 2006
 

Abstract
The optimal design of multiple tuned mass dampers (multiple TMD\'s) to suppress multi-mode structural response of beams and floor structures was investigated. A new method using a numerical optimizer, which can effectively handle a large number of design variables, was employed to search for both optimal placement and tuning of TMD\'s for these structures under wide-band loading. The first design problem considered was vibration control of a simple beam using 10 TMD\'s. The results confirmed that for structures with widely-spaced natural frequencies, multiple TMD\'s can be adequately designed by treating each structural vibration mode as an equivalent SDOF system. Next, the control of a beam structure with two closely-spaced natural frequencies was investigated. The results showed that the most effective multiple TMD\'s have their natural frequencies distributed over a range covering the two controlled structural frequencies and have low damping ratios. Moreover, a single TMD can also be made effective in controlling two modes with closely spaced frequencies by a newly identified control mechanism, but the effectiveness can be greatly impaired when the loading position changes. Finally, a realistic problem of a large floor structure with 5 closely spaced frequencies was presented. The acceleration responses at 5 positions on the floor excited by 3 wide-band forces were simultaneously suppressed using 10 TMD\'s. The obtained multiple TMD\'s were shown to be very effective and robust.

Key Words
multiple tuned mass dampers; optimal design; robustness design; closely-spaced natural frequencies structures; nonlinear programming.

Address
ennung Warnitchai; School of Civil Engineering, Asian Institute of Technology, P.O. Box 4 Klongluang,
Pathumthani 12120, Thailand
Nam Hoang; Department of Civil Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656, Japan

Abstract
Aging wiring in buildings, aircraft and transportation systems, consumer products, industrial machinery, etc. is among the most significant potential causes of catastrophic failure and maintenance cost in these structures. Smart wire health monitoring can therefore have a substantial impact on the overall health monitoring of the system. Reflectometry is commonly used for locating faults on wire and cables. This paper compares Time domain reflectometry (TDR), frequency domain reflectometry (FDR), mixed signal reflectometry (MSR), sequence time domain reflectometry (STDR), spread spectrum time domain reflectometry (SSTDR) and capacitance sensors in terms of their accuracy, convenience, cost, size, and ease of use. Advantages and limitations of each method are outlined and evaluated for several types of aircraft cables. The results in this paper can be extrapolated to other types of wire and cable systems.

Key Words
electrical wiring; reflectometry; nondestructive evaluation; aircraft maintenance.

Address
Cynthia Furse; University of Utah Department of Electrical and Computer Engineering, 50 S Campus Drive, Salt Lake City, Utah 84112, USA
You Chung Chung; Information and Communication Engineering Department, Daegu University, Kyungsan,
Kyungbuk 712-714, Korea
Chet Lo and Praveen Pendayala; University of Utah Department of Electrical and Computer Engineering, 50 S Campus Drive, Salt Lake City, Utah 84112, USA

Abstract
Bi-spectrum is a tool in the signal processing for identification of non-linear dynamic behvaiour in systems, and well-known for stationary system where components are non-linearly interacting. Breathing of a crack during shaft rotation is also exhibits a non-linear behaviour. The crack is known to generate 2X (twice the machine RPM) and higher harmonics in addition to 1X component in the shaft response during its rotation. Misaligned shaft also shows similar such feature as a crack in a shaft. The bi-spectrum method has now been applied on a small rotating rig to observe its features. The bi-spectrum results are found to be encouraging to distinguish these faults based on few experiments conducted on a small rig. The results are presented here.

Key Words
rotating machine; non-linear dynamics; cracked shaft; misaligned shaft; vibration experiments; bi-spectrum.

Address
Vibration Laboratory, Reactor Engineering Division, Bhabha Atomic Research Centre,
Mumbai 400 085, India

Abstract
The concept of structural vibration control is to absorb vibration energy of the structure by introducing auxiliary devices. Various types of structural vibration control theories and devices have been recently developed and introduced into mechanical systems. One of such devices is damper employing controllable fluids such as ElectroRheological (ER) or MagnetoRheological (MR) fluids. MagnetoRheological (MR) materials are suspensions of fine magnetizable ferromagnetic particles in a non-magnetic medium exhibiting controllable rheological behaviour in the presence of an applied magnetic field. This paper presents the modelling of an MR-fluid damper. The damper model is developed based on Newtonian shear flow and Bingham plastic shear flow models. The geometric parameters are varied to get the optimised damper characteristics. The numerical analysis is carried out to estimate the damping coefficient and damping force. The analytical results are compared with the experimental results. The results confirm that MR damper is one of the most promising new semi-active devices for structural vibration control.

Key Words
MagnetoRheological fluid; MR damper; Bingham plastic model; Newtonian fluid; vibration; nose landing gear.

Address
Dipak K. Maiti; Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur-721302, India
P. P. Shyju; Department of Civil Engineering, Malnad College of Engineering, Hassan, India
K. Vijayaraju; Airframe Directorate, Aeronautical Development Agency, Vimanapura, Bangalore-560017, India

Abstract
To calculate the shear capacity of concrete beams reinforced with fibre-reinforced polymer (FRP), current shear design provisions use slightly modified versions of existing semi-empirical shear design equations that were primarily derived from experimental data generated on concrete beams having steel reinforcement. However, FRP materials have different mechanical properties and mode of failure than steel, and extending existing shear design equations for steel reinforced beams to cover concrete beams reinforced with FRP is questionable. This paper investigates the feasibility of using artificial neural networks (ANNs) to estimate the nominal shear capacity, Vn of concrete beams reinforced with FRP bars. Experimental data on 150 FRP-reinforced beams were retrieved from published literature. The resulting database was used to evaluate the validity of several existing shear design methods for FRP reinforced beams, namely the ACI 440-03, CSA S806-02, JSCE-97, and ISIS Canada-01. The database was also used to develop an ANN model to predict the shear capacity of FRP reinforced concrete beams. Results show that current guidelines are either inadequate or very conservative in estimating the shear strength of FRP reinforced concrete beams. Based on ANN predictions, modified equations are proposed for the shear design of FRP reinforced concrete beams and proved to be more accurate than existing equations.

Key Words
neural networks; fibre-reinforced polymer; shear strength; RC beams.

Address
Department of Civil and Environmental Engineering, The University of Western Ontario, London,
Ontario, Canada N6A 5B9


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