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NEWSLETTERDynamic Testing of the London Millenium Bridge |
Fugro Structural Monitoring (FSM) featured strongly in a talk on the above subject by Tim Armitage of Ove Arup and Partners.
Tim described how FSM were contracted by Arup to provide instrumentation and testing services during trials of prototype dampers on the London Millennium Footbridge.
Arup had devised an innovative damping system to alleviate strong motions observed when very large numbers of pedestrians used the bridge.
Instrumentation
To quantify dynamic characteristics with different levels on damping on the bridge, FSM installed a grid of accelerometers orientated in both lateral and vertical directions.
In addition, load and displacement measurements were made on the prototype dampers, and environmental conditions were monitored. Data acquisition was performed using the FSM software package, SiMS-NT. SiMS-NT allows high-speed data acquisition simultaneously from a large number of sensors. It includes a range of analysis modules including frequency analysis, statistical analysis, fatigue cycle counting etc.
4kN Shaker
A unique feature of the project was a shaker capable of inducing a 4kN lateral cyclic load into the bridge. The shaker, designed and built by FSM in 6 weeks, comprised a 1 tonne mass which was finished and pulled along a 2m track by a hydraulic actuator. The shaker was used to quantify the modal properties of the bridge (natural frequencies, damping, mass and stiffness). FSM worked with the University of Sheffield who performed modal analysis using
Pedestrian Tests
A major part of the bridge testing was a series of controlled pedestrian tests. Arup devised and controlled a series of tests in which up to 300 people went through a sequence of walking routines in groups of 25 to quantify the interaction between pedestrians and the structure. The accelerometer signals were collected throughout the entire test sequence.
Outcome
Tim Armitage concluded by stating that the testing verified that the structural damping solution designed by Arup will cure the bridge motions. The FSM instrumentation quantified the engineering parameters with a high degree of reliability and accuracy. The specially built shaker was just one unique feature of this fascinating and innovative project.