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CONTENTS
Volume 20, Number 5, November 2017
 

Abstract
This paper explores different vibration control strategies for the cancellation of human-induced vibration on a structure with time-varying modal parameters. The main motivation of this study is a lively urban stress-ribbon footbridge (Pedro Gómez Bosque, Valladolid, Spain) that, after a whole-year monitoring, several natural frequencies within the band of interest (normal paring frequency range) have been tracked. The most perceptible vibration mode of the structure at approximately 1.8 Hz changes up to 20%. In order to find a solution for this real case, this paper takes the annual modal parameter estimates (approx. 14000 estimations) of this mode and designs three control strategies: a) a tuned mass damper (TMD) tuned to the most-repeated modal properties of the aforementioned mode, b) two semi-active TMD strategies, one with an on-off control law for the TMD damping, and other with frequency and damping tuned by updating the damper force. All strategies have been carefully compared considering two structure models: a) only the aforementioned mode and b) all the other tracked modes. The results have been compared considering human-induced vibrations and have helped the authors on making a decision of the most advisable strategy to be practically implemented.

Key Words
semi-active vibration control; dynamic behavior; time-varying modal-parameters; human-induced vibration; footbridge

Address
José M. Soria, Iván M. Díazand Jaime H. García-Palacios: Department of Continuous Medium Mechanics and Theory of Structures, ETS Ingenieros de Caminos,
Universidad Politécnica de Madrid, ES 28040, Madrid, Spain



Abstract
Thermoelasticity is a contactless technique for measuring stress distributions in structural elements stressed by dynamic loads. This work describes the characteristics, analyzes the main causes of uncertainty and illustrates a series of operative methods for reducing its effects. More specifically, the effects of the angle of view between the thermographic camera and the surface of the object are studied, along with those due to the heat transmission by conduction between the various parts of the thing being measured as a function of the stress frequencies. The analyses, both theoretical and experimental, are aimed at defining the operational limits and optimal measurement and test conditions in relation to the measurement uncertainty that is considered tolerable in the specific application.

Key Words
thermoelasticity; monitoring; uncertainty analysis; stress distribution measurement; thermography

Address
Roberto Marsili. Gianluca Rossi and Emanuela Speranzini:Department of Engineering, via G. Duranti, 93 – 06125 Perugia, Italy

Abstract
The US railroad network carries 40% of the nation\'s total freight. Railroad bridges are the most critical part of the network infrastructure and, therefore, must be properly maintained for the operational safety. Railroad managers inspect bridges by measuring displacements under train crossing events to assess their structural condition and prioritize bridge management and safety decisions accordingly. The displacement of a railroad bridge under train crossings is one parameter of interest to railroad bridge owners, as it quantifies a bridge\'s ability to perform safely and addresses its serviceability. Railroad bridges with poor track conditions will have amplified displacements under heavy loads due to impacts between the wheels and rail joints. Under these circumstances, vehicle-track-bridge interactions could cause excessive bridge displacements, and hence, unsafe train crossings. If displacements during train crossings could be measured objectively, owners could repair or replace less safe bridges first. However, data on bridge displacements is difficult to collect in the field as a fixed point of reference is required for measurement. Accelerations can be used to estimate dynamic displacements, but to date, the pseudo-static displacements cannot be measured using reference-free sensors. This study proposes a method to estimate total transverse displacements of a railroad bridge under live train loads using acceleration and tilt data at the top of the exterior pile bent of a standard timber trestle, where train derailment due to excessive lateral movement is the main concern. Researchers used real bridge transverse displacement data under train traffic from varying bridge serviceability levels. This study explores the design of a new bridge deck-pier experimental model that simulates the vibrations of railroad bridges under traffic using a shake table for the input of train crossing data collected from the field into a laboratory model of a standard timber railroad pile bent. Reference-free sensors measured both the inclination angle and accelerations of the pile cap. Various readings are used to estimate the total displacements of the bridge using data filtering. The estimated displacements are then compared to the true responses of the model measured with displacement sensors. An average peak error of 10% and a root mean square error average of 5% resulted, concluding that this method can cost-effectively measure the total displacement of railroad bridges without a fixed reference.

