Ambient vibration re-testing and operational modal analysis of the Humber Bridge

An ambient vibration survey of the Humber Bridge was carried out in July 2008 by a combined team from the UK, Portugal and Hong Kong. The exercise had several purposes that included the evaluation of the current technology for instrumentation and system identification and the generation of an experimental dataset of modal properties to be used for validation and updating of finite element models for scenario simulation and structural health monitoring. The exercise was conducted as part of a project aimed at developing online diagnosis capabilities for three landmark European suspension bridges. Ten stand-alone tri-axial acceleration recorders were deployed at locations along all three spans and in all four pylons during five days of consecutive one-hour recordings. Time series segments from the recorders were merged, and several operational modal analysis techniques were used to analyse these data and assemble modal models representing the global behaviour of the bridge in all three dimensions for all components of the structure. The paper describes the equipment and procedures used for the exercise, compares the operational modal analysis (OMA) technology used for system identification and presents modal parameters for key vibration modes of the complete structure. The results obtained using three techniques, natural excitation technique/eigensystem realisation algorithm, stochastic subspace identification and poly-Least Squares Frequency Domain method, are compared among themselves and with those obtained from a 1985 test of the bridge, showing few significant modal parameter changes over 23 years in cases where direct comparison is possible. The measurement system and the much more sophisticated OMA technology used in the present test show clear advantages necessary due to the compressed timescales compared to the earlier exercise. Even so, the parameter estimates exhibit significant variability between different methods and variations of the same method, while also varying in time and having inherent variability. (C) 2010 Elsevier Ltd. All rights reserved

Vibration characteristics of a suspension footbridge

Author's manuscript version. The final published version is available via doi:10.1006/jsvi.1996.0789. Copyright © 1997 Academic Press. All rights reserved.A suspension footbridge located in a tourist attraction in Singapore has a suspended span of 35 m and was designed for static pedestrian and wind loads. In common with other bridges of this type, it is a light, efficient structure and has a lively dynamic performance. Distributed parameter and finite element models were used to understand the vertical plane behaviour of the bridge and a prototype dynamic test using impact excitation was conducted to check the models and investigate the dynamic response. The first two vertical vibration modes were found to occur at the same frequency, 2 Hz, as the average pedestrian footfall. Response to pedestrians was simulated using linear and non-linear models of a moving excitation source. © 1997 Academic Press Limited

Ambient vibration studies for system identification of tall buildings

This is the peer reviewed version of the article, which has been published in final form at DOI 10.1002/eqe.215. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.The performance of a building under wind and seismic loads depends on stiffness and mass distribution, and may be estimated using finite element codes. Experience has, however, shown that such finite element models often fail to predict accurately the fundamental natural frequencies. Usually the frequencies will be underestimated, that is the building will turn out to be stiffer than anticipated, meaning the design would usually be conservative. On the other hand, effects like torsional eccentricity and foundation compliance may not be correctly modelled, which could be less desirable. A full understanding of linear performance under lateral loads can be obtained through experimental evaluation of the vibration modes. Traditionally only a limited range of modal analysis procedures and software has been applied to civil applications and the ‘special case’ where no input forces can be measured has been the usual situation for large civil structures. Recent developments in system identification, which is the set of procedures to build mathematical models of the dynamic structural systems based on measured data, have added significantly to the potential of ambient vibration or ‘output only’ testing. The aim of the research reported here has been to apply and evaluate the procedures on typical buildings. The procedures are briefly explained and two experimental programmes are then described; a long-term tremor monitoring exercise on a 280m office tower and an ambient vibration survey of a smaller office block. The different forms of response data are examined to study the performance of the analysis procedures and expose benefits and limitations in their use. There is a growing interest in output-only modal analysis procedures in civil engineering. The experience reported in this paper has shown that quick and reliable estimation of mode shapes and frequencies can be obtained, even with small amounts of data. Judgement of modal participation and damping ratios requires more detailed study yet the results are at least as convincing as existing and relatively limited frequency domain methods

Identification of unusual events in multi-channel bridge monitoring data

Peer reviewedPostprin

Vibration control of sensitive manufacturing facilities

Proceedings of the International Conference on Experimental Vibration Analysis for Civil Engineering Structures, 26-28 October 2005, Bordeaux, FranceAs demands increase for reducing feature sizes in computer processors, display units and media, the requirements for controlling vibration levels become more severe and begin to govern structural design. Design guidance is so far limited or non-existent and vibration control exercises are not standardised but case-specific. This paper presents experiences in vibration control of micro-electronics fabrication plants or ‘fabs’ subject to dynamic loading from pedestrians, wind, vehicles and other machinery, the lessons learnt and some suggestions on design

