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.

Detecting dynamic displacement of Tamar Bridge using remote sensors

Displacement measurement under dynamic excitations enables the evaluation of bridge performance and serviceability. However, measuring the dynamic displacement has been a challenging task, since conventional sensors are unfeasible. Recently, the emerging non-contact sensing techniques such as global positioning system (GPS), robotic total station (RTS), laser Doppler vibrometer (LDV) and vision-based system provide potential alternatives in the dynamic displacement monitoring of bridges.

VES members undertook a dynamic displacement monitoring test on the Tamar Bridge, using remote sensors. Tamar Bridge is a suspension bridge in southwest England, which spans the River Tamar between the City of Plymouth on the east bank, and the town of Saltash on the west bank.

01 pic of tamar bridge

This test investigated the feasibility and accuracy of three kinds of remote sensors, including GPS, robotic total station and the vision-based system. Back in July, VES members performed a similar test on Humber Bridge, and the information can be found in the following link. (https://veswordpresscom.wordpress.com/2015/09/23/mid-span-deflection-monitoring-of-humber-bridge-using-cameras-2/)

Compared with Humber Bridge, Tamar Bridge is shorter in span, and undergoes less movement, so it is more challenging to track movement with conventional sensors such as GPS and total station.

During the test at Tamar Bridge, a prefabricated target frame with a pattern of concentric circles was mounted on the hand rails at mid-span, together with GPS rover receiver and a circular prism. About 380 metres away, near the Tamar Bridge Office, a camera tracked the two dimensional movement of the target frame, and a robotic total station recorded the three dimensional coordinates of the prism reflector. The bridge was subjected to ambient excitations such as wind, vehicular loading and thermal effects.

The following two figures show 70 second signals of displacement in the temporal and frequency domains. Camera and GPS sensors obtained similar displacement time histories under traffic loads, but the camera measurement captured more dynamic components of the bridge motion.

EPSRC-funded doctoral research studentships

The Vibration Engineering Section at the University of Exeter is pleased to announce that it has two new EPSRC PhD studentships available.

The PhDs will be supervised by Professor Alex Pavic and Professor James Brownjohn. Dr Prakash Kripakaran will be a secondary supervisor on the first and Dr Victoria Goodwin on the second.

The deadline for applications is Monday, 4th January 2016. 

To find out more about the opportunities, please click on the links below:

EPSRC funded PhD in Engineering: Quantifying Resilience of the Built Environment to Extreme Weather Events Through Direct Field Measurement of Environmental Conditions and Structural Impacts Ref: 2046

EPSRC funded PhD in Engineering: Novel data driven approach using wearable sensors and personalised physics based models of human body motion to continuously monitor and mitigate risk of falls in real environments Ref: 2049