Abstract
With the main focus on safety, design of structures for vibration serviceability is often overlooked or mismanaged, resulting in some high profile structures failing publicly to perform adequately under human dynamic loading due to walking, running or jumping. A standard tool to inform better design, prove fitness for purpose before entering service and design retrofits is modal testing, a procedure that typically involves acceleration measurements using an array of wired sensors and force generation using a mechanical shaker. A critical but often overlooked aspect is using input (force) to output (response) relationships to enable estimation of modal mass, which is a key parameter directly controlling vibration levels in service.This paper describes the use of wireless inertial measurement units (IMUs), designed for biomechanics motion capture applications, for the modal testing of a 109m footbridge. IMUs were first used for an output-only vibration survey to identify mode frequencies, shapes and damping ratios, then for simultaneous measurement of body accelerations of a human subject jumping to excite specific vibrations modes and build up bridge deck accelerations at the jumping location. Using the mode shapes and the vertical acceleration data from a suitable body landmark scaled by body mass, thus providing jumping force data, it was possible to create frequency response functions and estimate modal masses.The modal mass estimates for this bridge were checked against estimates obtained using an instrumented hammer and known mass distributions, showing consistency among the experimental estimates. Finally, the method was used in an applied research application on a short span footbridge where the benefits of logistical and operational simplicity afforded by the highly portable and easy to use IMUs proved extremely useful for an efficient evaluation of vibration serviceability, including estimation of modal masses.
Highlights
Introduction and objectivesWith the main focus on safety, design of structures for vibration serviceability is often overlooked or mismanaged, resulting in some classic and public failures [1,2] to perform adequately under human dynamic loads due to walking, running or jumping.Designing for, assessing and improving vibration serviceability of footbridges for human dynamic loading requires reliable estimates of modal parameters, which may be obtained from numerical simulation or from full-scale testing
Provided the point where the inertial measurement units (IMUs) is attached to the body is such that it accurately captures the acceleration of the centre of mass, the simple product of body mass and IMU vertical acceleration can apparently provide a reliable estimate of the ground reaction force (GRF)
The paper describes the use of inertial measurement units (IMUs) for motion capture to identify ground reaction forces (GRFs) as well as for ambient vibration testing of footbridges in field conditions
Summary
With the main focus on safety, design of structures for vibration serviceability is often overlooked or mismanaged, resulting in some classic and public failures [1,2] to perform adequately under human dynamic loads due to walking, running or jumping. The numerical modelling route is the only option for a footbridge yet to be built, but for a structure being retrofitted (e.g. with a tuned mass damper) or that has been completed but not yet opened to the public, the most reliable values of modal parameters are recovered by in-situ testing. This approach was adopted for vibration serviceability assessment of Singapore's Helix Bridge [3] where modal parameters from a full-scale test using a pair of shakers and an array of wired accelerometers were used to create a modal model. It is believed the methodology opens up new possibilities for such studies through in-situ field testing
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