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Sheffield, United Kingdom

Fernandez P.,University of Oviedo | Reynolds P.,University of Sheffield | Reynolds P.,Full Scale Dynamics Ltd | Lopez-Aenlle M.,University of Oviedo
Experimental Mechanics | Year: 2011

In operational modal analysis (OMA) mode shapes can be obtained only with arbitrary normalization. There are many applications where mass normalized mode shapes are required, such as response prediction and stress analysis. A method to scale the mode shapes in OMA is to modify the dynamic behaviour of the structure by adding masses and then to use the modal parameters of both the original and modified structure. Several mass change methods have been proposed in recent years for estimating the scaling factors, where a distributed array of added masses are needed to obtain good results. In this work a new mass change approach based on performing several individual mass changes is presented. This approach requires only a small number of masses that are located at different points in each individual experiment. The results of the individual tests are then combined to estimate the scaling factors. The approach is developed and validated by measurements carried out on a 15-tonne prestressed concrete slab strip and a steel cantilever beam. The results show that a good accuracy can be obtained by this method when a proper mass change strategy is used. © 2010 Society for Experimental Mechanics. Source


Hudson E.J.,University of Exeter | Reynolds P.,University of Exeter | Reynolds P.,Full Scale Dynamics Ltd
Structural Control and Health Monitoring | Year: 2014

Active vibration control has shown great potential for reducing the response of floor structures and has the potential to realise significant material savings in slender designs through incorporation from the conceptual stage. However, different structural designs result in different modal properties, and these can significantly affect the effectiveness of an active vibration control implementation. This paper investigates the implications of these different structural designs. Two floor structures with known vibration serviceability issues are considered; both of these are fairly typical designs. Active control is then simulated on each floor in order to improve the response of the structure. A multipedestrian walking force model is developed and used to generate the response of each structure. This loading model is calibrated and verified using experimentally acquired data on the structures considered. It was found that the effectiveness of the control was localised to each structural panel, and therefore the structural design that yielded larger panels required fewer actuators to reduce the response over the entire structure. Copyright © 2013 John Wiley & Sons, Ltd. Source


Pereira E.,University of Alcala | Diaz I.M.,Technical University of Madrid | Hudson E.J.,University of Exeter | Reynolds P.,University of Exeter | Reynolds P.,Full Scale Dynamics Ltd
Engineering Structures | Year: 2014

Civil structures such as floor systems with open-plan layouts or lightweight footbridges can be susceptible to excessive levels of vibrations caused by human activities. Active vibration control (AVC) via inertial-mass actuators has been shown to be a viable technique to mitigate vibrations, allowing structures to satisfy vibration serviceability limits. It is generally considered that the determination of the optimal placement of sensors and actuators together with the output feedback gains leads to a tradeoff between the regulation performance and the control effort. However, the "optimal" settings may not have the desired effect when implemented because simplifications assumed in the control scheme components may not be valid and/or the actuator/sensor limitations are not considered. This work proposes a design methodology for multi-input multi-output vibration control of pedestrian structures to simultaneously obtain the sensor/actuator placement and the control law. This novel methodology consists of minimising a performance index that includes all the significant practical issues involved when inertial-mass actuators and accelerometers are used to implement a direct velocity feedback in practice. Experimental results obtained on an in-service indoor walkway confirm the viability of the proposed methodology. © 2014 Elsevier Ltd. Source


Brownjohn J.,University of Exeter | Brownjohn J.,Full Scale Dynamics Ltd | Racic V.,University of Sheffield | Racic V.,Polytechnic of Milan | Chen J.,Tongji University
Mechanical Systems and Signal Processing | Year: 2016

Floor vibrations caused by people walking are an important serviceability problem both for human occupants and vibration-sensitive equipment. Present design methodologies available for prediction of vibration response due to footfall loading are complex and suffer from division between low and high frequency floors. In order to simplify the design process and to avoid the problem of floor classification, this paper presents a methodology for predicting vibration response metrics due to pedestrian footfalls for any floor type having natural frequency in the range 1-20 Hz. Using a response spectrum approach, a database of 852 weight-normalised vertical ground reaction force (GRF) time histories recorded for more than 60 individuals walking on an instrumented treadmill was used to calculate response metrics. Chosen metrics were peak values of 1 s peak root-mean-square (RMS) acceleration and peak envelope one-third octave velocities. These were evaluated by weight-normalising the GRFs and applying to unit-mass single degree of freedom oscillators having natural frequencies in the range 1-20 Hz and damping ratios in the range 0.5-5%. Moreover, to account for effect of mode shape and duration of crossing (i.e. duration of dynamic loading), the recorded GRFs were applied for three most typical mode shapes and floor spans from 5 m to 40 m. The resulting peak values as functions of frequency i.e. spectra are condensed to statistical representations for chosen probability of being exceeded over a wide range of applications. RMS (acceleration) spectra show strong peaks corresponding to the first harmonic of pacing rate followed by clear minima at approximately 3.5 Hz, a second much smaller peak corresponding to the second harmonic and a steady decline with increasing frequency beginning around 5 Hz. One-third octave spectra show asymptotic trends with frequency, span and damping. A comprehensive validation exercise focusing on the acceleration RMS spectra was based on a representative range of floor samples for which modal properties had been identified and walking response studied during experimental campaigns of vibration serviceability evaluation. Due to the statistical approach an exact validation would not be possible, hence measured peak RMS values were matched to distributions for the equivalent idealised structure. In the vast majority of cases the measured values, intended to represent worst-case conditions, fitted the upper decile of the corresponding simulated spectra indicating consistency with the proposed approach. © 2015 Elsevier Ltd. All rights reserved. Source


Pavic A.,University of Exeter | Brownjohn J.M.W.,Full Scale Dynamics Ltd
Research and Applications in Structural Engineering, Mechanics and Computation - Proceedings of the 5th International Conference on Structural Engineering, Mechanics and Computation, SEMC 2013 | Year: 2013

This paper presents a vision for the future monitoring systems which will become normal requirements for management of bridges as key objects of national infrastructure in the UK and elsewhere. Rather than being pushed by authorities and legislation, we expect that bridge managers will recognize the clear business cases for investing in well-designed targeted monitoring. To support this proposition, the paper presents two case studied where state-of-the-art bridge monitoring technology was used or potentially could be used to: • Decide when to inspect and change bridge bearings, and • Decide when to close various traffic lanes to reduce probability of overstressing bridge structural components. © 2013 Taylor & Francis Group. Source

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