Dynamics of pedestrian timber bridges
Experimental and numerical analyses at various stages of Construction
Jens Bergenudd at the Department of Civil and Architectural Engineering will publicly defend his doctoral thesis on 15 December 2025.
What is the topic of your Doctoral Thesis?
Pedestrian bridges with resonance frequencies close to the pedestrian pacing frequencies can be susceptible to uncomfortable vibrations. These bridges therefore require accurate numerical and/or analytical predictions in the design phase to ensure that the comfort criteria are fulfilled. Previous research studies have shown the difficulty in creating accurate numerical models without experimental data. My thesis therefore investigates the dynamics of pedestrian timber bridges at various construction stages to discern the effects of different structural parts on the bridge dynamics. The main objectives were to:
- Perform in situ dynamic tests on five pedestrian timber bridges at different construction stages
- Evaluate the influence of different structural parts on the dynamic properties between the stages, i.e. the mode shapes, resonance frequencies and damping ratios
- Implement numerical models in the finite element (FE) software Abaqus and calibrate the models at each construction stage
Hopefully this will increase the knowledge regarding accurate modeling of different parts for pedestrian timber bridges.
Why did you choose this topic?
Increasing the construction of pedestrian timber bridges is an important step to create a more sustainable future and reach the EU goal of net zero carbon emissions by 2050. The possibility of prefabrication simplifies the assembly on site and ensures more time efficient construction. Lighter transports to the construction site due to the low weight of timber reduce carbon emissions. Furthermore, the research project involves detailed numerical modelling of the bridges and expertise from my supervisors Jean-Marc Battini and Roberto Crocetti, which has been an invaluable experience for my future career.
What are the most important results?
The experimental results showed that:
- The resonance frequencies generally increased at cold compared to warm conditions. This was attributed to several factors, such as the viscoelastic behavior of the asphalt layers where the asphalt stiffness increases at cold temperatures.
- The damping ratios, which measure the energy dissipation of structures, typically increased with an asphalt layer on the bridges due to absorbed strain energy and/or friction at the interface between the asphalt and timber deck. Especially for modes of vibration with large deformations of the asphalt layer, e.g. the first lateral modes increased from around 1 % to 2-4 %. These damping ratios were in many cases significantly higher than the values recommended by technical guidelines.
The most important factors in the numerical models of the five bridges to ensure calibrated results compared to the experimental results were the:
- Longitudinal stiffness at the supports for a girder and truss bridge
- Connection stiffnesses between the timber members for most bridges
- Stiffness of the pile foundations at the intermediate supports for a truss bridge
- Partial composite action of the mechanically connected arch segments for an arch bridge
- Reduced axial stiffness of the stays for a cable-stayed bridge
- Implementation of the asphalt stiffness at cold temperatures
The main conclusion is that detailed modelling of certain structural parts was necessary to achieve calibrated results for the bridges.
Did you come across something unexpected during your thesis research?
The densities of the timber species Norway spruce and Scots pine were estimated from experimental data and previous research. These values were unexpectedly high compared to the technical guidelines.
The damping ratios for the first lateral modes could increase to as much as 3-4 % with an asphalt layer on the bridge, which is significantly higher than the values of 1-1.5 % in the technical guidelines. There is undoubtedly potential for updated values in the guidelines for pedestrian timber bridges with and without an asphalt layer.
Who will benefit from your results? What kind the impact may it have on surrounding society?
The research project is funded by the Swedish Transport Administration (Trafikverket) and the J. Gustaf Richert foundation as a further development of our infrastructure and built environment. The project will hopefully increase the construction of pedestrian timber bridges which will benefit the future sustainability of our infrastructure.
What will you do next/where can one reach you?
The results from this doctoral thesis will hopefully generate new ideas for other research projects. After my PhD defense on the 15th of December, I will continue my career at the bridge division at WSP in Stockholm. I will gladly respond to any questions regarding my work in the future.