As part of the EPSRC-funded BAYOMALAW project, a team of researchers from Exeter and Liverpool travelled to Jiangyin in Jiangsu province, China to carry out a modal test of the Jiangyin Suspension Bridge.
Jiangyin Bridge has a single suspended span of 1,385m, with straight back stays. The concrete towers rise 191m above ground level and each have three hollow portal beams. The mid portal is illustrated by the bridge name in stylised Chinese characters, created by (and signed by) the former Chinese premier Jiang Zemin, who opened the bridge in 1999. At the time, the bridge was the world’s fourth longest span and the furthest downstream on the Yangtze River.
The 32.5m wide 3m deep steel box deck carries three traffic lanes (plus a narrow emergency lane) in each direction. There are 2.2m wide cantilevered walkways either side, although these are for maintenance as there is no pedestrian access to the bridge. These walkways were used by the test team for moving the loggers around.
The exercise was primarily an extreme test of the capabilities of two new technologies.
First, a new synchronised wireless logging system was created by James Bassitt (hardware) Vincent Ao and Emma Hudson (software). Four loggers were taken to China (as checked baggage) with a set of 12 force balance accelerometers and a set of short signal cables. These loggers are synchronised to a fraction of a microsecond before a measurement then distributed over the bridge deck and towers in a sequence of measurements to record ambient vibration signals.
Some 14 separate measurements were made over a period of three days (25th-27th April 2017). For each measurement, the master logger was left recording vertical and lateral vibrations continuously at hanger locations H67 and H71 on the east walkway (there are 85 pairs of hangers each side) while other (slave) loggers were ‘roved’ to record for at least an hour synchronously at other locations on the east and west sides and inside the south tower.
The weather conditions were mostly benign; cloudy, cold, hazy and breezy on the first two days, clearing to a fine sunny day for the tower measurements. Due to pollution and unpleasant atmospheric conditions resulting from the heavily industrialised area around Jiangyin, team members wore protective face masks most of the time.
The second major technology being evaluated was the operational modal analysis planning and evaluation procedure developed by Professor Ivan Au as part of the BAYOMALAW collaboration with the University of Liverpool.
BAYOMALAW stands for ‘Bayesian operational modal analysis law’, and the main aim of the project is to establish uncertainty laws for modal testing to optimise tests such as these. BAYOMALAW aims to establish the best measurement configurations and the resulting uncertainties in modal parameters in terms of natural frequencies, damping ratios and mode shapes. Jiangyin is an extreme test because of its ultra-low natural frequencies and its weak lateral response buried in quasi-static effects of deck rotation.
As part of the procedure, a ‘huddle test’ is used to check the self-noise and environmental noise of the measurement system. To do this, all sensors are huddled into one location and sense either vertical or lateral vibrations simultaneously.
The exercise required a large investment of resource into building, programming, delivering and operating the system on site – but preliminary analysis of the data shows that the exercise was a success and that everything worked. The test was funded by the EPSRC and supported by Tongji University and JSTI.