Structural dynamics helping to keep Australian roads safer, and smarter.
Insight

Structural dynamics helping to keep Australian roads safer and smarter

Dominik Duschlbauer Principal – Structural Dynamics, Acoustics and Vibration
Dominik Duschlbauer

Modern electronic freeway management systems rely heavily on cameras to help manage traffic flow, improve safety and create smarter roads.  Systems usually feature state-of-the-art Automatic Incident Detection (AID) technology to alert traffic controllers to incidents in real time - supporting more reliable journey times, reducing congestion, improving safety and ensuring quicker response time to incidents.  

Video-based AID systems are very sensitive to vibration.  High vibration risks damage to cameras, accelerates wear and tear, prevents the AID algorithm from functioning properly, makes direct observations of the camera feed difficult and can even induce motion-sickness in traffic controllers.  

On an elevated section on Melbourne’s Citylink roadway, the allowable vibration limits of cameras were regularly being exceeded.  SLR determined that the root cause was predominantly large trucks travelling at speed over the expansion joints of the elevated road deck spans.  The cameras were experiencing high accelerations and rotations because the camera support structures were amplifying road deck vibration, causing a ‘whip-lash’ effect.  Detailed analysis of the data collected during vibration surveys highlighted that maximum allowable vibration was typically exceeded hundreds of times every day, by factors of up to five.  

To alleviate these issues, SLR developed a modular tuned mass damper (TMD) system that could be retro-fitted to the existing cameras from an elevated work platform.  As such, all components were sufficiently small to allow for manual handling and a safe installation from an elevated work platform.  

The developed system was highly tuneable to accommodate the large range of problem frequencies which were found to span from 8 Hz to 16 Hz for short, gantry mounted poles.  For this purpose, SLR used rubber mounts of different stiffness but identical form-factors.  In addition, the mass was made up of small plates which were bolted together to give the correct mass as required for each gantry and type of rubber mount.  An important aspect of the on-site tuning was the ability to accurately and rapidly determine the whip-lashing frequency in order to determine the correct combination of rubber mounts and mass plates in one visit.  For this purpose, a customised software was developed to rapidly analyse the dynamic characteristics of each gantry with and without the tuned mass dampers.  This allows for immediate tuning and providing a before and after comparison to quickly visualise TMD performance and confirm that the correct mass was installed.  

The developed tuned mass damper system successfully reduced vibration to acceptable levels.  This meant that the existing gantries could be retained, eliminating the need to modify or replace the newly installed gantries.  SLR’s solution not only avoided a costly re-design of the gantries but our mitigation solution could be rolled out during partial road closures, minimizing the impact on traffic.  The developed tuned mass dampers also had a very small form-factor and did not significantly alter the gantries’ appearance.  Further, the final product is “safe-fail” in the sense that several components can fail without parts falling onto the road deck. 

View of the elevated freeway with two gantries spanning the inbound and outbound lanes
View of the elevated freeway with two gantries spanning the inbound and outbound lanes.