As part of the project, an experimental 20m long bridge, made entirely from fibre-reinforced polymer (FRP) sections, was constructed in the University of Warwick's structures lab. This bridge was designed to be lightweight and flexible in order to investigate both the future potential for FRP bridges and human-structure interaction problems.

Figure 1: Experimental FRP bridge

Bridge details

The bridge has a total length of 19.8m and width of 2.35m. Its constructed of a bolted truss, 0.627m in depth.

The truss chords and transverse beams are made of FRP 152x41x6.4mm channel sections. The vertical and diagonal elements are FRP 50.8x6.4mm boxes. The deck is 2.1m wide using FRP SafPlank product.

The bolts are stainless steel, with Nyloc nuts to reduce loosening of the connection due to vibrations.

The bridge sits on steel bearings and rollers to give a low friction and controllable boundary condition. Additionally, load cells are built into the supports to measure vertical reaction forces during testing.

The shallow depth of the truss, combined with low total mass (around 1400kg) results in a very flexible structure that is easy to excite from normal service conditions.

Finally, the supports can be moved to give span lengths between 14 and 19.6m, and therefore change the natural frequency of the structure.

Modal properties

The modal properties were both predicted using finite element during the design, and then measured when the bridge was completed. The first vertical bending mode was measured at 2.53Hz with a damping ratio of 0.94%. The first torsional mode was at 3.35Hz with damping ratio of 0.78%. Both these modes had modal masses under 700kg, resulting in very lively behaviour that is easily excited by typical pedestrian activities. Figure 2 shows the experimentally measured mode shapes.

Figure 2: Mode shapes

Response measurements

Figure 3 shows the measured acceleration as a single person walks at a set rate. As can be seen, at the faster rates large accelerations are caused, up to 6m/s² when walking at the resonant frequency. However, even slower walking causes noticeable levels of vibration, making this bridge perfect for considering human structure interaction problems.

Figure 3: Mid span acceleration at different pacing rates

Gallery

Pictures of the bridge.

Publications

• Russell, J., Mottram, J. T., Zivanovic, S. and Wei, X. (2019) Design and Performance of a Bespoke Lively FRP Footbridge. IMAC-XXXVII, Orlando, Florida, 28 - 31 January.
• Zivanovic, S. , Russell, J. and Racic, V. (2019) Vibration Performance of a Lightweight FRP Footbridge under Dynamic Excitation by Pedestrians. IMAC-XXXVII, Orlando, Florida, 28 - 31 January.