James Fisher Strainstall contribute to Highways Agency project with monitoring vibration levels on motorway bridge.

The M6 is the longest motorway in the United Kingdom and one of the busiest. In June 2012 JF Strainstall were commissioned to provide vibration monitoring on one of motorway bridges contribute to a Highways Agency project.

The task for our engineering team was to observe the levels of vibration and compare them with the governing standards. The purpose for this project was to assess the suitability for the installation and durability of a discontinuous variable message sign on the bridge.

JF Strainstall developed a purpose designed vibration monitoring system to manage the monitoring over three axes (lateral, fore and perpendicular) at high sampling rates with large amounts of data. The monitoring took place on Birmingham Box Phase 3 junctions 5-8 for a 24 hour period.

The use of a high accuracy, three axes accelerometer was required to achieve a high resolution and reach the required frequency range. The accelerometer utilised for this project was capable of handling acceleration levels up to 5g and sensitive to 0.001g. The range of frequency was 0.4Hz to 3kHz for lateral and fore axis and up to 6kHz on the vertical axis. Finally, the compact size of the equipment allowed non-invasive installation on the bridge structure as well as fast set-up and decommissioning of the equipment.

Customised software was generated using LabVIEW. The analysis of the raw data comprised an FFT analysis to identify the various frequencies that vibrations occurred, detection of the peak values and the calculation of statistical noise levels .

JF Strainstall were responsible for the installation and monitoring of the autonomous system over the 24 hour period. The large volume of data was then processed by the engineering team under the Highway Agency standards (TR 2130C) and British Standards (BS 50556:2011). The raw data and the analysis was sent for review by the Highways Agency.