Uncertainty Quantification and Management in Ambient Modal Identification

Lead Research Organisation: University of Exeter
Department Name: Engineering Computer Science and Maths

Abstract

The modal properties of a structure include primarily its natural frequencies, damping ratios and mode shapes. Their information is indispensable for design against dynamic loads such as wind, earthquake and human excitation. Uncertainty arises due to the lack of knowledge and modelling limitations and this generally increases project risk. Modal identification has long been recognised as an effective means for uncertainty mitigation in structural dynamics. Theoretically it is possible to identify the modal properties based on only the 'output' vibration response of structures without knowing the 'input' excitation. This type of test, called 'ambient vibration test', has now become the primary and most sustainable means for its high implementation feasibility, robustness and economy. In the absence of loading information and with data collected under uncontrolled field environment, however, the identification results have significant variability and low repeatability. This has limited the economic benefit of ambient vibration tests and undermined the scientific significance of their identification results. This has been well-recognised but there has been no quantitative account for its origin or how to control it.

This project aims at developing a comprehensive fundamental methodology for quantifying and managing the uncertainties of the modal properties of civil engineering structures identified from ambient vibration data. At the scientific core is a set of 'uncertainty laws', analogous to the laws of large numbers of data in classical probability, that expresses fundamentally the identification uncertainty of modal properties explicitly and quantitatively in terms of test configurations such as measurement noise, environmental load intensity and the number and location of sensors. Due to complexity of the problem, it is unlikely to obtain insightful results for general situations. The project aims at fundamental expressions with insights governing the dominant behaviour of the remaining identification uncertainty under realistic situations. The project objective is achieved through a comprehensive programme comprising fundamental theory development, extensive verification with synthetic, laboratory and field data, and knowledge transfer with industry. A practical guide for planning and performing ambient vibration test shall be produced incorporating scientific findings of the project and experience of the team members with input from industry partners.

Publications

10 25 50
publication icon
Au S (2016) Model validity and frequency band selection in operational modal analysis in Mechanical Systems and Signal Processing

publication icon
Au S (2018) Quantifying and managing uncertainty in operational modal analysis in Mechanical Systems and Signal Processing

publication icon
Faulkner K (2020) Tracking bridge tilt behaviour using sensor fusion techniques in Journal of Civil Structural Health Monitoring

publication icon
Raby AC (2019) Wolf Rock lighthouse: past developments and future survivability under wave loading. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

 
Description As one of the key objectives of the project, a Bayesian probabilistic theory with an explicit formula has been developed that fundamentally relates the identification uncertainty of modal properties to test configuration in an ambient vibration environment, covering commonly encountered situations of well-separated modes, close modes and multiple setups. Based on this, a method has been developed to allow the identification uncertainty in ambient vibration tests to be quantitatively managed, where factors such as data length, sensor quality and quantity can be assessed. This will allow practitioners to plan/budget ambient test campaigns in a risk-informed manner and improve efficiency by removing unnecessary conservatism previously applied due to lack of understanding of uncertainty. The method has been applied to test planning and understanding uncertainty of various full-scale structures, e.g., the Jiangyin Bridge in China, offshore lighthouses in UK, footbridge in Knowsley, Rainbow bridge in Japan and Jubilee bridge in Singapore.

The central theme of the project spins off a number of research work of theoretical and practical significance. For example, a method has been developed that allows sensor noise to be calibrated without precise alignment as required by existing methods. In addition to improving the accuracy of calibration results and safe-guarding them from bias due to potential misalignment, this allows noise calibration tests to be conveniently and robustly performed in the laboratory as well as field environment. A theory and method have been developed that address the issue of data synchronisation in ambient modal identification. A mobile system for distributed sensing and acquisition of high-fidelity ambient vibration data has been developed and applied to various test cases.
Exploitation Route The methods and findings in the project will allow practitioners to plan/budget ambient test campaigns in a risk-informed manner and improve efficiency by removing unnecessary conservatism previously applied due to lack of understanding of uncertainty.
Sectors Aerospace, Defence and Marine,Construction,Energy

 
Title Loggers 
Description Creation of autonomous loggers for vibration testing of large structures 
Type Of Material Data analysis technique 
Year Produced 2017 
Provided To Others? Yes  
Impact First time used for wireless measurements inside and outside bridge 
 
Title Mathematical tools 
Description Development of mathematical tools for predicting and managing uncertainty of ambient vibration measurements 
Type Of Material Data handling & control 
Year Produced 2017 
Provided To Others? Yes  
Impact First ever field test designed using this methodology 
 
