A team of researchers from the Vibration Engineering Section returned to the Mineral Line Bridge in Somerset, to monitor the deflections of the bridge under different loading conditions. The bridge forms part of the West Somerset Railway (WSR), a heritage railway line with 20 miles of track in the South West of England.
The team, consisting of Farhad Huseynov, Yan Xu and Karen Faulkner, also formed part of the research group which had previously gathered strain data and rotation data of the bridge under train loading conditions.
The purpose of repeating the test was to analyse the performance of the bridge under loading from the Flying Scotsman engine. The Flying Scotsman, one of the most well-known steam engines, was constructed in 1923 and was the first steam engine to officially record a speed of over 100 mph.
To achieve this speed, the engine itself measures 21.3 m in length and weighs 97.8 tonnes. This is significantly larger than the Raveningham Hall engine, and, with a length of 19.2 m and a weight of 76.4 tonnes, is the largest train belonging to the WSR. The aim of the testing was to measure deformations of the bridge under loading from the passing trains and to determine the effect of the larger Flying Scotsman locomotive on the bridge.
The Mineral Line Bridge is located on the outskirts of Watchet and was originally constructed to carry the Minehead route over the West Somerset Mineral Railway. The Mineral Railway now operates as a footpath and cycle path open to the public. The bridge opened in 1862, has a single span of 14.8 m and is constructed skewed at an angle of 60° to the pathway beneath.
A series of accelerometers were installed on the bridge deck at five test points, measuring at each abutment and at quarter-span, mid-span and three-quarter-span. The angle of rotation of the bridge deck at each test point was inferred from the accelerometer data. This rotation data was then used to determine the deflection of the bridge.
The Imetrum camera was used to measure deflections of the bridge under loading from the passing trains. Three Imetrum cameras were set up on tripods and targets were installed, one at mid-span on the bridge deck and two on the western abutment. The abutment deflection was monitored to gather information as part of a previous study, but the main focus of the testing on the day was to measure the deflection of the bridge at mid-span.
By comparing the results of the deflection calculated from the rotation data and measurements from the Imetrum camera, the team were able to verify the deflection measurements obtained from the accelerometers. There was good correlation between the two measurements, verifying the procedure used by the research team.
The results indicated increased deflections and rotations under loading from the Flying Scotsman, but was safely within the tolerances of the bridge.
The weather conditions were not ideal on the day, with periods of rainfall intermixed with sunny periods throughout the day. This required the equipment to be covered with plastic bags for parts of the day, though this was found to have a negligible effect on the data.
To find out what happened when VES last tested on the bridge, look at the previous blog post Structural Testing of a Heritage Railway Bridge