Opal IMU Sensors Used to Evaluate Bridge Design
Over the years, APDM Opal sensors have been used for a variety of applications. From measuring workload during robotic surgery, identifying safety hazards on construction sites, to characterizing human-spacesuit interaction, the use cases for Opals have proved endless. In a recent study out of the University of Exeter, titled “Footbridge system identification using wireless inertial measurement units for force and response measurements” (Brownjohn, et al.), researchers used Opals to evaluate the design of a 109 meter footbridge.
The bridge in question, Baker bridge, is used primarily for bicycle and pedestrian access to Sandy Park Stadium, the playing field of the Exeter Chiefs Rugby Club. On match days, the bridge experiences heavy pedestrian traffic, and “demonstrates a lively dynamic response.” As the team acknowledges early in the study, vibration serviceability is often overlooked in the design and construction of bridges of this sort, resulting in their failure to perform adequately under human dynamic loading.
Typically, bridges like Baker bridge will undergo what is called “Modal Testing,” before entering service. This procedure essentially involves the use of wired sensors to measure accelerations and forces put out by a mechanical shaker. However, the team states that the overlooked input (force) to output (response) relationships are critical, for they enable the estimation of modal mass, a key player in directly controlling vibration levels.
In this study, Opals were first used to only measure mode frequencies, shapes, and dampening ratios in a vibration study. Then, the team used them to also measure simultaneous measurement of body accelerations of a human subject jumping atop the bridge. The team was then able to estimate modal masses by reviewing this input-output relationship. These modal mass estimates were then validated using an instrumented hammer and known mass distributions – the estimates proved consistent.
This method was also used in an applied research application after a request was made by the National Gallery Singapore (NGS). The NGS wished to evaluate the vibration serviceability of one of two skybridges connecting the former Supreme Court and City Hall buildings. “Due to the national significance of the new museum, opened in November 2015, the bridge was investigated to confirm vibration serviceability to appropriate standards, including an estimation of modal properties in the unlikely event that retrofit needed to be considered.” Regular testing equipment would have required airfreight delivery and a significant amount of time and funding. This presented the Exeter team with an opportunity to test their methodology, and save the NGS the hassle of typical testing. In turn, both skybridges were tested, and both demonstrated acceptable vibration serviceability without mitigation.
This study is another great example of the Opal sensors’ ability to take long-term measurements in any environment. The team was able to simply transport the APDM equipment to Singapore in their carry-on baggage. Additionally, this study proved the benefits of logistical and operational simplicity afforded by Opal sensors. The team remarked that they were “highly portable…easy to use…extremely useful for efficient evaluation…”