Key Words
bridges; experiment; sensor/sensor placement; smart sensor; structural dynamics; structural health monitoring (SHM); serviceability; railroad bridges; tilt; displacements; accelerations

Address
Ali I. Ozdagli, Jose A. Gomez and Fernando Moreu: Department of Civil Engineering, University of New Mexico, MSC01 1070, 1 University of New Mexico,
Albuquerque, NM 87131, Mexico


Abstract
The safety of structures is closely associated with the structural out-of-plane behavior. In particular, long and slender beam structures have been increasingly used in the design and construction. Therefore, an evaluation of the lateral and torsional behavior of a structure is important for the safety of the structure during construction as well as under service conditions. The current contact measurement method using displacement meters cannot measure independent movements directly and also requires caution when installing the displacement meters. Therefore, in this study, a vision-based system was used to measure the in-plane and out-of-plane displacements of a structure. The image processing algorithm was based on reference objects, including multiple targets in Lab color space. The captured targets were synchronized using a load indicator connected wirelessly to a data logger system in the server. A laboratory beam test was to compare the displacements and rotation obtained from the proposed vision-based measurement system with those from the current measurement method using string potentiometers. The test results that the proposed vision-based measurement system could be applied successfully and easily to evaluating both the in-plane and out-of-plane movements of a beam including twisting rotation.

Key Words
vision; displacement; in-plane; out-of-plane; twisting

Address
Jong-Han Lee: Department of Civil Engineering, Daegu University, Gyeongsan 38453, Republic of Korea
Chi-Young Jung: Seismic Simulation Test Center, Pusan National University, Yangsan 50612, Republic of Korea
Eunsoo Choi: Department of Civil Engineering, Hongik University, Seoul 04066, Republic of Korea
Jin-Hwan Cheung: Department of Civil Engineering, Pusan National University, Busan 46241, Republic of Korea

Abstract
A vibrating double-layered nanoscale piezoelectric plate is developed accounting for the flexoelectricity and surface effects. The flexoelectricity is due to the coupling between electrical polarization and strain gradient. Applying Hamilton\'s principle, the governing equations and related boundary conditions are derived. Assuming suitable approximate functions, the governing equations are numerically solved for simply-supported and clamped boundary conditions. Obtained results indicate that both the flexoelectricity and surface effects possess notable impact on the vibration frequencies of the system. Only flexoelectricity yields a considerable difference between the present model and previous investigations on conventional piezoelectric nanoplates. Generally, a parametric study has been performed to examine the effects of surface elasticity, flexoelectricity, applied electric voltage, interlayer stiffness, geometrical parameters and boundary conditions on vibration frequencies of piezoelectric nanoplates.

Key Words
vibration; flexoelectric nanoplate; surface effect; double-layered nanoplate

Address
Mohammad Reza Barati: Aerospace Engineering Department & Center of Excellence in Computational Aerospace,
Amirkabir University of Technology, Tehran, Iran


Abstract
This research deals with the nonlocal temperature-dependent dynamic buckling analysis of embedded sandwich micro plates reinforced by functionally graded carbon nanotubes (FG-CNTs). The material properties of structure are assumed viscoelastic based on Kelvin–Voigt model. The effective material properties of structure are considered based on mixture rule. The elastic medium is simulated by orthotropic visco-Pasternak medium. The motion equations are derived applying Sinusoidal shear deformation theory (SSDT) in which the size effects are considered using Eringen\'s nonlocal theory. The differential quadrature (DQ) method in conjunction with the Bolotin\'s methods is applied for calculating resonance frequency and dynamic instability region (DIR) of structure. The effects of different parameters such as volume percent of CNTs, distribution type of CNTs, temperature, nonlocal parameter and structural damping on the dynamic instability of visco-system are shown. The results are compared with other published works in the literature. Results indicate that the CNTs have an important role in dynamic stability of structure and FGX distribution type is the better choice.

