Jonathan Newell finds out how remediation and monitoring is helping to protect museums and historic structures from induced vibration.
I was sure something wasn’t quite right whilst looking up at the Pergamon Altar on Berlin’s Museum Island in 1986. The city’s S-Bahn railway passes through the museum complex between the Pergamon Museum and the Bode Museum and each time a train passed, its vibration could be clearly and annoyingly felt. Surely over the years, decades or even centuries that pass, even such low levels of vibration would have a damaging effect on the exhibits or the building structure itself, wouldn’t they?
Thirty years on and a new railway is being built under London. Project Managers on the Crossrail project are taking every precaution to monitor ground vibration and displacements to ensure the work isn’t damaging the historic structures under which it’s progressing.
Both of these scenarios require careful measurement and the appropriate placement of sensors. In the case of the Pergamon Museum, this is to monitor the effects of the S-bahn and make decisions on the placement of artistic treasures or structural remediation work to prevent future damage.
The Vibration Monitoring Terminal Type 3680 (VMT) has recently been released by Brüel & Kjær. The VMT unit provides uninterrupted, tri-axial, real-time ground vibration measurement.
For sensitive construction projects similar to Crossrail, alarm levels are used for signalling when vibration levels reach a threshold beyond which there would be a risk of damaging nearby structures.
Commenting on the use of the terminal in such applications, Brüel & Kjær’s VMT Product Manager, Douglas Manvell told me, “Maximum levels are defined to prevent damage and the unit is set up to alert when vibration levels approach or exceed these limits. This is done in real-time so that operations that risk damaging the building can be stopped immediately.”
In addition to monitoring vibration during construction work, the unit also enables users to efficiently conduct background surveys prior to new construction as well as receive accurate data to evaluate vibration mitigation techniques.
In the case of structures which are already exposed to induced vibration, the VMT can be used for planning mitigation works and then monitoring the effectiveness of the remedial work that’s taken place.
The unit is very flexible in the way it can be positioned and used with a range of sensors to suit the application. A “spiked” sensor is supplied for embedding in earth for monitoring vibration through the ground but other options are available.
“We also have a mounting plate for affixing the sensor to the actual building or other structures such as building foundations or museum plinths,” explained Manvell.
With flexible sensor positioning, engineers can monitor structures, specific points of risk and places that may be showing signs of undesired resonances.
The VMT can be used in a range of vibration monitoring applications including the assessment of human responses to ground-borne vibration from road and rail traffic or monitoring background vibration to ensure sensitive equipment operates correctly in manufacturing and other environments.
According to Manvell, the system is used in hospitals and semiconductor manufacturing plants to help ensure patient comfort and avoid costly errors when using delicate equipment.
“For such applications, you need to have a flexible device you can rely on. The VMT does it all with minimum effort and provides real-time information for making critical decisions that avoid non-compliance and meet your social licence obligations,” said Manvell
No great shakes at the Pergamon
A 2010 study by a Stuttgart university monitored vibration and displacement levels of artefacts at the Pergamon Museum with surprising results. Concluding that the human annoyance level is lower than the building damage threshold, the researchers set a ground vibration Peak Particle Velocity (PPV) limit of 1mm/s, which wasn’t exceeded by the passing trains.