Pedestrian Induced Vibrations on Lightweight Bridges

Project Summary

Vibration serviceability is often the governing design criteria for slender, lightweight footbridges under pedestrian induced walking loads. Serviceability issues related to lightweight pedestrian bridges have attracted a lot of attention in the literature since the London Millennium Bridge incident, which involved large amplitude vibrations during its inauguration. Despite numerous studies into modeling walking-induced dynamic loading, the models remain relatively simplistic and lack thorough validation from experimental investigations. The main goal of the project is to develop a comprehensive experimental database of walking tests under different traffic scenarios on aluminum footbridges and subsequently investigate the pedestrian induced load models adopted by different design provisions. Moreover, this project aims to improve the existing design provisions and subsequently propose simple design tools for better prediction of pedestrian induced vibration of lively footbridges.

Experimental Testing

The experimental campaign involved testing several full-scale aluminum footbridges, ranging in spans from 3 m to 23 m, in the Structures laboratory at UW. These bridges were assembled from a patented bridge product called Make-A-Bridge® by MAADI Group. There are six main components to the Make-Abridge design, which are all fabricated from extruded T6061 aluminum through updated assembly in patent CA 268813 (Figure 1): top chord, bottom chord, diagonal, transversal, deck stringers, and decking. Mainly two spans were tested: 22.9 m and 12.2 m, which were instrumented with accelerometers, triaxial load cells and displacement transducers (Figure 2). The measurements were acquired using A/D data acquisition modules manufactured by Data Translation. Single and multiple pedestrians walking tests were conducted on these bridges (Figure 3).

Bridge Section
Figure 1: Basic assembly of Make-A-Bridge® specimen

Bridge Testing
Figure 2: (a) Modular aluminum bridge of span 12.2 m, and (c) Modular aluminum bridge of span 22.9 m, (c) Plan view of the bridge of length 22.9 m with locations for the instrumentation showing (d) accelerometers, (e) vibration data collection system (f) load cells, (g) displacement transducer

Pedestrian Loading
Figure 3: (a) One pedestrian and (b) groups of pedestrians walking on the 22.9 m bridge specimen during the experimental study carried out in the Structures Laboratory at the University of Waterloo

Experimental Results

On investigating the existing walking models and design provisions, significant disagreement was observed in predicted and measured responses (for example, see Figure 4 (a) or refer to [1] and [2]). Moreover the comparison study points to a need to harmonize the various guidelines and standards. Based on the experimental observations, several recommendations are being proposed to substantially improve these existing design methodologies (see Figure 4 (b) or refer to [2]). Besides, evaluation of the design provisions in a reliability-based framework points towards calibrating the provisions for higher reliability levels to achieve sufficient and uniform reliability indices under all possible traffic events (see Figure 5 or refer to [3]). Additionally, structural modifications to enhance the vibration performance are also being conducted.

Bridge Testing
Figure 4: (a) comparison of (a) measured and predicted peak accelerations, and (b) measured and recommended peak accelerations along with the comfort limits as proposed by guidelines in the lateral direction, where P stands for pedestrians (refer, [2]).

Pedestrian Loading
Figure 5: Minimum reliability levels (β) under different traffic events: (a) ISO 10137, (b) Eurocode 5, (c) British National Annex (group), (d) British National Annex (crowd), and (e) SETRA (red markers correspond to the design traffic of the particular PB class) (refer, [3])