SENSOR SELECTION -BUILDING VIBRATION
Vibration in Buildings and Structures – Selecting the correct Sensor for Measurement
Construction and demolition activities in urban areas requires consideration to how generated vibration from piling, excavation, blasting, movement of heavy plant and demolition may affect existing structures and services. Excessive vibration can cause nuisance to occupants and cosmetic damage to buildings and at higher levels it can cause more serious damage.
In order to evaluate the risks and monitor compliance it is commonplace to measure the vibration levels. Limits and guidelines are published in BS5228-2*, BS7385** and BS ISO 4866*** which indicate where vibration induced events may cause adverse response from people and damage to structures. These standards require that the vibration is measured in terms of peak vibration velocity, PPV.
It is possible to measure acceleration using an accelerometer and convert this to velocity, but it is not recommended for construction activities. With steady signals on a laboratory shaker it is possible, but in the real-world, vibration events are more complex and the process (integration) does not work so well.
The use of a sensor that gives a direct readout in terms of the metric specified in the standard is recommended. A direct reading vibration velocity sensor, also known as a geophone will give better results and less uncertainty, rather than trying to process the result. Geophones are routinely used for this reason
Advice on this subject is given in ISO 4866:2010, Section 8.2
“It is better to use an appropriate transducer to measure the required quantity directly and avoid the process of integration or differentiation.”
Figure 1 AVA Triaxial Geophone
Sensor mounting - It is better to have a free-standing sensor that can be directly mounted onto the structure to be monitored, as this provides direct coupling between source and instrument without the complexity of associated electronic correction networks. It also means that the sensor can be accurately calibrated to ensure ongoing compliance.
Power saving – Geophones will be more power efficient as energy required to make the complex conversions is not required.
For details on vibration measurement instrumentation:
*BS 7385-1: 1990 Evaluation and measurement for vibration in buildings. Part 1: Guide for measurement of vibrations and evaluation of their effects on buildings
**ISO 4866:2010 Mechanical vibration and shock – Vibration of fixed structures – Guidelines for the measurement of vibrations and their effects on structures
***BS 5228-2:2009 Code of practice for noise and vibration control on construction and open sites – Part 2: Vibration