Modal mass estimation for vibration serviceability assessment of footbridges

Recent high-profile failures of footbridges to carry pedestrians without excessive and uncomfortable vibration have shown the need for a better understanding of vibration serviceability of structures under pedestrian loads. Failure to predict excessive response may be due to misunderstanding the mechanism of pedestrian loading, but also due to inaccurate response calculations in code-based assessments during design. For a bridge expected to have a low natural frequency this assessment is a dynamic analysis using linear single degree of freedom models requiring realistic estimates of modal frequency, damping, shape and mass. If the design fails and a retrofit is necessary the same parameters are required, but are expected to be estimated to a higher accuracy by full-scale vibration measurement. Modal mass and modal damping are critical parameters for the assessment process yet are the most difficult to measure experimentally. This paper evaluates procedures for modal mass estimation via two case studies of problematic mass estimation through structural analysis end experiment. As well as traditional forced vibration testing, methods using calibrated footfall excitation are shown to be remarkably effective

Structural health monitoring of civil infrastructure

Structural health monitoring (SHM) is a term increasingly used in the last decade to describe a range of systems implemented on full-scale civil infrastructures and whose purposes are to assist and inform operators about continued 'fitness for purpose' of structures under gradual or sudden changes to their state, to learn about either or both of the load and response mechanisms. Arguably, various forms of SHM have been employed in civil infrastructure for at least half a century, but it is only in the last decade or two that computer-based systems are being designed for the purpose of assisting owners/operators of ageing infrastructure with timely information for their continued safe and economic operation. This paper describes the motivations for and recent history of SHM applications to various forms of civil infrastructure and provides case studies on specific types of structure. It ends with a discussion of the present state-of-the-art and future developments in terms of instrumentation, data acquisition, communication systems and data mining and presentation procedures for diagnosis of infrastructural 'health'

Full-scale performance evaluation of bridges using dynamic and static instrumentation

Paper presented at rhe International Conference on Bridge Management Systems - Monitoring, Assessment and Rehabilitation, 21-23 March 2006, Cairo, EgyptStructural Health Monitoring (SHM) is a popular research topic comprising a range of activities from sensor development to data mining. The essence of SHM is learning about the ‘state’ of a bridge by measurements of response parameters, along with loads such as temperature and wind. The state of the bridge is defined by a range of structural and response parameters. The aim of SHM is to check the state of the bridge against acceptance criteria (e.g. over-loaded or damaged) and to indicate changes to the state, signaling changes to the structure or the loading. To do this requires a combination of condition assessment, a detailed assessment or snap-shot of the structure including analytical modeling, inspection and dynamic testing, followed by long term but less detailed monitoring of performance using permanent instrumentation. Using few sensors and a well-developed understanding of the structure from the condition assessment, the long term monitoring serves to check that performance is within bounds, and provides indication of altered state, which can signal more detailed (condition) assessment to fully diagnose the likely fault. This paper describes condition assessment and long term monitoring of example bridges and how new technology is applied to improve the capability to detect and diagnose anomalous structural performance in real time in order to provide timely alerts for bridge operators to take action

Lateral loading and response for a tall building in the non-seismic doldrums

Author's accepted version. The version of record is available from the publisher via: doi:10.1016/j.engstruct.2005.04.021 Copyright © 2005 Elsevier Ltd. All rights reserved.The situation for building design against wind and earthquake effects in Singapore is apparently unique. There is no seismic design code as there is no local seismicity, yet the effects of significant regional earthquakes are frequently felt in many high rise buildings in Singapore. Whereas it has become clear that the strongest winds in Singapore originate from storms and squalls, design for wind by law requires use of an arbitrary design wind speed applied in a British loading code geared to cyclonic wind systems. A decade of monitoring of a 280m office has shown that distant strong earthquakes generate dynamic response typically an order of magnitude greater than due to the strongest winds occurring during the same period. The effect is greater for high rise apartment blocks and it is becoming clear that for extreme events with similar return periods, earthquake effects should govern design for lateral load in terms of dynamic base shears under such conditions. For the present building control authorities take code requirements to design for accidental eccentricity to be adequate to cover seismic loads and while there have been moves towards a more rational local code there remains an open question about the relationship of static and dynamic effects due to wind for both cyclonic and (thunder)storm winds. In this paper the evidence concerning the nature of the two forms of loading is presented and the various existing and potential code provisions examined

Dynamic performance of high frequency floors

startedmonth: JanuaryHospitals and micro-electronics fabrication facilities require ultra-low vibration environments, and to mitigate the effects of the governing vibration source, footfalls, the floors are typically designed to have high natural frequencies so that response takes the form of a series of transients that decay rapidly between successive foot impacts. For low frequency floors evaluation of performance is relatively simple, involving simulation of resonant response in modes up to no more than 10Hz with simplifications possible through dynamic amplification factors. For high frequency floors there appears to be no simple procedure for assessment due to the complex nature of the loading time history and various difficulties with the classical normal mode analysis approach. Two approaches are in use to assess the critical peak velocity, one simple empirical formula depending on floor frequency and stiffness, and a second more recently derived empirical formula for assessing modal contributions using an effective impulse dependent on floor and walker frequency. This paper studies the reliability of the two approaches and presents some of the difficulties in attempting to obtain a realistic prediction of the performance of a high frequency floor with pedestrian loads. Because of the highly localized effects of a sharp transient load and the energy dispersion that is generally underestimated, predictions are likely to be conservative. Generally this is good, but as vibration control is a major expense, more accurate predictions could result in significant economies