Description Ambient vibration test of Rainbow Bridge, Tokyo 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Vibration Engineering Section staff Ki Koo and James Bassitt travelled to Tokyo to join colleagues from University of Liverpool (Zuo Zhu), Nanyang Technological University Singapore (Ivan Au) and the University of Tokyo (led by Tomonori Nagayama) in an ambient vibration test of Tokyo's Rainbow Bridge. Rainbow Bridge is a conventional suspension bridge with a double-deck steel truss, having 570m longest span.
Rainbow Bridge has been studied previously including by researchers from University of Tokyo, and the aim of new study was to evaluate hardware and procedures developed for ambient vibration measurements and operational modal analysis as part of the EPSRC's BAYOMALAW project EP/N017803/1 and EP/N017897/1.
Access to the bridge was strictly limited to a window of six hours on 18th March 2019, so considerable effort was made to plan the measurements to make the best use of the available time, including management of technical risks. This included setting up in the Wind Tunnel Laboratory of the University of Tokyo, and an on-site 'huddle test' for function testing and cross-calibration of the wire-free autonomous loggers (OCXO boxes).
A large group of researchers from University of Tokyo participated, moving loggers to a precise schedule and standing guard on the public-access lower level walkway.
While not having an exceptionally long span, and with first vertical mode frequencies around 0.25Hz, Rainbow Bridge presented a particular challenge to operational modal analysis by providing unusually low signal levels for the low frequency modes (due in part to the heavy construction of the double-deck girder) and due to the first two vertical vibration modes occurring at very close frequencies. This is a unique phenomenon that occurs in suspension bridges for certain conditions of girder elastic stiffness compared to main cable geometric and elastic stiffness.

University of Tokyo's Bridge & Structures group has extensive expertise in advanced instrumentation technologies including wireless low-noise sensing and the collaboration provided an opportunity to share experiences and discuss technology advances.
Following the measurements, a joint seminar was held on 20th March to discuss related research:

Ki Koo (Exeter): Time-Synchronisation for distributed heterogeneous sensor systems using low-cost GPS module

James Bassitt (Exeter): STORMLAMP and the difficulties of field work in extreme environments

Haoqi Wang (University of Tokyo): Some examples on inverse analysis by data assimilation

Sui-Kui Au (NTU Singapore): Bayesian operational modal analysis

Zuo Zhu (University of Liverpool): Robust in-situ instrument noise calibration for field vibration testing
Year(s) Of Engagement Activity 2019
URL https://veswordpresscom.wordpress.com/2019/03/27/ambient-vibration-test-of-rainbow-bridge-tokyo/
 
Description BAYOMA Summer School 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Organised and ran a Short Course on Structural Vibrations and Testing, allowing Masters and PHD students, as well as representatives from industry, access to orur state of the art facilities. This took the form of a residential summer school with theory and practice sessions culminating in a modal test on a bridge.
Year(s) Of Engagement Activity 2018
URL https://veswordpresscom.wordpress.com/2018/08/06/students-from-all-over-the-uk-attend-structural-vib...
 
Description BAYOMA Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact BAYOMALAW Workshop on vibration monitoring and condition assessment

One Great George Street, London, Wednesday, 4th October 2019

4th October 2019

Sharing experiences in, and challenges with, vibration measurements, in relation to

· Modal identification (frequencies, damping ratios) to check as-built , retrofitted or under construction wind-sensitive long span/tall structures

· Diagnosing and mitigating excessive vibration response of wind-sensitive structures

· Characterising transmission paths and vibration response of building to human-induced and ground-borne vibrations

· Tracking long term performance of major infrastructure assets


30 delegates (equal from academic/industry, speakers travelling form Singapore (Au), Dubai (Denoon), Denmark (Brincker/Andersen), Portugal (Magalhaes), Japan (Kim), UK consultants CPP, BuroHappold, Waterman, Walsh, AKTII, WSP, COWI, Arqiva, Arup. Impact has been re-energised and new partnerships linking with other ongoing and developing research proposals.
Year(s) Of Engagement Activity 2019
URL https://veswordpresscom.wordpress.com/2019/10/21/engineers-from-all-over-the-world-attend-vibration-...
 
Description Eurodyn 2017 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Attended at and presented keynote address at Eurodyn 2017 in Rome in September 2017
Year(s) Of Engagement Activity 2017
 
Description JSTI training course delivery 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Research visit to the Jiangsu Transportation Institute (JSTI) in China to run a training course for bridge engineers on bridge inspection technologies, vibration measurement, data processing and structural modelling. Invited by JSTI to teach the course, in order to help improve technologies in bridge inspection and health monitoring of large span bridges, which will be beneficial for infrastructure management and maintenance in China.
Year(s) Of Engagement Activity 2017
 
Description JSTI workshop attendance 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Attended a workshop at JSTI in China on the evaluation of an existing health monitoring system in cable-supported bridges in Jiangsu and participated in a field test with JSTI on a bridge in Jiangsu. Invited by JSTI to attend the workshop, along with colleagues James Bassitt and Karen Faulkner, in order to help improve technologies in bridge inspection and health monitoring of large span bridges, which will be beneficial for infrastructure management and maintenance in China. JMWB and colleagues also participated in a bridge test.
Year(s) Of Engagement Activity 2017
 
Description Management Committee meeting 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Member of the EPSRC VSimulators Management Committee and chaired kick off meeting in November 2017. The meeting was about getting organised for managing the operation of and developing proposals for VSimulators.
Year(s) Of Engagement Activity 2017