Key Words
dynamic buckling; FG-CNT; nanocomposite sandwich micro plate; SSDT; viscoelastic

Address
Maryam Shokravi: Buein Zahra Technical University, Buein Zahra, Qazvin, Iran
Nader Jalili: Piezoactive Systems Laboratory, Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02115, USA


Abstract
The present article reported the thermal buckling strength of the sandwich shell panel structure and subsequent improvement of the same by embedding shape memory alloy (SMA) fibre via a general higher-order mathematical model in conjunction with finite element method. The geometrical distortion of the panel structure due to the temperature is included using Green-Lagrange strain-displacement relations. In addition, the material nonlinearity of SMA fibre due to the elevated thermal environment also incorporated in the current analysis through the marching technique. The final form of the equilibrium equation is obtained by minimising the total potential energy functional and solved computationally with the help of an original MATLAB code. The convergence and the accuracy of the developed model are demonstrated by solving similar kind of published numerical examples including the necessary input parameter. After the necessary establishment of the newly developed numerical solution, the model is extended further to examine the effect of the different structural parameters (side-to-thickness ratios, curvature ratios, core-to-face thickness ratios, volume fractions of SMA fibre and end conditions) on the buckling strength of the SMA embedded sandwich composite shell panel including the different geometrical configurations.

Key Words
buckling analysis; sandwich panel; SMA; HSDT; FEM

Address
ankaj V. Katariya, Subrata K. Panda, Chetan K. Hirwani and Kulmani Mehar: Department of Mechanical Engineering, NIT Rourkela, Rourkela-769008, Odisha, Sundergarh India
Omprakash Thakare: Department of Mechanical Engineering, G.D. Rungta College of Engineering, Bhilai, Chhatishgarh, India

Abstract
In this research, a newly developed nature-inspired optimization method, the Lion Pride Optimization algorithm (LPOA), is utilized for optimal design of composite steel box girder bridges. A composite box girder bridge is one of the common types of bridges used for medium spans due to their economic, aesthetic, and structural benefits. The aim of the present optimization procedure is to provide a feasible set of design variables in order to minimize the weight of the steel trapezoidal box girders. The solution space is delimited by different types of design constraints specified by the American Association of State Highway and Transportation Officials. Additionally, the optimal solution obtained by LPOA is compared to the results of other well-established meta-heuristic algorithms, namely Gray Wolf Optimization (GWO), Ant Lion Optimizer (ALO) and the results of former researches. By this comparison the capability of the LPOA in optimal design of composite steel box girder bridges is demonstrated.

Key Words
composite box girder; optimal design; lion pride optimization algorithm; constrained problems; meta-heuristic search

Address
A. Kavehand S. Mahjoubi: Centre of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science and Technology, Narmak,
Tehran, P.O. Box 16846-13114, Iran


Abstract
Magnetorheological (MR) damper is a type of controllable device widely used in vibration mitigation. This device is highly nonlinear, and exhibits strongly hysteretic behavior that is dependent on both the motion imposed on the device and the strength of the surrounding electromagnetic field. An accurate model for understanding and predicting the nonlinear damping force of the MR damper is crucial for its control applications. The MR damper models are often identified off-line by conducting regression analysis using data collected under constant voltage. In this study, a MR damper model is integrated with a model for the power supply unit (PSU) to consider the dynamic behavior of the PSU, and then a real-time nonlinear model updating technique is proposed to accurately identify this integrated MR damper model with the efficiency that cannot be offered by off-line methods. The unscented Kalman filter is implemented as the updating algorithm on a cyber-physical model updating platform. Using this platform, the experimental study is conducted to identify MR damper models in real-time, under in-service conditions with time-varying current levels. For comparison purposes, both off-line and real-time updating methods are applied in the experimental study. The results demonstrate that all the updated models can provide good identification accuracy, but the error comparison shows the real-time updated models yield smaller relative errors than the off-line updated model. In addition, the real-time state estimates obtained during the model updating can be used as feedback for potential nonlinear control design for MR dampers.

Key Words
nonlinear model identification; real-time; model updating; magnetorheological (MR) damper; power supply; unscented Kalman filter (UKF)

Address
Wei Song, Saeid Hayati and Shanglian Zhou: Department of Civil, Construction and Environmental Engineering,
The University of Alabama, Tuscaloosa, AL, USA 35487


Abstract
The adoption of cladding panels as dissipation device is a sort of passive control \"ante litteram\" for residential and commercial buildings. This paper gives details on the current technology outlining the difference between buffer panels and cladding panels. The discussion of robustness and resilience of the resulting system is afforded. It is shown that the strength of such solution, originally related to economy and light weight, is mainly associated with the respect of the main robustness requisites, as well as the short time it requires for removal and replacement (resilience).

Key Words
buffer panels; cladding panels; passive structural control; resilience; robustness

Address
Ugo Balzari: Building Façade Consultant, Milan, Italy
Andrea Balzari: Freelance Architect, Milan, Italy


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