Uncertainty Quantification and Management in Ambient Modal Identification

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.

The beneficiaries of this project are instrumentation consultants and vibration specialists (better field tests), structural design consultants (better design) and infrastructure owners (better structures and utilisation of health monitoring resources).

1. Improving field test quality and economy. The project leads to scientific guidance for planning and performing ambient vibration tests. This will improve the competitiveness of instrumentation contractors and vibration specialists in bidding and carrying out projects. Consultants with access to this expertise will be able to improve performance-based vibration serviceability design of tall buildings, long span bridges, stadia, etc. Currently field tests are planned and budgeted primarily based on experience. Low data quality directly translates into poor identification results and remedial measures (e.g. retesting) are costly or impossible. Over-conservatism on the other hand impairs project economy.

2. Better sensor utilisation. Sensors used in the aerospace/automotive industry under laboratory environment do not necessarily perform well in ambient vibration test of civil structures. It is commonly accepted that servo-accelerometers (originally used in seismic applications) consistently out-perform miniature piezoelectric/capacitive sensors (common in laboratory) in field tests, although the former tend to be heavier and more expensive. It is premature to exclude the latter, however, as recent progress in MEMS technology has revealed the opportunity for light-weight sensors with improving quality. The scientific account of sensor quality and configuration on identification precision in the project shall allow one to make suitable use of the whole spectrum of sensors, from low-cost MEMS sensors on smart phones to state-of-the-art servo-accelerometers.

3. Better efficiency in tall building design. The project resolves the precision and hence improves the scientific significance of modal identification results. This will have a positive impact on many application areas currently challenged by questionable identification precision. Tall building design is an example of frontier technological and high economic relevance. Governed by wind effects, multi-fold increase in design forces can result due to dynamic amplification, which is inversely proportional to structural damping. There is currently no accepted method for predicting damping. Values used in design are based on rule of thumb or historical databases derived from old generation estimation methods having questionable reliability. Uncertainty in damping directly carries over to design forces and hence a risk premium in the structural cost and reduced floor area (e.g. due to over-sized columns) when the only prudent choice is a conservative damping estimate. Field data abound but identified damping values show big scatter with low repeatability and there is a lack of information or understanding about their uncertainty. This project shall resolve the identification precision, in the long run contributing to establishing an engineering model for predicting damping in tall buildings.

4. Wind tunnel tests allow realistic estimation of wind load based on scaled-down models/terrains and they are regularly performed for tall building and long bridge designs. Aeroelastic models are recent advance that allow aerodynamic damping to be investigated, although they are more expensive than conventional high-frequency base-balance model tests. Aerodynamic damping values are identified under turbulent wind conditions and so they are subjected to the same uncertainty in ambient vibration tests. The project shall contribute to the advance of aeroelastic model tests by improving their planning, identification precision and enhancing their scientific value.