Modal testing and analysis of reconfigurable test-bed structure in the Structures Lab, University of Exeter

A reconfigurable test-bed structure in the Structures Lab at the University of Exeter has been designed to simulate and evaluate the dynamics of different types of civil structures subjected to pedestrian loading; therefore, it can be reconfigured into different sized floor panels and footbridges. Currently, the VES team is working on layout B, which is a 15m span by 2.5m wide layout suitable for studies into human-induced response and vibration control technologies applicable to relatively low frequency footbridge structures.

To measure the structural response, accelerometers and other transducers have been set up on the structure to provide signals that can be measured using a digital data acquisition system. In addition, modal testing has been carried out, by using a dynamic shaker to input a random signal to vibrate the structure whilst the responses were simultaneously measured over a prespecified test grid. The modal testing set up is shown in the following pictures.

After the modal testing, the structure’s dynamic properties were derived from curve fitting, including natural frequencies, damping ratios, modal masses and mode shapes. The first five mode shapes are shown below:

1st mode (f = 3.81 Hz, z = 1.01%)                     2nd mode (f = 5.14 Hz, z = 0.87 %)

3rd mode (f = 8.49 Hz, z = 0.88%)             4th mode (f = 12.37 Hz, z = 0.78 %)

Mode shape 5

5th mode (f = 18.61 Hz, z = 0.51 %)

The first mode is a pure bending mode. The second and third modes are torsional modes. The fourth and fifth modes are also higher order bending modes. These results are based on the vertical set up of the excitation source, since this is the direction of most relevance for this structure.

The structure will be used for further studies into human-structure interaction, human-induced vibration response and advanced technologies for vibration control.

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