1 . 1 Motivation for managing uncertainty of modal parameter ( MP ) estimates

36 Vibration testing of long span bridges is becoming a commissioning requirement, yet such exercises 37 represent the extreme of experimental capability, with challenges for instrumentation (due to frequency 38 range, resolution and km-order separation of sensor) and system identification (because of the extreme low 39 frequencies). 40 The challenge with instrumentation for modal analysis is managing synchronous data acquisition from 41 sensors distributed widely apart inside and outside the structure. The ideal solution is precisely synchronised 42 autonomous recorders that do not need cables, GPS or wireless communication. 43 The challenge with system identification is to maximise the reliability of modal parameters through 44 experimental design and subsequently to identify the parameters in terms of mean values and standard errors. 45 The challenge is particularly severe for modes with low frequency and damping typical of long span bridges. 46 One solution is to apply ‘third generation’ operational modal analysis procedures using Bayesian approaches 47 in both the planning and analysis stages. 48 The paper presents an exercise on the Jiangyin Yangtze River Bridge, a suspension bridge with a 1,385m 49 main span. The exercise comprised planning of a test campaign to optimise the reliability of operational 50 modal analysis, the deployment of a set of independent data acquisition units synchronised using precision 51 oven controlled crystal oscillators and the subsequent identification of a set of modal parameters in terms of 52 mean and variance errors. 53 Although the bridge has had structural health monitoring technology installed since it was completed, this 54 was the first full modal survey, aimed at identifying important features of the modal behaviour rather than 55 providing fine resolution of mode shapes through the whole structure. Therefore, measurements were made 56 in only the (south) tower, while torsional behaviour was identified by a single measurement using a pair of 57 recorders across the carriageway. The modal survey revealed a first lateral symmetric mode with natural 58 frequency 0.0536 Hz with standard error ±3.6% and damping ratio 4.4% with standard error ±88%. First 59 vertical mode is antisymmetric with frequency 0.11 Hz ± 1.2% and damping ratio 4.9% ± 41%. 60 A significant and novel element of the exercise was planning of the measurement setups and their necessary 61 duration linked to prior estimation of the precision of the frequency and damping estimates. The second 62 novelty is the use of the multi-sensor precision synchronised acquisition without external time reference on a 63 structure of this scale. The challenges of ambient vibration testing and modal identification in a complex 64 environment are addressed leveraging on advances in practical implementation and scientific understanding 65 of the problem. 66 Highlights 67 Ambient vibration test on 1385 m Jiangyin Yangtze River Bridge, by western academics. 68 Application of uncertainty laws of Bayesian operational modal analysis for test planning with and without 69 preliminary data. 70 Bayesian operational modal analysis of acceleration data, including challenging lateral vibration modes 71 Design and use in the field testing of precisely synchronised loggers not relying on GPS or wireless. 72